# Energycount — full text (tools, reference, guides & news) > UK energy & sustainability consultants (est. 2006, Poole, Dorset) and the free "Monkey" building-compliance calculators. Index: https://energycount.co.uk/llms.txt · Reference figures: https://energycount.co.uk/reference/ · Datasets: https://energycount.co.uk/data/ · Tools: https://energycount.co.uk/tools/ · Pricing: https://energycount.co.uk/pricing/ ## TOOLS & PRICING --- # U-Monkey — the free U-value calculator URL: https://energycount.co.uk/tools/u-value-calculator/ A free, browser-based U-value calculator with no sign-up and no account. Calculates to BS EN ISO 6946 using BR 443 conventions, with ISO 13370 for ground floors, and shows the U-value live as layers are added or changed. Elements covered: external, party and internal walls; roofs with insulation at rafter or at ceiling; flat roofs; exposed floors; solid ground floors (ISO 13370); and suspended ground floors. What it gives you free: the live U-value, a self-drawing cross-section of the build-up, pass/fail against the Part L limiting value and the notional dwelling value for the chosen nation, edition and type of work (England: the 2021 edition or the 2026 edition, i.e. the Future Homes Standard, in force 24 March 2027; its notional fabric for walls, roofs and floors is the same as 2021's — 0.18, 0.11 and 0.13 per SAP 10.3 Appendix R — so they show as one Notional / FHS check; Wales 2022 and 2026, Scotland, Northern Ireland and Ireland are built in), a 480+ item neutral database of real UK materials and products (no manufacturer favourites), and a Building Control-ready PDF report with the full layer breakdown, method references and a unique report ID. The PDF is free — there is no paywall on a U-value report. It also includes an interstitial condensation-risk screening to BS 5250 / BS EN ISO 13788 (the monthly Glaser method) built into the same build-up, so a wall that passes on U-value but fails on condensation shows up in one place. When a build-up does not pass, its "How to fix it" panel re-runs the usual remedies through the whole check (a vapour control layer added, upgraded or moved; a vapour-open underlay; a ventilated void under a roof underlay; a cold flat roof turned into a warm deck) and lists only those that pass, each with the rating it needs, such as a VCL of at least so many metres sd; where the remedy is a change of layers, one tap applies it to the build-up, with Undo. This is a screening to ISO 13788; it is NOT full or transient hygrothermal modelling to BS EN 15026, and Energycount does not offer that. It also calculates the thermal mass of every build-up to BS EN ISO 13786 (24-hour periodic heat-transfer matrix): areal heat capacity kappa in kJ/m2K (the SAP 10 thermal mass parameter input, in place of the Table 1e defaults of 100/250/450), decrement factor and time lag in hours. The figures print on the free PDF. Density and specific heat come from the material database with sources; U-Monkey does not estimate missing values. This is the ISO 13786 periodic method for comparing build-ups and for the SAP input, not a Part O dynamic overheating simulation (CIBSE TM59), which Energycount offers as a service. Paid extra: Monkey Pro (£10/month +VAT) adds project, client and revision references and your practice branding to the report, and in U-Monkey a calculation record page, a before-and-after report for upgrades, Home Energy Model (HEM) JSON export and one combined PDF per project. There is no separate per-report branding purchase. A standalone paid condensation report is £25 +VAT (the same on Monkey Pro; included on Pro Unlimited). Accuracy evidence: /tools/u-value-calculator/validation/ publishes U-Monkey's reference cases with the expected value, its source, the tolerance and the result for each, computed from the shipping engine when the site is built. Three rest on published figures (the Scottish Government's combined-method example 6.B.2, a timber-frame wall; the BS EN ISO 13370 suspended-floor model in Pelsmakers, Croxford and Elwell, Building Research & Information; and the 50 mm low-emissivity cavity, R 0.665, in the BBA certificate for Kingspan Thermawall TW50, whose foil is certified at emissivity 0.05); the rest are hand calculations written out in full, covering bridged layers, plasterboard on dabs, a ceiling-level loft with its roof space (BR 443 §8), low-emissivity, slightly and well-ventilated cavities, the ISO 6946 Annex F corrections (air gaps, wall ties, inverted roofs), slab-on-ground floors and a closed-form ISO 13786 thermal-mass limit. The condensation check is not on that page because no published ISO 13788 worked example is cited for it. Worked build-ups: 108 pages, one per common UK build-up (walls, pitched and flat roofs, floors), each with its typical U-value, a table of variants, what moves the number, what U-Monkey's condensation check shows and a link that opens the calculator pre-filled; they are listed with their typical U-values in /llms.txt and printed in full below under U-VALUE BUILD-UPS. Related: /tools/u-value-calculator/how-to-use/ (full tutorial), /tools/u-value-calculator/validation/ (how U-Monkey is tested), /tools/u-value-calculator/compare/ (how U-Monkey compares with the other kinds of U-value calculator), /tools/u-value-calculator/add-to-your-website/ (a free embeddable U-value box for other websites, generic materials, same engine), /tools/u-value-calculator/condensation-risk-calculator/, /tools/u-value-calculator/thermal-mass-calculator/ (decrement factor, time lag, kappa — ten UK build-ups compared), /tools/u-value-calculator/what-u-value-do-i-need/, and /tools/u-value-calculator/demo/ (28-second silent screencast with a written second-by-second description). Compliance regimes: the verdict can be judged against England (Approved Document L Vol 1 2021 / Vol 2 2021), Wales (AD L Vol 1 and Vol 2, 2022 editions, or the 2026 versions in force 4 March 2027), England's 2026 edition (the Future Homes Standard, in force 24 March 2027), Scotland (Section 6 of the Domestic or Non-domestic Technical Handbook, January 2025), Northern Ireland (Technical Booklets F1 and F2: 2022) or the Republic of Ireland (Technical Guidance Document L 2022, Dwellings and Buildings other than Dwellings) for dwellings or non-domestic buildings; ?rg=_ pre-selects it (e.g. /tools/u-value-calculator/app/?el=wall&rg=scotland_dwelling). Detail pages: /tools/u-value-calculator/scotland/, /tools/u-value-calculator/wales/, /tools/u-value-calculator/northern-ireland/, /tools/u-value-calculator/non-domestic/, /tools/u-value-calculator/ireland/. Elements: walls, pitched roofs (rafter or ceiling), flat roofs (with tapered and inverted-roof options), exposed floors, ground and suspended floors and heated basement walls and floors (BS EN ISO 13370), and party walls (U assigned by construction per SAP 10.2 Table 3.10: 0.5 / 0.2 / 0.0). BS EN ISO 6946 Annex F corrections for air gaps, mechanical fixings (wall ties, EWI fixings) and inverted roofs are applied and itemised; products whose declared conductivity depends on thickness switch band automatically. --- # ΨMonkey — the free psi value calculator URL: https://energycount.co.uk/tools/psi-calculator/ A free 2D thermal-bridge (psi value) calculator running in the browser. Numerical solution to BS EN ISO 10211 for linear thermal transmittance (Ψ) and the internal surface temperature factor f_Rsi, for use in SAP 10 and the Home Energy Model. 38 ready-made standard SAP junctions are built in: the E-series E1–E25 (external wall junctions), the party-wall P-series P1–P8, and the roof-window R-series R1–R4. A build-up assistant turns a layer list into a modelled junction on the grid, and Ψ and f_Rsi update live as thicknesses and materials change. You can also draw a junction by hand on the modelling canvas and set surface and edge boundary conditions. Accuracy evidence: the solver is validated against the BS EN ISO 10211 test reference cases and the results are published at /tools/psi-calculator/validation/ — the strongest trust asset on the site. Using calculated Ψ-values instead of SAP default y-values is usually the cheapest route to a Part L pass, because the defaults are deliberately punitive. Free: unlimited modelling, unlimited solving, Ψ and f_Rsi on screen. Paid: a Building Control-ready PDF report for ONE junction, £25 +VAT (the same on Monkey Pro; included on Pro Unlimited). No subscription is required — you can pay for a single junction. Related: /tools/psi-calculator/validation/, /tools/psi-calculator/guide/ (user guide), /tools/psi-calculator/free-alternative-to-autopsi/, and /tools/psi-calculator/demo/ (22-second silent screencast with a written second-by-second description). --- # WaterMonkey — the free Part G water calculator URL: https://energycount.co.uk/tools/part-g-water-calculator/ A free water efficiency calculator for Part G of the Building Regulations (Requirement G2 / regulation 36), running in the browser with no sign-up. It implements the official water efficiency calculation methodology for new dwellings and shows a live PASS/FAIL against either the 125 litres per person per day Building Regulations standard or the tighter 110 l/p/d optional requirement imposed by many planning conditions in England, or the Welsh requirement: 110 l/p/d for a newly built dwelling, 125 for one formed by a conversion. Inputs are the fittings schedule: WC flush volumes (single, dual or multiple), tap flow rates, bath capacity to overflow, shower flow rate, kitchen and utility sink taps, washing machine litres per kg dry load, dishwasher litres per place setting, plus optional waste disposal unit and water softener, and greywater recycling or rainwater harvesting where installed. Quick presets fill an entire dwelling type in one tap — typical 125 l/p/d, tight 110 l/p/d, very tight 90 l/p/d, or AD G default appliances — and the total recalculates instantly. Free: the full methodology and the live result, for as many dwelling types as you like. Paid: a Building Control-ready PDF report, £25 +VAT for the first dwelling type plus £15 +VAT for each additional type in the same report — so a scheme with several house types is one fee, not several. Note on London: a 105 l/p/d planning condition that excludes external water use is the same target as 110 l/p/d including it. Related: /tools/part-g-water-calculator/110-litres/, /tools/part-g-water-calculator/water-efficiency-calculator-spreadsheet/, /tools/part-g-water-calculator/shower-flow-rate/, and /tools/part-g-water-calculator/demo/ (28-second silent screencast with a written second-by-second description). --- # SoakMonkey — the BRE 365 soakaway calculator URL: https://energycount.co.uk/tools/soakaway-calculator/ A free soakaway sizing calculator working to BRE Digest 365 (Soakaway Design), the method Building Control and most LLFAs expect. Runs in the browser, no sign-up. It takes the percolation test results (trial pit length, width and effective storage depth, plus the time for the water level to fall from 75% to 25% effective depth on each of three fills), computes the infiltration rate f for each test and adopts the lowest for design, exactly as BRE 365 requires. It then runs the storm duration sweep against FSR rainfall data — impermeable area drained, rainfall ratio r, M5-60 rainfall, return period and a climate change uplift on rainfall — and reports the required storage volume, the half-empty (half-drain) time, and a DESIGN OK / not OK verdict. Four construction types are supported: filled pit, modular crate, concrete ring and trench, each dimensioned for the storage required. Free: the whole calculation and the sizing on screen, unlimited. Paid: a Building Control-ready PDF report, £25 +VAT. Related: /tools/soakaway-calculator/what-size-soakaway-do-i-need/, /tools/soakaway-calculator/percolation-test/, /tools/soakaway-calculator/bre-digest-365/, /tools/soakaway-calculator/soakaway-for-an-extension/, and /tools/soakaway-calculator/demo/ (17-second silent screencast with a written second-by-second description). --- # NutrientMonkey — the nutrient neutrality calculator URL: https://energycount.co.uk/tools/nutrient-neutrality-calculator/ A free nutrient budget calculator for residential development in nutrient neutrality catchments, implementing Natural England's nutrient budget methodology in the browser. All 27 Natural England catchments are built in, for nitrogen and for phosphorus, with NE's published default concentrations and the ×1.2 precautionary buffer. It builds the budget the way the planning application needs it: waste water from the new dwellings (number of dwellings, occupancy, water efficiency standard, drainage route and the receiving works' current and post-2030 total nitrogen permit), current land use, future land use, and any SuDS removal — then reports the pre-2030 and post-2030 budgets and the permanent credit requirement in kg per year, with a plain statement of how many credits are needed and how they can be secured (permanent credits, or temporary credits to 2030 plus permanent credits for the enduring baseline). Land-use export coefficients are catchment-, soil-, rainfall- and NVZ-specific; the tool says so on the page and asks you to confirm against the official NE calculator for your site. Free: the whole calculation on screen. Paid: a planning-ready PDF assessment, £79 +VAT. Related: catchment pages under /tools/nutrient-neutrality-calculator/, and /tools/nutrient-neutrality-calculator/demo/ (23-second silent screencast with a written second-by-second description). --- # Solar PV calculator — requirement L3 of the Future Homes Standard URL: https://energycount.co.uk/tools/solar-pv-calculator/ A free calculator for requirement L3 of Approved Document L 2026 (England, from 24 March 2027): the minimum on-site renewable electricity for a new dwelling. Runs in the browser, no sign-up. The rule (AD L1 paragraph 5.73(a)): the array's annual output, calculated in SAP 10.3, must at least equal that of a fixed benchmark array — 0.22 kWp/m² over 40% of the ground floor area (equation 5.1), orientated south-east to south-west, pitched at 45°, not overshaded. SAP 10.3 runs Part L compliance on UK-average weather, so the target is about 70.7 kWh a year per m² of ground floor (3,181 kWh for a 45 m² ground floor) and does not change with the actual roof or the region. The actual array must reach it: at a 35° pitch a south-facing slope needs about 93% of the benchmark kWp, east or west about 113%, north about 148%. Also covered: buildings containing flats (equations 5.2 and 5.3 — the whole-building and per-dwelling routes), the reasonably practicable roof area route (5.73(b) / 5.74(c)) with the Appendix B offsets, the 720 kWh a year exceptional-circumstances floor (720 ÷ storeys for each flat), and the open question of whether "south-east to south-west" is read as SE/SW (803 kWh a year per kWp) or due south (854). The BREL report from the approved SAP software is the final word. Related: /tools/solar-pv-calculator/how-many-solar-panels/ (panels needed by ground floor area and roof direction), /tools/solar-pv-calculator/flats/ (equations 5.2 and 5.3 worked through for three blocks), /tools/solar-pv-calculator/reasonably-practicable-roof-area/ (the Appendix B offsets and a panel-fit calculator for each roof slope), /tools/solar-pv-calculator/building-control-evidence/ (what Appendix B paragraph B9 asks for), /news/adl-2026-requirement-l3-solar-pv-faq/ (MHCLG's FAQ on requirement L3, explained), /guides/future-homes-standard/, /guides/solar-pv/. --- # Pricing — every calculator free, pay per report, or Monkey Pro URL: https://energycount.co.uk/pricing/ Every Energycount calculator is free to use in full, with no sign-up and no account. You pay only when you want a Building Control-ready PDF report, or for a subscription that brands your reports and cuts the report price. Pay-per-report list prices (all +VAT): - U-value report (U-Monkey) — free, on every tier. - Condensation risk report — £25 (the same on Monkey Pro; included on Pro Unlimited). - Thermal bridge / psi report (ΨMonkey) — £25 per junction (the same on Monkey Pro; included on Pro Unlimited). - Part G water calculation report (WaterMonkey) — £25, plus £15 for each additional dwelling type in the same report. Same price on every tier. - Soakaway report (SoakMonkey) — £25. Same price on every tier. - Nutrient neutrality assessment (NutrientMonkey) — £79. Same price on every tier. - Branding on a single U-value report without a subscription — £7, a one-off credit against your account. Subscriptions (+VAT): - Monkey Pro — £10/month or £100/year (two months free). Your logo and practice details on every report from every tool, project and client references on the page, no Energycount marketing; in U-Monkey also a calculation record page, a before-and-after report, HEM export and a combined project PDF. It changes no report's price. - Monkey Pro Unlimited — £49/month or £490/year (two months free). Everything in Monkey Pro, with thermal bridge and condensation reports included outright, however many you produce, plus the multi-junction schedule export in ΨMonkey. Part G, soakaway, nutrient, extension and renovation reports are charged at list price on every tier; only the branding changes. Two thermal bridge or condensation reports in a month and Pro Unlimited is already cheaper than paying per report. For comparison, AutoPSI's published pricing (June 2026) was £59/month or £649/year ex VAT for a single Professional thermal-bridging licence; Pro Unlimited is £49/month and covers every Energycount tool, and you can skip it entirely and pay £25 for the one junction you need. Reports carry a unique report ID that can be checked at /verify/. ## U-VALUE BUILD-UPS — 108 WORKED PAGES Each figure is for the build-up named, worked to BS EN ISO 6946 with BR 443 conventions (ground floors and basements to BS EN ISO 13370) exactly as the pre-filled calculator works it out; pass or fail is judged on the unrounded U-value. Link to the page rather than quoting a figure for a different build-up. --- # U-value of a cavity wall with full-fill insulation URL: https://energycount.co.uk/tools/u-value-calculator/cavity-wall-full-fill/ Typical U-value: 0.21 W/m²K — 150mm full-fill mineral wool, standard aircrete A brick-and-block cavity wall with 100mm full-fill mineral wool achieves a U-value of about 0.28 W/m²K — short of even the 0.26 limiting value, let alone the 0.18 W/m²K new-build notional target. That is why modern specifications use 150mm cavities (≈0.21), and reaching 0.18 takes enhanced batts (around λ 0.032) with thermal-grade blocks. Open the build-up below pre-filled and tune it to your own spec. Build-up, outside to inside: - 102.5mm brick outer leaf - 100–150mm full-fill mineral wool cavity batt - 100mm aircrete block inner leaf (mortar joints counted) - Plasterboard on dabs | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm full-fill, standard aircrete | ≈ 0.28 | Pre-2021 volume spec — misses even the 0.26 limit | | 150mm full-fill, standard aircrete | ≈ 0.21 | Passes the 0.26 limit; short of the 0.18 notional | | 150mm full-fill, thermal aircrete (λ0.11) | ≈ 0.20 | Thermal blocks alone barely move it | | 150mm λ0.032 batts, thermal aircrete (λ0.11) | ≈ 0.18 | Current new-build notional spec — inside the 0.18 (0.176) | What changes the number: The batt lambda (0.038 generic vs 0.032–0.035 enhanced), the block grade (dense 1.13 → aircrete 0.11 is a huge swing), and the cavity width do almost all the work. Mortar joints in the blockwork, the plaster dabs behind the plasterboard and wall ties nibble a little more — U-Monkey counts the mortar joints and the dabs for you, as BR 443 requires, and offers to add the ties. Q: Do I need a 150mm cavity to pass Part L? A: Broadly, yes for full-fill: 100mm of mineral wool comes out around 0.28 W/m²K, which misses even the 0.26 limiting value, while the notional dwelling assumes 0.18. A 150mm cavity clears the limit; reaching 0.18 takes enhanced batts (around λ 0.032) with thermal-grade blocks — or partial-fill PIR. --- # U-value of a cavity wall with PIR partial-fill insulation URL: https://energycount.co.uk/tools/u-value-calculator/cavity-wall-partial-fill-pir/ Typical U-value: 0.17 W/m²K — 100mm PIR + 40mm clear cavity A partial-fill cavity wall with 100mm PIR and a 100mm aircrete inner leaf achieves a U-value of about 0.17 W/m²K, inside the 0.18 notional value of Part L 2021 and the Future Homes Standard. 90mm shows as 0.18 but misses it once the mortar joints in the blockwork and the plaster dabs are counted, at 0.185, and the older 60mm-PIR spec manages about 0.24. Open it pre-filled below and adjust the board and blocks to your spec. Build-up, outside to inside: - 102.5mm brick outer leaf - ~40mm residual (clear) cavity - 60–100mm PIR partial-fill board - 100mm aircrete block inner leaf (mortar joints counted) - Plasterboard on dabs | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 60mm PIR + 40mm clear cavity | ≈ 0.24 | Common 2013-era spec | | 90mm PIR + 40mm clear cavity | ≈ 0.18 | Just misses the 0.18 notional once the mortar joints and dabs are counted (0.185) | | 100mm PIR + 40mm clear cavity | ≈ 0.17 | Current new-build notional spec | What changes the number: Board thickness and lambda dominate; keeping the residual cavity clean matters for both thermal performance and moisture. Swapping generic PIR (0.022) for phenolic (0.020) buys a little more, and the inner-leaf block grade still matters — as do its mortar joints, which U-Monkey counts for you (BR 443). Q: Partial-fill PIR or full-fill mineral wool — which is better? A: PIR gets more performance per millimetre (λ≈0.022 vs 0.038), so partial-fill hits 0.18 in a slimmer wall. Full-fill mineral wool is cheaper, more forgiving to install and better acoustically. Both are mainstream — model each in U-Monkey and compare like-for-like. --- # U-value of a timber frame wall URL: https://energycount.co.uk/tools/u-value-calculator/timber-frame-wall/ Typical U-value: 0.30 W/m²K — 140mm mineral wool between studs (15% timber) A brick-clad timber frame wall with 140mm mineral wool between studs achieves a U-value of about 0.30 W/m²K once the ~15% timber bridging is properly counted — not the ~0.24 a naive calculation suggests. Adding a 50mm PIR lining board under the plasterboard brings it to about 0.17. U-Monkey does the bridged (combined-method) calculation automatically. Build-up, outside to inside: - 102.5mm brick outer leaf - 50mm drained cavity - 9mm OSB sheathing - 140mm mineral wool between studs (~15% timber) - Polythene VCL (warm side) - Plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 140mm mineral wool between studs only | ≈ 0.30 | Bridging is why it is not ~0.24 | | 140mm + 50mm PIR lining under the plasterboard | ≈ 0.17 | Modern timber-frame spec | What changes the number: The stud fraction is the story: every timber path (λ0.13) short-circuits the wool (λ0.038). A continuous insulated lining breaks the bridge, which is why it is worth more than its thickness suggests. The VCL goes on the warm side — U-Monkey’s free condensation check will show you why. Q: Why is my timber frame U-value worse than the insulation thickness suggests? A: Because roughly 15% of the wall is timber, not insulation, and heat takes the easy path through the studs. The combined method of BS EN ISO 6946 averages the two paths — U-Monkey applies it automatically, which is why its answer differs from a simple resistance-adder. --- # U-value of a solid brick wall with internal insulation URL: https://energycount.co.uk/tools/u-value-calculator/solid-brick-wall-internal-insulation/ Typical U-value: 0.25 W/m²K — 215mm solid brick + 80mm PIR internally A bare 215mm solid brick wall has a U-value of roughly 2.2 W/m²K (2.1 once plastered). Lining it internally with 60mm PIR brings it to about 0.32, just short of the 0.30 renovation target; 70mm clears it at 0.28, and 80mm reaches a comfortable 0.25. Internal insulation moves the dew point, so run the free condensation check before you fix a spec. Build-up, outside to inside: - 215mm solid brick - 60–80mm PIR insulated lining - Plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Bare 215mm solid brick | ≈ 2.2 | The starting point, unplastered as pre-filled (2.1 with 13mm of plaster) | | + 60mm PIR lining | ≈ 0.32 | Just short of the 0.30 renovation target | | + 70mm PIR lining | ≈ 0.28 | Clears the 0.30 target | | + 80mm PIR lining | ≈ 0.25 | Comfortable pass, usual practice | What changes the number: Thickness versus lost room is the trade. The bigger risk is moisture: cold brickwork behind new insulation is exactly where interstitial condensation shows up, which is why U-Monkey’s built-in ISO 13788 screening check matters most on this wall. Airtight detailing at joists and reveals decides whether theory survives site. Q: Is internal wall insulation risky for condensation? A: It can be — insulation makes the brick colder, so vapour reaching it may condense. The screening check built into U-Monkey (ISO 13788, free) flags whether your build-up needs a vapour control layer or a different insulant. For unusual or historic walls, get a full hygrothermal assessment. Note: The pre-filled link opens as a retained solid wall being upgraded, judged against Approved Document L’s 0.30 for internal insulation (2021 edition, Table 4.3). --- # U-value of a one-and-a-half-brick (327.5mm) solid wall URL: https://energycount.co.uk/tools/u-value-calculator/one-and-a-half-brick-wall/ Typical U-value: 1.61 W/m²K — Bare 1½-brick (327.5mm) solid wall + internal plaster A bare one-and-a-half-brick solid wall — 327.5mm of brickwork (a brick’s length, 215mm, plus its width, 102.5mm, and the 10mm joint between them) — has a U-value of about 1.61 W/m²K with 13mm of dense plaster, calculated at BR 443’s conductivity for brickwork. That is far above any current standard, which is why these walls are almost always insulated: an 80mm PIR internal lining reaches about 0.24, and external insulation gets there without losing internal space. Internal insulation cools the brickwork and moves the dew point, so run the free ISO 13788 condensation screening before fixing a spec. Open the build-up below pre-filled and add your lining. Build-up, outside to inside: - 327.5mm solid brickwork (one-and-a-half brick) - 13mm dense plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Bare 1½-brick wall (uninsulated) | ≈ 1.61 | The starting point — needs insulating | | + 80mm PIR internal insulation (IWI) | ≈ 0.24 | The usual retrofit; see the solid-brick + internal insulation page | | + 110mm grey EPS external insulation (EWI) | ≈ 0.24 | Keeps internal space; the brick behind it counts as protected (λ 0.56). See the solid-brick + external insulation page | What changes the number: Almost nothing moves a bare solid wall except its thickness and brick density — soft, low-density bricks conduct a little less but weather worse. The only real fix is insulation, and on a solid wall that is a moisture decision as much as a thermal one: model the lining in U-Monkey and use its free ISO 13788 condensation screening before you commit. For unusual or historic fabric, get a full hygrothermal assessment. Q: What is the U-value of a 327.5mm (one-and-a-half-brick) solid wall? A: Bare, about 1.61 W/m²K with 13mm of dense plaster — 327.5mm is a brick’s length and width plus the joint between them. It needs internal or external insulation to approach the 0.30 renovation target; about 80mm of internal PIR brings a typical wall to around 0.24. Model yours free in U-Monkey and run the condensation check first. --- # U-value of a solid wall with external insulation URL: https://energycount.co.uk/tools/u-value-calculator/solid-brick-wall-external-insulation/ Typical U-value: 0.25 W/m²K — 110mm grey EPS + render on 215mm brick External wall insulation takes a 215mm solid brick wall from roughly 2.1 W/m²K to the 0.30 renovation target with 90mm of grey EPS, at 0.296 (shown as 0.30); 100mm gives about 0.27 and 110mm about 0.25, while 80mm, at 0.33, falls short. Behind the insulation the brick is taken at its protected λ, 0.56, as BR 443 directs once a wall has external insulation, rather than the 0.77 of an exposed wall — at 0.77 the same 90mm would just miss, at 0.303. EWI keeps the brick warm (kind to the wall and the dew point) at the cost of external appearance and detailing at eaves and openings. Build-up, outside to inside: - Thin-coat render - 90–110mm grey EPS - 215mm solid brick (protected by the insulation: λ 0.56) - Internal plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 80mm grey EPS + render | ≈ 0.33 | Misses the 0.30 renovation target | | 90mm grey EPS + render | ≈ 0.30 | Typical ECO-scheme spec — meets the 0.30 target (0.296) | | 100mm grey EPS + render | ≈ 0.27 | Clears the 0.30 renovation target | | 110mm grey EPS + render | ≈ 0.25 | Comfortable pass | | 90mm, with the brick at the exposed λ 0.77 | ≈ 0.30 | Just misses (0.303): the exposed value is for brick with nothing outside it | What changes the number: Board thickness and lambda (grey EPS 0.031 vs white 0.038) set the number, with one convention worth knowing: BR 443 §7 takes brick or block as exposed (λ 0.77 for brick) “except with an external insulation or cladding system when the protected value applies” (0.56). With the brick protected, BR 443 §4.2 also counts its mortar joints (17.3% of the wall at λ 0.88), which U-Monkey adds for you; net of them, the protected value is worth 0.005–0.008 here. U-Monkey suggests it when its generic brick (λ 0.77) has insulation, cladding or another leaf outside it — for a maker’s brick, use the maker’s own protected value. Unlike IWI, EWI keeps the masonry warm, so condensation risk is generally lower — but junctions, DPC level and fire classification of the system need real attention. Q: EWI or IWI for a solid wall? A: Thermally they can reach similar U-values. EWI keeps the wall warm and the rooms full-size but changes the outside and costs more; IWI is room-by-room and cheaper but eats space and needs careful moisture design. Model both in U-Monkey and run the condensation check on the IWI option. Note: The pre-filled link opens as a retained solid wall being upgraded, judged against Approved Document L’s 0.30 for external insulation (2021 edition, Table 4.3). The brick behind the insulation is taken as protected, λ 0.56 (BR 443 §7), with its mortar joints counted (§4.2). --- # What is the U-value of an uninsulated cavity wall? URL: https://energycount.co.uk/tools/u-value-calculator/uninsulated-cavity-wall/ Typical U-value: 1.67 W/m²K — Brick / 50mm clear cavity / dense block, no insulation A typical uninsulated cavity wall — brick, clear 50mm cavity, dense block, plaster — has a U-value of about 1.7 W/m²K, and older brick-brick versions are similar. That is roughly six times today’s new-build standard, which is why cavity fill is usually the cheapest big win in a retrofit. Build-up, outside to inside: - 102.5mm brick outer leaf - 50mm clear cavity - 100mm dense block inner leaf - Plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Brick / 50 cavity / dense block | ≈ 1.7 | Typical 1930s–70s wall | | Same wall, cavity filled (blown wool) | ≈ 0.5–0.6 | The classic retrofit step — see the retrofit cavity wall insulation page | What changes the number: The clear cavity contributes almost nothing (R≈0.18). Filling it transforms the wall; block density and any later lining decide the rest. Model your actual wall, then try the filled version — the difference is the business case. Q: How do I know if my cavity is already filled? A: A borescope inspection through a drilled joint is definitive; drill dust that comes out fibrous or beaded suggests fill. Homes built after the mid-1990s usually had insulation built in. If in doubt, model both cases in U-Monkey to see what is at stake. --- # U-value of a timber frame wall with PIR insulation outside the studs URL: https://energycount.co.uk/tools/u-value-calculator/timber-frame-wall-external-pir/ Typical U-value: 0.17 W/m²K — 140mm wool between studs + 60mm PIR over the sheathing A brick-clad timber frame wall with 140mm of mineral wool between the studs and 60mm of PIR fixed over the sheathing achieves a U-value of about 0.17 W/m²K, inside the 0.18 notional value of Part L 2021 and the Future Homes Standard. 50mm of PIR shows as 0.18 but misses it before rounding, at 0.1802. Without the board the same frame is about 0.30, because around 15% of the wall is timber: a continuous layer outside the studs covers every one of them, which is why it is worth more than its thickness suggests. Open the build-up below pre-filled and tune it to your frame. Build-up, outside to inside: - 102.5mm brick outer leaf - 50mm drained cavity - Breather membrane - 25–80mm PIR over the sheathing (continuous) - 9mm OSB sheathing - 140mm mineral wool between studs (~15% timber) - Polythene VCL (warm side) - Plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 140mm wool between studs, no board | ≈ 0.30 | The plain frame — see the timber frame wall page | | + 25mm PIR over the sheathing | ≈ 0.22 | Passes the 0.26 limiting value | | + 50mm PIR over the sheathing | ≈ 0.18 | Just misses the 0.18 notional before rounding (0.1802) | | + 60mm PIR over the sheathing | ≈ 0.17 | Inside the 0.18 notional value | | + 80mm PIR over the sheathing | ≈ 0.15 | Comfortably inside 0.18 | What changes the number: The studs set the base figure and the outside layer does the rest: timber (λ 0.13) conducts more than three times as well as the wool between the studs (λ 0.038), and a continuous board covers every stud. Foil-faced PIR is vapour-tight, so a board outside the frame needs to be thick enough to keep the sheathing behind it warm, with a sealed VCL on the warm side; as pre-filled, with 60mm, U-Monkey’s free condensation check finds no condensation in any month. The breather membrane is modelled over the board. Leave the foil face open to a clear cavity instead and U-Monkey treats that cavity as low-emissivity automatically (BR 443), which takes the 50mm version from 0.1802 to 0.172. Q: Should the PIR go outside the studs or inside them? A: Thermally the two are within about 0.01 W/m²K of each other: 50mm of PIR over the sheathing gives 0.180, and the same board as a lining under the plasterboard 0.173. What matters is that the layer is continuous. Outside, it keeps the studs and sheathing warm and leaves the service zone free; inside, it is the easier option on an existing wall. Wherever a foil-faced board goes, the vapour control on the warm side has to be sealed — model both positions in U-Monkey and run the free condensation check on each. --- # U-value of a solid stone wall with internal insulation URL: https://energycount.co.uk/tools/u-value-calculator/stone-wall-internal-insulation/ Typical U-value: 0.25 W/m²K — 500mm sandstone/granite + 80mm PIR internally A bare 500mm sandstone or granite wall with lime plaster has a U-value of about 2.4 W/m²K, and the same thickness of limestone about 1.8. Lining it internally with 80mm of PIR and plasterboard brings it to about 0.25, inside the 0.30 that Approved Document L sets for a wall being renovated; 70mm gets there at 0.28, while 60mm falls short at 0.32. A vapour-open lining needs more thickness: about 120mm of wood fibre with a lime finish to reach 0.30. Internal insulation leaves the stone colder and slower to dry, so on a traditional wall the moisture question matters as much as the U-value. Build-up, outside to inside: - 500mm solid sandstone or granite - 15mm lime plaster (existing) - 60–80mm PIR insulated lining - Plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Bare 500mm sandstone or granite, lime plaster | ≈ 2.4 | The starting point | | Bare 500mm limestone, lime plaster | ≈ 1.8 | Limestone conducts less than sandstone or granite | | + 120mm wood fibre, lime plaster finish | ≈ 0.30 | Vapour-open; just inside the 0.30 target (0.2998) | | + 60mm PIR lining | ≈ 0.32 | Short of the 0.30 renovation target | | + 80mm PIR lining | ≈ 0.25 | Comfortable pass | What changes the number: Stone type and thickness set the bare figure — sandstone and granite (λ 2.3) conduct more than limestone (λ 1.4), and a 600mm wall of the same stone is about 2.2 rather than 2.4 — but once the wall is lined, the lining decides the U-value: the same 80mm of PIR gives 0.24 on limestone and 0.25 on sandstone. The figures take the wall as solid stone at BR 443’s conductivity; a rubble-filled wall with lime mortar may conduct a little less. The moisture question is the harder one. U-Monkey’s free condensation check models vapour diffusion only, so it flags internal insulation on solid masonry for a full hygrothermal assessment to BS EN 15026 rather than reporting a clean pass; on a vapour-open lining with no vapour control it predicts condensation building up behind the insulation, because it cannot see the capillary drying such linings rely on. On a traditional or listed wall, check the pointing, render and rainwater goods first and get that assessment before choosing a lining. Q: What is the U-value of an uninsulated stone wall? A: It depends on the stone and the thickness. Calculated as solid stone with lime plaster, a 500mm sandstone or granite wall is about 2.4 W/m²K and a 500mm limestone wall about 1.8; at 600mm, sandstone or granite comes to about 2.2. All are far worse than the 0.70 threshold in Approved Document L Table 4.3, so a wall like this has to be improved when it is renovated. Model your own wall’s stone and thickness in U-Monkey, free. --- # U-value of a cavity wall with external wall insulation URL: https://energycount.co.uk/tools/u-value-calculator/cavity-wall-external-insulation/ Typical U-value: 0.22 W/m²K — 90mm grey EPS over a filled 50mm cavity wall A typical 1930s–70s cavity wall — brick, a 50mm cavity, dense block — is about 0.58 W/m²K once the cavity has been filled. 90mm of grey EPS external wall insulation over it brings the wall to about 0.22, well inside the 0.30 that Approved Document L sets for a wall upgraded with external insulation; 60mm clears that at 0.28, while 50mm only just misses it, at 0.301. The cavity fill still counts: the same 90mm over an empty cavity gives about 0.29. Build-up, outside to inside: - Thin-coat render - 50–90mm grey EPS - 102.5mm brick outer leaf (protected by the EWI: λ 0.56) - 50mm cavity, filled with mineral wool (λ 0.038) - 100mm dense block inner leaf - Plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Filled cavity wall, no EWI | ≈ 0.58 | The starting point after cavity fill | | + 50mm grey EPS + render | ≈ 0.30 | Misses the 0.30 renovation target (0.301) | | + 60mm grey EPS + render | ≈ 0.28 | Clears the 0.30 renovation target | | + 90mm grey EPS + render | ≈ 0.22 | Comfortable pass | | 90mm grey EPS over an empty cavity | ≈ 0.29 | The cavity fill is still doing work | What changes the number: Board thickness and lambda set the number: grey EPS (λ 0.031) does the job of white (λ 0.038) in less thickness — 90mm of white gives about 0.25 rather than 0.22. Behind the EPS the brick outer leaf is taken as protected (λ 0.56) rather than exposed (0.77, as it was before the work): BR 443 §7 takes brick behind an external insulation system as protected on a solid wall, and says that with one “all the masonry is protected” on a diaphragm wall. U-Monkey suggests the switch when you add the insulation. With most of the insulation outside the masonry the wall stays warm, and U-Monkey’s free condensation check finds no condensation in this build-up in any month. The pre-filled link opens it as an upgraded wall judged against that 0.30 (Upgrading existing, then External / internal insulation), not the 0.55 for cavity fill. Mechanical fixings through the boards add a small correction U-Monkey can include, and the detailing at eaves, sills, reveals and the DPC decides how much of the paper figure survives on site. Q: Is external insulation worth it on a cavity wall that is already filled? A: Thermally, yes: a filled 50mm cavity leaves the wall at about 0.58 W/m²K, and 60–90mm of grey EPS outside takes it to 0.28–0.22. It is a much bigger job than cavity fill — scaffolding, new render and reworked eaves, sills and pipework — so it makes most sense where the fill alone is not enough or the outside needs work anyway. Model your own wall in U-Monkey and compare it with and without the EWI. --- # U-value of a cavity wall with a dense concrete block inner leaf URL: https://energycount.co.uk/tools/u-value-calculator/cavity-wall-dense-block/ Typical U-value: 0.23 W/m²K — 150mm full-fill mineral wool, dense block inner leaf A brick and dense-block cavity wall with 150mm of full-fill mineral wool achieves a U-value of about 0.23 W/m²K: inside the 0.26 limiting value, but well short of the 0.18 notional value. The same wall with an aircrete inner leaf is about 0.21, because a dense block (λ 1.13) adds almost nothing to the wall’s resistance. With dense blocks the insulation has to do all the work: a 200mm cavity of full fill just reaches 0.18, and 100mm of PIR partial fill comes out at 0.19. Open the build-up below pre-filled and tune it to your spec. Build-up, outside to inside: - 102.5mm brick outer leaf - 100–200mm full-fill mineral wool cavity batt - 100mm dense concrete block inner leaf (mortar joints negligible, BR 443) - Plasterboard on dabs | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm full-fill, dense block | ≈ 0.32 | Misses the 0.26 limiting value | | 150mm full-fill, dense block | ≈ 0.23 | Passes the 0.26 limit; well short of the 0.18 notional | | 150mm λ0.032 batts, dense block | ≈ 0.20 | Better batts help, but not enough | | 200mm full-fill (wide cavity), dense block | ≈ 0.18 | Just inside the 0.18 notional (0.1796) | | 100mm PIR partial fill + 50mm clear cavity, dense block | ≈ 0.19 | Misses the 0.18 notional (0.187) | What changes the number: The inner leaf is the difference. A dense block is there for strength, mass and sound, not insulation, so the U-value has to come from the cavity. Its mortar joints are negligible: BR 443 lets them be ignored where block and mortar differ in resistance by less than 0.1 m²K/W, so U-Monkey adds none here (in aircrete, where the difference is larger, it counts them). Medium-dense blocks (λ 0.51) put the same wall at about 0.22, and aircrete at 0.21. The heavier leaf does store more heat: U-Monkey’s free thermal-mass figures give this wall an areal heat capacity of about 55 kJ/m²K, against 33 for the aircrete version. Q: Dense block or aircrete for the inner leaf? A: Thermally, aircrete: with 150mm of full fill the wall is about 0.21 with aircrete and 0.23 with dense block, because aircrete adds real resistance and dense block almost none. Dense block is stronger and heavier, better for sound and holds more heat. If the spec calls for dense blocks, put the extra thickness into the cavity or the board — model both inner leaves in U-Monkey and compare like for like. --- # U-value of a solid brick wall (9-inch, uninsulated) URL: https://energycount.co.uk/tools/u-value-calculator/solid-brick-wall-uninsulated/ Typical U-value: 2.10 W/m²K — 215mm solid brick + 13mm dense plaster A 215mm (9-inch) solid brick wall with 13mm of dense plaster has a calculated U-value of about 2.1 W/m²K, using BR 443’s conductivity for brickwork exposed to the weather (λ 0.77). That is the figure a U-value calculation, and so a Part L appraisal of the wall, starts from. Real walls usually do better: BRE measured 85 solid brick walls in English homes in 2014 and found an average of 1.57, and RdSAP 10 now assumes 1.7 for a wall 200–280mm thick. Whichever you use, it is far above the 0.70 at which Approved Document L counts a wall as one to upgrade, and about twelve times the 0.18 of a new-build wall. Build-up, outside to inside: - 215mm solid brickwork (9-inch, the bricks laid across the wall) - 13mm dense gypsum plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 215mm brick, unplastered | ≈ 2.23 | A garage or outbuilding wall | | 215mm brick + 20mm lime plaster | ≈ 2.09 | Lime instead of gypsum makes almost no difference | | 215mm brick + 13mm dense plaster | ≈ 2.10 | The calculated figure, as pre-filled | | RdSAP 10 default, solid brick 200–280mm | 1.7 | An assumed value for EPCs, down from 2.1 after BRE’s field measurements (CONSP:16, 2016) | | Measured: 85 solid brick walls (BRE, 2014) | 1.57 (mean) | In-situ heat-flux measurements; the same walls calculated at 1.90 on average | | One-and-a-half brick (327.5mm) + plaster | ≈ 1.61 | See the 1½-brick wall page | | Half brick (102.5mm) + plaster | ≈ 3.04 | Single-skin extensions and outbuildings | What changes the number: Nothing about a bare wall moves the number much except its thickness: 215 to 327.5mm takes it from 2.1 to 1.6. The calculation uses BR 443’s λ 0.77, the cautious value for brick that gets wet; drier brick, lime mortar and the air in the joints are why measured walls come out lower. Neither figure changes the answer, which is insulation — internal or external — and on a solid wall that is a moisture decision as much as a thermal one. U-Monkey opens this wall as an existing element, judged against the 0.70 threshold; switch the work type to “upgrade” and add a lining to see what reaches the 0.30 Approved Document L asks of an insulated wall. Q: Why is a solid wall 2.1 in one place and 1.7 in another? A: 2.1 is the calculation: BS EN ISO 6946 with BR 443’s conductivity for brick. 1.7 is RdSAP 10’s default for a solid brick wall 200–280mm thick (Table 13 of the June 2026 specification, version 4.0), which BRE proposed in 2016 after measuring 85 solid brick walls in English homes and finding an average of 1.57 W/m²K, against 1.90 calculated for the same walls and the 2.1 RdSAP then assumed. An EPC uses the default; a U-value calculation for Building Control uses the conductivities, so it gives the higher figure. Both say the wall loses heat roughly ten times faster than a new one. Note: The pre-filled link opens as an existing element, judged against Approved Document L’s 0.70 threshold (2021 edition, Table 4.3). Brick at BR 443’s λ 0.77 throughout; there is no cavity, so no air-gap correction or wall ties. --- # U-value of a solid stone wall (uninsulated) URL: https://energycount.co.uk/tools/u-value-calculator/stone-wall-uninsulated/ Typical U-value: 2.40 W/m²K — 500mm sandstone/granite + 20mm lime plaster A 500mm solid sandstone or granite wall with 20mm of lime plaster calculates at about 2.4 W/m²K with the stone at λ 2.3; limestone (λ 1.4) gives about 1.8, and a 600mm sandstone wall about 2.2. Measured walls usually do better than the calculation: the 21 stone walls in the SPAB’s in-situ study averaged 1.42 W/m²K, and of 57 traditional walls Historic Scotland measured, 44% came out below the range calculated for them and 42% within it. RdSAP 10 assumes about 1.7 for a 500mm sandstone or limestone wall and 1.9 for granite. Every one of those figures is far above the 0.70 at which Approved Document L counts a wall as one to upgrade. Build-up, outside to inside: - 500mm solid stone (sandstone or granite, rubble or ashlar) - 20mm lime plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 400mm sandstone/granite + lime plaster | ≈ 2.68 | | | 500mm sandstone/granite + lime plaster | ≈ 2.40 | As pre-filled | | 600mm sandstone/granite + lime plaster | ≈ 2.18 | | | 700mm sandstone/granite + lime plaster | ≈ 1.99 | | | 500mm limestone (λ 1.4) + lime plaster | ≈ 1.80 | Softer stone conducts less | | RdSAP 10, 500mm: sandstone or limestone / granite | ≈ 1.7 / 1.9 | Assumed values, from the stone-wall equations in RdSAP 10 (Table 12 of the June 2026 specification) | | Measured: 21 stone walls (SPAB, 2012) | 1.42 (mean) | The software over-estimated U in 77% of the SPAB’s cases | What changes the number: Thickness and the stone are what the calculation sees; it treats the wall as solid stone, though real walls are often two faces of stone with a rubble and lime core, which is one reason they measure better. The calculation is still what Building Control and a SAP calculation use (RdSAP, for EPCs, has its own defaults). For insulating it, the risk is moisture rather than heat: stone walls take in rain and dry to both sides, so internal insulation needs a vapour-open, capillary-active design or a full BS EN 15026 assessment. U-Monkey’s condensation check reports internal insulation on solid masonry as a screening result only for that reason. See the stone wall with internal insulation page for a lined version. Q: Should I use the calculated or the measured U-value for a stone wall? A: For a Building Regulations or SAP calculation, the calculated one: BR 443 and BS EN ISO 6946 are the agreed method, and RdSAP has its own defaults for EPCs. But know that it is often pessimistic for traditional walls: the SPAB found the calculation over-estimating the U-value in 77% of the walls it measured, and Historic Scotland found 44% of 57 walls below their calculated range and 42% within it, some far below. That matters when you weigh up insulating a stone wall, because the saving is the difference between the real U-value and the insulated one. An in-situ measurement (to BS ISO 9869) is the way to know your wall. Note: Stone at BR 443’s conductivities: sandstone and granite λ 2.3, limestone λ 1.4. The pre-filled link opens as an existing element, judged against Approved Document L’s 0.70 threshold (2021 edition, Table 4.3). --- # U-value of a cavity wall after cavity wall insulation (blown wool or bead) URL: https://energycount.co.uk/tools/u-value-calculator/cavity-wall-insulation-retrofit/ Typical U-value: 0.60 W/m²K — Brick / 50mm blown mineral wool / dense block / plaster Filling a typical 50mm cavity between brick and dense concrete block takes the wall from about 1.67 W/m²K to about 0.60 with blown mineral wool or white EPS bead at λ 0.040, or about 0.52 with graphite bead (λ 0.033). It is the biggest single improvement most cavity-walled homes can make, but it does not always reach the 0.55 Approved Document L gives for a cavity-insulated wall: λ 0.040 fill against dense block misses it, while graphite bead, a λ 0.034 wool or an aircrete inner leaf clear it. A 65mm cavity with the same wool reaches about 0.49. Build-up, outside to inside: - 102.5mm brick outer leaf - 50mm cavity, filled with blown mineral wool (λ 0.040) - 100mm dense concrete block inner leaf - 13mm dense plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Before: 50mm clear cavity | ≈ 1.67 | See the uninsulated cavity wall page | | 50mm blown mineral wool, λ 0.040 | ≈ 0.60 | As pre-filled: misses 0.55 | | 50mm white EPS bead, λ 0.040 | ≈ 0.60 | The same as λ 0.040 wool | | 50mm blown mineral wool, λ 0.034 | ≈ 0.53 | Clears 0.55 | | 50mm graphite EPS bead, λ 0.033 | ≈ 0.52 | Clears 0.55 | | 65mm cavity, blown wool λ 0.040 | ≈ 0.49 | A wider cavity does more than a better fill | | 50mm blown wool, aircrete inner leaf | ≈ 0.47 | Mortar joints counted (BR 443) | | 50mm blown wool, brick inner leaf | ≈ 0.57 | Older brick-and-brick cavity walls | What changes the number: Three things set the result: the cavity width, the fill’s λ (0.034 to 0.040 for blown wool, about 0.033 for graphite bead, nearer 0.040 for white bead) and the inner leaf, where aircrete is worth more than any upgrade of the fill. BR 443 applies no air-gap correction to a cavity filled by injection, so none is added. The figures leave out the wall ties: add them in U-Monkey (it offers to), because BR 443 counts metal ties through insulation — about +0.005 W/m²K for 4mm stainless wire ties at 2.5 per m², and about three times that for galvanised steel ties of the same size, which leaves a λ 0.034 fill inside 0.55 with little to spare (0.545). Approved Document L’s 0.55 applies only to walls suitable for cavity insulation (Table 4.3, note 5), and whether yours is depends on its exposure to wind-driven rain, the cavity’s width and condition, and the outer leaf — the installer’s survey decides that. Q: Is cavity wall insulation covered by the Building Regulations? A: Yes: inserting insulation into a cavity wall is building work in its own right (Building Regulations 2010, regulation 3(1)(e)), so it needs Building Control approval or an installer registered with a competent person scheme for cavity wall insulation, who certifies it. The thermal target in Approved Document L is the “improved” U-value for a cavity-insulated wall, 0.55 W/m²K (Table 4.3); where a fill cannot reach it, paragraph 4.13 accepts the best that is technically and functionally feasible with a simple payback of 15 years or less. Part C matters as much: the fill must suit the wall’s exposure to driving rain. Note: BR 443 (2019) §7 lists no air-gap correction for a cavity filled by injection after construction, so the air-gap correction is level 0 here. The pre-filled link opens as a retained wall being upgraded, judged against Approved Document L’s 0.55 for cavity insulation (2021 edition, Table 4.3). --- # Insulated plasterboard on a solid wall: what thickness reaches a U-value of 0.30? URL: https://energycount.co.uk/tools/u-value-calculator/insulated-plasterboard-solid-wall/ Typical U-value: 0.28 W/m²K — 215mm brick + plaster, 65mm PIR + 12.5mm board on dabs On a 215mm solid brick wall, insulated plasterboard fixed on dabs needs about 65mm of PIR core (a 77.5mm board) to meet the 0.30 W/m²K that Approved Document L asks of an internally insulated wall: that gives about 0.28. A 60mm core shows as 0.30 but is 0.302 before rounding, so it misses; 50mm gives about 0.35 and 40mm about 0.41. The plaster dabs behind the board bridge its 15mm air gap, which BR 443 counts and U-Monkey adds for you. Build-up, outside to inside: - 215mm solid brick - 13mm existing plaster - 15mm gap on plaster dabs (the dabs cover 20% of it, BR 443 §4.6.1) - 65mm PIR insulation core - 12.5mm plasterboard facing | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 25mm PIR core (37.5mm board) | ≈ 0.56 | | | 40mm PIR core (52.5mm board) | ≈ 0.41 | | | 50mm PIR core (62.5mm board) | ≈ 0.35 | | | 60mm PIR core (72.5mm board) | ≈ 0.30 | Misses: 0.302 before rounding | | 65mm PIR core (77.5mm board) | ≈ 0.28 | As pre-filled: meets 0.30 | | 80mm PIR core (92.5mm board) | ≈ 0.24 | | What changes the number: The core thickness and its λ do the work: a phenolic core (around λ 0.019) gets there thinner, and the products in U-Monkey’s library carry their own λ and, where certified, the emissivity of their facing. Room loss is the trade: about 92mm comes off every lined wall, and Approved Document L allows a lesser standard where meeting 0.30 would cut a room’s floor area by more than 5% (Table 4.3, note 6). The bigger question on a solid wall is moisture. Lining it makes the brick colder and wetter; as pre-filled, U-Monkey’s ISO 13788 screening finds no condensation, but it reports that as a screening result only, because PAS 2035 notes that an ISO 13788 check might not be sufficient for internal insulation on solid masonry; a BS EN 15026 hygrothermal assessment is the fuller test. Seal the board’s joints and edges, and treat window reveals and floor junctions as part of the job. Q: How thick is insulated plasterboard for a solid wall? A: For 0.30 W/m²K on a 215mm brick wall, a 77.5mm PIR board (65mm core + 12.5mm plasterboard) on dabs, with about 15mm of dab behind it, so roughly 92mm in front of the old plaster. A 62.5mm board gives about 0.35 and a 52.5mm board about 0.41. Phenolic boards reach the same U-value a little thinner. Open the build-up below and use the − / + buttons on a product to step through its real board sizes. Note: The pre-filled link opens as a retained solid wall being upgraded, judged against Approved Document L’s 0.30 for internal insulation (2021 edition, Table 4.3). The dab gap is BR 443’s 15mm air space with 20% plaster dabs; an ordinary foil-backed board gets no low-emissivity credit there. --- # U-value of a solid wall battened out with insulation URL: https://energycount.co.uk/tools/u-value-calculator/solid-wall-battened-insulation/ Typical U-value: 0.28 W/m²K — 50mm PIR between battens + 30mm PIR over, 215mm brick Battening out a 215mm solid brick wall with 50mm PIR between the battens and plasterboard over them gives about 0.48 W/m²K: the battens bridge 11.8% of the layer (BR 443). Add a continuous 30mm PIR layer across the battens and it reaches about 0.28, inside the 0.30 Approved Document L asks of an internally insulated wall; 25mm over the battens meets 0.30 with nothing to spare (0.2999). Mineral wool between the battens instead of PIR gives about 0.63. Build-up, outside to inside: - 215mm solid brick - 13mm existing plaster - 50mm PIR between 47 × 50mm battens (11.8% timber, BR 443 §4.6.2) - 30mm PIR across the battens (continuous) - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 50mm mineral wool between battens + plasterboard | ≈ 0.63 | Needs a vapour control layer | | 50mm PIR between battens + plasterboard | ≈ 0.48 | The battens bridge it | | 75mm PIR between 75mm battens + plasterboard | ≈ 0.35 | | | 50mm PIR between + 25mm PIR over | ≈ 0.30 | Meets 0.30 with nothing to spare (0.2999) | | 50mm PIR between + 30mm PIR over | ≈ 0.28 | As pre-filled | | 50mm PIR between + 37.5mm PIR over | ≈ 0.26 | | What changes the number: The continuous layer over the battens is what makes this work: it breaks the timber bridge and, as a second layer of insulation, takes BR 443’s air-gap correction to level 0. Thicker battens with more between them do less for the same depth. Use a foil-faced board or an insulated plasterboard over the battens with taped joints, or a vapour control layer: with mineral wool between the battens and nothing on the warm side, U-Monkey’s condensation check finds moisture building up year on year on the old plaster behind it. As pre-filled, with PIR, its ISO 13788 screening finds no condensation, and it reports that as a screening result only, because PAS 2035 notes that an ISO 13788 check might not be sufficient for internal insulation on solid masonry. The battens do leave a services zone, which insulated plasterboard on dabs does not. Q: Battens or insulated plasterboard on a solid wall? A: Thermally they end up close: 50mm PIR between battens plus 30mm over comes to about 0.28 W/m²K, the same as a 77.5mm insulated plasterboard on dabs, and both are about 90–95mm deep. Battens suit an uneven or damp-affected wall that needs a fixing background and a services void; dabs are quicker on a sound, flat plastered wall. Either way the joints and edges need sealing, and the moisture behaviour of the solid wall is the real design question. Note: BR 443 §4.6.2: 47mm battens at 600mm centres with top and bottom rails, 11.8% timber. The pre-filled link opens as a retained solid wall being upgraded, against Approved Document L’s 0.30 (2021 edition, Table 4.3). --- # U-value of a dormer cheek wall (loft conversion) URL: https://energycount.co.uk/tools/u-value-calculator/dormer-cheek-wall/ Typical U-value: 0.17 W/m²K — 100mm PIR between studs + 50mm PIR inside, tile-hung A tile-hung dormer cheek with 100mm PIR between the studs and a continuous 50mm PIR layer inside them reaches about 0.17 W/m²K, inside the 0.18 Approved Document L sets for a new wall in an existing home (a dormer’s cheeks count as walls, Table 4.2 note 2). 100mm between the studs alone gives only about 0.32, because the studs (15% of the wall, BR 443) bridge it; 25mm inside brings it to 0.22 and 40mm to 0.19, still short. 46mm is the least that clears it. Build-up, outside to inside: - Hanging tiles on battens (a ventilated space: it and the tiles are not counted) - Breather membrane - 12mm plywood sheathing - 100mm PIR between 100mm studs (15% timber) - 50mm PIR across the inside of the studs (continuous) - Plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm PIR between studs only | ≈ 0.32 | The studs bridge it | | 100mm between + 25mm PIR inside | ≈ 0.22 | | | 100mm between + 40mm PIR inside | ≈ 0.19 | Misses 0.18 (0.1895) | | 100mm between + 50mm PIR inside | ≈ 0.17 | As pre-filled: meets 0.18 | | 75mm between + 50mm PIR inside | ≈ 0.20 | Thinner studs, same lining: misses | | 100mm mineral wool between + 50mm PIR inside | ≈ 0.21 | | What changes the number: The continuous layer inside the studs matters more than the depth between them, because it covers the 15% of the wall that is timber. Space is the constraint in a dormer, so the inside layer is usually an insulated plasterboard or PIR with taped joints that doubles as the vapour control. As pre-filled, U-Monkey’s condensation check finds no condensation in any month. Dormer cheeks are lightweight, with little to store heat, which is worth knowing for a loft bedroom’s summer comfort: the thermal-mass figures in the app show it. Q: What U-values does a dormer need? A: In England a new dormer is a new element in an existing home, so Approved Document L’s Table 4.2 applies: 0.18 W/m²K for the cheeks, which count as walls, and 0.15 for the dormer’s roof (Table 4.2, note 2); the window needs 1.4. A flat dormer roof is a small warm roof, so see the warm flat roof page; the main roof slopes being converted are retained elements, with 0.16 for the roof in Table 4.3. Note: BR 443 counts nothing outside a well-ventilated air space, so the tiles and their batten space drop out and the breather membrane and plywood count. The pre-filled link opens as a new element in an existing home, judged against Approved Document L’s 0.18 (2021 edition, Table 4.2). --- # U-value of a garage conversion wall (insulating single-skin brick) URL: https://energycount.co.uk/tools/u-value-calculator/garage-conversion-wall/ Typical U-value: 0.28 W/m²K — 102.5mm brick, 25mm gap, 100mm PIR between studs A single-skin (102.5mm) brick garage wall lined with a stud wall — kept 25mm off the brick, 100mm PIR between the studs and plasterboard — comes out at about 0.28 W/m²K, inside the 0.30 Approved Document L asks of a retained wall insulated internally (Table 4.3; paragraph 4.12(b) names garage conversions). The bare brick is about 3.3. 75mm of PIR gives about 0.34 and misses, and so, by a whisker, does 89mm (0.301); 100mm PIR plus 25mm across the studs reaches about 0.20. A wall built in the old door opening is a new wall and needs 0.18. Build-up, outside to inside: - 102.5mm single-skin brick (or 100mm block) - 25mm clear gap, keeping the timber off the brick - 100mm PIR between 100mm timber studs (15% timber) - Plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Bare 102.5mm brick | ≈ 3.30 | The starting point | | 75mm PIR between studs | ≈ 0.34 | Misses 0.30 | | 89mm PIR between CLS studs | ≈ 0.30 | Misses: 0.301 before rounding | | 100mm PIR between studs | ≈ 0.28 | As pre-filled: meets 0.30 | | 100mm mineral wool between studs | ≈ 0.40 | Misses, and needs a vapour control layer | | 100mm PIR between studs + 25mm PIR across them | ≈ 0.20 | | | 100mm dense block instead of brick, same lining | ≈ 0.28 | The lining does the work | What changes the number: The lining does all the work; the brick is there to keep the weather out, and single-skin brick does not always manage that, which is why the studs usually stand clear of it on a gap. The 25mm gap counts as a low-emissivity air space because it faces the PIR’s foil, and the studs bridge 15% of the insulation, so a continuous layer across them gains more than deeper studs. With mineral wool between the studs and no vapour control, U-Monkey’s condensation check finds moisture building up year on year on the inside face of the brick. As pre-filled, with PIR, its ISO 13788 screening finds none, and it reports that as a screening result only, as PAS 2035 notes that an ISO 13788 check might not be sufficient for internal insulation on solid masonry. Q: What U-values do I need for a garage conversion? A: In England, Approved Document L treats the garage’s existing walls, floor and roof as retained elements (paragraph 4.12(b)): anything worse than the Table 4.3 threshold should be upgraded to its “improved” value — 0.30 for a wall insulated internally (0.55 where a cavity can be filled), 0.25 for the floor and 0.16 for the roof — so far as that is technically and functionally feasible with a simple payback of 15 years or less. New elements meet Table 4.2: 0.18 for the wall in the old door opening and 1.4 for new windows and doors. Building Control will ask for the calculations. Note: The pre-filled link opens as a retained wall being upgraded, judged against Approved Document L’s 0.30 for internal insulation (2021 edition, Table 4.3). Studs at 15% timber (BR 443). --- # U-value of a SIP wall (structural insulated panels) URL: https://energycount.co.uk/tools/u-value-calculator/sips-wall/ Typical U-value: 0.20 W/m²K — 142mm SIP (112mm PUR core, 4% timber), brick-clad A brick-clad wall of 142mm structural insulated panels — 112mm of polyurethane between two 15mm OSB skins — with a service void and plasterboard inside has a U-value of about 0.20 W/m²K with 4% of the core taken up by timber, and about 0.18 with none. That clears the 0.26 limiting value but not the 0.18 the notional dwelling uses for walls. The 172mm panel gives about 0.16. BR 443 sets no default timber fraction for SIPs: it asks for the bridging to be worked out from the panel’s own splines, plates and openings, so the timber is the number to check. Build-up, outside to inside: - 102.5mm brick outer leaf - 50mm cavity (the wall ties cross it empty: no correction) - Breather membrane - 15mm OSB/3 skin - 112mm polyurethane core (λ 0.024), 4% timber at the joints and plates - 15mm OSB/3 skin - 25mm service void on battens (11.8% timber, BR 443 §4.6.2) - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 142mm panel, no timber counted | ≈ 0.18 | What Kingspan’s TEK sheet quotes with brick | | 142mm panel, 4% timber in the core | ≈ 0.20 | As pre-filled (0.198) | | 142mm panel, 7.5% timber (solid timber splines) | ≈ 0.21 | | | 142mm panel + 25mm PIR in the service zone, 4% timber | ≈ 0.16 | Meets the 0.18 notional value | | 172mm panel (142mm core), 4% timber | ≈ 0.16 | Meets 0.18 (0.163); Kingspan’s sheet quotes 0.14 | | 172mm panel, 7.5% timber | ≈ 0.18 | Meets 0.18, just (0.178) | | Another maker’s 150mm panel: 128mm PUR core (λ 0.028), 11mm OSB, 4% | ≈ 0.20 | Thinner skins, weaker core | What changes the number: Two things set the figure: the core and the timber. A polyurethane core (λ 0.024 here) beats an EPS one (about λ 0.030), so an EPS-cored panel has to be about a quarter thicker for the same result. The timber is the variable to watch: splines at the panel joints, the sole and head plates and the framing round openings all put wood (λ 0.13) where the core would be. BR 443 (2019) asks for the bridging fraction to be worked out from the construction of the panel, using the same guidelines as a timber-frame wall (§4.4.1(iii)): leave out timbers outside the heat-loss wall area and a zone round windows and doors, and count every timber with no insulation behind it. Insulated splines keep it low; solid timber splines at every joint push it towards 7–8%. The core is foamed or bonded to its skins with sealed joints, so U-Monkey gives it BR 443’s air-gap level 0, and the ties holding the brick to the panel cross an empty cavity, so they need no correction. Q: Do SIPs meet the Future Homes Standard? A: They can, but not always with the standard panel. The Future Homes Standard’s notional dwelling (SAP 10.3 Appendix R) uses 0.18 W/m²K for walls, the same as Part L 2021, and that is a reference for setting the dwelling’s targets, not a limit: the limiting value for a wall stays at 0.26. A 142mm panel with brick comes out at about 0.20 once its timber is counted, so other parts of the house have to make up the difference; the 172mm panel (about 0.16), or 25mm of PIR in the service zone (about 0.16), gets under 0.18 on its own. Note: Panel as in Kingspan’s TEK specification: 15mm OSB/3 skins and a 112mm polyurethane core at λ 0.024. Its BBA certificate (02/S029, 2010 issue) gave 0.19–0.20 W/m²K for this wall for 0% to 4% timber, with the core then declared at λ 0.023 — about 0.01–0.02 above what U-Monkey calculates for that λ; the certificate does not show its working. The pre-filled build-up takes 4%. The SIP core takes air-gap level 0 (BR 443 §4.8.1). --- # U-value of an ICF wall (insulating concrete formwork) URL: https://energycount.co.uk/tools/u-value-calculator/icf-wall/ Typical U-value: 0.16 W/m²K — 2 × 100mm white EPS forms, 150mm concrete core, rendered An insulating concrete formwork wall with 100mm of white EPS either side of a 150mm concrete core, rendered outside and lined with plasterboard, has a U-value of about 0.16 W/m²K — the figure NSAI Agrément gives for that block (certificate 07/0277). Graphite (grey) EPS takes the same wall to 0.15, and 85mm grey forms round a 200mm core give 0.17. The concrete adds almost nothing: 150mm of it is worth about 0.07 m²K/W, against nearly 6 for the two layers of EPS. Thin forms are the trap: 50mm of XPS each side gives about 0.31, which misses even the 0.26 limiting value. Build-up, outside to inside: - 10mm render on the outer form - 100mm white EPS form (λ 0.034) - 150mm reinforced concrete core - 100mm white EPS form - 12.5mm plasterboard, screwed to the form’s plastic webs | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 2 × 100mm white EPS, 150mm core | ≈ 0.16 | As pre-filled; NSAI 07/0277 gives 0.16 | | 2 × 100mm grey EPS, 150mm core | ≈ 0.15 | NSAI 07/0277: 0.15 | | 2 × 85mm grey EPS, 200mm core | ≈ 0.17 | NSAI 07/0277: 0.17 | | 2 × 85mm white EPS, 200mm core | ≈ 0.19 | NSAI 07/0277: 0.19; misses 0.18 | | 2 × 75mm grey EPS, 150mm core | ≈ 0.19 | NSAI 07/0277: 0.20, with its allowance for the plastic ties | | 2 × 50mm XPS, 150mm core | ≈ 0.31 | Misses the 0.26 limiting value | | 150mm + 50mm XPS, 150mm core | ≈ 0.17 | A thicker outer form only | | 2 × 100mm white EPS behind a brick outer leaf and 50mm cavity | ≈ 0.15 | | What changes the number: All the insulation is in the two forms, so their thickness and grade decide the result: graphite EPS (λ 0.031) is about a tenth better than white (λ 0.034). The forms interlock and the concrete is cast against them, so there are no air voids to correct for: U-Monkey gives ICF BR 443’s air-gap level 0. The plastic webs that tie the two forms together conduct very little, and BR 443 treats non-metal ties as negligible (§4.8.2); the NSAI certificate does make a small allowance for them, which puts its figures up to about 0.01 above U-Monkey’s for the thinner blocks. What the wall does not do is give the room thermal mass: the inner form insulates the concrete from it, so the heat capacity facing the room is about 12 kJ/m²K, against about 160 for a dense-block inner leaf with wet plaster. Q: Is an ICF wall good for thermal mass? A: Not much, as the room sees it. The concrete core is heavy, but the inner EPS form separates it from the room, so very little of it takes part in the daily cycle: U-Monkey’s ISO 13786 figures give this wall an internal heat capacity of about 12 kJ/m²K, against about 160 for a brick-and-dense-block cavity wall with wet plaster. The mass still damps temperature swings through the wall itself — its decrement factor is very low — but for rooms that resist overheating, look to the floors and internal walls. Note: Forms and λ from NSAI Agrément certificate 07/0277 (Amvic, revised April 2026), whose Table 1 gives 0.16 / 0.15 for the 350mm block in white / grey EPS and 0.19 / 0.17 for the 370mm block; the XPS rows use Polarwall’s forms (BBA 14/5127). ICF forms take air-gap level 0 (BR 443 §4.8.1: no air voids within the insulation). --- # U-value of a light steel frame wall (SFS) URL: https://energycount.co.uk/tools/u-value-calculator/light-steel-frame-wall/ Typical U-value: 0.17 W/m²K — 100mm steel studs + mineral wool, 80mm PIR outside, brick A brick-clad light steel frame wall — 100mm steel C-studs at 600mm centres with mineral wool between them, 80mm of PIR outside the sheathing board and plasterboard inside — has a U-value of about 0.17 W/m²K by the method BR 443 sets for steel, BRE Digest 465. 60mm of PIR gives about 0.20 and 100mm about 0.15, as SCI’s design guide P402 tabulates them; P402 gives 0.18 for the 80mm wall. The steel is thin, but it conducts nearly 400 times better than timber, so BS EN ISO 6946’s plain average is replaced by a weighting, p, that Digest 465 works out from the frame: 0.65 for this wall. Build-up, outside to inside: - 102.5mm brick outer leaf - 50mm cavity - Breather membrane - 80mm PIR board, continuous outside the steel - 12mm cement particle sheathing board - 100mm steel C-studs at 600mm centres (1.2mm web: 0.2% steel) with 100mm mineral wool slab between - Polythene VCL - 15mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 60mm PIR outside the studs | ≈ 0.20 | SCI P402 Table 2.8: 0.20 | | 80mm PIR outside the studs | ≈ 0.17 | As pre-filled (p 0.65); P402: 0.18 | | 100mm PIR outside the studs | ≈ 0.15 | P402: 0.15 | | 80mm PIR, studs at 400mm centres | ≈ 0.18 | More steel, lower p (0.53): 0.1752 | | 80mm PIR, plain ISO 6946 average instead of Digest 465 | ≈ 0.17 | 0.1747 — not BR 443’s method for steel | | 80mm PIR, nothing between the studs (warm frame) | ≈ 0.23 | Normal ISO 6946 (p 0.5) | | Mineral wool between the studs only (cold frame) | ≈ 0.48 | p 0.41: the steel bypasses the wool | What changes the number: Most of the work is done by the board outside the steel, which no stud crosses; the wool between the studs is heavily short-circuited, which is why a cold frame — wool between the studs and nothing outside — is so poor. BR 443 (2019) §4.10 sorts steel walls by where the insulation is: outside the studs only is a warm frame and takes the normal ISO 6946 calculation; between them, or between them and outside (a hybrid, like this one), takes BRE Digest 465. Its p depends on how far apart the two ISO 6946 limits are and on the stud depth, centres and flange width, which U-Monkey asks for in the steel-frame panel that appears when a layer carries a steel bridge (here 50mm flanges, 600mm centres, 100mm deep): closer studs mean more steel and a lower p. Note the names: SCI calls this build-up a “warm frame”, where BR 443 calls it a hybrid. Rails, noggings and the brackets that carry cladding add bridging that a single figure cannot show, which is why published figures for similar walls vary. Q: Why does a steel frame wall need a different calculation? A: Because steel conducts so well that the two limits BS EN ISO 6946 averages end up far apart, and the plain average is then no longer reliable. BR 443 (2019) §4.10 therefore uses BRE Digest 465 for cold and hybrid frames: the same two limits, weighted by a factor p instead of half and half. Here that gives a slightly lower U-value than the plain average (0.166 against 0.175); with the wool between the studs only, Digest 465 is the harsher of the two. Only a warm frame, with all the insulation outside the steel, keeps the normal method — including a correction for the fixings that hold that insulation on. Note: Studs entered as BR 443 asks, as a bridged layer: a 1.2mm web at 600mm centres is 0.2% steel (λ 50). U-Monkey’s steel-frame panel supplies the flange width (50mm), the stud centres (600mm) and the stud depth (the bridged layer, 100mm) for Digest 465’s p. Digest 465 was derived from walls of about 0.20–0.45 W/m²K (Passivhaus Trust, 2022), so the app flags results below that range as less certain. --- # U-value of a timber frame wall with wood-fibre insulation URL: https://energycount.co.uk/tools/u-value-calculator/timber-frame-wall-wood-fibre/ Typical U-value: 0.17 W/m²K — 140mm studs + wood-fibre batts, 100mm wood-fibre render board A rendered timber frame wall with 140mm of wood-fibre batts between the studs (λ 0.036) and a 100mm wood-fibre render board outside them (λ 0.040), with OSB, a service void and plasterboard inside, has a U-value of about 0.17 W/m²K — inside the 0.18 notional value. The common 60mm board gives about 0.20, and 80mm shows as 0.18 but misses it, at 0.1803. Wood fibre insulates less well per millimetre than PIR, so the walls are thicker, but the board runs unbroken over the studs, which bridge 15% of the wall. Build-up, outside to inside: - 10mm lime-based render system - 100mm wood-fibre render board (λ 0.040) - 140mm wood-fibre batts between 140mm studs (15% timber) - 15mm OSB (racking board, vapour check and airtight layer) - 25mm service void on battens - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 40mm render board (type H, λ 0.043) | ≈ 0.23 | Type M starts at 60mm | | 60mm render board | ≈ 0.20 | A common thickness | | 80mm render board | ≈ 0.18 | Misses 0.18: 0.1803 | | 100mm render board | ≈ 0.17 | As pre-filled: meets 0.18 | | 120mm render board | ≈ 0.15 | | | 100mm board, 12.5% timber | ≈ 0.16 | BR 443 §4.4.1(ii) frame | | 100mm board, glass wool (λ 0.035) between the studs | ≈ 0.16 | Much the same | | Timber-clad: 60mm sarking board (λ 0.043), 140mm studs | ≈ 0.20 | Cladding and its batten space not counted | What changes the number: The studs and the board decide it. Studs bridge 15% of the wall by BR 443’s default (§4.4.1), or 12.5% with a single top plate, the sole plate below floor level, no mid-height noggings and junction studs no more than 38mm deep with insulation behind them; the board outside covers all of that timber, so each extra 20mm of it takes off about 0.013–0.023 W/m²K, less as it thickens. Wood-fibre batts and glass wool of similar λ give much the same figure, and the batts and the board sit together with no gap, which is BR 443’s air-gap level 0. What wood fibre changes is the wall’s behaviour rather than its U-value: it is vapour-open and holds moisture, so the wall is built to dry outwards, with the OSB inside as the vapour check and airtight layer and a render system made for wood-fibre boards outside. Q: Is wood fibre as good as PIR in a timber frame? A: Not per millimetre: wood-fibre boards run λ 0.036–0.043 against PIR’s 0.022, so it takes 100mm of wood-fibre board outside the frame to do what 60mm of PIR does — both walls come out at about 0.17 (see the timber frame wall with external PIR). Wood fibre is chosen for other reasons: it is vapour-open and hygroscopic, it can carry render directly, and it suits walls designed to let moisture out rather than keep it in. Note: Wood-fibre products as in U-Monkey’s library: STEICOflex 036 batts (λ 0.036) and STEICOprotect dry type M render board (λ 0.040, made 60–200mm). Studs at BR 443’s default 15%; the service-void battens at 11.8%. --- # U-value of an I-joist timber frame wall (Passivhaus-level) URL: https://energycount.co.uk/tools/u-value-calculator/i-joist-timber-frame-wall/ Typical U-value: 0.11 W/m²K — 300mm I-joists filled with cellulose, 60mm wood-fibre sheathing A timber frame wall of 300mm I-joists dense-filled with blown cellulose (λ 0.038), with 60mm of wood-fibre sheathing outside and OSB, a service void and plasterboard inside, has a U-value of about 0.11 W/m²K. 250mm I-joists give about 0.12 and 400mm about 0.08. The thin web is the point: BR 443 counts the flanges as 8.8% timber but the web as only 1.7%, so most of the depth is barely bridged — 300mm I-joists with no sheathing at all (0.124) do as well as solid studs at BR 443’s 15% default with the 60mm sheathing. Build-up, outside to inside: - Timber cladding on battens (a ventilated space: it and the cladding are not counted) - Breather membrane - 60mm wood-fibre sheathing board (λ 0.043) - 300mm I-joists at 600mm centres, dense-filled with blown cellulose: two 45mm flange zones (8.8% timber) and a 210mm web zone (1.7%) - 15mm OSB (airtight layer) - 25mm service void on battens - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 200mm I-joists | ≈ 0.14 | | | 250mm I-joists | ≈ 0.12 | | | 300mm I-joists | ≈ 0.11 | As pre-filled | | 350mm I-joists | ≈ 0.09 | | | 400mm I-joists | ≈ 0.08 | | | 300mm I-joists, no wood-fibre sheathing | ≈ 0.12 | 0.124 | | 300mm solid studs instead (BR 443’s 15% default), with the sheathing | ≈ 0.12 | 0.124 — the same as the I-joists without sheathing | What changes the number: BR 443 (2019) §4.4.2 treats an I-beam stud as three layers — flange, web, flange — each with its own timber fraction: 47mm flanges at 600mm centres are 8.8% and an 8–10mm web 1.7%. Because the web is so thin, deepening the joist adds insulation that is hardly bridged, which is why each extra 50mm still takes off about 0.01–0.02 W/m²K. Dense-blown cellulose fills the I-shape completely and runs continuously through the flange and web zones, so the air-gap correction is level 0. The OSB inside is usually the airtight layer, with the services kept in the void in front of it so the airtight line is not punctured. Q: Do I need a wall this good for the Future Homes Standard or Passivhaus? A: Not for the Future Homes Standard: its notional dwelling uses 0.18 W/m²K for walls (SAP 10.3 Appendix R) and the limiting value is 0.26, so 0.11 is far beyond both. Passivhaus certification sets limits on space heating demand, airtightness and primary energy rather than on U-values, so the wall is sized in the PHPP energy model together with the windows and the airtightness; deep, barely bridged timber walls like this one are one way of getting there. Note: BR 443 (2019) §4.4.2: an I-beam stud whose flange and web conductivities are within 20% of each other (softwood and OSB, both λ 0.13) is calculated as three bridged layers, with the fractions from its table for 47mm flanges and an 8–10mm web at 600mm centres. Cellulose at λ 0.038 (Warmcel, as in U-Monkey’s library). --- # U-value of an extension cavity wall (what reaches 0.18) URL: https://energycount.co.uk/tools/u-value-calculator/extension-cavity-wall/ Typical U-value: 0.17 W/m²K — 100mm cavity, λ 0.032 full fill, aircrete, 52.5mm insulated plasterboard An extension wall built with the house’s usual 100mm cavity — brick, 100mm of λ 0.032 cavity batts, 100mm aircrete blocks and plasterboard on dabs — comes out at about 0.25 W/m²K, well short of the 0.18 Approved Document L sets for a new wall in an existing home. Lining it with 52.5mm PIR insulated plasterboard (40mm of PIR) brings it to about 0.17, which meets; a 37.5mm board gives 0.19 and misses. The alternative is a wider cavity: 150mm of the same batts meets 0.18 with thermal-grade aircrete (0.176) but not quite with standard aircrete (0.181). Build-up, outside to inside: - 102.5mm brick outer leaf - 100mm cavity, fully filled with λ 0.032 glass mineral wool batts - 100mm aircrete block inner leaf (mortar joints counted) - 15mm gap on plaster dabs (20% dabs, BR 443 §4.6.1) - 40mm PIR insulated plasterboard core - 12.5mm plasterboard facing | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm cavity, λ 0.032 batts, plasterboard on dabs | ≈ 0.25 | Misses 0.18 | | + 25mm PIR insulated plasterboard (37.5mm board) | ≈ 0.19 | Misses: 0.1949 | | + 40mm PIR insulated plasterboard (52.5mm board) | ≈ 0.17 | As pre-filled: meets 0.18 | | 100mm cavity, dense block inner leaf, + 40mm PIR lining | ≈ 0.19 | Dense blocks undo it: 0.189 | | 150mm cavity, λ 0.032 batts, standard aircrete | ≈ 0.18 | Misses by a whisker: 0.181 | | 150mm cavity, λ 0.032 batts, thermal aircrete (λ 0.11) | ≈ 0.18 | Meets: 0.176 | | 150mm cavity, 100mm PIR + 50mm clear | ≈ 0.17 | | What changes the number: Matching the house usually means keeping its brick line and a 100mm cavity, and full fill can’t reach 0.18 there on its own: the lining has to do the rest, at the cost of about 40mm more room than plain plasterboard on each outside wall. Widening the cavity to 150mm puts the extra insulation where it costs no floor space, at the price of a wider wall and foundation, and the result then hangs on the inner leaf: standard aircrete 0.181, thermal-grade 0.176, dense block about 0.20. Mortar joints in the blockwork, the dabs behind the plasterboard and the wall ties all nibble at it; U-Monkey counts the mortar and the dabs for you, as BR 443 requires, and offers to add the ties. Q: What U-value does an extension wall need? A: In England, 0.18 W/m²K: an extension’s walls are new elements in an existing dwelling, so Approved Document L’s Table 4.2 applies (2021 edition; the 2026 edition, which takes effect on 24 March 2027, keeps 0.18 in its Table 3.2). The same table asks 0.15 of the roof, 0.18 of the floor and 1.4 of new windows and doors. A lesser standard may be appropriate for a wall where meeting it would reduce the floor area of the room by more than 5% (Table 4.2, note 3). Note: The pre-filled link opens as a new element in an existing home, judged against Approved Document L’s 0.18 (2021 edition, Table 4.2). Batts at λ 0.032 (Knauf DriTherm Cavity Slab 32 in U-Monkey’s library). --- # U-value of a wide cavity wall (200mm, 250mm, 300mm) URL: https://energycount.co.uk/tools/u-value-calculator/wide-cavity-wall/ Typical U-value: 0.13 W/m²K — 200mm cavity, 2 × 100mm λ 0.032 batts, thermal aircrete A brick-and-block wall with a 200mm cavity filled with two 100mm layers of λ 0.032 batts, a thermal-grade aircrete inner leaf and plasterboard on dabs has a U-value of about 0.13 W/m²K — comfortably inside the 0.18 notional value. A 250mm cavity gives about 0.11 and 300mm about 0.09; blown graphite EPS bead in a 300mm cavity gives about 0.10. Wall ties across a wide cavity add a little: stainless wire ties at 2.5 per m² take the 200mm wall from 0.134 to 0.135. Build-up, outside to inside: - 102.5mm brick outer leaf - 200mm cavity: two 100mm layers of λ 0.032 batts, joints staggered - 100mm thermal-grade aircrete inner leaf (λ 0.11, mortar joints counted) - 15mm gap on plaster dabs - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 150mm cavity, one layer of batts | ≈ 0.18 | Meets 0.18: 0.176 | | 200mm (2 × 100mm batts) | ≈ 0.13 | As pre-filled | | 250mm (2 × 125mm) | ≈ 0.11 | | | 300mm (2 × 150mm) | ≈ 0.09 | | | 300mm graphite EPS bead, blown | ≈ 0.10 | λ 0.033; a blown fill is air-gap level 0 | | 200mm batts, dense block inner leaf | ≈ 0.15 | | | 200mm batts + stainless wire ties (2.5/m²) | ≈ 0.14 | 0.135: under BS EN ISO 6946’s 3% rule | | 300mm batts + stainless wire ties | ≈ 0.10 | 0.095 | What changes the number: Each 50mm of λ 0.032 insulation takes off less than the last — 0.04 W/m²K from 150 to 200mm, under 0.02 from 250 to 300mm — so the wall’s thickness and its foundations, not the insulation, become the cost. Batts in two layers with staggered joints are BR 443’s air-gap level 0, and so is bead blown in once the walls are up (§7). Wide cavities need longer ties, and BR 443 asks for a correction where ties cross the insulation unless it is under 3% of the U-value: stainless wire ties stay under that here, and basalt-fibre or other ties below 1 W/mK need none at all. The inner leaf still counts: dense blocks instead of thermal aircrete cost about 0.013 at 200mm. Q: How wide does a cavity need to be for a U-value of 0.15? A: About 175mm of λ 0.032 batts with a thermal-grade aircrete inner leaf: 150mm gives 0.176, 175mm (100 + 75mm layers) 0.150 and 200mm 0.134. Getting down to about 0.10 takes nearly 300mm — 0.094 with two 150mm layers of batts or 0.097 with blown graphite bead, before ties. Note: Batts at λ 0.032 (Knauf DriTherm Cavity Slab 32, made up to 150mm thick, so a wider cavity takes two layers); bead at λ 0.033. Ties are not in the pre-filled build-up: U-Monkey offers to add them. --- # U-value of wood-fibre internal wall insulation on a solid brick wall URL: https://energycount.co.uk/tools/u-value-calculator/wood-fibre-internal-insulation/ Typical U-value: 0.49 W/m²K — 215mm brick, 60mm wood-fibre board, lime plaster A 215mm solid brick wall lined inside with 60mm of wood-fibre insulation board (λ 0.038) and lime plaster has a U-value of about 0.49 W/m²K, down from about 2.1. 80mm gives about 0.39 and 100mm about 0.33; it takes about 112mm to reach the 0.30 Approved Document L asks of an internally insulated wall. Most wood-fibre systems on solid walls stop well short of that on purpose: the thicker the lining, the colder and wetter the brick behind it, and the DEEP study, as Historic England reports it, found that targeting around 0.8 rather than 0.30 cut the moisture risk while keeping about two-thirds of the energy saving. Build-up, outside to inside: - 215mm solid brick - 15mm existing lime plaster - 60mm wood-fibre internal insulation board (λ 0.038), bonded to the wall - 10mm lime plaster finish | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Bare wall: 215mm brick, 15mm lime plaster | ≈ 2.12 | | | 40mm wood fibre | ≈ 0.66 | | | 60mm wood fibre | ≈ 0.49 | As pre-filled | | 80mm wood fibre | ≈ 0.39 | | | 100mm wood fibre | ≈ 0.33 | Misses 0.30 | | 120mm wood fibre (two boards) | ≈ 0.28 | Meets 0.30 | | 60mm board at λ 0.040 | ≈ 0.51 | | What changes the number: The board is bonded to the wall and finished in lime, so the wall can still dry to the inside: wood fibre is vapour-open and capillary-active, moving moisture back towards the room, which is the reason to choose it over a foil-faced board on an old wall. Thickness is the trade-off. The first 40–60mm does most of the good — at 60mm the wall loses under a quarter of the heat it did bare — while each extra 20mm saves less and leaves the brick colder. DEEP, the government-funded study of solid-wall insulation, found that targeting a U-value around 0.8 rather than 0.30 reduced the moisture risk while keeping about two-thirds of the energy saving, as Historic England reports it. The wall has to be in good order first: sound pointing, working gutters and no cement render or plastic paint holding water in. Q: Does wood-fibre insulation have to meet 0.30 under Part L? A: Approved Document L’s figure for a solid wall insulated internally is 0.30 W/m²K (Table 4.3), but it is not absolute. Where 0.30 is not technically or functionally feasible, the wall should reach the lowest U-value that is, and generally no worse than 0.7 (paragraph 4.13); a lesser standard may also be appropriate where 0.30 would cut a room’s floor area by more than 5% (note 6), or where the work complies with Part C on condensation. Paragraph 0.10 adds that historic and traditional dwellings should be improved only if it will not cause long-term deterioration of the fabric. On a solid wall, moisture risk is the usual reason a thinner lining is the right answer, and PAS 2035 notes that a BS EN ISO 13788 check alone may not be enough to judge it. Note: The pre-filled link opens as a retained solid wall being upgraded, judged against Approved Document L’s 0.30 (2021 edition, Table 4.3). Board at λ 0.038 (STEICOinternal, made 40–100mm, in U-Monkey’s library). U-Monkey’s ISO 13788 condensation check is a screening test here; a BS EN 15026 hygrothermal assessment is the fuller one. --- # U-value of external wall insulation with mineral wool URL: https://energycount.co.uk/tools/u-value-calculator/external-wall-insulation-mineral-wool/ Typical U-value: 0.27 W/m²K — 120mm mineral wool EWI on 215mm brick, fixings counted Rendered mineral-wool external wall insulation, 120mm thick (λ 0.036), on a 215mm solid brick wall gives a U-value of about 0.27 W/m²K once the steel fixings through it are counted — inside the 0.30 Approved Document L asks of an externally insulated wall. 110mm meets it too, at 0.29, and 100mm, at 0.32, does not. Mineral wool needs about a fifth more thickness than grey EPS for the same result (100mm of EPS at λ 0.031 gives 0.27), but it is non-combustible, which is why it is used where combustible insulation is not allowed. Build-up, outside to inside: - 10mm render system on mesh - 120mm dual-density mineral wool EWI slab (λ 0.036), with 6 steel screw fixings per m² - 215mm solid brick (protected by the insulation: λ 0.56) - 13mm plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm mineral wool | ≈ 0.32 | Misses 0.30 | | 110mm mineral wool | ≈ 0.29 | Meets: 0.294 | | 120mm mineral wool | ≈ 0.27 | As pre-filled: meets 0.30 | | 140mm mineral wool | ≈ 0.24 | | | 150mm mineral wool | ≈ 0.23 | | | 120mm, fixings left out | ≈ 0.26 | The fixings add about 0.01 | | 120mm slab at λ 0.039 | ≈ 0.29 | Meets: 0.291 | | 100mm grey EPS (λ 0.031), bonded, no fixings | ≈ 0.27 | For comparison | What changes the number: λ and the fixings set it. Mineral-wool EWI slabs run λ 0.034–0.039 against 0.030–0.032 for grey EPS, and mineral wool is mechanically fixed: each steel fixing through the insulation is a small thermal bridge, which BR 443 (§4.8.3) and BS EN ISO 6946 Annex F correct for. Here U-Monkey’s typical EWI screw fixings — 6 per m², 5mm stainless steel, λ 17 — add about 0.01; recessed fixings with insulating caps add less, so enter the system’s own fixing data where it is known. The rest is detailing: the insulation has to return into the window reveals, run down past the floor level at the base and up into the eaves, or those junctions undo a good part of the gain. Q: Why use mineral wool rather than EPS for external wall insulation? A: Fire. Mineral wool is non-combustible and EPS is not. In England, regulation 7(2) of the Building Regulations 2010 requires materials that become part of the external wall of a “relevant building” — one with a storey at least 18m above ground that contains dwellings, an institution or rooms for residential purposes — to be of European Classification A2-s1, d0 or A1, which rules out EPS there. Mineral wool is also chosen on lower buildings, at the cost of a thicker system for the same U-value. Note: The pre-filled link opens as a retained solid wall being upgraded, judged against Approved Document L’s 0.30 for external insulation (2021 edition, Table 4.3). Mineral wool at λ 0.036 (a dual-density EWI slab in U-Monkey’s library); fixings as the app’s “EWI screw fixings” preset, corrected by BS EN ISO 6946 Annex F.3. The brick behind the insulation is taken as protected, λ 0.56 (BR 443 §7), with its mortar joints counted (§4.2). --- # U-value of a knee wall in a loft conversion URL: https://energycount.co.uk/tools/u-value-calculator/knee-wall/ Typical U-value: 0.18 W/m²K — 100mm PIR between studs + 50mm PIR inside (with the eaves space taken as outside air; 0.16 with SAP’s allowance for it) A knee wall with 100mm of PIR between the studs and a continuous 50mm PIR layer inside them, finished in plasterboard, has a U-value of about 0.18 W/m²K taking the eaves space behind it as outside air — just inside the 0.18 Approved Document L sets for a new wall in an existing home (0.1796). 40mm inside gives 0.20 and misses on that basis. SAP 10.2 treats the eaves as an unheated space and adds a resistance of 0.5 m²K/W for it (Table 3.4), which takes the pre-filled wall to about 0.16 and the 40mm version to 0.179, inside 0.18; U-Monkey applies it when you pick the loft void under “On the other side”. Build-up, outside to inside: - (The ventilated eaves space behind: taken as outside air) - Breather membrane on the eaves face - 100mm PIR between 100mm studs (15% timber) - 50mm PIR across the inside of the studs (continuous) - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Uninsulated: plasterboard on studs | ≈ 2.27 | | | 50mm quilt between the studs | ≈ 0.84 | Many older rooms in roofs | | 100mm PIR between the studs only | ≈ 0.35 | The studs bridge it | | 100mm between + 25mm PIR inside | ≈ 0.23 | | | 100mm between + 40mm PIR inside | ≈ 0.20 | Misses (0.197); 0.179 with SAP’s Ru | | 100mm between + 50mm PIR inside | ≈ 0.18 | As pre-filled: 0.1796 | | 100mm mineral wool between + 50mm PIR inside | ≈ 0.22 | | | Pre-filled wall with SAP’s eaves allowance (Ru 0.5) | ≈ 0.16 | 1/(1/0.1796 + 0.5) = 0.1648 | What changes the number: The studs bridge 15% of the insulation between them, so the continuous layer inside does more per millimetre than extra depth: 100mm between the studs plus 25mm across them (0.23) beats 150mm between deeper studs (0.24). Many loft conversions avoid the knee wall as a thermal line altogether by insulating the roof slope right down to the eaves, which brings the eaves space inside; the knee wall then needs nothing, but the slope has to meet the roof figure all the way down. Where the knee wall is the line, the floor of the eaves space behind it — the ceiling of the room below — has to be insulated too, and the junctions sealed, or cold air from the ventilated eaves washes through the insulation. Q: Can I count the eaves space behind a knee wall? A: In a SAP calculation, yes: SAP 10.2 §3.3.5 gives an unheated-space resistance of 0.5 m²K/W (Table 3.4) for the insulated wall of a room in roof built into a roof insulated at ceiling level, for new dwellings and for conversions that create a room in roof, and BR 443 §5 says the effect of an unheated space can be incorporated into the U-value of the element. The pre-filled link takes the eaves as outside air, the cautious figure; pick “loft void behind the insulated wall of a room in roof” under “On the other side” and U-Monkey adds the 0.5. With Ru 0.5 the wall needs about 0.198 on its own to come out at 0.18, so ask your building control body which basis it will accept before thinning the lining. Note: The pre-filled link opens as a new element in an existing home, judged against Approved Document L’s 0.18 (2021 edition, Table 4.2). The eaves are taken as outside air (Rse 0.04); SAP’s eaves allowance (SAP 10.2 and 10.3, Table 3.4) is the separate row, 1/(1/U + 0.5) — in the app, “On the other side”: the loft void behind the insulated wall of a room in roof. --- # U-value of a party wall (0.0, 0.2 or 0.5) URL: https://energycount.co.uk/tools/u-value-calculator/party-wall/ Typical U-value: 0.00 W/m²K — Cavity fully filled and sealed at the edges A party wall’s U-value is assigned by its construction, not calculated from its layers: 0.5 W/m²K for an unfilled cavity with no effective edge sealing, 0.2 for an unfilled cavity effectively sealed at all its exposed edges, and 0.0 for a solid wall or a cavity that is fully filled and effectively sealed (SAP 10.2 Table 3.10; Approved Document L Table 2.1). The heat is not conducted to the neighbour: it is carried by air moving up through the cavity and out at the top — the party wall bypass, measured at Stamford Brook as an effective 0.50–0.63. Part L 2021’s notional dwelling assumes 0.0; the limiting value is 0.20. Build-up, outside to inside: - 12.5mm plasterboard on dabs - 100mm lightweight aggregate block - 75mm cavity fully filled with party wall roll, sealed at every exposed edge - 100mm lightweight aggregate block - 12.5mm plasterboard on dabs | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Solid wall (masonry, or a SIP) | 0.0 | SAP 10.2 Table 3.10 | | Cavity fully filled and effectively sealed at all exposed edges | 0.0 | As pre-filled | | Unfilled cavity, effectively sealed at all exposed edges | 0.2 | | | Unfilled cavity, no effective edge sealing | 0.5 | Worse than the 0.20 limiting value | | Measured, unfilled cavities (Stamford Brook, 2007) | 0.50–0.63 | The effective U-value of the bypass | | Home Energy Model: unfilled, unsealed, dry-lined | 0.6 | Behind wet plaster: 0.2 (HEM-TP-05) | What changes the number: Sealing and fill are the whole story. Sealing the cavity at the top, the bottom and the vertical edges — in line with the insulation of the roof, floor and external walls — stops the stack effect, and filling it stops air moving within it; only both together earn 0.0, and Approved Document L asks that the edge sealing be likely to be robust under normal site conditions (Table 2.1, note 1). Part E still applies: the fill and the seals must not spoil the wall’s sound insulation, and several Robust Details masonry separating walls (E-WM-17, E-WM-20 and E-WM-24, for example) now include a full-fill option with a named party wall roll. Under the Home Energy Model, one of the two routes in the 2026 edition of Approved Document L, the lining matters as well: an unfilled, unsealed cavity behind plasterboard on dabs counts as 0.6, behind wet plaster 0.2. Q: Do I have to fill a party wall cavity? A: To claim 0.0, yes: fully filled and effectively sealed at every exposed edge. Sealing alone gets 0.2, and an open, unfilled cavity 0.5, which is worse than Approved Document L’s 0.20 limiting value for a party wall (Table 4.1). The fill has to be compatible with the wall’s sound insulation under Part E, so use a Robust Details specification with a full-fill option, or a design that is sound-tested. Note: U-Monkey assigns a party wall’s U-value from the construction you pick, as SAP 10.2 (Table 3.10) and Approved Document L (2021 edition, paragraph 2.14 and Table 2.1) do: the layers are shown, and used for thermal mass, but do not change it. The pre-filled link opens with the cavity fully filled and sealed. --- # U-value of a basement wall (heated basement) URL: https://energycount.co.uk/tools/u-value-calculator/basement-wall/ Typical U-value: 0.16 W/m²K — 250mm concrete, 150mm XPS outside, 2.5m deep A heated basement’s 250mm concrete retaining wall with 150mm of XPS insulation outside and plasterboard on battens inside has a U-value of about 0.16 W/m²K, for a basement 2.5m deep under a typical house (30m perimeter, 50m² floor) with its floor insulated to match. The ground in front of the wall does part of the work: 100mm of XPS gives about 0.21 and 200mm about 0.12. The figure is an average over the buried depth, worked out with the floor by BS EN ISO 13370 — leave the basement floor uninsulated and the same wall comes out at about 0.12. Build-up, outside to inside: - (The ground: allowed for by BS EN ISO 13370 from the depth, the plan and the floor’s resistance) - 150mm XPS insulation outside the wall (λ 0.033) - Waterproofing (tanking) membrane - 250mm reinforced concrete retaining wall - 25mm service void on battens - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm XPS | ≈ 0.21 | | | 120mm XPS | ≈ 0.18 | | | 150mm XPS | ≈ 0.16 | As pre-filled | | 200mm XPS | ≈ 0.12 | | | 150mm XPS, basement floor not insulated | ≈ 0.12 | BS EN ISO 13370 links the two | | 150mm XPS, 2.0m deep | ≈ 0.16 | 0.162 | | 150mm XPS, 3.0m deep | ≈ 0.15 | | | 150mm XPS, clay soil (λ 1.5) | ≈ 0.15 | BR 443: clay is the commonest UK ground | | Uninsulated: concrete, plasterboard on dabs | ≈ 0.72 | | What changes the number: Depth, the floor and the ground move it as well as the insulation. The deeper the basement, the more ground lies between the wall and the outside air, so the average U-value falls with depth. BS EN ISO 13370 calculates the wall with the floor: an insulated floor leaves the ground beneath the basement colder, and the wall loses more heat into it, which is why U-Monkey asks for the floor’s resistance on the basement-wall element (R 4.6 here, the basement floor page’s 150mm of XPS). The soil matters too: U-Monkey uses 2.0 W/mK, sand or gravel, unless told otherwise, and BR 443 notes that clay (1.5) is the commonest UK ground. Insulating outside the wall keeps the concrete and the waterproofing warm and the room full size, and every layer of the wall counts, the concrete included (BR 443, 2006 edition, §10.1). Q: Does a basement wall need the same U-value as a wall above ground? A: Yes: Approved Document L has no separate row for basements, so a basement wall is a wall — 0.26 W/m²K limiting in a new dwelling (Table 4.1) and 0.18 for a new wall in an existing one (Table 4.2). What differs is the calculation: the U-value includes the ground, by BS EN ISO 13370, as SAP 10.2 and BR 443 require, so less insulation reaches the same figure than above ground. Note: Calculated to BS EN ISO 13370 for a heated basement: perimeter 30m, floor area 50m² (P/A 0.6), floor 2.5m below ground, wall 300mm thick above ground, soil λ 2.0, and the basement floor’s resistance R 4.6 m²K/W entered as the app’s floor R. XPS at λ 0.033 (Kingspan GreenGuard GG300 in U-Monkey’s library). --- # U-value of a cellar wall when converting a cellar (basement conversion) URL: https://energycount.co.uk/tools/u-value-calculator/cellar-conversion-wall/ Typical U-value: 0.25 W/m²K — 215mm brick cellar wall, cavity drain membrane, 62.5mm insulated plasterboard A 215mm brick cellar wall lined with a cavity drain membrane and 62.5mm PIR insulated plasterboard (50mm of PIR) on dabs has a U-value of about 0.25 W/m²K, for a cellar 2m deep under a typical house whose existing floor is upgraded as well (70mm of PIR under a screed gives about 0.21, inside the 0.25 Approved Document L asks of a retained floor). That is inside the 0.30 Approved Document L asks of a retained wall insulated inside; 40mm of PIR gives about 0.28 and 25mm about 0.35, which misses. The bare wall is about 0.84 — far better than the same brick above ground, because the ground is part of the calculation (BS EN ISO 13370). Build-up, outside to inside: - (The ground: allowed for by BS EN ISO 13370) - 215mm brick cellar wall - Studded cavity drain membrane (an 8mm air gap behind it) - 15mm gap on plaster dabs - 50mm PIR insulated plasterboard core - 12.5mm plasterboard facing | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Bare cellar wall: 215mm brick, lime plaster | ≈ 0.84 | | | 25mm PIR board | ≈ 0.35 | Misses 0.30 | | 40mm PIR | ≈ 0.28 | | | 50mm PIR | ≈ 0.25 | As pre-filled (0.246) | | 75mm PIR | ≈ 0.19 | | | 50mm PIR, cellar floor left uninsulated | ≈ 0.20 | BS EN ISO 13370 links wall and floor | | 50mm PIR, cellar 2.5m deep | ≈ 0.23 | | What changes the number: A converted cellar keeps water out by managing it: BS 8102’s Type C protection lets water through the old wall into the studded cavity drain membrane, which carries it to a drainage channel and pump, and the lining goes on the room side of the membrane — on a mesh-faced membrane, insulated plasterboard can be dabbed straight onto it. The ground does a lot of the work, so modest insulation goes a long way, and the result depends on the cellar’s depth, its plan and the floor: BS EN ISO 13370 links the wall and the floor, so insulating the floor raises the wall’s figure (0.20 with the floor left bare). The membrane is vapour-tight, so the room needs heating and ventilation that keep its humidity down, as the membrane makers stress. Q: What U-values does a cellar conversion need? A: In England, Approved Document L treats a cellar’s existing walls and floor like those of a loft or garage conversion — retained elements (paragraph 4.12(b)) — so a wall worse than 0.70 should be upgraded to 0.30 if insulated inside, and the floor to 0.25 (Table 4.3), so far as that is technically and functionally feasible with a simple payback of 15 years or less (paragraph 4.13). New elements meet Table 4.2. Both wall and floor are calculated with the ground, to BS EN ISO 13370, which is why thin linings can meet them. Note: Calculated to BS EN ISO 13370 for a heated basement: perimeter 30m, floor area 50m², floor 2.0m below ground, soil λ 2.0, with the floor’s resistance R 3.24 m²K/W (70mm of PIR under a 65mm screed laid over the existing floor, a floor U-value of about 0.21, inside the 0.25 for a retained floor) entered as the app’s floor R. A new floor, as where the cellar is dug out and relaid, should meet Table 4.2’s 0.18 instead: about 90mm of PIR here (0.17), which leaves the wall figures on this page unchanged. The pre-filled link opens as a retained wall being upgraded, judged against Approved Document L’s 0.30 (2021 edition, Table 4.3). --- # U-value of a hempcrete wall (hemp-lime) URL: https://energycount.co.uk/tools/u-value-calculator/hempcrete-wall/ Typical U-value: 0.23 W/m²K — 300mm cast hempcrete (λ 0.07) round a timber frame, lime finishes A 300mm cast hempcrete wall round a timber frame, lime-rendered outside and lime-plastered inside, has a U-value of about 0.23 W/m²K at λ 0.07 — inside the 0.26 limiting value for a new wall but short of the 0.18 notional value. The often-quoted 0.20 for 300mm needs λ 0.06, the figure given for Tradical Hemcrete at 275 kg/m³; denser mixes declared at 0.076–0.085 give 0.25–0.27, and the 0.27 misses the limit. 400mm at λ 0.07 gives about 0.17. Build-up, outside to inside: - 20mm lime render - 125mm cast hempcrete (λ 0.07) - 100mm hempcrete round 100mm timber studs (15% timber) - 75mm hempcrete covering the studs inside - 15mm lime plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 300mm at λ 0.06 | ≈ 0.20 | Tradical Hemcrete, 275 kg/m³ | | 300mm at λ 0.07 | ≈ 0.23 | As pre-filled | | 300mm at λ 0.076 | ≈ 0.25 | Tradical (France), 280 kg/m³ | | 300mm at λ 0.085 | ≈ 0.27 | Tradical (France), 320 kg/m³; misses the 0.26 limiting value | | 200mm at λ 0.07 | ≈ 0.34 | Misses the 0.26 limiting value | | 400mm at λ 0.07 | ≈ 0.17 | Meets 0.18 | | 300mm at λ 0.07, studs not counted | ≈ 0.22 | | What changes the number: λ is the whole question. Hempcrete’s conductivity rises with density — how hard it is tamped or sprayed, and the binder — and with moisture: declared values run from about 0.06 at 275 kg/m³ to 0.085 at 320 kg/m³, and Vicat’s data sheet advises adding 20–30% to dry values for the humidity of the air. IsoHemp’s certified design values for its blocks (λUi, UBAtc approval ATG 3169) are 0.071 and 0.080, so U-Monkey’s 0.07 sits at the good end. The timber frame barely matters: the studs (λ 0.13) are only about twice as conductive as the hempcrete round them, so counting them adds about 0.007. What hempcrete brings beyond its U-value is moisture buffering and a vapour-open wall, so keep the finishes lime-based. Q: Is 300mm of hempcrete enough for building regulations? A: It depends on the λ the mix can show in service. At λ 0.07, 300mm gives about 0.23 W/m²K, inside the 0.26 limiting value for a new wall (Approved Document L, Table 4.1); at 0.085, a denser mix, it gives about 0.27 and misses, and some makers advise adding 20–30% to dry λ values for the humidity of the air. So use the declared design value for your mix, not a dry laboratory figure — or build thicker: 400mm gives about 0.17 at λ 0.07. Either way 300mm is short of the 0.18 the notional dwelling uses, so the rest of the house has to make that up in the SAP calculation; for an extension, where each new wall should meet 0.18 (Table 4.2), 300mm misses even at λ 0.06 (about 0.20): it takes about 385mm at λ 0.07, or about 335mm at 0.06. Note: Hempcrete at λ 0.07 (U-Monkey’s library; typical cast hemp-lime at 275–330 kg/m³). Sources for the variants: Lime Technology’s figure for Tradical Hemcrete as published by the Limecrete Company (0.06 at 275 kg/m³) and BCB-Tradical’s 2016 brochure (0.076 / 0.085 at 280 / 320 kg/m³). Studs at BR 443’s default 15%, in the 100mm of wall they occupy. --- # U-value of a wall between a house and an integral garage URL: https://energycount.co.uk/tools/u-value-calculator/wall-to-integral-garage/ Typical U-value: 0.20 W/m²K — 75mm PIR partial fill, dense block garage leaf, integral garage (Ru 0.35) (with the integral garage counted as SAP 10.2 counts it; 0.21 facing outside air) A cavity wall between a house and its integral garage — a 100mm dense block garage leaf, a 50mm clear cavity, 75mm of PIR partial fill and an aircrete inner leaf with plasterboard on dabs — has a U-value of about 0.20 W/m²K with the garage counted the way SAP counts it, and 0.21 taking the garage as outside air. SAP 10.2 treats an integral garage as an extra thermal resistance, Ru, outside the wall: 0.35 m²K/W for a single garage that shares a side wall, an end wall and the floor with the house and sits outside its insulation (Table 3.1), applied as U = 1/(1/Uo + Ru). On a well-insulated wall that takes off only about 0.015; on a weak one it can decide a pass, as 50mm of PIR misses the 0.26 limiting value facing outside air (0.273) and meets it with the garage counted (0.249). An attached garage, as opposed to an integral one, is disregarded altogether. Build-up, outside to inside: - (The integral garage beyond: Ru 0.35, SAP 10.2 Table 3.1) - 100mm dense concrete block garage leaf - 50mm clear cavity - 75mm PIR partial-fill board - 100mm aircrete block inner leaf (mortar joints counted) - Plasterboard on dabs | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Facing outside air (or an attached garage, which SAP disregards) | ≈ 0.21 | Uo: 0.211 | | Single garage, fully integral: side wall, end wall and floor, outside the insulation (Ru 0.35) | ≈ 0.20 | As pre-filled: 0.197 | | Single garage, fully integral: one wall and floor, outside the insulation (Ru 0.25) | ≈ 0.20 | 0.2005; a half-integral double garage takes the same Ru | | Single garage, partially integral, displaced forward (Ru 0.30) | ≈ 0.20 | 0.199 | | Single garage, fully integral, inside the insulation (Ru 0.70) | ≈ 0.18 | 0.184: the insulated envelope goes round the garage | | 50mm PIR partial fill, Ru 0.35 | ≈ 0.25 | Meets the 0.26 limit only with the garage counted (0.273 without) | | Full fill: 100mm mineral wool (λ 0.032) between dense block and aircrete, Ru 0.35 | ≈ 0.23 | Fails U-Monkey’s condensation check: see below | | Single leaf: 100mm dense block + 82.5mm insulated plasterboard (70mm PIR), Ru 0.35 | ≈ 0.26 | Meets 0.26 only with the garage counted: 0.258 (0.284 without) | | Timber stud wall: 89mm mineral wool between the studs, VCL, plasterboard both sides, Ru 0.35 | ≈ 0.43 | 0.51 facing outside air | What changes the number: Pick the Ru that matches the plan. SAP 10.2 Table 3.1 covers single garages (about 3m × 6m) and Table 3.2 double garages (about 6m × 6m), both ventilated at 3 air changes an hour; the values depend on how much of the garage the house wraps round, and for a single garage roughly double when the house’s insulated envelope goes round the outside of it. The same Ru helps a weak wall more than a good one: 0.35 m²K/W is worth about 8mm of PIR (0.35 × λ 0.022), which takes 0.27 down to 0.25 but 0.21 only to 0.20. A garage much bigger or smaller than SAP’s typical one takes its own Ru from SAP’s formula (4), Ru = Ai / (Σ Ae·Ue + 0.33·n·V), which U-Monkey takes as a custom value under “On the other side”. The junctions at the garage are sheltered too: the SAP 10 conventions (5.12) multiply their Ψ-values by 0.83–0.94. And the wall, floor and door between the garage and the house have to provide the fire separation Part B asks for. Q: What Ru do I use for an integral garage in SAP? A: SAP 10.2 Table 3.1, for a single garage outside the house’s insulation: 0.35 m²K/W when it is fully integral, sharing a side wall, an end wall and the floor with the house, 0.25 when it is fully integral but shares one wall and the floor, and 0.30 when it is partially integral, displaced forward. When the house’s insulated envelope goes round the outside of the garage instead, they become 0.70, 0.55 and 0.60. Table 3.2 gives a double garage 0.35, 0.25 and 0.25 outside the insulation, and 0.60, 0.35 and 0.30 inside it. The values apply to the elements between the house and an integral garage only — an attached garage is disregarded — and the adjustment can be left out where the garage covers less than 10% of the house’s external wall area (or of its heat-loss floor area, for a garage under a floor). SAP 10.3 has the same tables. Note: Ru from SAP 10.2 (and 10.3) Table 3.1, applied as SAP’s formula (3), U = 1/(1/Uo + Ru), where Uo is the wall calculated as if it faced outside air; BR 443 §5 says the effect of an unheated space can be incorporated into the U-value of the element. The pre-filled link opens with “On the other side” set to this garage. The 50mm cavity beside the foil-faced board counts as a low-emissivity cavity (BR 443), worth about 0.01 here (0.197 against 0.207). Building control bodies may differ on whether the limiting values are checked with or without the garage, so agree the basis first. Condensation risk in a wall to a garage: U-Monkey’s free BS EN ISO 13788 check takes the garage as outside air, the cautious case, and the pre-filled wall passes it with no condensation in any month: the PIR’s foil facings stop vapour from the rooms on the warm side of the cavity, before it reaches the cold garage leaf. Fill the same cavity with 100mm of mineral wool instead and the check fails it. Condensation collects on the back of the dense block every winter, peaking at about 1.2 kg/m² in April, and about 830 g/m² of it is still there at the end of the year, because dense block resists vapour about four times as much as aircrete or brick (μ 40 against 10, from BS 5250). An aircrete garage leaf lets the same full-filled wall dry out each summer (a peak of about 220 g/m²); brick gets a conditional pass. A timber stud wall needs its vapour control layer on the house side of the wool: leave it out and the check finds over 3 kg/m² of condensation on the back of the garage-side board by March, which dries out in summer but is not something to build. The partial-fill pass rests on the foil facings being continuous, with the boards’ joints taped and their edges sealed: taken as a vapour check of only sd 5m, the PIR lets about 140 g/m² of seasonal condensation form, and at sd 2m the check fails. A real garage is milder than outside air, so these are screening results, not predictions — but they are a reason to keep vapour resistance on the warm side of the insulation, as the pre-filled wall does. --- # U-value of a wall to an unheated corridor or stairwell in flats URL: https://energycount.co.uk/tools/u-value-calculator/wall-to-unheated-corridor/ Typical U-value: 0.24 W/m²K — 215mm dense block + 82.5mm insulated plasterboard, corridor Ru 0.6 (with the corridor counted as SAP 10.2 counts it; 0.28 facing outside air) A 215mm dense blockwork wall between a flat and an unheated access corridor, lined on the flat’s side with 82.5mm insulated plasterboard (70mm of PIR), has a U-value of about 0.24 W/m²K with the corridor counted the way SAP 10.2 counts it, and about 0.28 taking the corridor as outside air. SAP adds the corridor as an extra resistance, Ru, outside the wall — 0.6 m²K/W for a corridor whose facing wall is exposed, with corridors above and below (Table 3.3) — using U = 1/(1/Uo + Ru). Next to an unheated stairwell the allowance is much bigger, Ru 2.1 to 2.5, which takes the same wall to 0.16–0.17. If the corridor is heated — with controlled fixed heaters, in SAP’s conventions (5.03) — the wall counts as a party wall instead, and SAP gives a solid one a U-value of 0. Build-up, outside to inside: - (The unheated access corridor beyond: Ru 0.6, SAP 10.2 Table 3.3) - 215mm dense concrete blockwork - 15mm plaster dabs - 82.5mm insulated plasterboard: 70mm PIR bonded to 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Facing outside air | ≈ 0.28 | Uo: 0.276, which misses the 0.26 limit | | Access corridor, facing wall exposed, corridors above and below (Ru 0.6) | ≈ 0.24 | As pre-filled: 0.237 | | Access corridor, facing wall exposed, corridor above or below (Ru 0.5) | ≈ 0.24 | 0.243 | | Access corridor, facing wall not exposed, corridors above and below (Ru 0.9) | ≈ 0.22 | | | Access corridor, facing wall not exposed, corridor above or below (Ru 0.7) | ≈ 0.23 | | | Unheated stairwell, facing wall exposed (Ru 2.1) | ≈ 0.17 | Inside the 0.18 notional value: 0.175 | | Unheated stairwell, facing wall not exposed (Ru 2.5) | ≈ 0.16 | | | 60mm PIR (72.5mm board), corridor Ru 0.6 | ≈ 0.26 | Misses 0.26 even with the corridor: 0.264 | | 50mm PIR (62.5mm board), corridor Ru 0.6 | ≈ 0.30 | | | Unlined: 215mm dense block plastered both sides, corridor Ru 0.6 | ≈ 0.99 | 2.43 facing outside air | What changes the number: Ru depends on which surfaces of the corridor lose heat. SAP 10.2 Table 3.3 asks whether the corridor’s facing wall — the one opposite the flat — is an external wall, and whether there are corridors both above and below or only above or below (SAP’s Figure 3.1 shows the layouts); a stairwell gets far more, Ru 2.1–2.5. SAP 10 recalculated these values at one air change an hour, where SAP 2012 had used three, because buildings had become more airtight (BRE consultation paper CONSP:09), so its figures are much higher than SAP 2012’s 0.28–0.43 for corridors and 0.82–0.90 for stairwells: an older assessment of the same wall shows a higher U-value. The lining does the work either way — the 215mm of block is worth about 0.19 m²K/W, the 70mm of PIR about 3.2. The SAP conventions also give a window onto an unheated corridor a reduced U-value (5.17) and multiply the Ψ-values of junctions at the corridor or stairwell by 0.71–0.85 (5.12). U-Monkey’s condensation check takes the corridor as outside air; it finds no condensation in any month, but rates internal insulation on solid masonry as a screening result only. Q: What U-value does a wall to an unheated corridor need? A: It is a thermal element: it separates the flat from an unconditioned part of the building (Building Regulations 2010, regulation 2(3)). In England, in a new block that means the 0.26 W/m²K limiting value for a wall (Approved Document L 2021, Table 4.1), and the notional dwelling that sets the energy targets gives “external walls including semi-exposed walls” 0.18 (Table 1.1). Where the common areas are unheated, Approved Document L also asks for their own fabric elements to meet the Section 4 standards (paragraph 0.4). SAP counts the corridor’s Ru in the energy calculation, and BR 443 §5 says the effect of an unheated space can be incorporated into the U-value of the element; whether your building control body checks the limiting value with it is worth agreeing first. The pre-filled wall meets 0.26 only with the corridor counted (0.237, against 0.276), and 60mm of PIR misses it even then (0.264). Note: Ru from SAP 10.2 (and 10.3) Table 3.3, applied as SAP’s formula (3), U = 1/(1/Uo + Ru), where Uo is the wall calculated as if it faced outside air. The pre-filled link opens with “On the other side” set to this corridor. The insulated plasterboard is modelled as 70mm of PIR bonded to 12.5mm plasterboard, on dabs bridging 20% of the gap (BR 443 §4.6.1). --- # U-value of a CLT (cross-laminated timber) wall URL: https://energycount.co.uk/tools/u-value-calculator/clt-wall/ Typical U-value: 0.16 W/m²K — 100mm CLT, 200mm wood-fibre board outside, rendered A 100mm cross-laminated timber wall with 200mm of wood-fibre insulation board outside it (STEICO Protect L, λ 0.037), a thin lime render and a plasterboard lining on battens has a U-value of about 0.16 W/m²K, with the fixings that hold the board on counted. 180mm of board gives about 0.18, inside the 0.18 notional value, and 160mm about 0.19. The panel itself does little: at λ 0.12, 100mm of CLT is worth about 0.83 m²K/W, against 5.4 for the wood fibre, so the board outside sets the figure. Build-up, outside to inside: - 10mm lime render - 200mm wood-fibre insulation board (STEICO Protect L, λ 0.037), screwed through to the panel (6 fixings/m²) - 100mm CLT panel (λ 0.12) - 25mm service void on battens (11.8% timber, BR 443 §4.6.2) - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 160mm wood-fibre board (STEICO Protect L) | ≈ 0.19 | Clears the 0.26 limit, misses the 0.18 notional | | 180mm | ≈ 0.18 | Inside 0.18: 0.175 | | 200mm | ≈ 0.16 | As pre-filled | | 240mm | ≈ 0.14 | | | 160mm Gutex Thermowall (λ 0.040) | ≈ 0.20 | | | 160mm graphite EPS (λ 0.031), rendered | ≈ 0.17 | | | 200mm wood fibre, CLT left exposed inside (no lining) | ≈ 0.17 | 0.168 | | 200mm on a 120mm panel | ≈ 0.16 | 0.158 | | 200mm, fixings left out | ≈ 0.16 | 0.157: the fixings add about 0.005 | What changes the number: The insulation outside the panel does nearly all the work, and because CLT is solid there are no studs to bridge it — only the screws that hold the boards on, which add about 0.005 here at six a square metre (BS EN ISO 6946 Annex F). Wood fibre (λ 0.037–0.040) needs a fifth to nearly a third more thickness than graphite EPS (λ 0.031) for the same figure; what it gives in return is a vapour-open build-up that lets the timber dry outwards. KLH and Stora Enso both declare their panels at λ 0.12 (to EN ISO 10456); taking 0.13, the figure for softwood generally, adds only about 0.002. Leaving the panel exposed inside costs the lining and the air behind it, about 0.007. Stora Enso’s own building-physics tables give 0.17–0.18 for a 100mm panel with 200mm of wood fibre at λ 0.038–0.039; with that board U-Monkey gives about 0.17. Height matters for the materials: in England, on a relevant building — a building with a storey at least 18m above ground, containing dwellings, an institution or rooms for residential purposes — regulation 7(2) of the Building Regulations limits the materials in an external wall to Euroclass A1 or A2-s1,d0, which rules out timber panels, wood fibre and EPS there. Q: Does a CLT wall need a vapour control layer? A: Not usually as a separate membrane. The panel is a vapour check in itself: KLH declares a vapour resistance factor of 46 (wet) to 300 (dry), so 100mm of panel resists vapour like 4.6–30m of still air, while the wood fibre outside is vapour-open and lets the wall dry outwards. U-Monkey’s free condensation check finds no condensation in any month in any of the walls on this page. The joints between panels and around windows and services still need sealing, because air leaking through them carries far more moisture than diffusion does. Note: CLT at λ 0.12 from KLH’s Declaration of Performance (2025); Stora Enso declares the same (ETA-14/0349). The board is STEICO Protect L dry from U-Monkey’s library, with its fixings counted as stainless screws at 6 per m² (BS EN ISO 6946 Annex F). The service battens are counted at 11.8% (BR 443 §4.6.2). --- # U-value of a straw bale wall URL: https://energycount.co.uk/tools/u-value-calculator/straw-bale-wall/ Typical U-value: 0.18 W/m²K — 450mm bales laid flat (λ 0.080), lime plaster both sides A straw bale wall of 450mm bales laid flat, with 25mm of lime plaster on each face, has a U-value of about 0.18 W/m²K (0.1797), just inside the 0.18 notional value, taking the λ of 0.080 that the German building approval for construction straw bales (2014, since lapsed) gave for heat flowing along the stalks. Which way the stalks run matters more than the size of the bale: set on edge, with the heat crossing the stalks (λ 0.052), a 350mm bale gives about 0.15 — better than the thicker bale laid flat. U-Monkey adds BR 443’s correction for air gaps in a single layer of insulation, about 0.009; most published figures leave it out. Build-up, outside to inside: - 25mm lime render - 450mm straw bales laid flat (stalks running through the wall: λ 0.080) - 25mm lime plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 450mm bales laid flat (λ 0.080, heat along the stalks) | ≈ 0.18 | As pre-filled: 0.1797 | | 480mm bales laid flat | ≈ 0.17 | | | 350mm bales on edge (λ 0.052, heat across the stalks) | ≈ 0.15 | Thinner, but better | | 360mm bales on edge | ≈ 0.15 | 0.149 | | 450mm at λ 0.065 (big bales, Passipedia) | ≈ 0.15 | | | 450mm at λ 0.09 (a 2001 UK guide) | ≈ 0.20 | | | 400mm EcoCocon straw panel (λ 0.0645) | ≈ 0.16 | EcoCocon quotes 0.150–0.152 without finishes or the air-gap correction | | 400mm EcoCocon panel + 60mm wood-fibre render board | ≈ 0.13 | EcoCocon: 0.127–0.129 with 60mm of wood-fibre board | What changes the number: The bale’s orientation and density set the λ, and the published values differ. In a two-string bale the stalks run along the bale’s width, across the strings, so a bale laid flat carries heat along the stalks and one on edge across them: the German approval for construction straw (DIBt Z-23.11-1595, 2014, lapsed in 2019) gave design values of 0.080 and 0.052 for the two, and Passipedia quotes the same pair, with 0.065 for big bales; a newer European assessment for one maker’s bales (ETA-17/0247) declares 0.048 across the stalks, and Bath University’s tests for the ModCell panel proposed 0.064. A 2001 UK guide quotes 0.09 and, in the same breath, 0.13 for walls over 450mm thick — which would need λ of about 0.06; at 0.09 a 450mm wall is about 0.20. The plaster adds little to the U-value but makes the wall airtight, which straw needs. Prefabricated straw panels such as EcoCocon’s, straw packed into a timber frame, work the same way. U-Monkey applies BR 443’s air-gap correction for a single layer of insulation, about 0.009 on these walls; makers’ figures usually leave it out, which accounts for most of the gap between EcoCocon’s 0.150–0.152 for its 400mm panel without finishes and U-Monkey’s 0.164 with lime plaster both sides. With a second layer, the wood-fibre board, the correction drops away and the figures agree: 0.128 here with 60mm of STEICO Protect H, against EcoCocon’s 0.127–0.129 with 60mm of wood-fibre board. Q: Is there a condensation risk in a straw bale wall? A: A small, seasonal one if it is plastered with lime. U-Monkey’s free condensation check finds condensation forming at the back of the outer render from December, peaking at about 100 g/m² in February and drying out fully in summer, and it stays acceptable across the range of vapour values it tests for the straw. A 25mm cement render outside, which resists vapour about twice as much as lime (μ 20 against 10 in U-Monkey’s defaults), raises the peak to about 590 g/m² in March, and straw that stays damp decays, so keep both coats vapour-open and the walls protected from driving rain with a good overhang and a raised plinth. Note: λ 0.080 / 0.052 from the German building approval for construction straw bales (DIBt Z-23.11-1595, 2014, Table 2: design values along and across the stalks; it lapsed in 2019), which Passipedia repeats; U-Monkey’s library carries both bale orientations. The air-gap correction is BR 443’s level 1, which U-Monkey applies to a single layer of insulation. --- # U-value of a cob wall URL: https://energycount.co.uk/tools/u-value-calculator/cob-wall/ Typical U-value: 0.82 W/m²K — 600mm cob (λ 0.6), lime plaster and render A 600mm cob wall, lime-rendered outside and lime-plastered inside, has a calculated U-value of about 0.82 W/m²K, taking λ 0.6 for the cob — the middle of a published range that runs from 0.45, the mean Goodhew & Griffiths give for Devon cob at 1450 kg/m³, to 0.73, a Devon Earth Building Association figure the SPAB used in its U-value research, which puts the same wall anywhere between 0.64 and 0.96. Measurement agrees: most of the SPAB’s in-situ readings on cob walls came out between 0.76 and 1.05, and RdSAP assumes 0.80 for a cob wall as built. Insulating one is about moisture as much as heat: 60mm of wood-fibre board inside gives about 0.36, and 250mm of hempcrete outside about 0.21. Build-up, outside to inside: - 20mm lime render - 600mm cob (earth and straw, λ 0.6) - 15mm lime plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 450mm cob | ≈ 1.03 | | | 500mm cob | ≈ 0.95 | | | 600mm cob | ≈ 0.82 | As pre-filled | | 700mm cob | ≈ 0.72 | | | 600mm at λ 0.45 (Devon cob mean, Goodhew & Griffiths 2005) | ≈ 0.64 | | | 600mm at λ 0.73 (Devon Earth Building Association, as the SPAB used) | ≈ 0.96 | | | 600mm + 60mm wood-fibre board inside, lime plaster | ≈ 0.36 | U-Monkey’s condensation check: screening only | | 600mm + 100mm wood-fibre board inside | ≈ 0.26 | Meets the 0.30 for an upgraded wall | | 600mm + 250mm hempcrete outside (λ 0.07) | ≈ 0.21 | A Plymouth study of this wall gives about 0.3 with its hempcrete | | 600mm + 40mm cork-lime render outside (λ 0.09) | ≈ 0.61 | λ from Secil’s data sheet FT 12004.04 (April 2024); an older distributor sheet gave 0.10 | What changes the number: The cob’s conductivity is the big unknown: it depends on the earth, the straw and above all the density and the moisture in the wall, and calculation is often pessimistic for traditional walls — the SPAB’s research found it overestimated the U-value in 77% of the walls it measured (Research Report 1, 2012). For an existing wall, a measured U-value (BS ISO 9869) is worth having before deciding how much insulation it needs. Whatever goes on has to let the wall keep drying: Historic England notes that materials highly resistant to the passage of water vapour or liquid water are not usually appropriate for a traditionally constructed building, and risk trapping moisture in the fabric. So the usual choices are vapour-open ones — wood fibre or cork inside with lime plaster, hemp-lime or cork-lime outside — on a wall kept dry at its base, which in historic earth buildings means a masonry plinth. Inside insulation needs the most care: U-Monkey’s check predicts seasonal condensation where the wood-fibre board meets the old plaster — about 1.1 kg/m² by March with 60mm, 1.7 kg/m² by April with 100mm — that dries out in summer, and calls it a screening result, because wood fibre and earth move water by capillary action, which a Glaser calculation can’t model; a BS EN 15026 assessment by a specialist is the next step. RdSAP assumes a 540mm cob wall and 0.80 W/m²K for one built before 1983, and gives 0.40 with 50mm of insulation added and 0.26 with 100mm. Q: Do I have to insulate a cob wall when I renovate it? A: Not necessarily. In England, renovating more than half of a wall — re-rendering it, say — normally brings in Approved Document L’s improved value for an existing wall, 0.30 W/m²K (Table 4.3), where that is technically, functionally and economically feasible (paragraph 4.13). But paragraph 0.10 says the energy efficiency of a historic or traditional dwelling should be improved only where that will not cause long-term deterioration of its fabric or fittings, and names cob among the vapour-permeable constructions this applies to; for a listed building or one in a conservation area, the standards apply only so far as they would not unacceptably alter its character or appearance (paragraphs 0.8–0.9). Agree what is reasonable with building control, and with the conservation officer where the building is listed. Note: Cob at λ 0.6 from U-Monkey’s library, inside the published range: Goodhew & Griffiths (2005) give 0.45 for Devon cob at 1450 kg/m³, and a Devon Earth Building Association figure of 0.73 that the SPAB used in its U-value research. The SPAB’s measured walls (Rye & Scott, Research Report 1, 2012) were about 400–620mm of cob. RdSAP figures are from RdSAP 10, Table 6 (England, age bands A–F). The pre-filled link opens as an existing element. --- # U-value of aerogel internal wall insulation URL: https://energycount.co.uk/tools/u-value-calculator/aerogel-internal-wall-insulation/ Typical U-value: 0.58 W/m²K — 20mm aerogel + foil-backed plasterboard on battens, solid brick A solid 9-inch brick wall (215mm, plastered) has a U-value of about 2.1 W/m²K; 20mm of aerogel bonded to 12.5mm foil-backed plasterboard, fixed on 25mm battens, takes it to about 0.58, and 10mm to about 0.82 — the figures Proctor’s own tables give for its Spacetherm Wallboard. That is the point of aerogel: a big cut in heat loss for little lost floor space. It is not magic per millimetre, though: at λ 0.0195 it is only about 11% better than PIR (λ 0.022), and on battens a PIR board of the same overall depth does as well; reaching the 0.30 Approved Document L sets for an upgraded wall takes about 55mm on battens. Its strengths are thinness where there is no room for battens or dabs, and a blanket that is itself non-combustible (Euroclass A1) — though Proctor notes that its laminated boards, the Wallboard included, no longer carry that rating. Build-up, outside to inside: - 215mm solid brick - 13mm plaster - 25mm battens (11.8% timber, BR 443 §4.6.2) - 20mm aerogel blanket (Spacetherm A1, λ 0.0195) - 12.5mm foil-backed plasterboard (vapour control: 78.5 MN·s/g) | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Bare wall: 215mm brick, 13mm plaster | ≈ 2.10 | Proctor’s base wall: 2.11 | | 10mm aerogel + 12.5mm plasterboard on 25mm battens | ≈ 0.82 | Proctor: 0.82 | | 15mm aerogel + plasterboard on battens | ≈ 0.68 | Proctor: 0.68; inside the 0.7 backstop | | 20mm aerogel + plasterboard on battens | ≈ 0.58 | As pre-filled; Proctor: 0.58 | | 30mm aerogel + plasterboard on battens | ≈ 0.45 | Proctor: 0.45 | | 55mm aerogel + plasterboard on battens | ≈ 0.29 | Meets the 0.30 for an upgraded wall; Proctor: 0.29 | | 10mm aerogel + plasterboard fixed straight to the wall (generic) | ≈ 0.96 | 22.5mm deep in all | | 20mm, fixed straight to the wall | ≈ 0.65 | 32.5mm deep | | 40mm, fixed straight to the wall | ≈ 0.39 | | | For comparison: 37.5mm PIR insulated plasterboard (25mm PIR) on dabs | ≈ 0.56 | 52.5mm deep, against 57.5mm for 20mm of aerogel on battens | | For comparison: 20mm PIR + foil-backed plasterboard on the same 25mm battens | ≈ 0.55 | The gap beside the foil counts as low-e | What changes the number: On battens the board gains a sheltered air gap, worth about another 3–4mm of aerogel even with the battens bridging it, so a battened board does better than the same board fixed straight to the wall (0.58 against 0.65 for 20mm) — Proctor fixes its Wallboard to timber straps. Depth for depth, though, the battens take away aerogel’s advantage: a 37.5mm PIR insulated plasterboard on dabs, 52.5mm deep in all, gives 0.56 against 0.58 for 20mm of aerogel on 25mm battens (57.5mm), and 20mm of PIR on the same battens gives 0.55, because the gap beside a foil-faced board counts as low-emissivity. Where aerogel earns its price is where there is no room for battens or dabs — window reveals, narrow rooms and listed interiors where the joinery limits how far a wall can come forward — and, as a bare blanket, in its make-up: Spacetherm A1 is Euroclass A1, non-combustible, though Proctor notes that its laminated boards, the Wallboard included, no longer carry that rating (its plasterboard is A2-s1,d0). In the government’s Thin Internal Wall Insulation trial (BEIS, 2021), a 14mm aerogel board took measured solid brick walls from 2.11 to 0.76, with no notable performance gap, and the time its joist ends spent at mould or rot risk in the modelling stayed far below that of 70mm of conventional internal insulation (2.8% of the time, against 11.3%, and 0.2% for the bare wall). Approved Document L asks an upgraded solid wall for 0.30 (Table 4.3), allows a lesser standard where meeting it would cut the room’s floor area by more than 5% (note 6), and adds that a thermal element once upgraded should generally be no worse than 0.7 (paragraph 4.13) — which 10mm on battens (0.82) misses and 15mm (0.68) meets. Q: Is 10mm of aerogel worth it on a solid wall? A: It roughly halves the heat loss through the wall: 10mm of aerogel on a 12.5mm board, fixed on battens, takes a 9-inch solid brick wall from about 2.1 to 0.82 W/m²K, and the 14mm aerogel board in the government’s thin internal insulation trial measured 0.76. That is short of the 0.30 Approved Document L sets for an upgraded wall in England, and of its general 0.7 backstop for an upgraded element (paragraph 4.13), which takes 15mm on battens (0.68); the guidance does allow a lesser standard than 0.30 where meeting it would reduce the room’s floor area by more than 5%. Even the thinnest boards leave the wall’s inside face warmer and less prone to mould: U-Monkey’s check puts the bare wall below the surface-mould limit (fRsi 0.58, against 0.75) and the 10mm board on battens comfortably above it. Note: Aerogel as Proctor’s Spacetherm A1 (λ 0.0195 at 23 °C), the blanket in its Spacetherm Wallboard, which is supplied on 12.5mm foil-faced plasterboard with a vapour control rating of 78.5 MN·s/g (Spacetherm brochure, August 2025); that foil is entered on the board here (sd 15.7m), because U-Monkey’s generic foil-backed board carries no vapour resistance of its own. Proctor’s tables, for 220mm of brick with 13mm plaster (2.11), are in the notes column. The pre-filled link opens as an upgraded solid wall. Condensation risk with aerogel internal insulation: Any internal insulation leaves the brick colder and slower to dry, and BS 5250 and current guidance treat a monthly condensation calculation as a screening for this, not a verdict: a solid wall is wetted by driving rain and moves water as liquid, which the calculation can’t see. U-Monkey’s free BS EN ISO 13788 check says so. For every aerogel row on this page it finds no interstitial condensation in any month, with the board’s foil facing as the vapour control layer, and rates the wall “screening only”, with a BS EN 15026 hygrothermal assessment by a specialist as the next step. The check’s surface-mould test is where thin boards shine: the bare wall fails it (fRsi 0.58, below the 0.75 limit for dwellings), and even 10mm of aerogel fixed straight to the wall passes (0.79). Before insulating, cure any damp and repoint or repair the outside so rain stays out — the BEIS trial found wetter bricks had higher U-values — and seal the board’s joints and edges, because that foil only works as a vapour control layer if it is continuous. --- # U-value of wood-fibre external wall insulation (EWI) on a solid wall URL: https://energycount.co.uk/tools/u-value-calculator/wood-fibre-external-wall-insulation/ Typical U-value: 0.26 W/m²K — 140mm wood-fibre EWI (STEICO Protect M), lime render, solid brick A 9-inch solid brick wall (about 2.1 W/m²K) with 140mm of render-carrying wood-fibre board (STEICO Protect M, λ 0.040) and a lime render system has a U-value of about 0.26 W/m²K with the board’s fixings counted, meeting the 0.30 Approved Document L sets for an upgraded wall. 120mm meets it too, at 0.297 (shown as 0.30), and 120mm of STEICO’s Protect L (λ 0.037) gives 0.28; 100mm of either falls short. Reaching 0.18 — the value for a new wall in an existing home — takes about 220mm of Protect L. Wood fibre is thicker than EPS or phenolic for the same figure, and vapour-open, which suits solid masonry. Build-up, outside to inside: - 10mm lime render system - 140mm wood-fibre render board (STEICO Protect M, λ 0.040), mechanically fixed (6 fixings/m²) - 215mm solid brick (protected by the insulation: λ 0.56) - 13mm plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Bare wall: 215mm brick, 13mm plaster | ≈ 2.10 | | | 100mm STEICO Protect M (λ 0.040) | ≈ 0.34 | Misses 0.30 | | 120mm Protect M | ≈ 0.30 | Meets 0.30: 0.297 | | 140mm Protect M | ≈ 0.26 | As pre-filled | | 160mm Protect M | ≈ 0.23 | | | 120mm STEICO Protect L (λ 0.037) | ≈ 0.28 | Meets 0.30 | | 220mm Protect L | ≈ 0.17 | Meets 0.18, the value for a new wall (200mm: 0.182) | | 140mm Gutex Thermowall (λ 0.040) | ≈ 0.26 | | | 140mm, fixings left out | ≈ 0.25 | The fixings add about 0.008 | What changes the number: The board’s thickness and grade decide the figure: STEICO’s render-carrying boards come as Protect M (λ 0.040, 60–200mm) and Protect L (λ 0.037, 80–240mm), and Gutex Thermowall matches Protect M. The fixings cost more here than through the thicker board on the CLT wall page: BS EN ISO 6946’s correction for a fixing that crosses the whole board grows as the board gets thinner, so at six a square metre they add about 0.008 here, against about 0.005 there — on a real job the number per square metre depends on the wind load and the system’s certificate, so check it. Wood fibre needs up to about 30% more thickness than EPS (λ 0.031–0.038) for the same figure, and nearly twice as much as phenolic board (λ 0.021), which matters at reveals, eaves and verges; in return it is vapour-open, so the brick can still dry outwards, and U-Monkey’s condensation check finds no condensation in any month for any thickness here. External insulation also puts the brick on the warm side: the surface-mould factor goes from 0.58 for the bare wall, a fail, to 0.94. Q: What U-value does external wall insulation need to meet? A: In England, Approved Document L asks for 0.30 W/m²K when an existing solid wall is insulated (Table 4.3, improved value, under “wall – internal or external insulation”), so far as that is technically, functionally and economically feasible with a simple payback of 15 years or less (paragraph 4.13). Insulating more than half of a wall counts as renovating it, which brings this in (regulation 23). With wood fibre on a 9-inch brick wall that means about 120mm of Protect M or Protect L. A new wall in an existing home — an extension — is held to 0.18 instead (Table 4.2), about 220mm of Protect L. In a conservation area, external insulation may also need planning permission, so check before you start. Note: Boards from U-Monkey’s library: STEICO Protect dry M (λ 0.040) and L (λ 0.037), Gutex Thermowall (λ 0.040). Fixings counted as stainless screws at 6 per m² through the full board depth (BS EN ISO 6946 Annex F). The brick behind the boards is taken as protected, λ 0.56 (BR 443 §7), with its mortar joints counted (§4.2). The pre-filled link opens as an upgraded solid wall, judged against Approved Document L’s 0.30 (2021 edition, Table 4.3). --- # U-value of a log cabin wall URL: https://energycount.co.uk/tools/u-value-calculator/log-cabin-wall/ Typical U-value: 1.97 W/m²K — 44mm solid logs A 44mm log cabin wall has a U-value of about 1.97 W/m²K, and a 70mm one about 1.41 — roughly eight times and five times the heat loss of a new house wall at the 0.26 limiting value. Logs alone can’t be made efficient by going thicker: even 120mm only reaches about 0.91. A cabin for use in winter needs insulation behind a lining: 50mm of PIR between battens inside 70mm logs takes it to about 0.42, 100mm to about 0.26, and 100mm with 25mm more across the battens to about 0.19. Retailers’ tables agree with these figures (one gives 1.93 for 44mm and 1.39 for 70mm). Build-up, outside to inside: - 44mm solid softwood logs (λ 0.13), interlocking | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 28mm logs | ≈ 2.59 | | | 34mm logs | ≈ 2.32 | | | 44mm logs | ≈ 1.97 | As pre-filled | | 58mm logs | ≈ 1.62 | | | 70mm logs | ≈ 1.41 | | | 92mm logs | ≈ 1.14 | | | 120mm logs | ≈ 0.91 | Passes the surface-mould check (from about 93mm) | | 44mm logs at λ 0.12 (spruce) | ≈ 1.86 | | | 70mm logs + 50mm PIR between battens + plasterboard | ≈ 0.42 | | | 70mm logs + 100mm PIR between battens + plasterboard | ≈ 0.26 | Just misses 0.26: 0.263 | | 70mm logs + 100mm PIR between battens + 25mm PIR over them | ≈ 0.19 | | What changes the number: A log is just timber, and timber is a fair insulator but no more: at λ 0.13, every 10mm of log is worth about 0.08 m²K/W, against 0.45 for 10mm of PIR. So thickness helps slowly — going from 44mm logs to 92mm only takes 1.97 to 1.14 — and the real gains come from insulation behind a lining, where the battens bridge it (11.8% here) unless a continuous layer covers them. Spruce, the usual species for imported cabins, is a little better than generic softwood (λ 0.12 in the BSW timber Declaration of Performance, against 0.13 for pine). The joints between logs matter as much as the logs: they shrink and settle, so draughts through gaps in the walls can lose more heat than conduction does. And thin logs are cold to the touch on the inside: U-Monkey’s surface-mould check fails every unlined wall up to about 92mm (fRsi under 0.75), so expect condensation and mould in corners if a thin cabin is heated and lived in through the winter. Q: Does a log cabin need building regulations approval? A: Often not, in England and Wales (Scotland and Northern Ireland have their own rules). A detached single-storey building of up to 30m², with no sleeping accommodation, is exempt from the Building Regulations if it is at least a metre from the boundary or built substantially of non-combustible material — which a log cabin isn’t — and so is any detached building of up to 15m² with no sleeping accommodation (Schedule 2, Class 6). Part P still applies to electrics fed from the house (regulation 9(3)). The energy efficiency requirements don’t apply to a stand-alone building, other than a dwelling, of under 50m² (regulation 21(3)(d)), even where other parts of the Regulations do. Add sleeping accommodation and the Class 6 exemption no longer applies: ask building control how it will assess the cabin for Part L before you build — as a new dwelling, it would need a SAP calculation and, in England, walls of 0.26 W/m²K or better. Note: Logs modelled as solid softwood at λ 0.13 (pine; 0.12 for spruce, BSW timber DoP) with surface resistances only; the air leakage through the log joints is not part of a U-value. Retail figures for comparison: Summerhouse24 gives 2.53, 1.93 and 1.39 for 28, 44 and 70mm walls. Battens in the lined rows at 11.8% (BR 443 §4.6.2). --- # U-value of a garden room (garden office) wall URL: https://energycount.co.uk/tools/u-value-calculator/garden-room-wall/ Typical U-value: 0.34 W/m²K — 75mm PIR between 100mm studs, VCL, timber cladding A typical garden room wall — timber cladding on a ventilated batten space, a breather membrane and 9mm OSB on 100mm studs, with 75mm of PIR between the studs, a vapour control layer and plasterboard — has a U-value of about 0.34 W/m²K. Filling the studs with 100mm of PIR only gets to about 0.32, because the studs carry the heat round the boards; 25mm more PIR across the inside of the studs takes it to about 0.22. Makers’ quoted wall figures run from 0.18 to 0.31, so ask for the build-up behind the number. Build-up, outside to inside: - 20mm timber cladding (not counted: outside a ventilated layer) - 25mm ventilated batten space - Breather membrane - 9mm OSB sheathing - 75mm PIR between 100mm studs (15% timber), 25mm air space behind - Polythene vapour control layer - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 50mm PIR between 100mm studs | ≈ 0.43 | | | 75mm PIR between the studs | ≈ 0.34 | As pre-filled | | 100mm PIR filling the studs | ≈ 0.32 | Only 0.02 better: the studs bridge it, and the low-e gap goes | | 100mm PIR between + 25mm PIR over the studs inside | ≈ 0.22 | Meets the 0.26 a new dwelling’s wall needs | | 100mm PIR between + 50mm PIR over the studs | ≈ 0.17 | Meets 0.18 | What changes the number: The studs are the weak point: at 15% of the wall (BR 443’s figure for timber frame) and six times as conductive as PIR, they carry a large share of the heat round the boards, so going from 75mm to 100mm between them gains only 0.02. Part of that is the 25mm gap beside the 75mm boards, which U-Monkey counts as a low-emissivity cavity because it faces the boards’ foil: without that credit — it needs a clean foil facing a sealed gap — the 75mm wall is 0.37 rather than 0.34, and filling the studs gains about 0.05. A continuous layer across the inside of the studs is what moves the figure — 25mm of PIR over 100mm between takes 0.32 to 0.22, and 50mm to 0.17. The cladding and its batten space count for nothing, because the space behind the cladding is ventilated (BS EN ISO 6946). The vapour control layer barely changes the U-value, and U-Monkey’s free condensation check finds no condensation in any month with or without it, because foil-faced PIR resists vapour on its own; but the boards only fill the spaces between the studs, so the membrane, taped at its laps, is what makes the wall airtight. Q: What U-value should a garden room have? A: Usually the Building Regulations don’t set one, in England and Wales at least (Scotland and Northern Ireland have their own rules): a detached garden room or office that isn’t a dwelling and has a floor area under 50m² is outside the energy efficiency requirements (regulation 21(3)(d)), and many are exempt from most of the Regulations (Schedule 2, Class 6), though Part P still applies to electrics fed from the house (regulation 9(3)). That leaves it to you. For a room heated every working day, aim for what a new house in England would meet: about 0.26 W/m²K or better for walls (Approved Document L’s limiting value), and, to keep running costs down, nearer 0.18 — the notional dwelling’s figure, from which a new home’s energy target is set — the second needs a layer of insulation across the studs, not just between them. A garden room with sleeping accommodation is a different matter: it loses the Class 6 exemption, and whether the energy requirements then apply depends on whether it counts as a dwelling, so ask building control before you build. Note: Studs at BR 443’s 15% for timber frame, bridging both the PIR and the air space behind it. The cladding and its batten space are outside a well-ventilated layer and not counted (BS EN ISO 6946 6.9.4). Makers’ published figures for comparison: Green Retreats 0.31, Warwick Buildings 0.29 (50mm) and 0.21 (70mm), Executive Garden Rooms 0.18 (140mm in a 200mm wall). --- # U-value of an SFS infill wall with rainscreen cladding URL: https://energycount.co.uk/tools/u-value-calculator/sfs-rainscreen-wall/ Typical U-value: 0.16 W/m²K — 150mm NyRock Rainscreen 032 over 100mm SFS with frame slab, example certified brackets (with the χ values certified for one low-conductivity bracket system; 0.44 with BR 443’s default where no χ is known) A steel frame system (SFS) infill wall — 100mm steel studs at 600mm centres with 100mm of ROCKWOOL NyRock Frame Slab 032 between them, 12.5mm Siniat Weather Defence sheathing, and 150mm of NyRock Rainscreen 032 outside behind ventilated cladding — has a U-value of about 0.14 W/m²K before the cladding brackets are counted, and about 0.16 with the χ values certified for one low-conductivity bracket system. BR 443 says brackets that pass through the insulation must be counted: from the bracket system’s own point thermal transmittances (χ) where they exist, or by adding a conservative 0.30 W/m²K where they don’t — which takes the same wall to 0.44 and fails the 0.26 limiting value at any thickness. So the U-value of a rainscreen wall is as much a question of which brackets, and whose numbers, as of insulation. Build-up, outside to inside: - Rainscreen cladding (8mm fibre cement) and its ventilated cavity: not counted, BR 443 §4.8.5 - 150mm ROCKWOOL NyRock Rainscreen 032 (λ 0.032), pierced by the cladding brackets (Σχ·n 0.014 W/m²K) - Breather membrane - 12.5mm Siniat Weather Defence sheathing board (λ 0.25, A1) - 100mm steel C-studs at 600mm centres (1.2mm web: 0.2% steel) with 100mm NyRock Frame Slab 032 between them - Polythene vapour control layer - 15mm fire-rated plasterboard (modelled as high-density plasterboard) | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | No brackets counted (U0): 150mm rainscreen slab | ≈ 0.14 | Where BR 443 does not let you stop: brackets through the insulation must be counted | | Example certified brackets (Σχ·n 0.014): 100mm rainscreen slab | ≈ 0.20 | Meets the 0.26 limit; misses the 0.18 notional | | Example certified brackets: 150mm rainscreen slab | ≈ 0.16 | As pre-filled (0.156) | | Example certified brackets: 200mm rainscreen slab | ≈ 0.13 | | | Brackets with no calculated χ: BR 443’s +0.30 default, 150mm | ≈ 0.44 | Fails 0.26 — and no thickness of slab can pass with 0.30 added | | Brackets with no calculated χ: BR 443’s +0.30 default, 200mm | ≈ 0.42 | | | 150mm studs + 150mm frame slab, 150mm rainscreen, example certified brackets | ≈ 0.14 | | | Warm frame: nothing between the studs, 150mm rainscreen, example certified brackets | ≈ 0.21 | All the insulation outside the steel: plain ISO 6946, no Digest 465 weighting (BR 443 §4.10) | | RainScreen Duo (λ 0.035) 150mm instead, example certified brackets | ≈ 0.16 | 0.164 | | Studs at 400mm centres (0.3% steel), 150mm, example certified brackets | ≈ 0.16 | 0.163 | What changes the number: The brackets come first. BR 443 (2019) §4.8.5 says the brackets or rails that fix rainscreen cladding “must be taken into account” where they penetrate the insulation, in one of four ways: a detailed calculation of the whole wall; a linear thermal transmittance for a rail; a point thermal transmittance (χ, W/K) for each bracket, added as U = U0 + Σχ·n with n brackets per m²; or, where no calculated result exists, U0 plus a conservative 0.30 W/m²K. The Annex F fixings correction of BS EN ISO 6946, used for wall ties and screws, is not the method for them. A χ comes from a three-dimensional calculation (BS EN ISO 10211) by the bracket or subframe maker, and how much it adds varies far more than the insulation does: the Passive House Institute’s certificate for the BILDA rainscreen system, for heavy stone cladding, gives Σχ·n 0.0139 W/m²K (the pre-filled figure); ROCKWOOL’s own NyRock Rainscreen 032 tables, modelled in 3-D with FastFrame brackets and 8mm cladding, read 0.18 at 145mm and 0.17 at 160mm, about 0.035 above U-Monkey’s U0; and Hilti’s rainscreen brochure takes one wall from 0.161 without brackets to anywhere between 0.165 and 0.357 with them, depending on the bracket system. The steel studs are BRE Digest 465’s hybrid frame — insulation between and outside the studs — which U-Monkey’s light steel frame panel applies, weighting the upper and lower limits by p; keeping most of the insulation outside the steel keeps the studs warm (“the majority of the insulation should be external”, SCI P283), and the warm frame row, with nothing between the studs, shows the other extreme. Results below about 0.20 sit outside the range of walls Digest 465 was derived from, and the calculator says so. Slab edges, head tracks and window reveals are linear thermal bridges, not part of the U-value — model them in ΨMonkey. And in England, on a relevant building — one with a storey at least 18m above ground that contains dwellings, an institution or rooms for residential purposes — regulation 7(2) of the Building Regulations limits the materials in an external wall to A2-s1,d0 or A1: the mineral wool and the Weather Defence board here qualify; PIR and phenolic rainscreen boards (Kooltherm K15 is C-s2,d0) do not. Q: What χ value should I use for rainscreen brackets? A: The one your bracket or subframe supplier calculates for your brackets, cladding weight and spacing, to BS EN ISO 10211 — normally one χ per bracket type (fixed and sliding points, or supporting and restraining brackets) with the number of each per m², summed as Σχ·n and added to the wall’s U-value (BR 443 §4.8.5(c)). As a sense of scale, the Passive House Institute–certified BILDA system lists χ 0.0353 W/K for its supporting brackets and 0.0053 for its restraining brackets — at 0.32 and 0.49 per m² for stone cladding, 0.0139 W/m²K in all. With no calculated χ, BR 443 §4.8.5(d) adds 0.30 W/m²K, enough on its own to fail Approved Document L’s 0.26 limit. In U-Monkey, enter brackets in the rainscreen layer’s fixings sheet: “Certified χ per fixing or bracket”, or “Rainscreen brackets, no χ” for the default. Note: The brackets are entered as the Passive House Institute component certificate for the BILDA Rainscreen System gives them for heavy (stone) cladding: supporting brackets at 0.12 and 0.20 per m² (χ 0.0353 W/K) and restraining brackets at 0.49 per m² (χ 0.0053 W/K), Σχ·n 0.0139 W/m²K, entered as their mean χ (0.01715 W/K) over 0.81 brackets per m² — use your own system’s figures. The certificate’s χ values were calculated for its reference façade, with 240mm of insulation at λ 0.035, and it was valid to the end of 2025; a bracket’s χ changes with the insulation depth and the wall behind it, so the figures here are an example, not a property of this wall. The steel frame is BRE Digest 465’s hybrid case in U-Monkey’s light steel frame panel (50mm flanges, 600mm centres). The cladding and its ventilated cavity are disregarded and the outside surface resistance becomes 0.13 (BS EN ISO 6946 §6.9.4; BR 443 §4.8.5). Condensation risk in an SFS rainscreen wall: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled wall, and it stays that way with every assumed vapour value tested across its plausible range and the vapour control layer at a third of its rated resistance, one at a time and all together: the polythene layer inside, the vapour-open sheathing (μ 10) and membrane outside, and two-thirds of the insulation outside the steel. The check is one-dimensional, through the insulation between the studs; the studs themselves and the brackets are two- and three-dimensional effects it cannot see. Keep the vapour control layer continuous and sealed at the head and base tracks and around openings, and keep the bulk of the insulation outside the frame — a cold frame, with wool only between the studs, is where condensation on the steel becomes a risk. --- # U-value of a solid aircrete wall (thin-joint blockwork) URL: https://energycount.co.uk/tools/u-value-calculator/thin-joint-aircrete-wall/ Typical U-value: 0.20 W/m²K — 100mm grey EPS external insulation on 215mm Solar-grade aircrete, thin joints A single leaf of 215mm Solar-grade aircrete (λ 0.11) laid on 2–3mm thin joints and rendered has a U-value of about 0.49 W/m²K — nearly twice the 0.26 limiting value for a new dwelling — so a solid aircrete wall needs insulating: 100mm of grey EPS external wall insulation (λ 0.032) takes it to about 0.20, and 150mm to 0.16. The thin joints matter on the bare wall: laid on conventional 10mm mortar joints, the same wall reads 0.59, because BR 443 counts mortar as a bridge whenever the block is far more insulating than the joints. Behind 100mm of external insulation the joints cost only about 0.01. Build-up, outside to inside: - 8mm render on the insulation - 100mm grey EPS external wall insulation (webertherm XM EPS 70 grey, λ 0.032), screw fixings counted - 215mm H+H Celcon Solar-grade aircrete (λ 0.11) on thin joints (1.9% mortar at λ 0.88) - 13mm lightweight plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Rendered only: 215mm Solar block, thin joints | ≈ 0.49 | Fails the 0.26 limit for a new dwelling | | Rendered only, 10mm mortar joints instead | ≈ 0.59 | BR 443’s standard 6.7% of joints | | Rendered only: 300mm Mannok Aircrete Super (λ 0.12), thin joints | ≈ 0.40 | The widest above-ground aircrete block made as standard | | 215mm Solar + 60mm grey EPS | ≈ 0.26 | Just misses the 0.26 limit (0.261) | | 215mm Solar + 100mm grey EPS | ≈ 0.20 | As pre-filled: meets 0.26, misses the 0.18 notional | | 215mm Solar + 120mm grey EPS | ≈ 0.18 | Just over 0.18 (0.181) | | 215mm Solar + 150mm grey EPS | ≈ 0.16 | Meets the 0.18 notional and the 0.18 for an extension | | Standard-grade block (λ 0.15) + 100mm grey EPS | ≈ 0.22 | | | 215mm Solar + 100mm mineral wool EWI (λ 0.036) | ≈ 0.21 | Non-combustible insulation | | 215mm Solar + 100mm grey EPS, 10mm mortar joints | ≈ 0.21 | The joints cost about 0.01 behind insulation (0.212) | | 215mm Solar + 100mm grey EPS, fixings left out | ≈ 0.20 | The screws add about 0.006 (0.196 without them) | What changes the number: Three things move a solid aircrete wall: the block’s grade, its joints and the insulation added to it. Grades run from about 0.11 (H+H Solar, Thermalite Turbo) through 0.15 (standard) to 0.18 for the high-strength blocks, and most makers stop at 215mm above ground; Mannok makes a 300mm Aircrete Super (λ 0.12) as standard, which still gives only about 0.40 rendered on thin joints (0.48 on 10mm joints) — reaching 0.26 with a block alone would take about 450mm at λ 0.11. The joints are the subtle one. BR 443 §4.2 counts mortar joints as a bridge unless the block and mortar differ by less than 0.1 m²K/W, which aircrete fails by a wide margin, and gives 6.7% of joints for 10mm mortar around a 440 × 215mm block; it says nothing specific about thin-joint work, so the joints here are its general formula at 3mm around H+H’s 630 × 215mm face: 1.9% of the wall, at BR 443’s mortar λ (0.88 behind insulation, 0.94 where the block is only rendered). A thin-joint mortar’s λ is rarely published: a European thin-bed mortar declares 0.76 and Wienerberger’s Porotherm thin-layer mortar (for clay blocks) 1.17, which would move the rendered-only wall by about 0.01 either way and the insulated one by under 0.002. The block λ needs care on a rendered-only wall: BR 443 takes masonry as exposed unless it has external insulation or cladding, and the 0.11 here is the maker’s declared maximum with no stated moisture basis. External insulation is the realistic route — it keeps the block dry and warm, and covers the thin joints, which can’t be raked out to key a render (H+H recommends a stipple bonding coat or a metal lath). Approved Document L sets 0.26 for the walls of a new dwelling (Table 4.1) and 0.18 for a new wall in an existing one (Table 4.2). Q: Can a solid aircrete wall meet Building Regulations without insulation? A: Not at any thickness the makers produce. 215mm of Solar-grade aircrete (λ 0.11) on thin joints, rendered and plastered, comes to about 0.49 W/m²K, and Mannok’s 300mm Aircrete Super, also on thin joints, about 0.40, against the 0.26 Approved Document L sets for a new dwelling’s walls; 0.26 would need about 450mm of Solar-grade block (λ 0.11). That is why solid aircrete walls are built with external insulation — 100mm of grey EPS gives about 0.20 — or lined inside (H+H’s thin-joint brochure gives 0.24 for a 200mm solid wall with 62.5mm of phenolic insulated plasterboard). Note: The thin joints are entered as a bridge at BR 443 §4.2’s general fraction for 3mm joints around a 630 × 215mm block face (0.019), with its mortar λ (0.88 behind external insulation, 0.94 on a rendered-only wall), because BR 443 has no thin-joint figure; without it, U-Monkey adds its standard 6.7% for 10mm joints. The block is H+H’s Celcon Plus in Solar grade (λ 0.11, the declared maximum on NBS Source, 460 kg/m³). External insulation fixings as U-Monkey’s “EWI screw fixings” preset (6 per m², λ 17, 19.6mm²). Condensation risk in a solid aircrete wall with external insulation: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled wall, and it stays that way with the EPS tested across BS 5250’s published range of vapour resistance: the insulation outside keeps the block warm, and the block is vapour-open. The render does the weathering, so the system’s details at openings and at the base track matter more than the calculation does. --- # U-value of a shipping container wall URL: https://energycount.co.uk/tools/u-value-calculator/shipping-container-wall/ Typical U-value: 0.23 W/m²K — 100mm closed-cell spray foam, battened service void, plasterboard A shipping container’s side wall lined with 100mm of closed-cell spray foam (BASF Walltite, λ 0.025), taken at its thinnest point, then a battened service void and plasterboard, has a U-value of about 0.23 W/m²K — inside the 0.26 limiting value for a new dwelling; 125mm gives 0.19 and 50mm only 0.42. The steel adds almost nothing to the U-value: its part in the calculation is to be vapour-tight on the cold side, and that is why U-Monkey’s condensation check fails every version on this page insulated on the inside — 271 g/m² of condensate is still in the wall at the end of the year with the foam alone, about 30 g/m² with a polythene vapour control layer added — and passes the same container insulated on the outside, or inside behind a sealed foil vapour control layer about as tight as the steel. Build-up, outside to inside: - 2mm corrugated Corten side panel (steel, λ 50): thermally negligible, vapour-tight - 100mm closed-cell spray foam bonded to the steel, counted at its thinnest point (the trough air is not counted) - 25mm service void on battens (11.8% timber, BR 443 §4.6.2) - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 50mm closed-cell foam | ≈ 0.42 | | | 75mm closed-cell foam | ≈ 0.30 | | | 100mm closed-cell foam | ≈ 0.23 | As pre-filled: meets the 0.26 limit | | 125mm closed-cell foam | ≈ 0.19 | A self-build reported 0.2 for 125mm of the same foam | | 100mm foam + a polythene VCL behind the service void | ≈ 0.23 | Same U-value; far less condensate (below) | | 75mm PIR between 75mm timber studs on a 25mm gap, VCL | ≈ 0.36 | The studs bridge the boards through their full depth | | 50mm foam + 50mm PIR between studs, VCL | ≈ 0.26 | Just misses 0.26 (0.263) | | Insulated outside: 100mm PIR under ventilated timber cladding | ≈ 0.22 | The steel stays warm: the condensation check passes it | What changes the number: The corrugations and the frame are the conventions to settle. Foam sprayed onto corrugated steel is thickest in the troughs; U-Monkey takes it at its thinnest point, as BR 443 does where profile ribs compress insulation (§4.9.2, “at its thinnest point”, ignoring what spills into the ribs), and counts nothing for the air in the profile (as BR 443 §4.7.7 does for the troughs of a metal deck). The corner posts and the top and bottom rails are heavy steel sections that bridge the insulation in lines: they are linear thermal bridges, outside the U-value of the panel, and BS EN ISO 6946 leaves insulation penetrated by metal outside its method altogether (BR 443 §2) — so line a container with timber studs or battens, not steel ones. Thickness then does the work: 50mm of foam gives 0.42, 100mm 0.23, 125mm 0.19. Claimed figures are worth checking against that: a self-build container house sprayed with 125mm of the same foam reported about 0.2, which U-Monkey reproduces, while a container hotel was reported at 0.2 with 50mm, which it would take far more foam to achieve. To be lived in, a container has the figures of a new dwelling to meet — Approved Document L sets 0.26 for walls (Table 4.1), and the notional dwelling 0.18; a small detached garden office of up to 30m² with no sleeping accommodation can be exempt from most of the Building Regulations (Schedule 2, Class 6), though Part P still applies to electrics fed from the house (regulation 9(3)) — see the garden room pages. Q: Does spray foam stop condensation in a shipping container? A: It reduces it, but by the standard screening calculation it does not stop it. Closed-cell foam bonded to the steel leaves no air gap behind it and resists vapour (around μ 60), but the steel outside is completely vapour-tight, so any vapour that reaches it has nowhere to go: U-Monkey’s BS EN ISO 13788 check leaves 271 g/m² in a wall lined with 100mm of foam at the end of the year, and about 30 g/m² with a sealed polythene vapour control layer behind the service void as well — still a fail, because it builds up year on year. Two versions pass: insulation outside the container, under ventilated cladding, which keeps the steel warm; or a foil vapour control layer behind the foam rated at about sd 5,300 m or more and sealed at every lap and penetration, which seals the foam between two vapour-tight layers. Note: Closed-cell foam at BASF Walltite’s declared λ 0.025, measured at its thinnest point over the corrugations. The side panel is Corten (weathering) steel, taken at λ 50 like any mild steel (BR 443); its thickness makes no difference to the U-value. The battened service void gets BR 443’s batten bridge (11.8% timber) automatically. Condensation in an insulated shipping container: U-Monkey’s free BS EN ISO 13788 check fails every row on this page that is insulated on the inside, because the steel skin is a vapour barrier on the cold side. With 100mm of closed-cell foam and nothing else, 271 g/m² of condensate is still in the wall at the end of the year (peak 307 g/m²); 125mm of foam leaves 219 g/m²; a sealed polythene vapour control layer behind the service void brings it down to about 30 g/m² a year, and PIR boards between timber studs behind a vapour control layer to about 6.5 g/m² — smaller, but still building up year on year, which the standard counts as a fail. Two arrangements pass. Insulation outside the steel — 100mm of PIR under ventilated timber cladding passes with every assumed vapour value tested across its range, the steel taken with its laps unsealed. And inside, a foil vapour control layer about as tight as the steel: behind 100mm of foam, one rated at about sd 5,300 m or more passes even at a third of its rating, because almost no vapour then reaches the foam (U-Monkey does not count the trace the arithmetic leaves between two layers that tight). Whatever the insulation, seal the vapour control layer and every service penetration, and expect the steel frame members at the corners to stay colder than the panels: they are where surface condensation and mould show first. --- # U-value of a park home wall with external insulation URL: https://energycount.co.uk/tools/u-value-calculator/park-home-wall/ Typical U-value: 0.25 W/m²K — 100mm mineral wool external insulation on a timber-frame park home (BBA reference wall) RdSAP 10 gives the walls of a park home as built a U-value of 1.7 W/m²K before 1983, 1.2 for 1983–1995, 0.7 for 1996–2005 and 0.6 from 2006 (Table 11). External insulation is the usual upgrade: on the reference wall in the BBA certificate for webertherm’s park home system — a 75mm timber frame filled with insulation, 6mm sheathing and render — 100mm of webertherm MFD mineral wool (λ 0.036) gives about 0.25 W/m²K, 70mm about 0.30, the figure Ofgem’s park home insulation measure asked for, and 170mm about 0.19. Those are U-Monkey’s figures with the certificate’s own fixings (7 per m² at χ 0.004 W/K), and they agree with the certificate’s table to within 0.01. Build-up, outside to inside: - 6.5mm render (λ 1.0, as the certificate takes it) - 100mm webertherm MFD dual-density mineral wool (λ 0.036), 7 fixings per m² at χ 0.004 W/K - Breather membrane - 6mm timber sheathing (λ 0.13) - 75mm timber frame filled with insulation (λ 0.038, 15% timber) - Polythene vapour control layer - 2 × 12.5mm plasterboard (λ 0.25, as the certificate takes it) | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | As built before 1983 (RdSAP 10 default) | 1.7 | RdSAP 10 Table 11, age band F | | As built 1983–1995 | 1.2 | Age band G | | As built 1996–2005 | 0.7 | Age band I | | As built from 2006 | 0.6 | Age band K | | 50mm mineral wool EWI (BBA reference wall) | ≈ 0.34 | BBA: 50mm for 0.35 | | 70mm | ≈ 0.30 | BBA: 70mm for 0.30 — Ofgem’s post-installation 0.3 | | 80mm | ≈ 0.28 | BBA: 80mm for 0.28 (and 0.27) | | 100mm | ≈ 0.25 | As pre-filled; BBA: 100mm for 0.25 | | 170mm | ≈ 0.19 | BBA: 170mm for 0.18 — U-Monkey adds BR 443’s air-gap correction to both insulation layers; without it, 0.179 | | 100mm grey EPS (λ 0.032) instead | ≈ 0.24 | | What changes the number: A park home starts from very little: RdSAP 10 takes the walls as built at 1.7 W/m²K before 1983 and 0.6 for homes built since 2006. Insulation goes on the outside because the inside is too thin to lose, and the maker’s certificate gives the reference wall for its system — here BBA 91/2691 Product Sheet 7 for webertherm’s mechanically fixed mineral wool slabs, whose table U-Monkey reproduces: 50mm → 0.35, 70mm → 0.30, 100mm → 0.25. The fixings count: five per slab, about seven per m², each worth χ 0.004 W/K by the certificate, add 0.028 to every row, which U-Monkey takes as a certified point thermal transmittance. Grey EPS (λ 0.032) does a little better than the mineral wool at the same thickness, but the mineral wool slabs are non-combustible (A1), and the boards’ reaction to fire matters in a park home: in England the model site licence standards keep homes at least 6m apart where practicable, or 5.25m where a home’s facing walls have been clad in Class 1 fire-rated materials (Model Standards 2008, standard 2), and every millimetre of insulation comes off that gap. RdSAP adds insulation to an existing park home wall as 1/U = 1/U(existing) + R (§1.18.1), so a 1.7 wall with R 2.75 added reads about 0.30. Q: Do park homes have to meet Building Regulations U-values? A: Not the national Building Regulations: the BBA certificate for the webertherm system notes that it “is not subject to the national Building Regulations”, because park homes are built to the Mobile Homes Act 2013, the 2008 Model Standards and BS 3632 (revised in 2023 with “improved thermal transmittances”). In practice the target comes from the scheme paying for the work: Ofgem’s Park Home Insulation measure asked for walls, roof and floor of 1.7 W/m²K or worse before and 0.3 or better after, which 70mm of mineral wool reaches on the certificate’s reference wall. Note: The pre-filled wall is the reference wall in BBA 91/2691 Product Sheet 7 (issue 2, 4 January 2024), Table 9, with the certificate’s own λ values — including 0.25 for the plasterboard and 1.0 for the render — and its fixings: EJOT SW8-R anchors, five per 1200 × 600mm slab (about 7 per m²), χ 0.004 W/K each, entered as certified point thermal transmittances. The certificate’s table uses 6mm sheathing, but asks for at least 12mm plywood or cement particle board on site (Table 12). RdSAP 10 specification (BRE, version 4.0, 7 June 2026), Table 11, for the as-built rows. Condensation risk with park home external insulation: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled wall, and it stays that way with every assumed vapour value tested across its plausible range and the vapour control layer at a third of its rated resistance, one at a time and all together: the mineral wool (μ 1) and render outside let the frame dry outwards. The BBA asks for the timber frame to be checked for moisture before work starts, and for the skirting air gap and the vents under the home to stay clear. --- # U-value of the wall between a house and a conservatory URL: https://energycount.co.uk/tools/u-value-calculator/wall-to-conservatory/ Typical U-value: 0.60 W/m²K — Filled 50mm cavity wall, the conservatory disregarded (SAP 10.2) (with the conservatory disregarded, as SAP 10.2 does; 0.55–0.58 counting it as an unheated space (BR 443 §5)) The wall between a house and a thermally separated conservatory keeps the U-value it had as an outside wall: SAP 10.2 calculates the dwelling “as if it were not present” (§3.3.3), so a filled 50mm cavity wall stays at about 0.60 W/m²K and a solid 9-inch brick wall at about 2.1. BR 443 does allow an unheated space such as a conservatory to be counted as an extra resistance, Ru — about 0.05–0.07 m²K/W for an older, leaky conservatory and 0.15 for a well-glazed, airtighter one, by SAP’s own formula (4) — which would take the cavity wall to 0.58 or 0.55: a small effect, and not one SAP uses. Take the doors out or extend the heating into the conservatory, and that wall stops being the boundary at all. Build-up, outside to inside: - (The conservatory beyond: disregarded, as SAP 10.2 §3.3.3 does for a thermally separated one) - 102.5mm brick outer leaf - 50mm cavity filled with blown mineral wool - 100mm dense concrete block inner leaf - 13mm plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Filled 50mm cavity, the conservatory disregarded (SAP 10.2) | ≈ 0.60 | As pre-filled: how SAP and RdSAP count it | | Same, counting an older double-glazed conservatory (Ru 0.07) | ≈ 0.58 | Ru from SAP formula (4) for the example below | | Same, with a well-glazed, airtighter conservatory (Ru 0.15) | ≈ 0.55 | | | Unfilled 50mm cavity | ≈ 1.67 | | | Unfilled 50mm cavity, Ru 0.15 | ≈ 1.34 | | | Solid 215mm brick, plastered | ≈ 2.10 | | | Solid 215mm brick, Ru 0.15 | ≈ 1.60 | Where an Ru makes most difference | | A new separating wall: 100mm PIR partial fill, aircrete, plasterboard on dabs | ≈ 0.17 | Meets the 0.18 for a new wall in an existing dwelling (Table 4.2) | What changes the number: Two rules meet here. SAP 10.2 §3.3.3 says a conservatory is thermally separated when it is divided from the house by walls, floors, windows and doors with U-values similar to (or, for a new conservatory, no worse than) the dwelling’s own exposed elements, and new doors and windows draught-stripped to match; then “the calculation should be undertaken as if it were not present” — the wall behind it counts as an outside wall, and the conservatory’s floor area is left out. BR 443 §5, the U-value conventions, lists conservatories among the unheated spaces whose effect can be added to a wall as a resistance Ru, from Ru = Ai / (ΣAe·Ue + 0.33·n·V). For an example conservatory 3.5 × 3.0m with 2.4m walls under a roof averaging 2.5m (an 8.4m² wall to the house, 22.8m² of glazed walls, a 10.5m² roof, 26m³ of air), older double glazing at 2.8 and a polycarbonate roof at 2.4 with SAP’s default three air changes an hour give Ru ≈ 0.07 (single glazing, at about 4.8, gives 0.05); modern double glazing at 1.4, a roof at 1.5 and one air change an hour give about 0.15. Either way the effect on a filled cavity wall is a few hundredths, and on a solid brick wall larger (2.10 → 1.60); SAP leaves it out regardless. Approved Document L exempts a new conservatory of up to 30m² at ground level from the energy requirements if the wall, door or window between it and the house is kept (or replaced with one), and the heating is not extended into it (paragraph 0.14; regulation 21); remove that separation or heat the conservatory, and regulation 21(4) brings it within them — see GlazeMonkey’s conservatory guide. Q: Do I need to insulate the wall between my house and conservatory? A: Not for SAP or an EPC: a thermally separated conservatory is disregarded, so the wall behind it keeps the U-value it had as an outside wall — about 0.60 W/m²K for a filled cavity wall, 2.1 for solid brick — and improving it helps exactly as much as improving any outside wall. The conservatory itself only buffers it a little (an Ru of about 0.05–0.15 m²K/W). The wall matters most when it is taken away: knock through, or extend the heating into the conservatory, and the conservatory’s own glazing, roof and dwarf walls become the outside of the house, with the Building Regulations’ energy requirements applying to them (regulation 21(4)). Note: Calculated as an external wall, as SAP 10.2 §3.3.3 does for a thermally separated conservatory; the Ru rows add BR 443 §5’s unheated-space resistance, U = 1/(1/Uo + Ru), for the example conservatory in “What changes the number” (SAP 10.2 formula (4), the floor left out as it says). The pre-filled link opens the wall facing outside air; U-Monkey takes an Ru under “On the other side” → “Another unheated space”. --- # U-value of calcium silicate internal wall insulation URL: https://energycount.co.uk/tools/u-value-calculator/calcium-silicate-internal-insulation/ Typical U-value: 0.54 W/m²K — 50mm calcium silicate board on 20mm lime-cork levelling plaster, solid brick A solid 9-inch brick wall (about 2.1 W/m²K) lined with 50mm of calcium silicate board — redstone Clima Redboard pro, λ 0.059 — over a 20mm lime-cork levelling plaster comes to about 0.54 W/m²K, which is what its UK distributor’s table gives; on the old lime plaster, without the insulating levelling coat, the same 50mm gives 0.75, just short of the 0.7 that Approved Document L asks of an upgraded wall as a minimum, and 60mm (0.67) is needed. Calcium silicate is chosen for moisture, not for its λ: it is strongly capillary-active, alkaline and non-combustible, and at 0.059 it would take about 174mm to reach the 0.30 Table 4.3 sets — beyond the 120mm its approval covers. Multipor mineral board (λ 0.040) reaches 0.30 at about 116mm. Build-up, outside to inside: - 215mm solid brick (old gypsum plaster and paint removed) - 20mm Diathonite Thermactive.037 lime-cork levelling plaster (λ 0.037) - 5mm board adhesive (modelled as lime plaster) - 50mm redstone Clima Redboard pro calcium silicate board (λ 0.059, μ 3) - 5mm lime finish on mesh | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Bare wall: 215mm brick, 15mm lime plaster | ≈ 2.12 | | | 25mm board, on the old lime plaster | ≈ 1.10 | | | 50mm board, on the old lime plaster | ≈ 0.75 | Just misses the 0.7 backstop | | 60mm board, on the old lime plaster | ≈ 0.67 | Meets the 0.7 backstop | | 80mm board, on the old lime plaster | ≈ 0.55 | | | 100mm board (two layers), on the old lime plaster | ≈ 0.46 | | | 20mm lime-cork levelling coat + 30mm board | ≈ 0.66 | Distributor’s table: 0.66 | | 20mm lime-cork levelling coat + 50mm board | ≈ 0.54 | As pre-filled; distributor: 0.54 | | 20mm lime-cork levelling coat + 80mm board | ≈ 0.43 | Distributor: 0.42 | | 50mm board at the rated λ 0.062, on the old plaster | ≈ 0.78 | The German “rated value”, about 5% above the declared one | | 50mm SkamoWall (λ 0.068), on the old plaster | ≈ 0.82 | | | 500mm sandstone, 30mm levelling coat + 50mm board | ≈ 0.49 | Distributor: 0.48 for 500mm of stone | | Multipor M4 mineral board (λ 0.040): 60mm | ≈ 0.50 | UK stockist’s table: 0.51 | | Multipor M4: 100mm | ≈ 0.34 | Stockist: 0.34 | | Multipor M4: 120mm | ≈ 0.29 | Meets the 0.30 (0.290); stockist: 0.29 | | Multipor M4: 120mm at the rated λ 0.042 | ≈ 0.30 | Just over 0.30 (0.302) | What changes the number: The levelling coat is the hidden variable. The distributor’s table (Ecological Building Systems: 30mm → 0.66, 50mm → 0.54, 80mm → 0.42, 100mm → 0.37 on 220mm brick) includes 20mm of lime-cork insulating plaster under the boards, worth R 0.54 m²K/W — about as much as 32mm of the board itself; on an ordinary lime base the same boards give 0.75 at 50mm and need 60mm for the 0.7 backstop. The λ basis is the second: the board’s European approval (ETA-19/0559) declares λD(23,50) 0.059, and the German “rated value” printed on data sheets is about 5% higher (0.062); sellers’ tables use either. SkamoWall, another board stocked in the UK, is 0.068 (its newer variants 0.073), and a seller’s “U 2.64 W/m²K” for a 25mm board is just the board’s own conductance (0.066 ÷ 0.025, at λ 0.066), with no wall and no surface resistances. Multipor is a different, much lighter mineral board (90 kg/m³): better λ (0.040), very vapour-open (μ 2), and far less absorbent — at most 2 kg/m² short-term water uptake in its approval, against 17–28 for the calcium silicate boards — and, like wood fibre, it can reach 0.30, at about 116mm, though that takes some 13cm off the room. Approved Document L asks an upgraded solid wall for 0.30 (Table 4.3), allows less where that would cut the room’s floor area by more than 5% (note 6), and expects an upgraded element to be no worse than 0.7 (paragraph 4.13); on traditional buildings it asks for improvements that won’t harm the fabric (paragraph 0.10), and Historic England reports the DEEP study’s finding that targeting around 0.8 rather than 0.30 cut the moisture risk while keeping about two-thirds of the energy saving. Strip gypsum plaster, paint and salts first, fix damp and rain penetration, and finish with breathable coatings only. Q: Why is the U-value of calcium silicate board so modest? A: Because its thermal conductivity is about 0.059 W/mK — some 2.7 times that of PIR — so 50mm on a 9-inch brick wall gives only about 0.75 W/m²K, or 0.54 with the 20mm lime-cork levelling coat its distributor uses under it. It is not chosen as insulation per millimetre: it is a strongly capillary-active, alkaline, non-combustible board, sold to keep the wall’s inner face dry and free of mould. If the U-value matters more, Multipor mineral board (λ 0.040) does about as well as wood fibre: 100mm gives about 0.34, 120mm 0.29 (wood-fibre boards 0.33 and 0.28). Note: Calcium silicate at its declared λD(23,50) 0.059 W/mK (ETA-19/0559, 180–187 kg/m³, μ 3); the distributor’s table is for 220mm brick with 20mm of levelling plaster. U-Monkey’s figures use 215mm brick, Diathonite Thermactive.037 as the levelling coat (λ 0.037, μ 3) and a 5mm adhesive bed and 5mm finish entered as lime plaster (neither publishes a λ). Multipor rows: 5mm light mortar (λ 0.18) under and over the board, on 15mm of lime plaster; the stockist’s table is for 225mm brick (its λ basis isn’t stated). The pre-filled link opens as an upgraded solid wall. Condensation risk with calcium silicate boards: U-Monkey’s free BS EN ISO 13788 check does not pass these boards. For the pre-filled wall it leaves 1,415 g/m² of condensate at the end of the year (peak 3,440 g/m²), against the brick, and almost every board and Multipor row on this page fails the same way; the three that don’t — 25mm of board, and 50mm of the boards at λ 0.062 and 0.068 on the old plaster — still build up 2.7–3.6 kg/m² over the winter before drying out in summer, which it rates “screening only”. The boards are so vapour-open (μ 2–3) that a vapour-diffusion calculation sends a winter’s worth of moisture to the cold brick behind them. The check also says why that is not the last word: a Glaser calculation “models vapour diffusion only: it does not model the capillary transport that carries moisture through brick or stone”, and calcium silicate is chosen precisely because it moves liquid water by capillary action (water absorption coefficient about 46 kg/m²√h). So the calculation that matters here is a transient hygrothermal one to BS EN 15026 (WUFI or DELPHIN), which the board’s distributor recommends instead of the Glaser method, and which the STBA recommends for any internal insulation on a solid wall; U-Monkey rates internal insulation on solid masonry no better than “screening only” for the same reason. Its surface-mould test is clear: the bare wall fails it (fRsi 0.58, below 0.75), and the pre-filled lining passes comfortably (0.87). --- # U-value of cork insulation on a solid wall URL: https://energycount.co.uk/tools/u-value-calculator/cork-insulation-wall/ Typical U-value: 0.50 W/m²K — 60mm expanded cork board inside a solid brick wall, lime plasters Expanded cork board (Amorim ICB, λ 0.040) 60mm thick, bonded inside a solid 9-inch brick wall and finished in lime plaster, gives a U-value of about 0.50 W/m²K, from about 2.1 — about the same as 60mm of wood fibre, and the 0.49 the ASBP’s specification generator gives for 60mm of cork. 40mm (0.67) is the thinnest standard board that meets the 0.7 Approved Document L sets as a minimum for an upgraded wall, and its 0.30 takes about 120mm, inside (0.29) or outside (0.29). Cork earns its place on what differs from wood fibre: it does not take up water by capillary action, and it is more vapour-resistant (μ 20, against 3–5). Build-up, outside to inside: - 215mm solid brick - 15mm lime plaster (existing, or a hydraulic lime levelling coat) - 10mm lime-cork adhesive (modelled as lime plaster) - 60mm Amorim expanded cork board (ICB, λ 0.040, μ 20) - 10mm meshed lime base coat - 3mm lime top coat | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Internal: 40mm cork | ≈ 0.67 | Meets the 0.7 backstop | | Internal: 50mm | ≈ 0.58 | | | Internal: 60mm | ≈ 0.50 | As pre-filled; ASBP: 0.49 | | Internal: 80mm | ≈ 0.41 | | | Internal: 100mm | ≈ 0.34 | | | Internal: 120mm | ≈ 0.29 | Meets the 0.30 (0.293) | | Internal: 60mm at the declared λ 0.039 | ≈ 0.50 | 0.495 | | External: 100mm cork, lime base coat | ≈ 0.33 | | | External: 120mm | ≈ 0.29 | Tŷ-Mawr’s external design for 0.30 or better | | External: 140mm | ≈ 0.25 | | | External: 160mm | ≈ 0.23 | | | 500mm sandstone or granite + 60mm cork inside | ≈ 0.52 | Fails the condensation check (about 650 g/m² left at the year end) | | 500mm limestone + 60mm cork inside | ≈ 0.49 | Fails the condensation check (about 400 g/m² left) | What changes the number: Thickness does the work, as with any board, and cork’s λ (Amorim declares 0.039 for its EU label and 0.040 under ACERMI; 0.040 is used here) puts it level with wood fibre: 60mm takes a plastered 9-inch brick wall from about 2.1 to 0.50, 120mm to 0.29. What sets cork apart is the moisture behaviour. Amorim’s expanded corkboard “does not absorb water by capillary action” and has a vapour resistance factor of 20 — about 1.2m of equivalent air at 60mm, four to seven times less open than 60mm of wood fibre — so it neither wicks moisture away like a capillary-active board nor lets vapour through as freely. That is part of why it is also used outside: Tŷ-Mawr’s external design puts 120mm of cork on a lime levelling coat, with insulated fixings at about 8 per m² and a meshed lime-cork base coat, for 0.30 or better — U-Monkey gives 0.29 with the fixings left out (add them in the fixings sheet to suit the fixing chosen). The board is Euroclass E (Amorim’s external system is B), so in England it can’t form part of the external wall of a relevant building — one with a storey at least 18m above ground that contains dwellings, an institution or rooms for residential purposes — inside or out (regulation 7(2)). Approved Document L asks an upgraded solid wall for 0.30 (Table 4.3), allows a lesser standard where internal insulation would take more than 5% of the room’s floor area (note 6), and generally no worse than 0.7 (paragraph 4.13); outside, note 6 doesn’t apply. Inside or out, cure damp and keep driving rain out first — Tŷ-Mawr calls external protection from driving rain essential for internal insulation — and use lime adhesives and vapour-permeable paints. Q: Is cork better than wood fibre for insulating a solid wall? A: Thermally they are level: λ 0.039–0.040 for cork, 0.038–0.040 for wood-fibre boards, so 60mm of either takes a 9-inch brick wall to about 0.49–0.50 W/m²K. The differences are in moisture and use. Wood fibre is capillary-active and very vapour-open (μ 3–5); cork does not take up water by capillary action and is more vapour-resistant (μ 20). In U-Monkey’s screening check, 60mm of cork inside a 9-inch brick wall leaves seasonal condensation that dries out each summer (on 500mm of stone it builds up instead), and 120mm of cork outside the wall none at all. Both are used outside as well as in, on lime-based systems. Note: Amorim expanded corkboard (ICB) at λ 0.040 and μ 20 (Amorim technical features; ACERMI 0.040, EU declared 0.039). The adhesive (Secil Isovit, an NHL and cork mix) publishes no λ and is entered as lime plaster, like the base and top coats. External rows: 3mm lime top coat and 10mm base coat outside the cork, 10mm adhesive, 215mm brick (protected by the cork outside it: λ 0.56, BR 443 §7, mortar joints counted, §4.2), 13mm plaster inside, with the fixings left out (Tŷ-Mawr names no fixing type). The pre-filled link opens as an upgraded solid wall. Condensation risk with cork internal insulation: For the pre-filled wall, U-Monkey’s free BS EN ISO 13788 check finds seasonal condensation at the back of the cork, where it meets the lime adhesive — a peak of 463 g/m² in March — that dries out fully over the summer, and rates the wall “screening only”: internal insulation on solid masonry needs a BS EN 15026 hygrothermal assessment before the result can be relied on, because a monthly calculation can’t see driving rain or liquid moisture moving through the brick. It also flags that the condensation forms against a bio-based insulant, which a prolonged wetting can damage. On 500mm of stone the same 60mm fails instead: condensate builds up year on year (about 650 g/m² left at the end of the year on sandstone or granite, 400 on limestone). The same cork outside the wall — 120mm on the brick, under a lime render — gets a straight pass: no condensation in any month. That is the general pattern for solid walls, and the reason internal insulation is the one that needs the extra care: keep the rain out, the joints and edges sealed, and the finishes vapour-permeable. --- # U-value of insulating (thermal) plaster on a solid wall URL: https://energycount.co.uk/tools/u-value-calculator/insulating-plaster-solid-wall/ Typical U-value: 0.64 W/m²K — 50mm lime-cork insulating plaster (Diathonite Evolution) on solid brick Insulating plaster laid straight onto a solid 9-inch brick wall (about 2.1–2.2 W/m²K) gives about 0.64 W/m²K at 50mm of Diathonite Evolution (λ 0.045), applied in two coats — inside the 0.7 Approved Document L sets as a minimum for an upgraded wall, which takes about 44mm of it. How far a thermal plaster goes depends almost entirely on its λ, and products sold under the same name range from 0.037 to 0.12: 50mm of Diathonite Thermactive.037 gives 0.55, close to the 0.54 reported for a mews-house retrofit with it, while 50mm of an NHL insulating render at λ 0.12 only roughly halves the heat loss (1.15). None reaches the 0.30 of Table 4.3 at a practical thickness: it would take about 107mm of the best and well over 300mm of the weakest. Build-up, outside to inside: - 215mm solid brick (old gypsum plaster removed) - 50mm Diathonite Evolution lime-cork insulating plaster (λ 0.045, μ 4), in two coats - 5mm lime finish | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Bare 215mm brick, no plaster | ≈ 2.23 | | | 15mm ordinary lime plaster | ≈ 2.09 | | | Diathonite Evolution (λ 0.045): 30mm | ≈ 0.89 | | | Diathonite Evolution: 40mm (one coat) | ≈ 0.74 | Just misses the 0.7 backstop; 40mm is the most one coat takes | | Diathonite Evolution: 50mm (two coats) | ≈ 0.64 | As pre-filled: meets the 0.7 backstop | | Diathonite Evolution: 60mm | ≈ 0.56 | Mesh needed above 60mm | | Diathonite Thermactive.037 (λ 0.037): 30mm | ≈ 0.79 | | | Thermactive.037: 50mm | ≈ 0.55 | A mews-house retrofit reported 2.1 → 0.54 | | Baumit NHL Thermo (λ 0.075): 40mm | ≈ 1.01 | | | Baumit NHL Thermo: 80mm (its maximum) | ≈ 0.66 | Meets the 0.7 backstop only at its maximum build | | Secil ecoCork Lime (λ 0.09): 40mm | ≈ 1.11 | | | Womersley’s NHL2 insulating render (λ 0.12): 50mm | ≈ 1.15 | About 49% less heat loss than the bare wall, against a claimed “up to 60%” | What changes the number: The product is the whole story. Thermal plasters sold for solid walls run from λ 0.037 (Thermactive.037, natural hydraulic lime with recycled cork, expanded amorphous silica, perlite and pumice) and 0.045 (Diathonite Evolution, cork, clay and lime) through 0.075 (Baumit NHL Thermo) and 0.09 (Secil ecoCork Lime) to 0.12 for NHL renders with a lightweight mineral aggregate — a threefold spread that matters more than any thickness choice: 50mm takes a 9-inch brick wall to 0.55 with the best and 1.15 with the weakest. Only Secil’s is a measurement to EN 12667: Diasen cite EN 1745, whose values can be tabulated rather than tested, Baumit no method at all, and EN 998-1’s T1 class only caps a plaster’s λ at 0.10 — so treat the figures as approximate. Claims are worth checking against the physics: Diasen’s 0.222 m²K/W per centimetre is right; a 50mm NHL2 render taking “up to 60%” off a solid wall’s heat loss needs a λ nearer 0.07 than the 0.12 Carrington Lime lists for the same render (U-Monkey: 49% on bare brick); and a lime plaster sold as cutting heat loss by “more than 75%” at 50mm manages it only if its claimed λ of 0.04 holds, and then only on a poorer wall (U-Monkey: 74% on 9-inch brick, 76% on 500mm of sandstone). The other limits are per coat: Diathonite takes 15–40mm a coat with mesh above 60mm in total, Baumit’s NHL Thermo at least 30mm a coat and 80mm in all, Secil’s ecoCork 20mm a coat. Where thermal plasters earn their place is uneven stone and brick, reveals and junctions — they go on in full contact with the wall, leaving no gap for air to move behind them, and Historic England lists insulating plasters (“typically lime based”, with cork, hemp or aerated lime) among its internal insulation options. Approved Document L’s 0.30 for an upgraded wall (Table 4.3) is out of reach; the 0.7 backstop (paragraph 4.13) and the 5% floor-area allowance (note 6) are the realistic tests, and Historic England reports the DEEP study’s finding that targeting around 0.8 rather than 0.30 cut the moisture risk while keeping about two-thirds of the energy saving. Q: Does insulating plaster actually work? A: Yes, within limits set by its λ. On a 9-inch brick wall of about 2.2 W/m²K, 50mm of a good lime-cork plaster (λ 0.037–0.045) brings it to about 0.55–0.64 — a 70–75% cut, inside the 0.7 Approved Document L expects of an upgraded wall — and even 30mm lifts the inside surface clear of the mould-risk threshold (fRsi 0.80, against 0.75). 50mm of a lime render with a lightweight mineral aggregate (λ about 0.12) roughly halves the loss (1.15). No thermal plaster reaches the 0.30 the regulations ask of an upgraded wall at a practical thickness: about 107mm of the best, over 300mm of the weakest. Note: Plasters at their makers’ declared λ: Diathonite Evolution 0.045 (UNI EN 1745, T1; μ 4) and Thermactive.037 0.037 (μ 3) from Diasen’s data sheets; Baumit NHL Thermo ≤ 0.075 (rated 0.08); Secil ecoCork Lime 0.09 (EN 12667, technical data sheet April 2024); Womersley’s NHL2 0.12 (the figure Carrington Lime lists for the same Cornerstone render). Each on 215mm brick with a 5mm lime finish. The mews-house figure is the ASBP’s account of a Prewett Bizley retrofit. The pre-filled link opens as an upgraded solid wall. Condensation risk with insulating plaster: U-Monkey’s free BS EN ISO 13788 check fails the pre-filled wall: 1,215 g/m² of condensate is still against the brick at the end of the year (peak 3,850 g/m²). How much the plaster insulates and how vapour-open it is both count: 50mm of Thermactive.037 (R 1.35, μ 3) leaves 2,681 g/m², while 40mm of Baumit’s NHL Thermo (R 0.53, and more vapour-resistant) gets “screening only”, seasonal condensation that dries out. The Glaser method moves moisture by vapour diffusion alone, so the more a plaster insulates, the colder the brick behind it, and the more vapour-open it is, the more of a winter’s vapour reaches that cold brick. The check says itself that this is not the whole picture: it “does not model the capillary transport that carries moisture through brick or stone”, and lime plasters in full contact with the wall move liquid water both ways. A BS EN 15026 hygrothermal assessment is the right tool, as for any internal insulation on solid masonry, and the usual precautions apply — fix damp and rain penetration first and keep every finish vapour-permeable. The surface-mould test, which is what most people buy a thermal plaster for, is clear: the bare wall fails it (fRsi 0.56), 30mm of Diathonite Evolution passes it (0.80), and 50mm of the NHL2 render only just misses it (0.748, against 0.75). --- # U-value of brick-slip external wall insulation URL: https://energycount.co.uk/tools/u-value-calculator/brick-slip-external-insulation/ Typical U-value: 0.28 W/m²K — 110mm grey EPS external insulation finished in brick slips, solid brick, certified fixings External wall insulation finished with brick slips on a solid 9-inch brick wall needs about 110mm of grey EPS (Wetherby Epsitherm 70E, λ 0.032) to meet the 0.30 W/m²K Approved Document L asks of an upgraded wall: U-Monkey gives 0.28 with the system’s certified fixings counted (7 per m² at χ 0.004 W/K), and 100mm misses, at 0.31. The BBA certificate’s table puts 100mm at 0.30. Like U-Monkey, it takes the brick behind as protected (λ 0.56), as BR 443 does once a wall has external insulation, and its figures are close to a calculation with no air-gap correction: U-Monkey gives 0.299 on that basis, and adds BR 443’s default correction otherwise. The slips themselves add almost nothing: what sets them apart from render is the finish, the weight and the fixings. Build-up, outside to inside: - 15mm brick slips on adhesive, pointed (λ 0.77, as the BBA certificate takes them) - 110mm Wetherby Epsitherm 70E grey EPS (λ 0.032), 7 fixings per m² at χ 0.004 W/K - 215mm solid brick (protected by the insulation: λ 0.56) - 13mm plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 80mm Epsitherm 70E (λ 0.032) | ≈ 0.36 | | | 90mm | ≈ 0.33 | The maker’s web page says 90mm meets refurbishment standards “in the majority of cases” | | 100mm | ≈ 0.31 | Misses 0.30 (0.306); the BBA table’s 100mm has no air-gap correction | | 110mm | ≈ 0.28 | As pre-filled: meets 0.30 | | 120mm | ≈ 0.27 | | | 130mm | ≈ 0.25 | BBA: 130mm for 0.25 | | 90mm Epsitherm 90E (λ 0.030) | ≈ 0.32 | | | 100mm Epsitherm 90E | ≈ 0.29 | Meets 0.30 (0.292) | | 100mm mineral wool (λ 0.036) | ≈ 0.33 | Non-combustible insulation, for taller buildings | | 120mm mineral wool | ≈ 0.29 | Meets 0.30 (0.292) | | 140mm mineral wool | ≈ 0.26 | | | 100mm EPS, fixings left out | ≈ 0.28 | The fixings add 0.028 | | 100mm EPS, no air-gap correction (the BBA table’s basis) | ≈ 0.30 | Meets 0.30 (0.299); BBA: 0.30 | | 100mm EPS with the brick at the exposed λ 0.77 | ≈ 0.31 | Misses (0.312): the exposed value is for brick with nothing outside it | What changes the number: Brick slips are a finish, not insulation: 15mm of slip on adhesive is worth about R 0.02 m²K/W, the same as a thin render, so the insulation and its fixings set the U-value. The BBA certificate for Wetherby’s Epsibrick system (14/5137 Product Sheet 1) counts seven galvanised fixings per m² at χ 0.004 W/K each, 0.028 on every row, and U-Monkey takes them as certified point thermal transmittances; left out, 100mm reads 0.28. The conventions explain the rest. BR 443 takes solid masonry as protected once it has external insulation, and U-Monkey and the certificate’s table both use protected brick (λ 0.56): the table gives 100mm for 0.30, 130mm for 0.25 and 190mm for 0.18. With the brick protected, BR 443 §4.2 counts its mortar joints (17.3% at λ 0.88, about +0.002), and U-Monkey also adds BR 443’s default air-gap correction (level 1) to the boards; without the correction it gives 0.299, 0.244 and 0.182 for the table’s 100, 130 and 190mm, close to the certificate. Taking the brick at its exposed value (0.77) instead would add about 0.006 at 100mm. 110mm meets 0.30 on any of these bases. Mineral wool (λ 0.036) needs about 120mm, but in England it is the insulation for a relevant building — one with a storey at least 18m above ground that contains dwellings, an institution or rooms for residential purposes: the EPS system is classed B-s1,d0, which regulation 7(2) rules out there, and the mineral wool versions are A2-s1,d0 or A1. The fixing pattern is the system’s own, and “for application to second storey floors and above” the certificate recommends that the designer considers at least one stainless-steel fixing per m² and fire barriers, in line with BRE’s fire guidance for external insulation (BR 135). Planning can be the reason for slips at all: householder permitted development asks for external materials “of a similar appearance” to the existing house (Schedule 2, Part 1, Class A, condition A.3(a)), and in a conservation area permitted development excludes cladding in stone, render, timber, plastic or tiles (A.2(a)), so check with the council; Historic England considers external insulation unlikely to be acceptable on a listed building. Q: How thick does brick-slip external insulation need to be to meet Building Regulations? A: About 110mm of grey EPS (λ 0.032) on a 9-inch solid brick wall, counting the system’s fixings: U-Monkey gives 0.28 W/m²K against the 0.30 Approved Document L sets for an upgraded wall (Table 4.3). The system’s BBA certificate puts 100mm at 0.30; U-Monkey reads 0.31 there (0.306), because it adds BR 443’s default air-gap correction, which the certificate’s figures appear to leave out: without it, 0.299. Both take the brick behind as protected (λ 0.56). With higher-grade EPS (λ 0.030) 100mm reaches 0.29; with mineral wool (λ 0.036) it takes about 120mm. Note: The slips, the insulation and the fixings follow BBA 14/5137 Product Sheet 1 (Wetherby Epsibrick EWI system; a 2014 issue, read in a council planning file): 15mm brick slips at λ 0.77, Epsitherm 70E λ 0.032 (90E 0.030), seven galvanised fixings per m² at χ 0.004 W/K, entered as certified point thermal transmittances. The brick behind the insulation is taken as protected, λ 0.56 (BR 443 §7), as the certificate takes it, with its mortar joints counted (§4.2); one row shows the exposed 0.77. Mineral wool rows: K Systems’ dual-density slab (λ 0.036) with the same fixings. The pre-filled link opens as an upgraded solid wall. Condensation risk with brick-slip external insulation: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled wall, and it stays that way with the slips, the EPS and the brick each tested across BS 5250’s published range of vapour resistance, one at a time and all together: insulation outside keeps the brick warm. The slip-and-adhesive skin is cementitious and less vapour-open than a lime render, which is why Historic England favours lime renders and rainscreens on traditional walls. --- # U-value of a tile-hung wall URL: https://energycount.co.uk/tools/u-value-calculator/tile-hung-wall/ Typical U-value: 1.28 W/m²K — 1930s tile-hung cavity wall: brick both leaves, 50mm empty cavity The tiles on a tile-hung wall don’t count in its U-value: BR 443 (§4.7.1) treats the space behind tile hanging as a well-ventilated cavity, so the tiles, the battens and that space are left out, and the outside surface resistance rises from 0.04 to 0.13. A 1930s tile-hung cavity wall — brick both leaves and a 50mm empty cavity — comes out at about 1.28 W/m²K, against 1.44 for the same wall below the tiles. Filled with 50mm of blown mineral wool it is about 0.54, inside the 0.55 Approved Document L asks of cavity fill. Re-hanging the tiles over 100mm of PIR on a 9-inch solid wall gives about 0.21, inside the 0.30 for external insulation. Build-up, outside to inside: - Clay plain tiles on 38 × 25mm battens (a well-ventilated space: disregarded) - 102.5mm brick outer leaf (λ 0.77, BR 443’s outer-leaf value) - 50mm unventilated cavity - 102.5mm brick inner leaf - 13mm plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 1930s tile-hung cavity wall: brick both leaves, 50mm empty cavity | ≈ 1.28 | As pre-filled | | The same, with the outer leaf taken as protected brick (λ 0.56) | ≈ 1.20 | BR 443 §7 gives the protected value behind cladding to a solid wall, not to an outer leaf | | The same wall untiled (ground floor) | ≈ 1.44 | Outside surface 0.04 | | 50mm blown mineral wool cavity fill (λ 0.040) | ≈ 0.54 | Meets 0.55 for cavity fill (0.540) | | 9-inch solid brick, tile-hung | ≈ 1.57 | A solid wall behind cladding: protected brick (BR 443 §7), its mortar joints counted (§4.2) | | 9-inch solid brick, tile-hung, brick at 0.77 | ≈ 1.77 | | | 9-inch wall re-hung over 100mm PIR, 6 stainless screws per m² through it | ≈ 0.21 | Meets 0.30; of the usual board thicknesses, 70mm is the thinnest that does (0.284) | | 9-inch wall re-hung over 120mm wood fibre (λ 0.041), screws as above | ≈ 0.30 | Meets 0.30, just: 0.296 | | Counted the wrong way: tiles and a still air space, outside surface 0.04 | ≈ 1.13 | Understates the U-value by 0.15 | What changes the number: The tiles are cladding, not insulation. BR 443 takes the air behind tile hanging as well ventilated (§4.7.1), at the outside temperature, so everything outside it drops out, and because the cladding shelters the wall from the wind the outside surface resistance becomes 0.13 rather than 0.04 (§4.7.3); counting the tiles and a still air space instead understates the U-value by about a tenth. Behind the tiles the brick stays drier, but BR 443 gives brick its protected λ, 0.56, behind “an external insulation or cladding system” only in a solid wall (§7); for the outer leaf of a cavity wall its value is 0.77 “unless otherwise specified” (§3), and that is what U-Monkey uses here. Taken as protected, the wall would be 1.20. A solid 9-inch wall behind tiles does take the protected value. From there the usual routes apply. A 50mm cavity can be filled (0.54 with blown mineral wool at λ 0.040), and the tiles keep driving rain off the outer leaf, which is what tile hanging has long been used for. Stripping and re-hanging the tiles is the chance to put insulation behind them: on a 9-inch wall, 100mm of PIR gives about 0.21 with the screws through it, and 120mm of wood fibre about 0.30. Or insulate inside and leave the tiles alone. Q: Do I have to insulate when I re-hang tile hanging? A: Possibly. The Building Regulations count the replacement of a layer of a wall as renovating it (regulation 2(1)), and renovating more than half of a wall’s area brings in the upgrade Approved Document L asks for — 0.30 for external insulation — so far as that is technically, functionally and economically feasible, with a simple payback of 15 years or less (regulation 23; Approved Document L, paragraphs 4.12 and 4.13). Whether stripping and re-hanging the tiles counts that way is for building control, so ask before you start; if it does, 70–100mm of PIR or about 120mm of wood fibre behind the new tiles is the usual answer on a 9-inch wall. Note: Clay tiles on battens are entered as cladding outside a well-ventilated air space, so U-Monkey disregards them and uses an outside surface resistance of 0.13 (BS EN ISO 6946 §6.9.4; BR 443 §4.7.1, §4.7.3). The outer leaf behind the tiles is taken at BR 443’s outer-leaf value, λ 0.77 (§3): its protected value behind cladding, 0.56, is written for solid walls (§7), and one row shows it. A solid wall behind tiles is taken as protected. Screws through insulation behind re-hung tiles are U-Monkey’s “EWI screw fixings” preset (6 per m², stainless). The link opens as an existing cavity wall being filled, judged against Approved Document L’s 0.55 (2021 edition, Table 4.3). Condensation and mould with a tile-hung wall: The empty 1930s wall fails U-Monkey’s surface-mould check — fRsi 0.69 against the 0.75 a home needs — as most uninsulated walls do; filling the cavity lifts it to 0.87. The filled wall gets a conditional pass in the BS EN ISO 13788 check: seasonal condensation against the back of the outer leaf, peaking at about 490 g/m² in March, that dries out over the summer — the usual Glaser result for a filled cavity. The check itself says it can’t settle this one alone: the result turns on the brick’s vapour resistance, which BS 5250 gives only as a wide range, and a Glaser calculation doesn’t model rain or the liquid moisture that brick carries, so its figures are indicative. Insulation behind re-hung tiles, PIR or wood fibre, passes with no condensation in any month: it keeps the whole wall warm. Keep the batten space ventilated and drained, with an underlay behind the battens. --- # U-value of built-up (twin-skin) metal wall cladding URL: https://energycount.co.uk/tools/u-value-calculator/built-up-metal-wall/ Typical U-value: 0.25 W/m²K — 180mm glass mineral wool (λ 0.040) on rails and brackets at SCI P312’s defaults, sealed steel liner (by SCI P312, with its default rails every 1.2m and 0.8 brackets per m²; 0.22 counting neither, which BR 443 does not allow) Built-up (twin-skin) metal wall cladding — a 0.4mm steel liner sealed at its laps, rails and brackets at SCI P312’s default spacing (rails every 1.2m, 0.8 brackets per m²), 180mm of glass mineral wool cladding roll (λ 0.040) and a 0.7mm profiled steel outer sheet — has a U-value of about 0.25 W/m²K, inside the 0.26 Approved Document L Volume 2 sets for a wall. 160mm reads 0.28 and fails. The rails and brackets add about 0.02: a layer-by-layer sum gives 0.22 at 180mm, but BR 443 says metal cladding “needs to take account of … any metallic components within the insulation”, and for rails and brackets it gives SCI P312 as the alternative to a numerical calculation. Build-up, outside to inside: - 0.7mm profiled steel outer sheet - 180mm glass mineral wool cladding roll (Knauf Insulation FactoryClad Roll 40, λ 0.040) — crossed by 40 × 40mm rails of 1.2mm steel every 1.2m and 0.8 brackets per m² (60mm² each) on thermal break pads, SCI P312’s defaults - 0.4mm steel liner sheet, laps sealed — the air and vapour control layer | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 140mm, SCI P312 defaults | ≈ 0.31 | Fails 0.26 | | 160mm | ≈ 0.28 | Fails 0.26 (0.276) | | 180mm | ≈ 0.25 | As pre-filled: meets 0.26 (0.247) | | 200mm | ≈ 0.22 | | | 220mm | ≈ 0.21 | Still short of the 0.18 the notional building takes for a side-lit building (0.205) | | 160mm, rails every 1.8m and 0.46 brackets per m² | ≈ 0.26 | Still fails (0.264); Tata Steel’s Trisobuild wall table gives 0.26 at 160mm at this spacing | | 180mm, rails every 1.8m and 0.46 brackets per m² | ≈ 0.24 | 0.237; Tata Steel gives 0.23 | | 180mm FactoryClad Roll 35 (λ 0.035) | ≈ 0.22 | 0.221 | | 180mm, rails and brackets not counted | ≈ 0.22 | Not allowed: BR 443 §4.9 (0.223) | What changes the number: Steel that crosses the insulation is what a plain BS EN ISO 6946 sum cannot handle, and built-up cladding has it at every rail and bracket. BR 443 (2019) §4.9: “The U-value of metal-clad walls and roofs needs to take account of joints between panels and any metallic components within the insulation, including through fixings”. It refers to MCRMA Technical Paper 18 for the calculation, its wall table asks for numerical analysis, and §4.9.1 adds that “the method given in Steel Construction Institute (SCI) Technical Note P312 … can be used as an alternative for rail-and-bracket systems”. SCI P312 (2002) is what U-Monkey uses when Metal cladding is set to Built-up: the quilt is split into the rail zone, bridged by the rail steel and by the air the rail squeezes out of the quilt (“twice the width of the rail”), and the rest, crossed only by the brackets, each costing ΔU = 1.6·λ·A·n·(R_i/R_ub)²/d_i on its cross-section A and number per m² n; the two resistance limits are weighted by P312’s p (its equation 3, based on the one for light steel framing) rather than averaged, and the brackets must sit on thermal break pads. U-Monkey reproduces P312’s worked example (a 170mm wall, U 0.255). The method is for rail-and-bracket spacers: Z-spacers and standing-seam clips need the system maker’s figures. What moves the number: the depth and λ of the quilt first — a 0.035 roll saves about 0.03 at 180mm — then the brackets and rails, which the steelwork spacing and the wind load set (P312’s method covers rails at least 1m apart and up to 1.3 brackets per m²). Makers’ own 3-D figures sit close: Tata Steel’s Trisobuild wall, at 1.8m steelwork and 1.2m bracket spacing, gives 0.26 at 160mm and 0.23 at 180mm, against U-Monkey’s 0.26 and 0.24 at the same spacing. Fill the void without over-compressing the quilt; where shallow liner ribs press it (no deeper than 25mm, at least 250mm apart) BR 443 §4.9.2 adds a correction, and otherwise says to “measure the insulation thickness at its thinnest point”. Approved Document L Volume 2 sets 0.26 for a wall in both its 2021 and 2026 editions; for a new building the real test is the whole-building calculation — the BER and BPER against the TER and TPER — where the NCM notional building has 0.18 walls if it is side-lit and 0.26 if top-lit, the rooflit shed. Q: Can I work out a twin-skin wall’s U-value layer by layer? A: Not for compliance. BR 443 (2019) §4.9 says metal-clad walls need the metal within the insulation taking into account, and its wall table asks for numerical analysis, with SCI P312 as the alternative for rails and brackets. Layer by layer, this wall reads 0.22 at 180mm; by P312, 0.25. In U-Monkey, enter the outer sheet, the quilt at its full depth (as one layer or as the rolls that make it up) and the liner, set Metal cladding to Built-up, and type in your system’s rail spacing and brackets per m² — or use the system maker’s own calculated value. Note: The pre-filled link opens in the England · non-domestic regime with Metal cladding set to Built-up (SCI P312) at the method’s Table 2 defaults — rails 40 × 40mm at 1.2m centres, 0.8 brackets per m² — with its worked example’s 1.2mm rail steel (one thickness crossing the quilt) and 60mm² bracket (50 × 1.2mm), steel at λ 60 as P312 takes it; no liner ribs pressing the quilt. The air round the rails takes BS EN ISO 6946’s 0.18 for horizontal heat flow; the quilt BR 443’s air-gap level 1. Condensation risk in built-up wall cladding: U-Monkey’s free BS EN ISO 13788 check gives the pre-filled wall a conditional pass: no interstitial condensation in any month while the steel liner works as a sealed air and vapour control layer, but condensate builds up year on year if its laps, fixings and penetrations are left unsealed — the check tests the liner unsealed as well as sealed. With steel on both faces nothing can dry out through the outer sheet, so the liner is what keeps the quilt dry. A perforated (acoustic) liner lets vapour through. With no vapour control layer the same wall collects kilograms of water per m² over a winter in the check, and Knauf’s guidance is plain: “Where a perforated liner sheet is used then a separate vapour control layer should always be installed.” Which one depends on the building’s humidity class and the liner, so take the system maker’s advice. U-Monkey’s check takes the outer sheet as vapour-tight, so it asks a lot of that layer: a polythene VCL still leaves condensate building up year on year, a foil VCL (sd in the thousands of metres) does not. --- # U-value of an insulated composite panel wall URL: https://energycount.co.uk/tools/u-value-calculator/composite-panel-wall/ Typical U-value: 0.19 W/m²K — Kingspan QuadCore AWP, 100mm PIR core, at its declared U-value (the maker’s declared value, joints and fixings included; a layer-by-layer sum reads 0.17) An insulated composite wall panel with a 100mm PIR core — Kingspan’s QuadCore AWP, λ 0.018 — has a declared U-value of 0.19 W/m²K, well inside the 0.26 Approved Document L Volume 2 sets for a wall; 74mm (0.25) is the thinnest that meets it, and 120mm (0.15) beats the 0.18 the NCM notional building takes for a side-lit building. For a composite panel the maker’s declared value is the one to use: BR 443 says “numerical analysis is needed to take account of joints between panels”, and a layer-by-layer sum of steel, core and steel reads low — 0.17 for this panel, and 0.37 for the 45mm one, declared at 0.46. U-Monkey takes a composite panel at the declared U-value you enter. Build-up, outside to inside: - Kingspan QuadCore AWP wall panel: steel outer face, 100mm QuadCore PIR core (λ 0.018), steel inner face, side joints with a factory-applied seal - Entered as one layer at its declared U-value, 0.19 W/m²K (QuadCore AWP data sheet, sixth issue, 3 July 2023; the April 2026 LEC sheet has the same table): R = 1/0.19 − (0.13 + 0.04) = 5.09 m²K/W | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | QuadCore AWP, 70mm core | 0.27 | Fails 0.26 | | QuadCore AWP, 74mm core | 0.25 | The thinnest that meets 0.26 | | QuadCore AWP, 80mm core | 0.23 | | | QuadCore AWP, 100mm core | 0.19 | As pre-filled | | QuadCore AWP, 120mm core | 0.15 | Beats the side-lit notional 0.18 | | QuadCore AWP, 150mm core | 0.12 | | | Tata Steel Trisomet, 100mm core | 0.20 | Datasheet (2025): BS EN 14509, computer modelled to BS EN ISO 10211 | | Joris Ide JI SF Wall 1000, 100mm PIR | 0.21 | Stated basis “EN 14509:2013” | | Stone wool core: Joris Ide JI Vulcasteel Wall, 175mm | 0.25 | A2-s1,d0 in all but two of its coatings; the thinnest of its sizes to meet 0.26 (240mm: 0.18) | | Layer by layer: steel, 100mm core at λ 0.018, steel | ≈ 0.17 | What BR 443 says not to do — no joints, no fixings (0.175; air gaps level 0, as a foamed core has none) | | Layer by layer at 45mm | ≈ 0.37 | Declared 0.46: the thinner the panel, the more the joints matter | What changes the number: A composite panel is the one common envelope U-Monkey does not calculate from its layers, because BR 443 (2019) says it cannot be done that way: its wall table says numerical analysis is needed “to take account of joints between panels, if it is thinner at joints or if metal fully or partly penetrates insulation, and to take account of profiled sheets”, and §4.8.3 rules out the ISO 6946 fixings correction “when both ends of the metallic fixing are in direct contact with metal sheets, including composite panels bounded by metal sheets”. Panel makers declare U-values to BS EN 14509, the panel standard, whose Annex A.10 adds the longitudinal joint by a two-dimensional calculation or a tabulated joint factor, and many model them to BS EN ISO 10211; the metal cladding makers’ own guidance had the quoted value including “the thermal bridge at side laps and any through fixings” (MCRMA/EPIC Technical Paper 17, 2007, since withdrawn). How much the joint costs depends on its design: a 2020 finite-element study at RWTH Aachen put a standard joint at about 11–17% of the U-value of 60–140mm panels, and found a leaky joint could add about 0.20 W/m²K through air leakage — which is why side joints are sealed on the liner side. So compare panels on their declared values: at 100mm, QuadCore AWP (λ 0.018) declares 0.19, Tata Steel’s Trisomet 0.20 and Joris Ide’s SF Wall 0.21. A stone wool core (λ about 0.040–0.045), chosen for its reaction to fire, needs nearly twice the depth: Joris Ide’s Vulcasteel wall panel, A2-s1,d0 in all but two of its coatings, reaches 0.25 at 175mm. Where the fire rules for a building restrict combustible materials in its external walls, the core’s class decides which panel can be used: check it against the rules that apply. Approved Document L Volume 2 sets 0.26 for a wall in its 2021 and 2026 editions; for a new building the test is the whole-building calculation — the BER and BPER against the TER and TPER — where the NCM notional building has 0.18 walls if side-lit and 0.26 if top-lit. Q: Can I calculate a composite panel’s U-value from its core thickness and λ? A: Not for compliance. BR 443 needs a numerical analysis of the joints and any fixings, which is what the maker’s declared value (BS EN 14509) contains, and the layer-by-layer figure differs from it: 0.17 for a 100mm QuadCore AWP core against the declared 0.19, and 0.37 against 0.46 at 45mm. Take the declared U for your panel and core thickness from its data sheet. In U-Monkey, pick the maker’s panel from the materials — Kingspan’s QuadCore, Tata Steel’s Trisomet and Joris Ide’s panels are there — and its core thickness, and the declared U fills itself in; for any other panel, add “Insulated composite panel (the maker’s declared U-value)” and type its U into the row. Any lining you add inside is then calculated with it. Note: The pre-filled link opens in the England · non-domestic regime with the panel as one layer, “Kingspan QuadCore AWP wall panel (100mm core)”, at its declared U-value: U-Monkey takes the panel’s resistance from the U you give it, R = 1/U − (Rsi + Rse), here 1/0.19 − 0.17 = 5.09 m²K/W. Pick another core thickness in its row and U-Monkey fills in Kingspan’s declared U for it; Tata Steel’s Trisomet and Joris Ide’s panels are in the library too, and for any other maker’s panel add the generic one and type its declared U. Declared values are rounded to two places, and the result is no more precise than that. Condensation in composite panels: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in the panel: with steel on both faces nothing diffuses in, and panel makers’ declarations of performance give the water vapour permeability as “Impermeable”. A lining added inside a 100mm panel — 50mm of mineral wool and plasterboard — also stays clear, because the panel keeps its inner face warm. What a one-dimensional check cannot see is the joints. Vapour and air get through them if anywhere — “condensation near the joint is by far more probable than at the centre of the panel”, a 2020 study at ZAG Ljubljana found — so the side-joint seal belongs on the liner (warm) side, and the outer one is “for weather tightness only” (SteelConstruction.info). For a pool, a kitchen or a cold store, take the maker’s advice on joint sealing for the humidity. --- # U-value of rainscreen cladding on a concrete or blockwork wall URL: https://energycount.co.uk/tools/u-value-calculator/rainscreen-concrete-wall/ Typical U-value: 0.23 W/m²K — 200mm RainScreen Duo Slab on 200mm concrete, one aluminium bracket per m² at Hilti’s χ 0.064 W/K (with one aluminium bracket per m² at the χ Hilti publishes for it; 0.17 for the insulation alone, 0.47 with BR 443’s default where no χ is known) A concrete wall with rainscreen cladding — 200mm reinforced concrete lined inside with plasterboard on dabs, 200mm of ROCKWOOL RainScreen Duo Slab (λ 0.035) outside it and fibre-cement panels on a ventilated subframe — has a U-value of about 0.17 W/m²K for the insulation alone, and about 0.23 once the aluminium cladding brackets are counted at one per m² with the point thermal transmittance Hilti publishes for its standard bracket into concrete (χ 0.064 W/K). Stainless steel brackets (χ 0.011) give 0.18; with no calculated χ, BR 443 adds 0.30 and the wall reads 0.47, failing Approved Document L Volume 2’s 0.26 at any thickness. On a rainscreen the bracket choice moves the U-value more than the insulation does. Build-up, outside to inside: - Fibre cement rainscreen panels (8mm) and their ventilated cavity: not counted, BR 443 §4.8.5 - 200mm ROCKWOOL RainScreen Duo Slab (λ 0.035), pierced by aluminium helping-hand brackets with plastic isolators, one per m² (χ 0.064 W/K each: Hilti’s calculated value for its MFT-MFI medium bracket into 180mm concrete) - 200mm reinforced concrete wall (λ 2.3) - Plasterboard on dabs (BR 443 §4.6.1: 20% plaster dabs) | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Insulation alone, no brackets counted (U0) | ≈ 0.17 | Where BR 443 does not let you stop: brackets through insulation must be counted | | 150mm, aluminium brackets, 1 per m² | ≈ 0.28 | Fails 0.26 (0.280) | | 200mm, aluminium, 1 per m² | ≈ 0.23 | As pre-filled (0.233) | | 250mm, aluminium, 1 per m² | ≈ 0.20 | 0.203 | | 200mm, aluminium, 1.5 per m² | ≈ 0.26 | Fails 0.26 (0.265) | | 200mm, aluminium, 2 per m² | ≈ 0.30 | | | 200mm, Hilti’s large aluminium bracket (χ 0.108), 1 per m² | ≈ 0.28 | Heavier cladding, bigger brackets | | 200mm, stainless steel brackets (χ 0.011), 1 per m² | ≈ 0.18 | 0.179 | | 200mm, stainless, 2 per m² | ≈ 0.19 | 0.190; ROCKWOOL’s own 3-D figure for 210mm on 200mm concrete with stainless brackets is 0.18 | | 200mm, thermally separated brackets (χ 0.0035), 1 per m² | ≈ 0.17 | 0.172 | | 200mm, brackets with no calculated χ: BR 443’s +0.30 | ≈ 0.47 | Fails 0.26 — no thickness passes with 0.30 added | | 200mm NyRock Rainscreen 032 instead, aluminium, 1 per m² | ≈ 0.22 | 0.220 | What changes the number: BR 443 (2019) §4.8.5 treats a rainscreen in two parts. The cladding and its ventilated cavity drop out, and the outside surface resistance becomes 0.13; but “the effect of brackets or rails fixing the cladding to the wall behind must be taken into account” wherever they cross insulation — by a calculation of the whole wall, a linear thermal transmittance for rails, a point thermal transmittance χ for each bracket (U = U0 + Σχ·n), or, with no calculated result, “a conservative default value of 0.30 W/m²K”. Few bracket makers publish χ. Hilti’s technical manual for ventilated façades (2019, calculated to BS EN ISO 10211) is the fullest public set, and shows the range: about 0.064 W/K for its medium aluminium bracket with a polypropylene isolator into 180mm of concrete, 0.108 for the large one, 0.010–0.032 for stainless steel brackets and 0.001–0.0035 for brackets with a polyamide thermal separation; into 300mm of brick the same aluminium brackets lose roughly half as much. The count per m² comes from the subframe layout — rail spacing times bracket spacing, set by the cladding’s weight and the wind load — so heavy cladding costs twice. Insulation makers’ tables come from 3-D calculations with one named bracket system: ROCKWOOL gives 0.18 for 210mm of RainScreen Duo Slab on 200mm of concrete with stainless steel brackets, and 0.18 for 200mm of NyRock Rainscreen 032 on 150mm of concrete with Ash & Lacy’s AxiAL aluminium brackets, which come with isolation pads — use such figures for the system modelled, not yours. The concrete itself adds little resistance, but what the brackets fix into changes their χ, as the brick figures above show, so use the χ for your backing wall; slab edges, window reveals and parapets are junction Ψ-values on top. Approved Document L Volume 2 sets 0.26 for a wall (2021 and 2026), and the NCM notional building 0.18 for a side-lit building. Where the fire rules for a building restrict combustible materials in its external walls, check the insulation’s reaction-to-fire class against them. Q: Aluminium or stainless steel rainscreen brackets? A: For the U-value, stainless or thermally separated. On this wall one aluminium bracket per m² with an isolator (χ 0.064 W/K) adds 0.064 W/m²K, taking 200mm of insulation from 0.17 to 0.23, while a stainless bracket (χ 0.011) adds 0.011, giving 0.18; Hilti’s polyamide-separated brackets add about 0.0035. The χ is the bracket maker’s, for your backing wall and insulation depth. In U-Monkey, enter it with your number of brackets per m² in the insulation layer’s fixings sheet (“Certified χ per fixing or bracket”); with no χ, pick “Rainscreen brackets, no χ” for BR 443’s +0.30. Note: Brackets as Hilti’s Technical Manual for ventilated façades (Section 6, U-value, 2019) gives them for its MFT-MFI medium aluminium bracket with a polypropylene isolator into 180mm concrete with λ 0.035 insulation: χ 0.0638 W/K at 100mm, 0.0642 at 160mm and 0.0634 at 200mm (interpolated in the source), entered as 0.064 at one bracket per m² — a round figure to show the scale; enter your own system’s χ and count. Stainless rows use its VTR stainless bracket (0.017 at 100mm, 0.0106 at 200mm), the thermally separated row its polyamide-separated brackets (about 0.001–0.0035). The cladding and its ventilated cavity are disregarded and Rse becomes 0.13 (BS EN ISO 6946 §6.9.4; BR 443 §4.8.5); the plaster dabs are bridged as BR 443 §4.6.1 directs. Opens in the England · non-domestic regime. Condensation risk with rainscreen on concrete: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled wall: the concrete is on the warm side and the mineral wool outside it is vapour-open to the ventilated cavity, so any vapour that reaches it passes out. The check is one-dimensional, through the insulation; the brackets are three-dimensional thermal bridges it cannot see. Keep the insulation tight round them, and treat the slab edges and openings as junctions. --- # U-value of a precast concrete sandwich panel URL: https://energycount.co.uk/tools/u-value-calculator/precast-concrete-sandwich-panel/ Typical U-value: 0.15 W/m²K — 80mm outer leaf, 150mm PIR, 150mm structural inner leaf, glass-fibre composite ties (with composite or plastic ties, which need no correction (λ below 1 W/mK)) A precast concrete sandwich panel — an 80mm outer leaf, 150mm of PIR insulation and a 150mm structural inner leaf, the leaves connected by glass-fibre composite ties — has a U-value of about 0.15 W/m²K, well inside Approved Document L Volume 2’s 0.26 for a wall and the 0.18 the NCM notional building takes for a side-lit building. The 230mm of concrete adds only 0.10 m²K/W; the insulation and the ties decide it. Ties below 1 W/mK, like glass-fibre composite connectors and the plastic ties FP McCann uses, need no correction (BR 443 §4.8.2), but steel connectors, which other precasters use, can matter a great deal: in a 2012 study of a similar panel, steel connectors took the measured U-value from 0.139 to 0.260. Build-up, outside to inside: - 80mm reinforced concrete outer leaf (λ 2.3) - 150mm PIR insulation (λ 0.022), the leaves connected by glass-fibre reinforced composite ties (λ below 1 W/mK: no correction, BR 443 §4.8.2) - 150mm reinforced concrete structural inner leaf, power-floated finish | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm PIR | ≈ 0.22 | 0.217 | | 120mm PIR | ≈ 0.18 | 0.184: misses the side-lit notional 0.18, unrounded | | 150mm PIR | ≈ 0.15 | As pre-filled (0.150) | | 200mm PIR | ≈ 0.12 | 0.116 | | 150mm PIR at air-gap level 2 | ≈ 0.18 | 0.178: a 2020 Estonian study recommends level 2 for sandwich panels with ties | | 150mm PIR at level 0 (two layers, staggered joints) | ≈ 0.14 | 0.141 | | 200mm mineral wool slab (λ 0.038) | ≈ 0.19 | A non-combustible core (0.190) | | 150mm XPS (λ 0.034) | ≈ 0.22 | 0.222 | What changes the number: The calculation itself is the plain BS EN ISO 6946 sum — both leaves and the insulation are continuous — and two things decide whether it holds. The ties. BR 443 (2019) §4.8.2 says ties with a conductivity below 1 W/mK “have very little effect on the calculated U-value”, and Ancon puts it plainly for its basalt-fibre wall ties: “Fixings with a lambda value of less than 1.0W/mK are excluded from u-value calculations”. Glass-fibre composite connectors (Thermomass, whose maker says steel conducts heat “at a rate over 50 times greater”) and Schöck’s Thermoanker (λ 0.7) are in that class, and FP McCann’s outer skins hang on “proprietary plastic ties”. Steel is another matter: a University College Dublin study (2012) of a 90 / 120 / 150mm phenolic-cored panel measured 0.139 W/m²K with no connectors, 0.150 with fibre-reinforced polymer ones and 0.260 with steel ones in a hot box, and a 2020 Estonian study of stainless steel diagonal ties found they added 8% with the insulation tight to them and 45–56% where 10mm air gaps formed beside them — against 4–39% by the ISO 6946 method — and recommended air-gap level 2 for such panels. With stainless anchors, get their χ from the maker or a BS EN ISO 10211 calculation. The leaves: Halfen gives 70mm as the minimum facing layer under the German national annex to Eurocode 2, with the load-bearing layer at least 50% thicker; UK projects have used 80mm outside, 200–450mm of insulation and 150–195mm inside (FP McCann’s University Locks panels are 80 / 250 or 450 / 195mm), and a precaster’s design guide for Wates gives 80 / 230 / 80mm at 0.145 with a stone wool core and steel connectors. That core is the choice where the fire rules for a building restrict combustible materials in its external walls, as PIR is combustible. The inner leaf also gives the room a lot of thermal mass: U-Monkey’s BS EN ISO 13786 figure for this panel is κ about 280 kJ/m²K. Q: Do the ties in a sandwich panel affect its U-value? A: Plastic and glass-fibre composite ties barely do — below 1 W/mK BR 443 needs no correction for them. Steel can: one study measured a panel’s U-value rising from 0.139 to 0.260 W/m²K with steel connectors, and stainless diagonal ties added up to 56% where air gaps formed round them. With steel ties, enter their point thermal transmittance (χ) from the maker in the insulation layer’s fixings sheet, and consider air-gap level 2. Note: Ties taken as glass-fibre composite (λ below 1 W/mK), so no fixings correction (BS EN ISO 6946 Annex F; BR 443 §4.8.2); the PIR at BR 443’s air-gap level 1 for a single layer of boards. Opens in the England · non-domestic regime. Condensation risk in a precast sandwich panel: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in the pre-filled panel, and it stays that way with each assumed vapour value tested across its plausible range: the dense concrete inner leaf and the PIR keep vapour out of the core. A mineral wool core is vapour-open: 200mm of it shows seasonal condensation against the back of the outer leaf in winter (a peak of 127 g/m²) that dries out in summer — a pass under BS EN ISO 13788, but water held against the concrete outer leaf, which a one-dimensional check cannot follow further. --- # U-value of 270mm loft insulation URL: https://energycount.co.uk/tools/u-value-calculator/loft-insulation-270mm/ Typical U-value: 0.15 W/m²K — 270mm quilt (170 over + 100 between joists) The standard loft build-up — 100mm of mineral wool between the joists plus 170mm laid over (270mm total) — achieves a U-value of about 0.15 W/m²K with λ 0.040 quilt, inside the 0.16 limiting value. Check the roll’s lambda: with λ 0.044 loft roll the same 270mm is 0.162, just over, and needs 300mm. The 0.11 notional needs about 365mm of λ 0.040 quilt, or a board upgrade. Build-up, outside to inside: - (The ventilated loft, the roof timbers and the tiles: one roof-space resistance, R 0.2 — BR 443 §8) - 170–200mm mineral wool quilt laid over joists - 100mm quilt between joists (12.8% timber, BR 443 §4.5.1) - Plasterboard ceiling | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 270mm total (100 between + 170 over), λ 0.040 | ≈ 0.15 | The standard recommendation | | 270mm total of λ 0.044 loft roll | ≈ 0.16 | Just over the 0.16 limit (0.162) — the roll’s lambda matters | | 300mm total (100 + 200 over), λ 0.040 | ≈ 0.13 | Common top-up | | ~365mm total, λ 0.040 | ≈ 0.11 | New-build notional territory | What changes the number: Only the layer between the joists is bridged — the continuous quilt over the top is the efficient part, which is why "more over the joists" beats "thicker between". Squashing quilt under boarding ruins it; if you need storage, raise the boards on legs tall enough to clear the full depth of quilt with an air gap above it. Q: Is 270mm of loft insulation still enough? A: With λ 0.040 quilt, yes: it comes out at about 0.15, inside the Part L limiting value of 0.16, and it remains the standard advice for existing homes. Check the roll’s lambda, though: at λ 0.044 the same 270mm is 0.162, just over, and needs 300mm. New-build notional performance (0.11) needs substantially more — around 365mm of λ 0.040 quilt — or higher-performing boards at ceiling level. Note: Joists at BR 443’s default 12.8% (§4.5.1), and everything above the insulation — the ventilated loft, the roof timbers and the tiles — as one roof-space resistance, R 0.2 (BS EN ISO 6946 Table 9, BR 443 §8). RdSAP’s own table, which also allows for the joists and the roof space, gives 0.16 for 270mm of mineral wool. Condensation risk in a cold loft: Insulating at ceiling level leaves the loft as a cold, ventilated void, so the question is not only the U-value but where the moisture from the rooms below ends up. BS 5250:2021 is clear that a condensation calculation is not the tool for this: it models vapour diffusing through the ceiling, but most of the moisture that reaches a cold loft arrives on air leaking through it, so the standard relies on prescriptive rules instead. U-Monkey’s free BS EN ISO 13788 check follows it. As pre-filled, the ceiling itself shows no condensation in any month, and 400mm of quilt makes no difference, but the check reports a cold pitched roof as outside the scope of a calculation rather than as a pass. Boarding is still worth modelling, because the check can see it. Lay chipboard straight onto the insulation and it predicts condensate building up under the boards year on year, a fail; raise them clear on legs with a ventilated air gap and that clears. What decides a cold loft in practice is air leakage, so seal round the hatch, pipes and downlights until the ceiling works as an air and vapour control layer, and keep the eaves ventilation clear when you top up. --- # U-value of an uninsulated loft (a ceiling with no insulation) URL: https://energycount.co.uk/tools/u-value-calculator/uninsulated-loft/ Typical U-value: 2.50 W/m²K — Plasterboard ceiling with no insulation above it A plasterboard ceiling with no insulation over it has a U-value of about 2.5 W/m²K. That includes the roof above it: BR 443 counts the ventilated loft, the roof timbers and the tiles as one resistance, 0.2 m²K/W for a roof with felt or an underlay (BS EN ISO 6946 Table 9); leave it out and the ceiling alone would be 5.0. RdSAP assumes 2.3 for a roof with no insulation. 100mm of mineral wool between the joists brings it to about 0.43, and 270mm to about 0.15 — a sixth of the heat loss for the first 100mm, a seventeenth for 270mm. Build-up, outside to inside: - (The ventilated loft, the roof timbers and the tiles: one roof-space resistance, R 0.2 — BR 443 §8) - 12.5mm plasterboard ceiling | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Plasterboard ceiling, tiles on felt or an underlay | ≈ 2.50 | As pre-filled (roof space R 0.2) | | Plasterboard ceiling, tiles with nothing under them | ≈ 3.85 | Roof space R 0.06 | | Plasterboard ceiling, roof boarded and felted | ≈ 2.00 | Roof space R 0.3 | | 20mm lime plaster ceiling (lath and plaster), felted roof | ≈ 2.71 | The laths themselves not counted | | RdSAP 10 default, no insulation | 2.3 | An assumed value, allowing for the roof space | | + 100mm mineral wool between the joists | ≈ 0.43 | See the 100mm loft insulation page | | + 270mm (100 between + 170 over) | ≈ 0.15 | See the 270mm loft insulation page | What changes the number: With nothing on the ceiling, the roof above is most of the resistance there is, which is why the roof type (chosen in the box above the build-up in U-Monkey) moves the figure from 2.0 to 3.9. Every millimetre of insulation then swamps it: at a 20°C difference an uninsulated ceiling loses about 50 W per square metre, 100mm of quilt cuts that to about 8.5 and 270mm to about 3. Air leaking through the hatch, downlights and pipe holes is not in a U-value at all — seal them as you insulate, and keep the eaves ventilation clear. Q: Does an uninsulated loft count as a thermal element to upgrade? A: Yes. At about 2.5 W/m²K it is far worse than the 0.35 threshold for a roof in Approved Document L (Table 4.3), so if the roof is renovated — re-covered over more than half its area, for example — it should be brought to the improved value of 0.16 so far as that is technically and functionally feasible with a simple payback of 15 years or less (paragraph 4.13). Note: BR 443 (2019) §8 gives everything above the insulation of a ceiling-level loft one resistance from BS EN ISO 6946 Table 9: 0.2 for tiles or slates on felt, boards or an underlay, 0.06 with nothing beneath them, 0.3 for a boarded and felted roof or one with a foil underside; U-Monkey adds it, with Rse 0.04 beyond it. The pre-filled link opens as an existing roof, judged against Approved Document L’s 0.35 threshold (2021 edition, Table 4.3). --- # U-value of 100mm loft insulation (and what a top-up achieves) URL: https://energycount.co.uk/tools/u-value-calculator/loft-insulation-100mm/ Typical U-value: 0.43 W/m²K — 100mm quilt between the joists, nothing over them 100mm of mineral wool quilt between the ceiling joists gives a U-value of about 0.43 W/m²K with λ 0.040 quilt, or 0.45 with older λ 0.044 roll. That is worse than the 0.35 at which Approved Document L counts a roof as an element to upgrade. Topping it up with 170mm laid over the joists (270mm in all) brings it to about 0.15, and 200mm over (300mm) to about 0.13. RdSAP assumes 0.40 for 100mm, and uses the same figure for an older loft whose insulation is unknown, noting that most lofts have at least 100mm. Build-up, outside to inside: - (The ventilated loft, the roof timbers and the tiles: one roof-space resistance, R 0.2 — BR 443 §8) - 100mm mineral wool quilt between 100mm ceiling joists (12.8% timber, BR 443 §4.5.1) - 12.5mm plasterboard ceiling | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 50mm quilt between the joists | ≈ 0.72 | | | 75mm quilt between the joists | ≈ 0.53 | | | 100mm quilt, λ 0.040 | ≈ 0.43 | As pre-filled | | 100mm loft roll, λ 0.044 | ≈ 0.45 | | | 100mm, joists at 600mm centres (35mm trussed rafters, 6.8%) | ≈ 0.39 | Fewer, thinner joists | | + 170mm over the joists (270mm) | ≈ 0.15 | Meets the 0.16 improved value | | + 200mm over the joists (300mm) | ≈ 0.13 | | | + 300mm over the joists (400mm) | ≈ 0.10 | See the 400mm page | What changes the number: While all the insulation is between the joists, the joists matter: BR 443’s default is 47mm timbers at 400mm centres plus 1% for other timbers, 12.8% of the layer, and a trussed-rafter roof at 600mm centres has about half that. Once quilt is laid over the joists the timber is covered and the top layer works at full value, which is why a top-up does so much more than its thickness suggests, and why BR 443 drops the air-gap correction to level 0 with a second layer in place. Lay the top-up across the joists, not along them; do not squash it under boards — raise them on legs tall enough to clear the quilt with an air gap above it — and keep it back from the eaves vents. Q: Is it worth topping up 100mm of loft insulation? A: Yes: going from 100mm to 270mm cuts the heat through the ceiling by about two-thirds, from about 0.43 to 0.15 W/m²K. For a 50m² loft at a 20°C difference that is about 430 W down to 150 W, all the time the heating is on. The Energy Saving Trust gives 270mm of mineral wool as the minimum to aim for. Note: Joists at BR 443’s default 12.8% (§4.5.1) and the roof space above as R 0.2 (BS EN ISO 6946 Table 9, BR 443 §8). The pre-filled link opens as an existing roof, judged against Approved Document L’s 0.35 threshold (2021 edition, Table 4.3). --- # U-value of 400mm loft insulation URL: https://energycount.co.uk/tools/u-value-calculator/loft-insulation-400mm/ Typical U-value: 0.10 W/m²K — 100mm between + 300mm over the joists 400mm of mineral wool quilt — 100mm between the joists and 300mm over them — gives a U-value of about 0.10 W/m²K with λ 0.040 quilt, better than the 0.11 of the new-build notional roof. With λ 0.044 roll it is 0.109, just inside 0.11. 350mm gives 0.11 when rounded but is 0.114, so it misses the notional figure; λ 0.040 quilt needs about 365mm to meet it. Each 100mm added saves less than the last: 200 to 300mm saves twice as much heat as 300 to 400mm. Build-up, outside to inside: - (The ventilated loft, the roof timbers and the tiles: one roof-space resistance, R 0.2 — BR 443 §8) - 300mm mineral wool quilt laid over the joists - 100mm quilt between 100mm ceiling joists (12.8% timber) - 12.5mm plasterboard ceiling | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 270mm (100 between + 170 over) | ≈ 0.15 | See the 270mm page | | 300mm (100 + 200) | ≈ 0.13 | | | 350mm (100 + 250) | ≈ 0.11 | Misses the 0.11 notional: 0.114 | | 400mm (100 + 300), λ 0.040 | ≈ 0.10 | As pre-filled | | 400mm of λ 0.044 loft roll | ≈ 0.11 | Meets 0.11 (0.109) | | 450mm (100 + 350) | ≈ 0.09 | | | 500mm (100 + 400) | ≈ 0.08 | | What changes the number: Past about 300mm the quilt over the joists does nearly all the work and the U-value falls slowly: each extra 50mm of λ 0.040 quilt saves less than 0.02 W/m²K. What spoils deep loft insulation in practice is compression (boards laid straight on it), gaps at the eaves and around the hatch, and quilt pushed into the eaves so that it blocks the ventilation; BS 5250 wants the roof space ventilated, and the condensation check in U-Monkey reports a cold loft as outside the scope of a calculation for that reason. RdSAP uses 0.11 for any loft with 400mm or more. Q: Is 400mm of loft insulation too much? A: Not thermally: it takes a loft to about 0.10 W/m²K, inside the 0.11 a new-build roof is modelled at, and there is no condensation penalty provided the loft stays ventilated and the ceiling is well sealed. The return per millimetre is small by then, though — going from 300 to 400mm saves about 0.03 W/m²K, against 0.28 for the first top-up from 100 to 270mm — so check the joists can take the weight of boarding raised above it, and that the hatch and eaves can be insulated and ventilated properly. Note: Joists at BR 443’s default 12.8% (§4.5.1) and the roof space above as R 0.2 (BS EN ISO 6946 Table 9, BR 443 §8). A second layer covers the joists, so the air-gap correction is level 0. --- # U-value of insulation between rafters (loft conversion) URL: https://energycount.co.uk/tools/u-value-calculator/pitched-roof-rafter-insulation/ Typical U-value: 0.19 W/m²K — 120mm PIR between + 25mm under rafters A loft-conversion roof with 120mm PIR between the rafters and 25mm continuous PIR beneath them achieves a U-value of about 0.19 W/m²K; moving to 100mm between plus 50mm under reaches about 0.17. The continuous under-layer matters more than its thickness suggests because it breaks the rafter bridge. Build-up, outside to inside: - Tiles + ventilated batten space - Breather membrane - 100–120mm PIR between rafters (~12% timber) - 25–50mm continuous PIR under rafters - Plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 120mm between + 25mm under | ≈ 0.19 | Common conversion spec | | 100mm between + 50mm under | ≈ 0.17 | Better bridge-breaking for the same total | What changes the number: Rafter depth limits what fits between, and a 50mm ventilated air path (or a suitable membrane strategy) must be kept above the insulation. The under-rafter layer is continuous, so shifting thickness from "between" to "under" usually wins. Q: How much headroom will rafter insulation cost me? A: Typically 40–65mm below the rafters (continuous board plus plasterboard). That is exactly the trade the between/under split controls — model both splits in U-Monkey and check the headroom against the U-value you need. Condensation risk at rafter level: Insulate along the rafters and the cold face of the insulation sits just below the underlay, so the layers either side of it — the vapour control inside, the underlay outside — decide whether moisture gets trapped. As pre-filled, with foil-faced PIR between and under the rafters, U-Monkey’s free BS EN ISO 13788 check, with the roof surface taken 2 K below the air for night-sky cooling as the standard asks, finds no condensation in any month, though the margin is tight, at about 98% of saturation inside the outer board in December. It calls that a conditional pass, for a reason worth knowing: BS 5250 treats an underlay above sd 0.05 m as high-resistance, which needs a ventilated void beneath it, and the generic breather is entered at exactly 0.05 m. Enter your underlay’s declared value; most modern breather membranes are well under it. The pass also relies on the under-rafter PIR acting as the air and vapour control layer, so every joint needs taping and every penetration sealing. Swap the PIR for mineral wool and the membranes take over. Under a vapour-permeable (LR) underlay with nothing on the warm side, the check will not call it a pass: BS 5250:2021 expects an air and vapour control layer on the warm side of a warm pitched roof, and the calculation itself predicts heavy seasonal condensation under the underlay. Add a vapour control layer behind the plasterboard and that drops to under 2 g/m² that dries out each summer. Under a traditional 1F bitumen felt with no VCL, condensate builds up year on year on the underside of the felt. A ventilated 50mm gap between felt and insulation clears the condensation, but BS 5250 wants the VCL as well, and only with both does the check pass it. --- # U-value of insulation over the rafters (warm pitched roof) URL: https://energycount.co.uk/tools/u-value-calculator/pitched-roof-insulation-over-rafters/ Typical U-value: 0.14 W/m²K — 100mm PIR over + 100mm mineral wool between rafters A warm pitched roof with 100mm of PIR laid over the rafters and 100mm of mineral wool between them achieves a U-value of about 0.14 W/m²K, inside the 0.15 for a new roof on an extension and the 0.16 limiting value. 120mm over the rafters reaches about 0.13, and 150mm meets the 0.11 notional value. Because the boards run over the rafters, the timber bridges only the wool between them. The boards go on from outside, so this is usually done on a new roof or when a roof is stripped and re-covered. Build-up, outside to inside: - Tiles on battens and counter-battens (ventilated, not counted) - Breather membrane - 80–150mm PIR over the rafters (continuous) - 100mm mineral wool between the rafters (~12% timber) - Polythene VCL (warm side) - Plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm PIR over, rafter void left empty | ≈ 0.20 | The boards alone fall short of the 0.16 limit | | 80mm PIR over + 100mm wool between | ≈ 0.16 | Just misses the 0.16 limiting value before rounding (0.164) | | 100mm PIR over + 100mm wool between | ≈ 0.14 | Inside the 0.15 for a new roof on an extension | | 120mm PIR over + 100mm wool between | ≈ 0.13 | Comfortable pass | | 150mm PIR over + 100mm wool between | ≈ 0.11 | Meets the 0.11 notional value (0.108) | What changes the number: Thickness over the rafters is worth more than depth between them, because the layer is continuous. It also earns BR 443’s level 0 for air gaps — insulation in two layers, one covering the rafters and any gaps in the other — which U-Monkey applies automatically. With foil-faced PIR above the wool, the vapour control layer on the warm side is essential: take it out and U-Monkey’s free condensation check will not pass the roof, because BS 5250 expects an air and vapour control layer on the warm side of a warm pitched roof. As pre-filled it finds no condensation in any month, a conditional pass that relies on the breather membrane being vapour-open, so enter your underlay’s declared value. The counter-battens are fixed through the boards into the rafters, so check the fixing lengths against the thickness you choose. Q: Is insulation over the rafters better than between them? A: Per millimetre, yes: it is continuous, so the rafters bridge only what sits between them, and it costs no headroom inside. The catch is access — it goes on from outside, so it suits a new roof or one being re-covered, and it raises the roof line, which changes the eaves and verge details. Where the roof covering is staying put, insulation between and under the rafters is the usual route; see the rafter-level insulation page and compare the two in U-Monkey. --- # U-value of a room-in-roof sloping ceiling URL: https://energycount.co.uk/tools/u-value-calculator/room-in-roof-sloping-ceiling/ Typical U-value: 0.15 W/m²K — 50mm PIR between + 100mm PIR under rafters, vent gap kept An existing room-in-roof slope with 100mm rafters, 50mm of quilt between them and a plasterboard ceiling has a U-value of about 0.79 W/m²K. Under an old bitumen felt, a high-resistance underlay, BS 5250 asks for a ventilated gap between the felt and the insulation — usually 50mm — which leaves 50mm of the rafter depth to insulate. With 50mm of PIR there and 100mm of PIR under the rafters the slope reaches about 0.15, inside the 0.16 that Approved Document L sets for a roof being renovated; 75mm under the rafters gives only 0.19. Re-covering the roof with a vapour-permeable underlay lets you fill the rafters instead: 100mm of PIR between and 60mm under reaches 0.16. Build-up, outside to inside: - Tiles on battens (ventilated, not counted) - Bitumen felt underlay, type 1F (not counted) - 50mm ventilated gap under the felt (not counted) - 50mm PIR between 100mm rafters (~12% timber) - 75–100mm PIR under the rafters (continuous) - Polythene VCL (warm side) - Plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | As built: 50mm quilt between the rafters | ≈ 0.79 | Renovating it brings in Table 4.3’s 0.16; worse than the 0.35 threshold too | | 50mm PIR between + 50mm under | ≈ 0.24 | Well short of the 0.16 renovation target | | 50mm PIR between + 75mm under | ≈ 0.19 | Still short of 0.16 | | 50mm PIR between + 100mm under | ≈ 0.15 | Meets the 0.16 for a renovated roof | | Vapour-permeable underlay, no gap: 100mm PIR between + 60mm under | ≈ 0.16 | Meets 0.16 (0.159) with 40mm less lost headroom | What changes the number: The ventilated gap is what makes this slope hard. BS EN ISO 6946 disregards it and everything above it, so only the insulation inboard of the gap counts, and in shallow rafters that is not much. Every millimetre under the rafters comes off the room — 100mm of PIR and the plasterboard bring the ceiling 112.5mm further in — which is why the underlay matters: replace the old felt with a vapour-permeable one when the roof is re-covered and the full rafter depth can be filled. Either way, BS 5250 expects an air and vapour control layer on the warm side, and U-Monkey’s free condensation check will not pass the slope without one; as pre-filled, with the VCL behind the plasterboard, it finds next to no condensation, a conditional pass that depends on the assumed vapour values, so enter your products’ declared figures. The other parts of a room in the roof — the stud walls, the flat ceiling and the insulation in the eaves voids — are separate elements: model each one. Q: Do I have to reach 0.16 when I insulate a room in the roof? A: If the work renovates the roof — stripping the sloping ceiling back to the rafters, for example — Approved Document L Table 4.3 asks for its improved value of 0.16 W/m²K where that is technically, functionally and economically feasible; where it is not, the slope is improved as far as is feasible within a 15-year simple payback. That holds whatever the slope starts from: the table’s 0.35 threshold is for a roof kept in a conversion (paragraph 4.12(b)), and at about 0.79 this one would be upgraded then too. Headroom is usually what limits it: model the options in U-Monkey and agree the spec with building control before you start. --- # U-value of a warm flat roof URL: https://energycount.co.uk/tools/u-value-calculator/warm-flat-roof/ Typical U-value: 0.14 W/m²K — 150mm PIR warm deck A warm flat roof with 120mm PIR above the deck achieves a U-value of about 0.18 W/m²K; 150mm reaches about 0.14, which is why 150mm is the common specification for new extensions. "Warm" construction — insulation above the deck, VCL below the insulation — is the default because it keeps the structure warm and dry. Build-up, outside to inside: - Waterproof membrane - 120–150mm PIR - Vapour control layer - 18mm deck (OSB/ply) - Firrings/void + plasterboard ceiling | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 120mm PIR warm roof | ≈ 0.18 | Short of the 0.16 limiting value — a new roof needs about 140mm | | 150mm PIR warm roof | ≈ 0.14 | The usual extension spec | What changes the number: Board thickness is nearly everything. The VCL sits below the insulation on the warm side — get that wrong and the deck gets wet, which the free condensation check will show. Tapered schemes (falls cut into the insulation) are handled by U-Monkey’s tapered mode. Q: Warm roof or cold roof for my extension? A: Warm (insulation above the deck) is the modern default: simpler ventilation, structure stays warm, condensation risk is lower. Cold roofs need a ventilated void and careful detailing. U-Monkey models both — but if you have the build-up height, warm wins. Condensation risk in a warm flat roof: A warm deck puts vapour-tight waterproofing on the cold side of the insulation, so moisture that reaches it has nowhere to go — which is why the vapour control layer under the insulation matters. As pre-filled, U-Monkey’s free BS EN ISO 13788 check fails it. With the roof surface taken 2 K below the air every month, the standard’s simplified method for roofs, a little condensation builds up under the waterproofing through the winter, peaking at about 7 g/m² in April, and about 3.5 g/m² of it is still there at the end of the year, so it accumulates. The waterproofing is entered at BS 5250’s figure for roofing felt laid in bitumen, sd 90 m, and a 500-gauge polythene VCL, sd 50 m, is simply less vapour-resistant than the roof above it. The fix is the one warm-deck specifications use: a high-performance vapour control layer, such as a foil-cored bitumen VCL. Left on the generic felt, the check asks for a VCL rated at least sd 2,400 m, because the result has to hold with the waterproofing anywhere up to BS 5250’s sd 200 m for a reinforced bitumen membrane; enter your membrane’s declared value and that falls, to about 950 m for a declared 90 m. Swap the PIR for mineral wool slabs and it fails by far more with the polythene VCL. Model a cold deck instead — insulation between the joists, deck on top — and it fails outright, with or without a VCL at the ceiling; even a cold deck that clears the calculation is reported as outside its scope, because BS 5250:2021 relies on prescriptive rules for cold roofs. BS 5250 counts a warm deck with a sealed vapour control layer among the arrangements experience shows are safe, and the calculation is harder on it than experience is: its simplified roof method leaves out the sun, which dries a roof in summer. At colder or wetter stations than Manchester it asks for more, and at some none of the standard fixes clears it, so run it at your own. --- # Flat roof insulation thickness — about 150mm PIR for 0.15, 140mm for 0.16 URL: https://energycount.co.uk/tools/u-value-calculator/flat-roof-insulation-thickness/ Typical U-value: 0.15 W/m²K — 140mm PIR warm deck To reach the common 0.15 W/m²K target, a warm flat roof needs roughly 150mm of PIR; the 0.16 limiting value needs about 140mm, and the 0.11 new-build notional pushes past 200mm. U-Monkey’s "Hit a target U-value" solver does this backwards calculation for you against your exact build-up. Build-up, outside to inside: - Waterproof membrane - PIR (thickness = the question) - VCL - 18mm deck - Ceiling | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Target 0.16 (limiting value) | ~140mm PIR | Minimum for a new thermal element | | Target 0.15 | ~150mm PIR | The usual extension spec | | Target 0.11 (notional) | ~210mm PIR | New-build performance | What changes the number: Lambda sets the exchange rate: phenolic (0.020) saves ~10% thickness over PIR (0.022); mineral wool (0.038) nearly doubles it. Upstands, rooflights and falls all eat into the build-up height — solve for the real target before the joists are ordered. Q: Can U-Monkey tell me the thickness directly? A: Yes — build the roof, press "Hit a target U-value…", pick the target, and it solves the insulation thickness for you. Free, in the browser. Does the thickness change the condensation risk?: Hardly — in a warm deck the layer order does the work. Run this build-up through U-Monkey’s free BS EN ISO 13788 check at 100mm, 140mm and 210mm of PIR and the result barely moves: condensation under the waterproofing peaks at about 7 g/m² in spring and 3–4 g/m² is left at the end of the year, so with the waterproofing at BS 5250’s figure for bitumen felt, sd 90 m, over a 500-gauge polythene VCL, it fails at every thickness. What moves it is the vapour control, not the insulation: a high-performance VCL clears it, and the check’s How to fix it gives the rating it needs, lower once you enter the membrane’s declared value. Where thickness does matter is when it is split to save height. Put 60mm of PIR above the deck and 150mm of mineral wool between the joists below it, leave the vapour control layer on the deck, and the check puts heavy condensation on the underside of the deck every winter, more than dries out by summer: a fail, with its own timber check warning about the deck. Move the VCL down to the ceiling and the amount falls to a few grams, but it still does not clear each year, so that fails too. Solve the thickness first, then check the build-up you will actually build. --- # U-value of a cold flat roof (cold deck) URL: https://energycount.co.uk/tools/u-value-calculator/cold-flat-roof/ Typical U-value: 0.14 W/m²K — 175mm quilt between joists + 70mm PIR under, ventilated void A cold flat roof — insulation between the joists, a ventilated void above it, deck and waterproofing on top — needs far more insulation than it looks. With 175mm of mineral wool between the joists it reaches only about 0.28 W/m²K, because BS EN ISO 6946 ignores the ventilated void and everything above it, and the joists bridge the whole insulation layer. Adding 70mm of PIR under the joists brings it to about 0.14, inside the 0.15 a new roof on an extension has to meet. But BS 6229:2018, the flat roofing code of practice (revised in 2025), noted that this construction is not now recommended, and BS 5250:2021 says it should not be used where the void spans more than 5m, can’t be cross-ventilated from both sides, or can’t have a sealed vapour control layer — which is why most new flat roofs are built warm. Build-up, outside to inside: - Waterproof membrane on an 18mm deck (OSB/ply) - 50mm ventilated void, cross-ventilated from side to side - 175mm mineral wool quilt between 225mm joists (~12% timber) - 70mm PIR under the joists (continuous) - Sealed vapour control layer - Plasterboard ceiling | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm quilt between joists | ≈ 0.45 | Older cold roof — re-covering more than half of it brings in Table 4.3’s 0.16 where feasible | | 175mm quilt between joists | ≈ 0.28 | Joist-depth insulation alone — well short of a new roof | | 175mm quilt + 50mm PIR under | ≈ 0.16 | Just over the 0.16 limiting value for new dwellings (0.163) | | 175mm quilt + 70mm PIR under | ≈ 0.14 | Meets the 0.15 for a new roof on an extension | What changes the number: The ventilated void is the catch. BS EN ISO 6946 ignores it and every layer above it, so the deck and waterproofing add nothing, and the quilt between the joists is bridged by timber through its full depth — which is why a continuous layer under the joists does more per millimetre than extra depth between them. The void itself has to stay open: BS 5250:2021 asks for at least 50mm clear above the insulation, cross-ventilation equal to a continuous 25mm opening on each side, a sealed air and vapour control layer on the warm side, and no cold-deck void spanning more than 5m. Q: Can I still build a cold flat roof? A: You can, but only as a last resort. BS 6229:2018, the flat roofing code of practice (revised in 2025), noted that this type of roof is not now recommended: a sealed vapour control layer and enough cross-ventilation are hard to achieve and keep. BS 5250:2021, the code of practice on moisture in buildings, says it should not be used where the void spans more than 5m, can’t be ventilated at both ends, or can’t have a sealed air and vapour control layer. Warranty providers may refuse it on new work too: LABC Warranty, for one, does not accept cold deck flat roofs over 3m². Where one is unavoidable — for example an existing roof being re-covered with no height to add insulation above the deck — it needs a sealed air and vapour control layer on the warm side, a clear 50mm air gap above the insulation and cross-ventilation equal to a continuous 25mm opening on each side. If you can gain the height, converting it to a warm deck is the better answer. Condensation risk in a cold flat roof: A cold deck leaves the vapour-tight waterproofing on a deck that sits outside the insulation, so everything rests on the void between them staying ventilated. By the standard’s convention a well-ventilated void counts as outside air, so a condensation calculation takes the ventilation on trust, and BS 5250:2021 draws the conclusion: calculation methods don’t model air movement and are not suitable for cold flat roofs, which it treats by prescriptive rules instead (and BS 6229:2018, since revised, noted they are not now recommended). U-Monkey’s free BS EN ISO 13788 check follows it. As pre-filled, with a 50mm void open to outside air, it finds no condensation, with or without the vapour control layer, but it reports the roof as outside the scope of a calculation rather than as a pass. What the check can show is what happens when the ventilation isn’t there — blocked vents, a void too shallow, a span too long to cross-ventilate. Model the void as unventilated, with just the quilt between the joists and no vapour control, and condensate builds up under the deck year on year, far faster than it can dry: the app’s own timber check flags the OSB. Add a sealed VCL at the ceiling and the peak drops to about 45 g/m², but it still builds up year on year. Keep the pre-filled foil-faced PIR under the joists as well and it falls to a few grams, and even that does not quite clear each year with the waterproofing at BS 5250’s figure for bitumen felt. If you have to build a cold roof, the ventilation and a continuous, sealed VCL are not optional. --- # U-value of an inverted flat roof URL: https://energycount.co.uk/tools/u-value-calculator/inverted-flat-roof/ Typical U-value: 0.14 W/m²K — 250mm XPS in two layers over the waterproofing, at its design λ, with a WFRL An inverted flat roof with 250mm of XPS laid over the waterproofing of a concrete deck, in two layers and taken at the design λ BR 443 asks for in an inverted roof, achieves about 0.14 W/m²K, provided a certified water flow reducing layer (WFRL) goes over the boards. Leave it out and the rain that runs under the insulation carries heat away: BS EN ISO 6946 adds a correction for it — about 0.06 W/m²K at Manchester’s rainfall for these rebated boards under gravel — and the same roof comes out near 0.20. In an inverted roof it is that correction, not condensation, that decides how much insulation you need. Build-up, outside to inside: - Gravel ballast or paving (not counted in the U-value) - Water flow reducing layer (WFRL) over the boards - 250mm XPS in two layers, 150 + 100mm, joints staggered (Bauder XPS 300 Inverted, design λ 0.035) - Waterproofing membrane — under the insulation, on the warm side - 150mm reinforced concrete deck - Plaster soffit | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 250mm XPS (150 + 100) + certified WFRL (f·x 0.001) | ≈ 0.14 | Meets the 0.15 for a new roof on an extension | | 250mm XPS, no WFRL, gravel ballast (f·x 0.03, rebated boards) | ≈ 0.20 | Same boards, with rain running underneath | | 300mm XPS (2 × 150), no WFRL (f·x 0.03) | ≈ 0.17 | Still short of 0.15: more board doesn’t replace the WFRL | | 300mm XPS (2 × 150) + certified WFRL | ≈ 0.12 | | What changes the number: The rain correction is the number to watch. BS EN ISO 6946 adds ΔU = p × f·x × (R1/RT)², where p is the site’s average daily rainfall from October to May, f·x is how much of it reaches the waterproofing and how much heat it takes with it, and R1/RT is the share of the roof’s thermal resistance that sits above the membrane. Without a certified value, f·x is 0.04 for butt-jointed boards under open gravel, 0.03 for rebated boards — Bauder’s are rebated on all four sides — and 0.02 under a green roof or a cast concrete parking deck. A certified water flow reducing layer brings it down to around 0.001, the value this build-up opens with, but take the figure from your system’s certificate. U-Monkey fills p from the nearest Met Office station, so pick yours: the correction scales with the rain. The figures here take Bauder XPS 300 Inverted at its design λ, 0.035, not the 0.033 it declares: BR 443 (§4.8.6, after ETAG 031) asks for the design value in an inverted roof, which allows for the moisture the boards take up in use. Laid in two layers with staggered joints, as here, the boards take BR 443’s air-gap level 0; one layer of butt-jointed boards takes level 1, about 0.01 worse (rebated boards in one layer qualify for level 0 too — choose Perfect under Fitting quality). Q: Is there a condensation risk in an inverted roof? A: Low, as a rule. The waterproofing is vapour-tight, and in an inverted roof it sits under the insulation on the warm side, so it stays above the dew point and does the job a vapour control layer does in a warm deck. U-Monkey’s free check on this build-up finds no interstitial condensation in any month. What a monthly screening can’t model is cold rain running under the boards; the U-value correction above accounts for the heat it carries away. Note: Bauder XPS 300 Inverted at its design λ, 0.035 (it declares 0.033), in two layers with staggered joints: BR 443’s air-gap level 0. The table and the pre-filled link use Manchester’s October–May rainfall (2.16 mm/day), U-Monkey’s default station. The rain correction scales with rainfall, so wetter sites lose more: pick your nearest station in the app. Condensation risk in an inverted roof: Turning the roof upside down turns the condensation question around: the vapour-tight waterproofing sits under the insulation, on the warm side, where a warm deck would have its vapour control layer. As pre-filled, this roof passes U-Monkey’s free BS EN ISO 13788 condensation check with no condensation in any month, with the roof surface taken 2 K below the air as the standard asks, and it stays clear at the upland Eskdalemuir station and at humidity class 4. The margin is tight, at about 97% of saturation inside the XPS in December, but the verdict holds with the assumed vapour values for the XPS and the membrane tested across their ranges, one at a time and together. The same 250mm of XPS over the waterproofing of an 18mm plywood deck on timber joists passes too. What the screening can’t see is cold rainwater running between the boards and the membrane — it works month by month, in steady state — which is why the rain correction in the U-value, and the water flow reducing layer that keeps it small, matter more here than the condensation check. --- # U-value of an uninsulated flat roof (and a warm-deck upgrade) URL: https://energycount.co.uk/tools/u-value-calculator/uninsulated-flat-roof/ Typical U-value: 1.83 W/m²K — Felt, 18mm deck, empty 150mm joist void, plasterboard An old flat roof with no insulation — felt on an 18mm deck, an empty 150mm void between the joists and a plasterboard ceiling — has a U-value of about 1.8 W/m²K with the void sealed. If the void is ventilated to the outside, BR 443 disregards the deck and the felt above it and the roof is about 3.9. Re-roofing it as a warm deck with 150mm of PIR over the existing deck brings it to about 0.14, inside the 0.16 Approved Document L asks of a renovated roof; 140mm gives 0.15, and 120mm about 0.17, which misses. Build-up, outside to inside: - Bitumen felt waterproofing - 18mm plywood deck - 150mm void between the joists, sealed (12% timber) - 12.5mm plasterboard ceiling | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Void sealed, plywood deck | ≈ 1.83 | As pre-filled | | Void sealed, chipboard deck | ≈ 1.86 | | | Void ventilated to outside | ≈ 3.85 | The deck and felt are then disregarded | | + 100mm PIR warm deck over the old deck | ≈ 0.20 | | | + 120mm PIR warm deck | ≈ 0.17 | Misses the 0.16 improved value | | + 140mm PIR warm deck | ≈ 0.15 | Meets 0.16 (0.152) | | + 150mm PIR warm deck | ≈ 0.14 | Meets 0.16 | What changes the number: With no insulation, the air in the joist void is most of the resistance, and ventilating it to the outside roughly doubles the U-value. Insulated, the board thickness is nearly everything; the old deck, void and ceiling add only a little. The usual upgrade is to strip the felt and lay a new vapour control layer, the PIR and a new membrane on the existing deck, which keeps the structure warm and the void dry; the existing void must then not be ventilated from outside. The warm flat roof page explains why the vapour control layer under the boards decides the condensation result. Adding 140–150mm raises the roof edge, and Approved Document L allows a lesser standard where upstand heights or the deck’s load capacity make 0.16 impractical (Table 4.3, note 4). Q: Do I have to insulate a flat roof when I re-cover it? A: Usually, yes. Replacing the covering of more than half of a roof is renovating a thermal element (Building Regulations 2010, regulation 23), so the whole roof should be brought up to Approved Document L’s 0.16 W/m²K (Table 4.3) so far as that is technically, functionally and economically feasible — paragraph 4.13 uses a simple payback of 15 years or less. An uninsulated flat roof is far worse than the 0.35 threshold, and a warm deck over the old deck is normally the practical way to do it. Note: Joists at 12% timber. The pre-filled link opens as an existing roof, judged against Approved Document L’s 0.35 threshold (2021 edition, Table 4.3); switch the work type to “upgrade” and add the warm deck to check it against 0.16. --- # U-value of a room-in-roof flat ceiling (collar ceiling) URL: https://energycount.co.uk/tools/u-value-calculator/room-in-roof-flat-ceiling/ Typical U-value: 0.15 W/m²K — 100mm PIR between joists + 82.5mm insulated plasterboard The flat ceiling at the top of a loft room — 100mm of PIR between the ceiling joists or collars and 82.5mm PIR insulated plasterboard (70mm of PIR) under them — has a U-value of about 0.15 W/m²K, meeting the 0.15 Approved Document L sets for a new roof element in an existing home (0.146). A 62.5mm board gives 0.17 and misses. Where the small loft above can be reached, quilt is the cheaper route: 100mm between the joists and 200mm over them gives about 0.13. BR 443 calculates this ceiling as a loft, with the roof above it counted as one resistance (R 0.2). Build-up, outside to inside: - (The small loft above and the roof over it: one roof-space resistance, R 0.2 — BR 443 §8) - 100mm PIR between 100mm ceiling joists or collars (12.8% timber, BR 443 §4.5.1) - 70mm PIR under the joists (continuous) - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm PIR between the joists only | ≈ 0.31 | | | + 25mm PIR under (37.5mm board) | ≈ 0.21 | | | + 50mm PIR under (62.5mm board) | ≈ 0.17 | Misses 0.15 | | + 70mm PIR under (82.5mm board) | ≈ 0.15 | As pre-filled: 0.146 | | 150mm PIR between 150mm joists + 25mm under | ≈ 0.16 | Misses 0.15 (0.163) | | 100mm quilt between + 200mm quilt over the joists | ≈ 0.13 | Where the loft above can be reached | What changes the number: BR 443 (2019) §8 says an insulated flat ceiling near the top of the pitch is calculated as a roof insulated at ceiling level, so U-Monkey treats it as a loft: the void above, the roof timbers and the tiles count as one resistance from BS EN ISO 6946 Table 9 (0.2 for tiles on felt or an underlay), and the joists take BR 443’s 12.8%. The layer under the joists covers them and does more per millimetre than extra depth between them. The small loft above is a cold roof space, so keep it ventilated, seal the ceiling well, and carry the insulation unbroken round the corner where the flat ceiling meets the sloping one — that junction is the usual weak point. Q: What U-value does the flat ceiling of a loft conversion need? A: In England, 0.15 W/m²K treated as a new roof element in an existing home (Approved Document L 2021, Table 4.2); some building control bodies ask for 0.15 across the whole roof of a loft conversion — CNC Building Control’s 2025 loft conversion guide does. Where the existing roof is treated as a retained element being upgraded instead, Table 4.3’s improved value is 0.16, with a lesser standard possible where meeting it would limit headroom (note 3). The pre-filled build-up meets both. Note: Ceiling joists at BR 443’s default 12.8% (§4.5.1) and the roof space above as R 0.2 (BS EN ISO 6946 Table 9, BR 443 §8). The pre-filled link opens as a new element in an existing home, judged against Approved Document L’s 0.15 (2021 edition, Table 4.2). --- # U-value of a wood-fibre sarking roof (a vapour-open warm pitched roof) URL: https://energycount.co.uk/tools/u-value-calculator/wood-fibre-sarking-roof/ Typical U-value: 0.14 W/m²K — 100mm wood-fibre sarking over 200mm rafters filled with wood-fibre batts A pitched roof with 100mm of wood-fibre sarking board over the rafters (λ 0.043) and 200mm of wood-fibre batts between them (λ 0.036), with a plasterboard ceiling, has a U-value of about 0.14 W/m²K — inside the 0.15 for a new roof on an extension and the 0.16 limiting value for a new house. On existing 150mm rafters the same board gives about 0.17, short of the 0.16 Approved Document L asks of a renovated roof. The common 60mm board over 150mm of batts gives about 0.20. Build-up, outside to inside: - Tiles on battens and counter-battens (ventilated, not counted) - 100mm wood-fibre sarking board over the rafters (λ 0.043) - 200mm wood-fibre batts between 200mm rafters (12% timber) - (Airtight, vapour-control membrane: not counted) - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 60mm sarking + 150mm between | ≈ 0.20 | | | 60mm sarking + 200mm between | ≈ 0.16 | 0.1646 | | 80mm sarking + 200mm between | ≈ 0.15 | Misses 0.15: 0.152 | | 100mm sarking + 150mm between (150mm rafters) | ≈ 0.17 | Misses the 0.16 for a renovated roof | | 100mm sarking + 200mm between | ≈ 0.14 | As pre-filled | | 120mm sarking (λ 0.041) + 200mm between | ≈ 0.13 | | What changes the number: The board over the rafters counts most: it is continuous, while the rafters bridge 12% of the batts between them, and wood fibre’s λ (0.036–0.043) means it takes about twice the thickness of PIR for the same effect. Deep rafters therefore help a lot — on 150mm rafters even 100mm of sarking only reaches about 0.17. The boards go on from outside, so this is usually done on a new roof or when a roof is stripped and re-covered, with counter-battens over the boards to keep a ventilated space under the tiles, which BR 443 leaves out of the calculation. The whole build-up is vapour-open, so the airtight, vapour-control layer on the inside decides the condensation risk. Q: What U-value does a re-roofed roof need? A: Replacing the covering of more than half of a roof is renovating a thermal element (Building Regulations 2010, regulation 23), so in England the roof should be brought to Approved Document L’s 0.16 W/m²K (Table 4.3) so far as that is technically and functionally feasible with a simple payback of 15 years or less (paragraph 4.13). With wood fibre that usually means deepening shallow rafters or using thicker boards: 100mm of sarking over 150mm rafters gives about 0.17, over 200mm rafters about 0.14. Note: Wood fibre as in U-Monkey’s library: STEICOuniversal dry sarking board (λ 0.043, made 35–100mm) and STEICOflex 036 batts; rafters at 12% timber. The pre-filled link opens as a new element in an existing home, judged against Approved Document L’s 0.15 (2021 edition, Table 4.2). --- # U-value of a solid (tiled) conservatory roof URL: https://energycount.co.uk/tools/u-value-calculator/solid-conservatory-roof/ Typical U-value: 0.15 W/m²K — 150mm PIR between rafters + 40mm under, lightweight tiles A solid replacement conservatory roof with 150mm of PIR between the rafters and 40mm of PIR under them, tiled outside and plasterboarded inside, has a U-value of about 0.15 W/m²K (0.146), meeting the 0.15 Approved Document L sets for a new roof element in an existing home. 140mm between and 40mm under shows as 0.15 but misses, at 0.152, and 100mm between with 50mm under gives about 0.17. Makers of proprietary lightweight roof systems quote their own U-values, so ask for the calculation behind the figure for the build-up you are buying. Build-up, outside to inside: - Lightweight roof tiles on battens (ventilated, not counted) - Breather membrane - 150mm PIR between 150mm rafters (12% timber) - 40mm PIR under the rafters (continuous) - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm PIR between + 25mm under | ≈ 0.22 | | | 100mm between + 50mm under | ≈ 0.17 | Misses 0.15 | | 120mm between + 50mm under | ≈ 0.16 | Misses: 0.155 | | 140mm between + 40mm under | ≈ 0.15 | Misses: 0.152 | | 150mm between + 40mm under | ≈ 0.15 | As pre-filled: meets (0.146) | | 150mm between + 50mm under | ≈ 0.14 | | What changes the number: The rafter depth decides how much fits between, and the layer under the rafters covers the 12% that is timber, so the two together set the figure. A conservatory roof is small and full of junctions — the ring beam on the old frame, the box gutter, the wall plate against the house — and those details, with the windows and the walls below, decide how warm the room feels as much as the roof’s U-value does. Many proprietary systems use aluminium or steel beams, which conduct far better than timber; for those, use the U-value in the system’s own certificate or registered detail, which should allow for the metal. Q: Do I need building regulations approval to put a solid roof on a conservatory? A: Usually, yes. A conservatory of up to 30m² at ground level is exempt from the Building Regulations (Schedule 2, Class 7) while it stays thermally separate from the house and the house’s heating is not extended into it (Approved Document L 2021, paragraph 0.14). LABC’s guidance on solid conservatory roofs (2013, updated 2018) says that work which significantly reduces the glazing or the translucency of the roof loses that exemption, so the new roof needs approval and should meet current standards as a new thermal element — 0.15 W/m²K in Table 4.2 — with the structure checked for the extra weight and the existing walls and floor left no worse than before. Note: The pre-filled link opens as a new element in an existing home, judged against Approved Document L’s 0.15 (2021 edition, Table 4.2). Rafters at 12% timber; the tiles and their batten space lie outside a ventilated layer and are not counted (BR 443). --- # U-value of a boarded loft (loft boarding over insulation) URL: https://energycount.co.uk/tools/u-value-calculator/loft-boarding/ Typical U-value: 0.15 W/m²K — 270mm quilt under loft boards on a raised floor, 50mm air gap A loft with 270mm of mineral wool quilt — 100mm between the joists and 170mm over them — has a U-value of about 0.15 W/m²K, and boarding it for storage costs nothing as long as the boards are raised clear of the quilt: BR 443 counts everything above the insulation as roof space, so the figure stays at 0.15. Lay the boards on the joists instead and only the 100mm between them is left, about 0.43 — nearly three times the heat loss. Squashing the full 270mm into the joist depth is little better: the National Physical Laboratory measured the quilt’s resistance falling from 6.11 to 3.09 m²K/W, which gives about 0.38. Build-up, outside to inside: - (The ventilated loft and the roof over it: one roof-space resistance, R 0.2 — BR 443 §8) - 18mm loft boards on a raised floor (part of the roof space: not counted) - 50mm air gap under the boards, open to the loft at the sides - 170mm mineral wool quilt over the joists - 100mm quilt between the joists (12.8% timber, BR 443 §4.5.1) - 12.5mm plasterboard ceiling | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 270mm of quilt, no boards | ≈ 0.15 | 0.149 | | 270mm under boards raised clear, 50mm air gap | ≈ 0.15 | As pre-filled: the same | | Boards on the joists, 100mm of quilt between them only | ≈ 0.43 | The quilt over the joists taken out | | Boards on the joists, 270mm squashed into the joist depth | ≈ 0.38 | NPL: R 6.11 → 3.09 | | 330mm under raised boards (100 between + 230 over) | ≈ 0.12 | The most LoftZone’s StoreFloor allows over the joists | | 350mm under boards on 300mm legs (100 + 250, 50mm gap) | ≈ 0.11 | Legs this tall on 100mm joists, with the gap kept | What changes the number: What matters is whether the quilt keeps its thickness. BR 443 (2019) §8 gives a loft insulated at ceiling level one resistance for everything above the insulation — the boards, the air under them, the ventilated loft and the roof — so boards held clear of the quilt change nothing, and U-Monkey (from engine v3.9) counts them that way. Squashing is the problem: compressed quilt loses resistance even though its λ improves, and boards on the joists usually mean the quilt over them has been flattened or taken out. Raised floors come in two kinds: LoftZone’s StoreFloor, which holds the boards 279mm above the joists and has a BBA certificate (15/5269), and loft legs such as Loft Leg’s 175mm leg, sold for boards “above 270mm of quilt”, and its 300mm XL. On 100mm joists a 175mm leg puts the boards 275mm above the ceiling, so 270mm of quilt leaves almost no gap; the BBA certificate asks for a 50mm air gap between quilt and boards, kept open at opposite sides, and with that gap a 175mm leg leaves room for about 125mm of quilt over the joists (about 0.18 in all), a 300mm leg for about 250mm (0.11). Quilt pressed up against the boards is the case U-Monkey’s condensation check fails. The joists have to carry the load too: the certificate takes ceiling joists as typically good for 0.25 kN/m², and more only where an engineer confirms it. Q: Does loft boarding reduce the effectiveness of insulation? A: Only if it squashes the insulation or takes its place. Boards raised clear of the quilt, with an air gap under them, leave the U-value unchanged — BR 443 counts everything above the insulation as roof space — but boards laid on the joists leave room for only the quilt between them: about 0.43 W/m²K, against 0.15 with 270mm. Compressing 270mm of quilt into the joist depth roughly halved its thermal resistance in tests by the National Physical Laboratory for LoftZone (6.11 to 3.09 m²K/W). The Energy Saving Trust’s advice is the same: if you use mineral wool, don’t squash it. Note: Joists at BR 443’s 12.8% (§4.5.1), and the boards, the air under them, the loft and the tiles as one roof-space resistance, R 0.2 (BS EN ISO 6946 Table 9, BR 443 §8). The squashed row takes the NPL’s measured R of 3.09 for 270mm compressed to 100mm (λ 0.0324). The pre-filled link opens as an upgraded existing roof, judged against Approved Document L’s 0.16 (2021 edition, Table 4.3). Condensation risk under loft boards: Boards over loft insulation sit on its cold side, so they are a condensation question as much as a heat one. Lay them on the joists, straight onto the quilt, and U-Monkey’s free BS EN ISO 13788 check fails the ceiling: moisture from the rooms condenses on the underside of the boards and builds up year on year, to around 9 kg/m² by the end of the first year in the calculation. Raise them on a raised floor or legs, with an air gap open to the loft, and that goes: the boards become part of the ventilated roof space, which is what the BBA certificate’s 50mm gap, open at opposite sides, is for. A loft insulated at ceiling level is a cold pitched roof, which BS 5250:2021 treats by prescriptive rules rather than a calculation, because most of the moisture that reaches a loft gets there on air leaking through the ceiling. So U-Monkey reports the pre-filled loft as outside the scope of a calculation rather than as a pass, with no condensation in the ceiling itself in any month. Keep the eaves ventilation clear when you add quilt, seal round the loft hatch, pipes and downlights, and don’t lay polythene or foil-backed sheet over the insulation. --- # U-value of a green roof (sedum or extensive green roof) URL: https://energycount.co.uk/tools/u-value-calculator/green-roof/ Typical U-value: 0.14 W/m²K — 150mm PIR warm deck under the green roof (landscaping not counted) A green roof’s U-value comes from the roof under it: BR 443 says to calculate it as for any other flat roof, disregarding the landscaping, so the substrate, plants and drainage layers count for nothing. A warm deck with 150mm of PIR under a sedum roof has a U-value of about 0.14 W/m²K, inside the 0.15 for a new roof on an extension; 120mm gives about 0.17. Built as an inverted roof, with the insulation over the waterproofing, some of the rain reaches the waterproofing and carries heat away: 200mm of XPS (at its design λ, as BR 443 asks for inverted roofs) under a green roof gives about 0.21 with Manchester’s rainfall, and about 0.17 under a certified water flow reducing layer. Build-up, outside to inside: - Sedum blanket, substrate and drainage layers (not counted: BR 443 disregards the landscaping) - Root-resistant waterproofing (EPDM here) - 150mm PIR board - Vapour control layer - 18mm plywood deck - 150mm joists (12% timber), unventilated void - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm PIR warm deck | ≈ 0.20 | | | 120mm | ≈ 0.17 | | | 140mm | ≈ 0.15 | Misses 0.15: 0.153 | | 150mm | ≈ 0.14 | As pre-filled: 0.144 | | 160mm | ≈ 0.14 | | | Inverted: 200mm XPS (2 × 100) on a concrete deck, green roof f·x 0.02, Manchester rain (2.16 mm/day) | ≈ 0.21 | The rain adds 0.040 | | Inverted: 200mm XPS at 3 mm/day (a wetter site) | ≈ 0.22 | | | Inverted: 200mm XPS under a water flow reducing layer (f·x 0.001) | ≈ 0.17 | | | Inverted: 250mm XPS (150 + 100), f·x 0.02, Manchester | ≈ 0.18 | | | Inverted: 300mm XPS (2 × 150), f·x 0.02, Manchester | ≈ 0.15 | Misses 0.15: 0.154 | What changes the number: In a warm deck the board does all the work, as on any flat roof, and the green roof sits outside the waterproofing. For a roof that isn’t inverted, BR 443 says to leave every kind of roof garden out of the calculation until a method has been defined; for an inverted green roof, to proceed as for other roofs, disregarding the landscaping, with a design λ for it only where an accredited certificate gives one. The GRO Green Roof Code (2021 edition) says a green roof “cannot as yet be quantified” to influence the insulation a building needs, and Kingspan’s GreenGuard tables give a green roof the same U-value as a gravel-ballasted one. In an inverted roof the insulation’s design λ counts, not its declared one (BR 443 §4.8.6, after ETAG 031) — Bauder’s XPS 300 Inverted is 0.035 against 0.033 declared — and so does the rain correction. BR 443 takes f·x as 0.02 under a roof garden or green roof — less than the 0.03–0.04 for boards under an open covering such as gravel, because less of the rain reaches the waterproofing — and a certified water flow reducing layer brings it down to about 0.001 (BBA guidance takes 2.5% of the rain as the minimum). The weight is a structural question: an extensive sedum system with 80mm of substrate weighs about 130 kg/m² saturated (Bauder’s system summary), and the GRO Code puts extensive roofs at typically under 250 kg/m². Q: Does a green roof add insulation? A: Not in a U-value calculation. BR 443 leaves roof gardens out of the calculation for a roof that isn’t inverted, and for an inverted one says to calculate it as for any other roof, disregarding the landscaping, unless an accredited certificate gives the landscaping a design λ; the GRO Green Roof Code says its effect on the insulation needed “cannot as yet be quantified”. So the roof under it has to meet the U-value on its own: in England 0.15 W/m²K for a new roof on an extension, 0.16 for a new house (Approved Document L 2021, Tables 4.2 and 4.1). A green roof’s benefits lie elsewhere — holding back rainwater, wildlife, and shielding the waterproofing from sun and frost. Note: Warm deck: joists at 12%, the void between them unventilated. Inverted rows: Bauder XPS 300 Inverted at its design λ, 0.035, in two layers (BR 443’s air-gap level 0), on a 150mm reinforced concrete deck, with U-Monkey’s rain correction (BS EN ISO 6946 Annex F.4), f·x 0.02 for a green roof (BR 443 §4.8.6) and Manchester’s October–May rainfall, 2.16 mm/day, U-Monkey’s default station — pick yours in the app. The landscaping is left out throughout. --- # U-value of a thatched roof URL: https://energycount.co.uk/tools/u-value-calculator/thatched-roof/ Typical U-value: 0.28 W/m²K — 300mm water reed, plasterboard under the rafters A 300mm coat of water reed thatch with a plasterboard ceiling under the rafters has a U-value of about 0.28 W/m²K, taking the λ of 0.09 that Dorset Council’s thatch guidance gives for reed, from CIBSE Guide A; the thatch on its own is about 0.30. Thatch is a good insulator for a roof covering — 450mm of reed, or 350mm of long straw (λ 0.07), gives about 0.19, close to the 0.2 that guidance quotes — but a 300mm coat falls well short of the 0.16 Approved Document L asks of a renovated roof. Over an unused loft, 270mm of quilt at the ceiling gives about 0.15, as in any loft. Build-up, outside to inside: - 300mm water reed thatch (λ 0.09) on battens - 100mm rafter void, unventilated (12% timber) - 12.5mm plasterboard under the rafters | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 300mm water reed on its own | ≈ 0.30 | The thatch and its surfaces only | | 300mm water reed, plasterboard under the rafters | ≈ 0.28 | As pre-filled | | 350mm water reed | ≈ 0.24 | | | 450mm water reed | ≈ 0.19 | Dorset Council: 0.2 | | 350mm long straw (λ 0.07) | ≈ 0.19 | Dorset Council: 0.2 | | 450mm long straw | ≈ 0.16 | Meets 0.16: 0.155 | | 300mm reed + 100mm wood-fibre batts between the rafters | ≈ 0.17 | | | 300mm reed over a loft: 270mm of quilt at the ceiling | ≈ 0.15 | The thatch becomes part of the roof space (R 0.2) | What changes the number: The material and the thickness decide it. Water reed is the more conductive thatch (λ 0.09 in the CIBSE figures Dorset Council quotes), long straw and combed wheat reed about 0.07, and a new coat is typically 280–330mm of water reed or 400mm or more of long straw (East Anglian Master Thatchers Association guidance), thinning as it weathers. Under the coat, a room in the roof gains a little from the rafter void and the ceiling, and wood-fibre batts between the rafters take 300mm of reed to about 0.17 — but Historic England’s advice is that insulation at rafter level is “generally neither necessary nor appropriate” under thatch, and that where the roof space isn’t lived in, insulation at ceiling level may be considered. Over a loft, BR 443 gives everything above the ceiling insulation one roof-space resistance from BS EN ISO 6946 Table 9, whose rows are for tiled and sheeted roofs; there is none for thatch, and U-Monkey takes 0.2, the row for a covering with boards or felt under it, which gives 300mm of reed almost no credit — a cautious figure. RdSAP 2012, which does count the thatch, gave 0.35 for an uninsulated thatched roof and 0.12 with 270mm in the loft (Table S9; RdSAP 10 has since replaced it). Q: Do I have to insulate a thatched roof when it is re-thatched? A: Possibly, but not at any cost. In England, re-thatching adds or replaces a layer of the roof, so doing more than half of it counts as renovating a thermal element (Building Regulations 2010, regulations 2 and 23), which brings in Approved Document L’s 0.16 W/m²K (Table 4.3) where that is technically, functionally and economically feasible with a simple payback of 15 years or less (paragraph 4.13). For a historic or traditional building, Approved Document L adds that energy efficiency should be improved only where it won’t cause long-term deterioration of the fabric or fittings (paragraph 0.10), and listed buildings and those in conservation areas need only meet the standards so far as that won’t unacceptably alter their character or appearance (0.8–0.9). Historic England advises against insulating at rafter level under thatch. Agree the approach with building control, and with the conservation officer where the building is listed. Note: Thatch λ from Dorset Council’s “Dorset Model” guidance (reed 0.09, straw 0.07, citing CIBSE Guide A); the East Anglian Master Thatchers Association gives the same for water reed and long straw. Rafters at 12%, the rafter void unventilated. The loft row counts everything above the ceiling insulation as one roof-space resistance (BR 443 §8), with the joists at 12.8%. The pre-filled link opens as an existing element. Condensation risk under thatch: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month in any of the rafter-level rows here. It still rates them “not a pass”, because BS 5250:2021 expects an air and vapour control layer on the warm side of a roof insulated at the slope, and a calculation can’t see the warm, moist room air that leaks up through a ceiling, which is how most moisture gets into a roof. Over a loft it treats the roof as a cold pitched roof, which BS 5250 handles by prescriptive rules instead of a calculation. What matters most in practice is keeping room air out of the thatch: seal the ceiling’s joints, the loft hatch, pipes and light fittings. A fire barrier over the rafters — Dorset Council’s guidance asks for a 30-minute one on new thatched buildings and extensions within 12m of a boundary — changes both results, so add it as a layer and run the check again. Historic England warns that such a board may leave a ventilated void under the thatch; if that void is ventilated, the thatch sits outside it and counts for nothing in a U-value calculation (about 2.1 for this roof, against about 0.26 with the void sealed), so any insulation meant to count would have to go under the board — and whether to add it at all needs the care Historic England asks for. --- # U-value of spray foam insulation in a pitched roof URL: https://energycount.co.uk/tools/u-value-calculator/spray-foam-roof/ Typical U-value: 0.31 W/m²K — 150mm open-cell spray foam between rafters Open-cell spray foam (λ 0.039) sprayed 150mm deep between the rafters, under a breathable underlay, with the air and vapour control layer its BBA certificate requires and a plasterboard ceiling under it, gives a pitched roof a U-value of about 0.31 W/m²K — well short of the 0.16 Approved Document L asks of a renovated roof. 200mm, the most its BBA certificate allows, gives about 0.24, and 150mm with 30mm more between battens under the rafters about 0.25, the figure the certificate gives for that build-up. Closed-cell foam is better per millimetre (λ about 0.025–0.028) but far less vapour-open. And before choosing either, know that surveyors and mortgage lenders treat spray foam with caution: the RICS says its presence could affect a surveyor’s opinion of value. Build-up, outside to inside: - Tiles on battens (outside a ventilated layer: not counted) - Breathable (LR) roofing underlay - 150mm open-cell spray foam between 150mm rafters (12% timber) - Air and vapour control layer (AVCL), as the certificate requires - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm open-cell between the rafters | ≈ 0.44 | | | 150mm open-cell between the rafters | ≈ 0.31 | As pre-filled | | 175mm open-cell (deeper rafters) | ≈ 0.27 | | | 200mm open-cell (deeper rafters) | ≈ 0.24 | The most BBA 23/7062 allows | | 150mm open-cell + 30mm between battens under the rafters | ≈ 0.25 | BBA 23/7062 Table 6: 0.25 | | 100mm closed-cell (λ 0.026) | ≈ 0.35 | | | 125mm closed-cell | ≈ 0.29 | | | 150mm open-cell + 50mm PIR under the rafters | ≈ 0.18 | The foil-faced board, taped, is the AVCL | | For comparison: 150mm PIR between the rafters + 50mm under | ≈ 0.14 | | What changes the number: Spray foam’s appeal is that it fills round the rafters and seals the roof in one pass, but it is not a high performer: open-cell foam is about as conductive as mineral wool (λ 0.039), and with the rafters bridging 12% of it, 150mm only reaches 0.31. A continuous layer under the rafters helps most, as in any rafter roof: 50mm of PIR takes 150mm of foam to about 0.18. The BBA certificate for Huntsman’s H2Foam Lite E (23/7062) limits it to roofs with a breathable underlay, requires an air and vapour control layer under it on the warm side, asks for a pre-installation survey that includes a condensation risk assessment and timber below 20% moisture content, and caps it at 200mm. Closed-cell foams such as BASF’s Walltite (NSAI 13/0377, λ 0.025–0.028) and Huntsman’s Heatlok HFO Pro (Kiwa, 0.025) insulate better but resist vapour far more, and the RICS warns that foam can seal in moisture unless adequate precautions are taken. Both kinds hide the timbers: the RICS consumer guide (2023) notes that a roof structure covered in foam “cannot be fully seen or inspected”, the Property Care Association’s inspection protocol (2023) treats foam sprayed onto the tiles or onto a high-resistance felt as higher risk, and LABC Warranty decided in 2022 not to accept spray foam in roofs. Q: Will spray foam insulation stop me selling or remortgaging? A: It can make it harder. The RICS’s consumer guide (2023) says spray foam is a factor that could affect a surveyor’s opinion of value and that a surveyor may ask for an independent specialist report; lenders that do accept it usually want one, made to the Property Care Association’s inspection protocol, and some won’t lend at all. Have the specialist report, the product’s BBA or Kiwa certificate and the installer’s paperwork ready. Removing the foam is possible but disruptive, and the timbers then need inspecting — the very thing the foam had prevented. Note: Open-cell foam as Huntsman’s H2Foam Lite (λ 0.039, and μ 4.4 for the condensation check, BBA 23/7062), closed-cell as U-Monkey’s generic entry (λ 0.026, μ 60). Rafters at 12%, battens under them at 11.8%; the tiles and their batten space lie outside a ventilated layer and are not counted. The pre-filled link opens as an upgraded existing roof, judged against Approved Document L’s 0.16 (2021 edition, Table 4.3). Condensation risk with spray foam: Open-cell foam is vapour-open, so the layer under it decides the condensation risk. The BBA certificate requires an air and vapour control layer on the warm side, and with one, as pre-filled, U-Monkey’s free BS EN ISO 13788 check finds only a trace of seasonal condensation at the underlay (about 1 g/m² in December) and gives a conditional pass, depending on the underlay staying low-resistance (sd 0.05m or less). Leave the AVCL out — as many installations have — and the check predicts about 1.2 kg/m² of condensation on the underside of the underlay by February, evaporating in summer, and rates the roof “not a pass”: BS 5250:2021 asks a warm pitched roof for an AVCL on the warm side, and the calculation can’t see the room air leaking into the roof, which is how most moisture gets there. Closed-cell foam resists vapour far more (U-Monkey takes μ 60, against 4.4 for this open-cell foam), and without an AVCL the check predicts only about 55 g/m² with 100mm of it, but still rates the roof “not a pass”. Foil-faced PIR under the rafters, taped at its joints, can do the AVCL’s job: with 50mm of it under 150mm of foam the check finds no condensation in any month. --- # U-value of a cold flat roof converted to a warm roof URL: https://energycount.co.uk/tools/u-value-calculator/flat-roof-cold-to-warm-conversion/ Typical U-value: 0.13 W/m²K — 120mm PIR above the deck, existing 100mm wool kept (hybrid) A cold flat roof with 100mm of mineral wool between the joists and a ventilated void has a U-value of about 0.45 W/m²K. Converting it to a warm roof — PIR above the deck under new waterproofing, the vents sealed — with 120mm of PIR and the old wool kept takes it to about 0.13, well inside the 0.16 Approved Document L asks of a renovated roof. Keeping the wool makes it a hybrid roof, and the deck stays warm only while enough of the insulation is above it: BS 5250’s rule of thumb is no more than a third of the thermal resistance under the vapour control layer, which takes about 110mm of PIR over 100mm of wool, counting the insulation alone. Take the wool out and 150mm of PIR above the deck gives about 0.14. Build-up, outside to inside: - New single-ply waterproofing - 120mm PIR above the deck - Vapour control layer on the deck - 18mm OSB deck - 50mm void, vents sealed (12% timber) - 100mm existing mineral wool between the joists (12% timber) - 12.5mm plasterboard ceiling | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Before: cold roof, 100mm mineral wool between the joists, ventilated void | ≈ 0.45 | Everything above the void is ignored | | Hybrid: 70mm PIR above the deck, the wool kept | ≈ 0.18 | Too little above: U-Monkey flags the deck as cold | | Hybrid: 90mm PIR above | ≈ 0.16 | The deck stays warm, but misses the one-third rule | | Hybrid: 110mm PIR above | ≈ 0.14 | One-third rule just met | | Hybrid: 120mm PIR above | ≈ 0.13 | As pre-filled | | Hybrid: 140mm PIR above | ≈ 0.12 | | | Warm roof, wool taken out: 140mm PIR above the deck | ≈ 0.15 | Meets 0.16: 0.153 | | Warm roof, wool taken out: 150mm PIR above the deck | ≈ 0.14 | | What changes the number: Once the deck is warm, the insulation above it does the work, and the wool below adds a little if it can stay. BS 5250:2021 calls for a condensation risk calculation whenever insulation is split above and below the deck, and gives a rule of thumb that no more than a third of the thermal resistance should be on the warm side of the vapour control layer (as quoted in industry guidance on hybrid roofs); Premier Guarantee’s technical manual asks for at least as much resistance above the deck as below, with a condensation calculation. U-Monkey checks the deck itself: with 70mm of PIR over the wool the underside of the deck falls to about 11.4 °C in February, below the room air’s 11.9 °C dew point, so it reports a cold deck; with 110mm or more the deck stays more than a degree above it (13.3 °C). Bauder advises that the existing insulation may need to come out, and that the ventilation under the deck has to be sealed off — a void left vented under a warm deck puts the new insulation outside the ventilation, where it does nothing for the rooms below. Height and load are the usual limits: where the frame can’t carry the load or the upstands aren’t high enough, Approved Document L allows a lesser standard for a flat roof (Table 4.3, note 4). Q: Should I convert my cold flat roof to a warm roof? A: Usually, when it needs re-covering. BS 6229:2018, the flat roofing code of practice (revised in 2025), noted that cold deck roofs are not now recommended, and BS 5250:2021 says they should not be used where the void spans more than 5m, can’t be cross-ventilated from both sides or can’t have a sealed vapour control layer — so a longer span being refurbished should be converted to a warm or inverted roof. In England, replacing the waterproofing over more than half the roof is renovating a thermal element (regulations 2 and 23), which brings in the 0.16 W/m²K of Approved Document L’s Table 4.3 where it is technically, functionally and economically feasible. A warm conversion meets that with about 140mm of PIR above the deck, or less if the wool below can stay as a properly checked hybrid. Note: Joists at 12%, bridging both the old wool and the sealed void above it. The “before” row has the void open to outside air, so the deck and waterproofing above it are not counted (BS EN ISO 6946 6.9.4). The one-third rule is as BS 5250:2021 Table 4 footnote B is quoted by Rob Firman in AT Journal 148 (Winter 2023), reproduced on Designing Buildings (“Hybrid flat roofs”); Premier Guarantee’s rule is from its technical manual (version 12). The pre-filled link opens as an upgraded existing roof, judged against Approved Document L’s 0.16 (2021 edition, Table 4.3). Condensation risk in a converted flat roof: Putting insulation above the deck moves the deck to the warm side, which is the point of the conversion — but only if enough of the insulation is above it. U-Monkey’s free BS EN ISO 13788 check on the pre-filled roof finds a few grams of seasonal condensation under the new waterproofing (about 6.5 g/m² in March) that evaporates in summer, and gives a conditional pass that depends on three assumed values: the membrane’s vapour resistance, the PIR’s foil facings and the vapour control layer as installed. It also notes that the deck stays above the room air’s dew point (13.7 °C against 11.9 °C in February) only while the insulation above it is at least as modelled. With 70mm of PIR over the wool, the check reports a cold deck instead, outside the scope of a calculation: room air leaking into the joist zone would condense on the underside of the deck, and the calculation, which can’t see air leakage, already finds about 280 g/m² there in February. The same applies to a vented void left open under the new deck. Seal the old ventilation, put the vapour control layer on the deck under the new insulation, and don’t thin the insulation above the deck on site. --- # U-value of a garden room (garden office) roof URL: https://energycount.co.uk/tools/u-value-calculator/garden-room-roof/ Typical U-value: 0.20 W/m²K — 100mm PIR warm deck, EPDM A garden room roof built as a warm deck — 100mm of PIR above an 18mm OSB deck, under EPDM, with a vapour control layer and a plasterboard ceiling — has a U-value of about 0.20 W/m²K; 120mm gives about 0.17 and 75mm about 0.26. Many garden rooms put the PIR between the joists instead, under an EPDM-covered deck, which is cheaper and lower: 100mm that way gives about 0.27, because the joists bridge it. That cold deck build-up is the one BS 6229:2018 (since revised) said is not now recommended, and U-Monkey’s condensation check reports it as a cold deck — the warm deck is the better roof for a room that will be heated. Build-up, outside to inside: - EPDM waterproofing - 100mm PIR above the deck - Vapour control layer - 18mm OSB deck - 150mm joists (12% timber), unventilated void - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 50mm PIR warm deck | ≈ 0.36 | | | 75mm PIR warm deck | ≈ 0.26 | 0.259 | | 100mm PIR warm deck | ≈ 0.20 | As pre-filled | | 120mm PIR warm deck | ≈ 0.17 | | | 75mm PIR between the joists, EPDM on the deck (cold deck, unventilated) | ≈ 0.33 | U-Monkey’s check: cold deck | | 100mm PIR between the joists (cold deck, unventilated) | ≈ 0.27 | U-Monkey’s check: cold deck | What changes the number: Where the insulation goes matters as much as how much. Above the deck it is continuous; between the joists it is bridged by them (12% here), so 100mm between the joists (0.27) does about as well as 75mm above the deck (0.26). The bigger difference is the deck’s temperature. In a warm deck the OSB sits inside the insulation, at about room temperature; in a cold deck it sits outside it, at about 2 °C in U-Monkey’s check for a February in Manchester, well below the room air’s dew point, so any warm air that gets past the ceiling condenses on it. BS 5250:2021 asks for a cold deck to have a ventilated void at least 50mm deep above the insulation, with cross-ventilation equal to a continuous 25mm opening on each side, and a sealed vapour control layer; a garden room’s roof is rarely deep enough to give the void and still insulate well, which is why the warm deck is the sound choice. Q: Warm or cold deck for a garden room roof? A: Warm. The cold deck — insulation between the joists, waterproofing straight onto the deck above them — is the build-up BS 6229:2018 (revised in 2025) noted is not now recommended, because the deck is cold and any room air that reaches it condenses, and BS 5250:2021 only accepts it with a ventilated void at least 50mm deep above the insulation, cross-ventilated on both sides, and a sealed vapour control layer. Unventilated, U-Monkey’s condensation check reports it as a cold deck, outside the scope of a calculation. A warm deck with 100mm of PIR above the deck gives about 0.20 W/m²K and keeps the deck at room temperature; its check is a conditional pass, with a few grams of seasonal condensation under the waterproofing. Note: Joists at 12%, the void between them unventilated. In England and Wales, detached (stand-alone) garden rooms of under 50m² that are not dwellings are outside the energy efficiency requirements (Building Regulations 2010, regulation 21(3)(d)), so no U-value is required; the figures are for comparing roofs. --- # U-value of a warm flat roof on a profiled metal deck URL: https://energycount.co.uk/tools/u-value-calculator/metal-deck-warm-roof/ Typical U-value: 0.18 W/m²K — 150mm PIR on a profiled steel deck, 5 galvanised fasteners per m², PVC single ply A warm flat roof on a profiled (trapezoidal) steel deck — a polythene vapour control layer on the deck, 150mm of PIR fixed through to it with five 4.8mm galvanised steel fasteners per m², and a mechanically fixed PVC single-ply membrane — has a U-value of about 0.18 W/m²K, just inside the 0.18 that Approved Document L Volume 2 sets for a new flat roof on a building other than a dwelling (0.176). The fasteners matter more than on any house roof: they add about 0.023, so the same roof held down with telescopic tube fasteners, whose steel screw sits recessed in a plastic tube and which BR 443 lets you leave out, reads 0.15 — and 140mm with galvanised fasteners misses, at 0.19. The air in the deck’s troughs counts for nothing (BR 443 §4.7.7). Build-up, outside to inside: - 1.5mm PVC single-ply membrane, mechanically fixed (Bauder Thermofol U 15, sd 30m) - 150mm PIR roof board (λ 0.022): board and membrane fasteners through to the deck, five 4.8mm galvanised steel screws per m² - Polythene air and vapour control layer on the deck crowns, laps sealed - 0.7mm trapezoidal galvanised steel deck (the troughs are not counted, BR 443 §4.7.7) | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 120mm PIR, 5 galvanised fixings per m² | ≈ 0.22 | | | 140mm PIR, galvanised fixings | ≈ 0.19 | Misses 0.18 (0.188); a table without the air-gap term reads 0.18 here | | 150mm PIR, galvanised fixings | ≈ 0.18 | As pre-filled: meets 0.18 (0.176) | | 140mm PIR, stainless fixings | ≈ 0.17 | Stainless (λ 17) costs about a third as much as galvanised steel | | 140mm PIR, telescopic tube fasteners | ≈ 0.16 | No fixings correction (BR 443 §4.8.3) | | 150mm PIR, telescopic tube fasteners | ≈ 0.15 | 0.153 | | 160mm as 2 × 80mm, staggered, telescopic fasteners | ≈ 0.13 | Two layers with staggered joints: no air-gap correction either | | 150mm PIR, 10 galvanised fixings per m² (a corner zone) | ≈ 0.20 | Corners and edges carry more fasteners for wind uplift | | 150mm tissue-faced PIR (λ 0.024), galvanised fixings | ≈ 0.19 | Boards for bonded or torch-on membranes, λ by thickness band | | 200mm stone wool roof board (λ 0.039), galvanised fixings | ≈ 0.22 | Non-combustible | What changes the number: On a metal deck the fasteners are part of the U-value. BR 443 (2019) §4.8.3 counts both the board fixings and the membrane fixings, each by BS EN ISO 6946 Annex F — λ of the fastener, its cross-section, the number per m² and how deep it runs into the insulation — and five 4.8mm galvanised screws per m² through 150mm of PIR add about 0.023 W/m²K. The same clause lets one kind off: “No correction need be applied in the case of fixings in a flat roof where the metal part … is recessed by at least 50% of the length of the fixing and the density of fixings … does not exceed 15 fixings per square metre” — the telescopic tube fastener. Sika, which makes one, says standard steel fasteners “can add in excess of 10mm to the required thickness”; here they make the difference between 150mm and 130mm for 0.18. Stainless screws (λ 17) cost about a third of the galvanised correction. The density comes from the membrane maker’s wind-uplift calculation and is higher at edges and corners. Published tables differ on the air-gap term: NSAI Agrément 23/0436 (Unilin) gives 140mm with five galvanised insulation fixings per m² for 0.18, which U-Monkey matches with no air-gap correction (0.178); with BR 443’s level 1 for a single layer of butt-jointed boards it reads 0.188, while two layers with staggered joints, or boards with lapped or sealed joints, take level 0. The air in the profile troughs counts for nothing, “No allowance for these should be made in U-value calculations” (§4.7.7), and composite panels and twin-skin metal roofs are outside this kind of calculation — BR 443 sends them to numerical analysis. Approved Document L Volume 2 sets 0.18 for a new flat roof (Table 4.1; the 2026 edition keeps it). Over-roofing or re-covering more than half of an existing roof is renovation (regulation 23), and the renovated roof should reach 0.18 too (Table 4.2, column (b)) where that is technically, functionally and economically feasible, with a lesser standard allowed where the frame’s load-bearing capacity or the upstand heights prevent it — common when an old metal roof is over-roofed; the table’s 0.35 threshold is for a roof kept through a change of use or of energy status. Q: Do roof fasteners really change the U-value? A: On a warm roof over a metal deck, yes — enough to decide a pass. Five 4.8mm galvanised steel fasteners per m², the board and membrane fixings together, add about 0.023 W/m²K to 150mm of PIR (0.153 becomes 0.176), so 140mm, which would pass without them, reads 0.19 against Approved Document L Volume 2’s 0.18. Stainless fasteners add about a third as much; telescopic tube fasteners, with the steel recessed at least halfway down a plastic tube, need no correction at all at up to 15 per m² (BR 443 §4.8.3). In U-Monkey, open the insulation layer’s fixings sheet and pick the galvanised, stainless or telescopic preset, then set your own density. Note: Fasteners as U-Monkey’s “Roof fasteners, galvanised” preset: 4.8mm steel screws (18.1mm², λ 50), five per m² for board and membrane fixings together, through the full board; enter your own density from the wind-load calculation. Telescopic rows use the “Telescopic fasteners” preset, which records BR 443’s exemption. The pre-filled link opens in U-Monkey’s England · non-domestic regime (Approved Document L Volume 2), at BR 443’s air-gap level 1 for a single layer of boards. Condensation risk on a metal deck: U-Monkey’s free BS EN ISO 13788 check gives the pre-filled roof a conditional pass: a trace of seasonal condensation (a peak under 0.1 g/m²) that dries out in summer, but condensate builds up year on year if the PIR, the polythene vapour control layer and the deck all sit at the unfavourable ends of their tested ranges at once — the deck taken with its laps unsealed. On a profiled deck the vapour control layer does the work, so it must be sealed at every lap, penetration and upstand, and the fasteners that pass through it make that harder. The membrane moves the result: a PVC single ply (sd about 30m) lets the roof dry upwards a little, while a polyolefin or bitumen membrane is ten times tighter and needs a stronger vapour control layer below it. For a pool, a sports hall or anything else with high humidity, raise the humidity class and run the check again. --- # U-value of a roof terrace with paving on pedestals URL: https://energycount.co.uk/tools/u-value-calculator/roof-terrace/ Typical U-value: 0.14 W/m²K — 2 × 120mm inverted-roof XPS under a water flow reducing layer, paving on pedestals (with a water flow reducing layer over the insulation; 0.20 without one) A roof terrace built as an inverted roof — two layers of 120mm extruded polystyrene (Bauder XPS 300 Inverted, design λ 0.035) over hot-melt waterproofing on a 200mm concrete slab, under a water flow reducing layer, with paving on pedestals above — has a U-value of about 0.14 W/m²K, inside the 0.15 Approved Document L sets for a new roof on an existing dwelling. The reducing layer is what makes that true: paving on pedestals is an “open covering” to BR 443, so without a tested reducing layer the cold rainwater running under the boards adds a correction of f·x 0.03 (rebated boards) or 0.04 (butt-jointed) — 0.20 and 0.22 for the same 240mm at Manchester’s winter rainfall, and 0.26 at Eskdalemuir’s. The paving and pedestals are left out of the calculation. Build-up, outside to inside: - Paving (20mm porcelain or 40–50mm concrete) on adjustable pedestals with spreader plates: not counted (BR 443 §4.8.6) - Water flow reducing layer, vapour-open, loose-laid: no thermal value; f·x 0.001 (the BBA minimum) - 2 × 120mm Bauder XPS 300 Inverted, joints staggered (design λ 0.035) - 10mm hot-melt waterproofing (the vapour control layer, on the warm side) - 200mm reinforced concrete slab - 13mm plaster soffit | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 2 × 100mm XPS under a water flow reducing layer (f·x 0.001) | ≈ 0.17 | Misses the 0.16 and the 0.15 | | 2 × 120mm XPS under a water flow reducing layer | ≈ 0.14 | As pre-filled | | 2 × 120mm, no reducing layer, rebated boards (f·x 0.03) | ≈ 0.20 | Paving on pedestals is an open covering | | 2 × 120mm, no reducing layer, butt-jointed boards (f·x 0.04) | ≈ 0.22 | | | 2 × 120mm, no reducing layer, rebated boards, at Eskdalemuir (4.41 mm/day) | ≈ 0.26 | Twice Manchester’s October–May rainfall | | 2 × 120mm + reducing layer, at Eskdalemuir | ≈ 0.14 | With the layer, the rainfall hardly matters (0.144) | | 2 × 150mm XPS + reducing layer | ≈ 0.11 | Just over the 0.11 of the notional dwelling (0.115) | | Hybrid: 120mm PIR under the membrane + 50mm XPS over it (f·x 0.03) | ≈ 0.15 | Saves height; just misses 0.15 (0.151) | | Warm-roof terrace: 150mm PIR under the membrane, pedestals on pads | ≈ 0.15 | Just meets 0.15 (0.1497); see the condensation note | What changes the number: The rain correction is the terrace’s own variable. BR 443 §4.8.6 and the BBA’s Information Bulletin 4 add ΔU = p·f·x·(R1/RT)² for the rainwater that runs between the boards and the waterproofing, where p is the average daily rainfall from October to May (2.16 mm at Manchester, 4.41 at Eskdalemuir), and f·x is 0.02 for roof gardens and green roofs, 0.03 for rebated boards and 0.04 for butt-jointed boards under an “open covering” such as gravel or paving — and paving on pedestals is an open covering. A tested water flow reducing layer laid over the boards cuts f to the BBA’s minimum 2.5% (f·x 0.001), which takes the correction from about 0.06 to almost nothing. The boards are taken at their design λ, not the declared one (ETAG 031-1, as BR 443 says): Bauder’s XPS 300 is 0.035 for inverted use. Ravago gives 175mm of its XPS X ULTRA for 0.15 on the same slab, at its declared λ 0.027 — U-Monkey’s library holds Ravatherm boards at their declared values, so use a design figure in an inverted row. The other limit is height at the door: LABC Warranty asks for upstands 150mm above the finished roof except at door access, and an overflow at least 25mm below the sill, with a finished fall of 1 in 80; a hybrid build-up, PIR under the waterproofing and a thinner XPS layer over it, saves 70mm here for about the same U-value. LABC Warranty also rules out a cold deck under a terrace that forms part of the roof, and asks for spreader plates under the pedestals. Approved Document L sets 0.16 for the roof of a new dwelling (Table 4.1) and 0.15 for a new roof on an existing one (Table 4.2); the notional dwelling assumes 0.11. Q: Do I need a water flow reducing layer on a roof terrace? A: For the U-value, almost always. With paving on pedestals the insulation sits under an open covering, so rain runs between the boards and the waterproofing all winter and BR 443 adds a correction for it: 240mm of XPS that is 0.14 W/m²K under a tested reducing layer is 0.20 without one (rebated boards, Manchester rainfall) and 0.26 in a wet part of the country. Meeting 0.15 without the layer would take about 400mm of XPS instead of 240mm, because the correction grows as the insulation gets thicker — more than any terrace door threshold has room for. The layer must be the vapour-open kind made for inverted roofs, and the terrace needs drainage at both levels — above the layer and at the waterproofing. Note: Inverted-roof XPS at Bauder’s design λ (0.035 for XPS 300 Inverted), as BR 443 §4.8.6 asks. The rain correction is U-Monkey’s inverted-roof option, at Manchester’s October–May mean rainfall (2.16 mm/day) unless a row says otherwise; the hot-melt layer is marked as the waterproofing. The reducing layer, paving and pedestals carry no thermal resistance. The pre-filled link opens with the inverted roof switched on at f·x 0.001. Condensation risk on a roof terrace: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month in the inverted rows on this page, with every assumed vapour value tested across its plausible range: the hot-melt waterproofing is a vapour barrier on the warm side of the insulation, and the boards above it stay cold and dry. The reducing layer on top must be vapour-open, so the boards can breathe outwards. The warm-roof variant is different. With 150mm of PIR on a polythene vapour control layer under a 10mm bitumen membrane, the check finds condensate building up year on year (3 g/m² still there at the end of the year): under a membrane that tight, a warm roof needs a high-performance vapour control layer, such as a reinforced bitumen or aluminium-cored sheet. The hybrid has a conditional pass, so run it with your own products. --- # U-value of a SIP roof (structural insulated panels) URL: https://energycount.co.uk/tools/u-value-calculator/sip-roof/ Typical U-value: 0.17 W/m²K — 172mm SIP (142mm PUR core), 4% timber, tiled, 25mm service void A pitched roof of 172mm structural insulated panels — a 142mm polyurethane core (λ 0.024) between 15mm OSB skins, with 4% timber at the joints, eaves and ridge — tiled on counter-battens and lined with plasterboard on a 25mm service void has a U-value of about 0.17 W/m²K: short of the 0.16 Approved Document L sets for the roof of a new dwelling and the 0.15 for a new roof on an extension. The timber fraction is the variable: with no timber the same panel gives 0.15, the figure in Kingspan’s low-pitched roof note (which states no timber fraction), and with 8% 0.19. 25mm of PIR in the service zone takes it to 0.14, and 50mm to 0.13. Build-up, outside to inside: - Tiles on battens and 25 × 50mm counter-battens: ventilated, not counted - Vapour-permeable underlay on the panel - 15mm OSB/3 outer skin - 142mm polyurethane core (Kingspan TEK, λ 0.024), 4% timber at the joints, eaves and ridge - 15mm OSB/3 inner skin - 25mm service void on battens - 12.5mm plasterboard | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 142mm panel (112mm core), no timber | ≈ 0.19 | Kingspan TEK’s certificate: 0.18–0.20 for its typical roof | | 142mm panel, 4% timber | ≈ 0.21 | | | 172mm panel (142mm core), no timber | ≈ 0.15 | Kingspan’s low-pitch note: 0.15 | | 172mm panel, 4% timber | ≈ 0.17 | As pre-filled | | 172mm panel, 8% timber (solid splines, eaves and ridge plates) | ≈ 0.19 | | | 172mm panel + 25mm PIR in the service zone, 4% timber | ≈ 0.14 | Meets 0.15 (0.144); Kingspan’s note: 0.13 | | 172mm panel + 50mm PIR in the service zone, 4% timber | ≈ 0.13 | Kingspan’s note: 0.11 | | Hemsec 200mm panel (178mm core, λ 0.028), 4% timber | ≈ 0.16 | Hemsec’s data sheet: 0.15 for the panel alone | What changes the number: A SIP roof’s U-value turns on how much timber runs through the core. BR 443 says the panel’s resistance is calculated separately, with the bridging fraction set by how the panel is made, and gives no SIP default: Kingspan’s TEK certificate takes 0–1% timber for its roof (0.18–0.20 for the 142mm panel), U-Monkey’s SIP wall page uses 4%, and solid timber splines at every joint, with eaves and ridge plates, push it higher (the 8% row). Each extra 1% costs about 0.005. Panel depth matters next — 142mm to 172mm takes the same timber fraction from 0.21 to 0.17 — and then the service zone: 25mm of PIR on the inner skin, behind the battens, brings the 172mm panel to 0.14, and 50mm to 0.13 — the board’s foil facing also makes the 25mm void behind it a low-emissivity air space; Kingspan’s low-pitched roof note gives 0.13 and 0.11 for those, with no timber fraction stated. The bonded core has no air gaps to correct for (BR 443 §4.8.1, level 0, which U-Monkey applies to a SIP core); once a separate board is added in the service zone, behind the inner skin, U-Monkey brings back level 1 — a cautious reading of BR 443’s level 0 for double-layer insulation. Other panels differ by core: Hemsec’s polyurethane cores are λ 0.028–0.030 by thickness, its 200mm panel 0.15 on its own. Approved Document L sets 0.16 for the roof of a new dwelling (Table 4.1) and 0.15 for a new roof on an existing one (Table 4.2), and the notional dwelling assumes 0.11, which even the 50mm service-zone layer only approaches (0.12 with no timber). Q: What timber fraction should I use for a SIP roof? A: The one your panel system actually has — BR 443 sets the fraction “according to the construction of the panel” and gives no default. Panels joined with insulated cassette joints carry very little timber (Kingspan’s TEK certificate assumes 0–1% for its roof); solid timber splines at every joint, plus eaves and ridge plates and trimmers round roof windows, add a good deal more. On a 172mm panel, no timber gives 0.15 W/m²K and 8% gives 0.19, so take the figure from the system’s own calculation, and enter it in U-Monkey as the bridge through the core. Note: Panel core at Kingspan’s declared λ 0.024 (TEK Building System specification manual; the 2010 BBA certificate used 0.023), with 15mm OSB/3 skins at λ 0.13. The timber is entered as a bridge through the core. U-Monkey applies BR 443 §4.8.1’s level 0 to a bonded SIP core, and level 1 once a separate insulation layer is added in the service zone. The service-zone rows use Kingspan Thermapitch TP10 (λ 0.022), whose foil facing has a declared emissivity of 0.05 (BBA 14/5133), so the 25mm void behind it counts as a low-emissivity air space (BS EN ISO 6946 Annex D), as U-Monkey applies automatically. The tiles and counter-battens sit outside a ventilated batten space and are not counted. Condensation risk in a SIP roof: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled roof, but does not give it a pass: as a warm pitched roof with no separate air and vapour control layer, it falls outside the cases BS 5250:2021 covers, and a calculation cannot see the air leakage that guidance guards against. Kingspan’s BBA assessment found no vapour control layer necessary for its typical tiled roof under a very vapour-open underlay; its low-pitched roof note, on the other hand, asks for a polythene vapour control layer and a humidity class no higher than 3. Either way, the joints are what matter in a SIP roof: the panels resist vapour well, but air leaking through unsealed joints carries moisture straight to the cold outer skin. Seal every joint as the system’s details show, and follow the panel maker’s certified build-up. --- # U-value of a ceiling insulated from below with insulated plasterboard URL: https://energycount.co.uk/tools/u-value-calculator/ceiling-insulation-from-below/ Typical U-value: 0.22 W/m²K — 62.5mm insulated plasterboard under an existing ceiling, 100mm quilt above Where a loft can’t be reached or cleared, a ceiling can be insulated from below: 62.5mm of PIR insulated plasterboard (50mm of PIR) screwed through the existing plasterboard ceiling into the joists, under the 100mm of quilt already between them, takes the roof from about 0.43 W/m²K to 0.22 — the old quilt still counts, and the loft and roof above count as one resistance (BR 443 §8). Reaching the 0.16 Approved Document L asks of a renovated roof takes about 90mm of PIR, or a 92.5mm phenolic board (0.16); 82.5mm of PIR gives 0.18. Headroom is the usual limit, and the new board becomes the room’s vapour and air barrier, so its joints have to be taped and sealed. Build-up, outside to inside: - (Loft, roof timbers and covering above: one resistance, R 0.2 — BR 443 §8, BS EN ISO 6946 Table 9) - Existing 100mm mineral wool quilt between 100mm ceiling joists (12.8% timber) - Existing 12.5mm plasterboard ceiling, kept - 62.5mm PIR insulated plasterboard (50mm PIR + 12.5mm board), screwed through to the joists, joints taped | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | As it is: 100mm quilt between the joists, plasterboard ceiling | ≈ 0.43 | Worse than 0.35, the threshold for upgrading a roof kept in a conversion | | + 37.5mm insulated plasterboard (25mm PIR) | ≈ 0.28 | | | + 52.5mm (40mm PIR) | ≈ 0.24 | | | + 62.5mm (50mm PIR) | ≈ 0.22 | As pre-filled | | + 82.5mm (70mm PIR) | ≈ 0.18 | | | + 72.5mm phenolic board (60mm, λ 0.019) | ≈ 0.18 | Kingspan K118 or similar | | + 92.5mm phenolic board (80mm, λ 0.019) | ≈ 0.16 | Meets the 0.16 for a renovated roof (0.156) | | Only 50mm of old quilt + 62.5mm board | ≈ 0.27 | | | No quilt above + 82.5mm board | ≈ 0.29 | The old ceiling counts as part of the roof space here, so this is a little cautious | What changes the number: Three things carry the result: the board, what is already in the loft, and the loft itself. The board does most of it — each 10mm of PIR is worth about 0.02 on this roof — and a phenolic core (λ 0.019) goes further per millimetre, which is why the board that reaches 0.16 under 100mm of old quilt is a 92.5mm phenolic one. The old quilt still counts, bridged by the joists (12.8%, BR 443 §4.5.1); with only 50mm left, the same 62.5mm board gives 0.27. Everything above the quilt — the ventilated loft, the roof timbers and the tiles — is taken as one resistance, 0.2 m²K/W for a felted or boarded roof, as BR 443 §8 does for any loft. BR 443 also lets the air-gap correction drop to level 0 where a second insulation layer covers the joists; U-Monkey keeps level 1 here, because the old plasterboard separates the two layers, and reads 0.21 at level 0. Adding a layer counts as renovating the roof (regulation 2(1)), and renovating more than half of it brings in regulation 23: the whole roof to 0.16 (Approved Document L, Table 4.3) “in so far as that is technically, functionally and economically feasible” — on a ceiling, headroom and door heads usually set the thickness, and paragraph 4.13 asks for the lowest U-value that is practical, generally no worse than 0.7. Q: Can I insulate my loft from below? A: Yes — by fixing insulated plasterboard to the underside of the existing ceiling, screwed through into the joists. Under 100mm of existing quilt, 62.5mm of PIR board takes the roof from about 0.43 to 0.22 W/m²K, and a 92.5mm phenolic board reaches the 0.16 Approved Document L asks of a renovated roof. Keep the loft ventilated at the eaves, tape the board’s joints and seal round lights and pipes, because the new board becomes the ceiling’s vapour and air barrier — and don’t fix it over an existing vapour check such as foil-backed plasterboard or gloss paint, which Celotex warns against for its boards. Note: Insulated plasterboard entered as its two layers, PIR (λ 0.022) and 12.5mm plasterboard, as U-Monkey’s other insulated-plasterboard pages enter it; phenolic rows at λ 0.019 (Kingspan K118, Unilin SR019/TB-MF). The roof space above the loft is BS EN ISO 6946 Table 9’s 0.2 m²K/W for a felted or boarded roof (U-Monkey’s default; 0.06 with nothing under the tiles). The pre-filled link opens as an upgraded element. Condensation risk when insulating a ceiling from below: A loft with insulation at ceiling level is a cold, ventilated roof, which BS 5250:2021 handles with prescriptive guidance rather than a calculation — so U-Monkey’s free BS EN ISO 13788 check reports it as outside a calculation’s scope, with the Glaser result (no interstitial condensation in any month) shown for information only. The guidance is what to follow: keep the eaves ventilation open, make the new board the air and vapour control layer by taping its joints and sealing every light, pipe and the loft hatch, and don’t fix it over an existing vapour check — Celotex allows its insulated plasterboard directly under an existing ceiling only where there is no vapour check layer, such as gloss paint or foil-backed plasterboard, already there. --- # U-value of a garage conversion roof (flat roof insulated from below) URL: https://energycount.co.uk/tools/u-value-calculator/garage-conversion-roof/ Typical U-value: 0.16 W/m²K — 100mm PIR between 150mm joists + 72.5mm insulated plasterboard, 50mm ventilated gap Insulating a flat garage roof from inside as part of a garage conversion — 100mm of PIR between 150mm joists, leaving a 50mm ventilated gap under the deck, and 72.5mm of PIR insulated plasterboard (60mm PIR) under the joists with its joints taped — gives a U-value of about 0.16 W/m²K: the 0.16 Approved Document L asks of a roof kept in a garage conversion (paragraph 4.12(b), Table 4.3), and the figure Celotex’s data sheet gives for it. 150mm of PIR between the joists alone, a common specification, reaches only 0.22, because the joists bridge it through its full depth. Building control guides usually prefer a warm deck where the roof is being re-covered anyway: 150mm of PIR over the deck gives 0.14. Build-up, outside to inside: - Existing felt on an 18mm deck: outside the ventilated gap, not counted - 50mm ventilated gap under the deck, cross-ventilated - 100mm PIR between 150mm joists at 400mm centres (11.7% timber) - 72.5mm PIR insulated plasterboard (60mm PIR + 12.5mm board) under the joists, joints taped as the vapour control layer | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | As it is: felt and deck on empty joists, plasterboard ceiling | ≈ 1.83 | Far worse than the 0.35 threshold | | 150mm PIR between 150mm joists only, 50mm vented gap above (a common spec) | ≈ 0.22 | The joists bridge it: misses 0.16 | | 100mm PIR between + 50mm PIR under the joists | ≈ 0.17 | Celotex: 0.17 | | 100mm between + 60mm under | ≈ 0.16 | As pre-filled (0.159; 0.1605 at BR 443’s 12.75%); Celotex: 0.16 | | 100mm between + 65mm under | ≈ 0.15 | Celotex: 0.15 | | 175mm joists: 120mm between + 50mm under | ≈ 0.15 | Celotex: 0.15 | | Warm deck instead: 140mm PIR over the deck, joists left empty | ≈ 0.15 | Meets 0.16 (0.152) | | Warm deck: 150mm PIR over the deck | ≈ 0.14 | Meets the 0.15 for a new roof too | What changes the number: The joists decide how much the between-joist insulation is worth: they run through its full depth (11.7% at 400mm centres on Celotex’s sheet), so 150mm of PIR between them only reaches 0.22, while 100mm between plus a continuous 60mm under them reaches 0.16 — the layer under the joists covers the timber. The gap above the insulation is the other rule: a cold flat roof needs at least 50mm of ventilated space under the deck, cross-ventilated (Celotex asks for “a 50mm minimum ventilation gap above any original insulation”), which is why only 100mm of PIR fits between 150mm joists. Everything above that gap — deck and felt — is disregarded, as BS EN ISO 6946 does for layers outside a well-ventilated air space. Celotex’s garage-conversion data sheet gives, for 150mm joists with 100mm between, 0.19, 0.17, 0.16 and 0.15 with 40, 50, 60 and 65mm of PIR in the insulated plasterboard below, which U-Monkey reproduces. The regulations make the target: a garage roof kept in a conversion is a retained element (Approved Document L paragraph 4.12(b)), to be upgraded to 0.16 if it is worse than 0.35 (Table 4.3), with a lesser standard allowed where the frame’s load-bearing capacity or the upstand heights prevent it (note 4); a new or rebuilt roof is a new element at 0.15 (Table 4.2). Building control guides such as CNC’s say that “a warm roof construction is the best option” where it can be done — insulation over the deck, the void below left unventilated — and BS 6229, as IKO summarises it, no longer recommends cold decks. Q: What U-value does a garage conversion roof need? A: In England, 0.16 W/m²K for a flat garage roof that is kept and upgraded: Approved Document L treats elements retained in a garage conversion as upgrades (paragraph 4.12(b)) and asks for 0.16 where the roof is worse than 0.35 (Table 4.3). If the roof is rebuilt, the new roof needs 0.15 (Table 4.2). From inside, 100mm of PIR between 150mm joists with 72.5mm insulated plasterboard below meets 0.16 on Celotex’s figures, and 77.5mm (65mm of PIR) leaves a margin; a warm deck with 150mm of PIR over the existing deck meets 0.15. Note: Joists at 400mm centres taken at 11.7% timber, the figure on Celotex’s Flat Roofing Application Data Sheet “Garage Conversion – Between and Under Joist” (November 2018); BR 443’s general rule, width over spacing plus 0.01 (12.75% for 47mm joists), gives 0.1605 for the pre-filled roof, a whisker over 0.16, so 65mm under the joists (0.155) is the specification with a margin; insulated plasterboard entered as PIR (λ 0.022) and 12.5mm plasterboard. Everything outside the ventilated gap is disregarded (BS EN ISO 6946 §6.9.4). Warm-deck rows: felt, PIR and a polythene vapour control layer on the old 18mm deck, the joist void below unventilated. The pre-filled link opens as an upgraded element. Condensation risk in a converted garage roof: A ventilated cold flat roof is one of the constructions BS 5250:2021 covers with prescriptive guidance rather than a calculation, so U-Monkey’s free BS EN ISO 13788 check reports the pre-filled roof as outside a calculation’s scope, with the Glaser result (no interstitial condensation in any month) shown for information. The guidance is the ventilated gap above the insulation — open at both ends, which is hard where the joists run across a garage — and a continuous, taped vapour control layer at the ceiling. The warm-deck rows are calculated, and they show the vapour control layer has to suit the membrane: with polythene under a bitumen felt, the check finds condensate building up (3.5 g/m² still there at the end of the year). Under a felt or bitumen membrane, use a high-performance vapour control layer, as the warm flat roof page explains. --- # U-value of a warm flat roof on a concrete deck URL: https://energycount.co.uk/tools/u-value-calculator/warm-flat-roof-concrete-deck/ Typical U-value: 0.14 W/m²K — 150mm foil-faced PIR in two layers (2 × 75mm) on a 150mm concrete slab, telescopic fasteners A warm flat roof on a 150mm concrete slab needs about 150mm of foil-faced PIR (λ 0.022) for the 0.15 W/m²K Approved Document L sets for a new roof on an existing home — laid in two staggered layers and held with telescopic fasteners, or bonded. As 2 × 75mm it gives 0.14; the same 150mm in a single layer gives 0.1507, which shows as 0.15 but misses, because BR 443’s default air-gap correction (level 1) applies to a single layer of butt-jointed boards, and none (level 0) to two staggered layers. Plain screws count too: five galvanised ones per m² through both layers take the two-layer roof to 0.164, which misses. 160mm in one layer meets it (0.142), and the 0.16 for a new home takes about 140mm in two layers (0.151). The slab itself adds almost nothing: 200mm of concrete instead of 150mm changes the U by 0.0005. Build-up, outside to inside: - 1.5mm PVC single-ply membrane (Bauder Thermofol U 15, sd 30m) - 2 × 75mm foil-faced PIR roof board (λ 0.022), joints staggered, held with telescopic tube fasteners (5 per m²) - Bitumen air and vapour control layer, fully bonded to the deck - 150mm reinforced concrete slab - 13mm plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm PIR, one layer | ≈ 0.22 | | | 120mm, one layer | ≈ 0.18 | Misses 0.18: 0.184 | | 140mm, one layer | ≈ 0.16 | Misses 0.16: 0.1604 | | 150mm, one layer | ≈ 0.15 | Shows 0.15 but misses it: 0.1507 | | 160mm, one layer | ≈ 0.14 | Meets 0.15 | | 150mm as 2 × 75mm, joints staggered | ≈ 0.14 | As pre-filled: two layers take air-gap level 0, and telescopic fasteners no correction | | 2 × 75mm, 5 galvanised screws per m² through both layers | ≈ 0.16 | Misses 0.15 (0.164): fixed as Unilin’s NSAI table assumes | | 2 × 75mm, 5 stainless screws per m² | ≈ 0.15 | Meets 0.15, just (0.149) | | 180mm as 2 × 90mm | ≈ 0.12 | | | 150mm in one layer, 5 galvanised screws per m² through it | ≈ 0.17 | The screws add 0.022 | | 150mm in one layer, 5 stainless screws per m² | ≈ 0.16 | They add 0.008 | | 150mm in one layer, telescopic tube fasteners | ≈ 0.15 | Steel recessed at least half the fixing length, up to 15 per m²: no correction (BR 443 §4.8.3) | | 150mm in one layer on a 200mm slab | ≈ 0.15 | The deck barely matters | | Stone wool (λ 0.039), 150 + 60mm | ≈ 0.18 | Meets 0.18 (a non-domestic flat roof) | | Stone wool, 150 + 100mm | ≈ 0.15 | Misses 0.15 by a hair (0.1501); ROCKWOOL’s table gives 150 + 105mm | What changes the number: Thickness, and whether it goes down in one layer or two. BR 443 (§4.8.1) adds an air-gap correction of level 1 to a single layer of boards — about 0.009 here — unless the joints are lapped or sealed or the boards are dimensionally tight to 5mm, and level 0 to double-layer insulation, so two staggered layers of 75mm beat one of 150mm. Fixings are the other lever. On a concrete deck the boards are bonded, or held with telescopic fasteners whose steel is recessed at least half the fixing length, which need no correction at up to 15 per m² (BR 443 §4.8.3); five galvanised screws per m² straight through 150mm of PIR add 0.022, stainless ones 0.008. The deck hardly matters: 200mm of reinforced concrete (λ 2.3) instead of 150mm improves the U by 0.0005, and a 50mm screed laid to falls by about 0.001 — just enough to take a single 150mm layer from 0.1507 to 0.1497. Makers’ and building-control tables are worked to two decimal places with no air-gap correction: Unilin’s NSAI certificate for FR/ALU on concrete (23/0436) puts 160mm with galvanised fixings, or 150mm with stainless, at 0.15, and U-Monkey reproduces both on that basis; a Hertfordshire building-control guide lists 150mm for 0.15. Stone wool (λ 0.039; A2-s1, d0) needs about 255mm: ROCKWOOL’s table for HardRock Multi-Fix gives 150 + 105mm for 0.15. Q: Can PIR go on the flat roof of a block of flats over 18m? A: Yes, on the roof itself. Regulation 7(2) of the Building Regulations 2010 requires materials in the external walls and “specified attachments” of a relevant building — one with a storey at least 18m above ground containing dwellings, an institution or rooms for residential purposes — to be Euroclass A2-s1, d0 or A1. A flat roof isn’t an external wall, so that doesn’t reach it, and regulation 7(3)(b) makes clear that even the parts of a roof connected to an external wall are exempt, unless pitched at more than 70° beside a space people use. A balcony is a specified attachment, so it is caught; a terrace over a heated room is a grey area to settle with building control. The roof covering still has to meet Approved Document B’s requirements for external fire performance. Note: Foil-faced PIR at λ 0.022: Unilin Thin-R FR/ALU (NSAI Agrément 23/0436) in U-Monkey’s library; any foil-faced flat-roof board at 0.022 gives the same. The boards are held with telescopic tube fasteners, 5 per m² (steel recessed at least half the fixing length, no more than 15 per m²), which BR 443 §4.8.3 exempts from the fixings correction; bonded boards give the same. Fixed as Unilin’s NSAI table assumes — five 4.8mm galvanised screws per m² through both layers — the same roof is 0.164 and misses 0.15; with stainless screws it is 0.149. Stone wool: ROCKWOOL HardRock Multi-Fix DD (λ 0.039). The link opens as a new roof on an existing home, judged against Approved Document L’s 0.15 (2021 edition, Table 4.2); a new home’s roof is 0.16 (Table 4.1). Condensation risk with a warm roof on concrete: For the pre-filled roof, U-Monkey’s free BS EN ISO 13788 check finds a trace of seasonal condensation under the PVC membrane — a peak of 2.1 g/m² in February — that evaporates in summer: a conditional pass, which holds as long as the vapour values are not all at the unfavourable ends of their tested ranges at once. The bitumen vapour control layer is the one to rely on — the boards’ foil is broken at every joint — so it has to be fully bonded and sealed at laps and upstands, and a new slab or screed can only dry downwards once it is on, so let it dry first. Stone wool is vapour-open, which puts all the load on that layer: with U-Monkey’s generic bitumen vapour control layer (sd 90m) the 250mm stone-wool roof fails, with about 7 g/m² of condensate still there at the end of the year, so enter the declared value of the layer you will use before relying on it. --- # U-value of tapered roof insulation URL: https://energycount.co.uk/tools/u-value-calculator/tapered-roof-insulation/ Typical U-value: 0.15 W/m²K — Foil-faced PIR falling 1:60 from 100 to 200mm over a 6m run (tapered boards over a flat base board), on a 150mm concrete deck A tapered PIR scheme (λ 0.022) falling 1:60 over a 6m run, from 100mm at the outlet to 200mm, on a 150mm concrete deck averages about 0.15 W/m²K (0.147) — within the 0.15 Approved Document L sets for a new roof on an existing home and the 0.16 for a new home. That is for the usual build-up, tapered boards over a flat base board: laid as a single layer of tapered boards it takes BR 443’s default air-gap correction and averages 0.156, which misses 0.15, so start it at 110mm (0.146). The average has to be taken over the area, as BS EN ISO 6946 (Annex E) does, not by averaging the thickness: the thin end loses more heat than the thick end saves, so 80 → 180mm, which averages 130mm, gives 0.170 — not the 0.162 that the average thickness suggests. BS 6229 adds a limit of its own: no point on a heated roof worse than 0.35, which on this deck needs at least 58mm of foil-faced PIR at the thinnest point (59mm as a single layer). Build-up, outside to inside: - 1.5mm PVC single-ply membrane - Tapered foil-faced PIR (λ 0.022) over a flat base board, 1:60, 100 → 200mm in total over a 6m run (U-Monkey’s tapered option, laid as two layers) - Bitumen air and vapour control layer - 150mm reinforced concrete slab - 13mm plaster | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 60 → 160mm (1:60 over 6m) | ≈ 0.20 | Thinnest point 0.34, just inside BS 6229’s 0.35 | | 80 → 180mm | ≈ 0.17 | Thinnest point 0.26; averaging the thickness (130mm) would suggest 0.162 | | 100 → 200mm | ≈ 0.15 | As pre-filled: 0.147; thinnest point 0.21 | | 100 → 200mm as one layer of tapered boards | ≈ 0.16 | BR 443’s level 1 air-gap correction: 0.156, misses 0.15; thinnest point 0.22 | | 110 → 210mm as one layer | ≈ 0.15 | Meets 0.15 (0.146) | | 120 → 220mm | ≈ 0.13 | | | 80 → 230mm (1:60 over 9m) | ≈ 0.15 | A longer run lowers the average, but the outlet still sets the thinnest point (0.26) | | 30 → 130mm | ≈ 0.29 | Fails BS 6229 at the outlet: 0.62 at the thinnest point | What changes the number: The fall, the run and the thickness at the outlet. A fall of 1:60 adds about 17mm per metre of run, so a 6m run adds 100mm and a 9m run 150mm; the outlet thickness sets the BS 6229 check and much of the average. Bauder’s summary of BS 6229:2025 says the old advice to design for 1:40 to achieve 1:80 has gone: 1:80 is the minimum completed fall on most flat roofs, and a design fall of 1:60 a reasonable starting point in many cases. U-Monkey’s tapered option takes a one-way fall over a rectangle — BS EN ISO 6946’s Annex E formula, U = ln(1 + R₁/R₀)/R₁, with R₀ the total resistance at the thinnest point and R₁ the extra resistance at the thickest — which suits a single-fall roof; hips, valleys and sumps form triangles, which Annex E handles with formulas of their own, and a whole roof is the area-weighted average of its parts, so makers design each scheme to the project. The pre-filled scheme is tapered boards over a flat base board, double-layer insulation, so BR 443 sets its air-gap correction at level 0; a single layer of tapered boards takes level 1, about 0.009 more averaged over the fall (0.156 at 100 → 200mm), and needs 59mm rather than 58mm at the outlet for BS 6229’s 0.35 — U-Monkey’s tapered option asks which it is. Fixings are a correction of their own, which the tapered option leaves to you: telescopic fasteners need none (BR 443 §4.8.3), but five galvanised screws per m² through a 100 → 200mm taper would add about 0.02, stainless ones about 0.01. Q: Can you average the thickness of tapered insulation to get its U-value? A: No. Heat flows in proportion to 1/R, not to thickness, so the U-value has to be averaged over the area: BS EN ISO 6946 Annex E gives the formulas, and BR 443 (§8) sends tapered layers to it. Averaging the thickness flatters the result — 80 → 180mm of PIR on a concrete deck gives 0.170 by Annex E, against 0.162 from its 130mm average thickness — and Gradient Roofing made the same point in 2016 with a 1:60 scheme for 0.18 that needed 80 → 200mm by the area method, where averaging the thickness said 70 → 190mm would do. Approved Document L judges the area-weighted average; BS 6229 separately caps any point at 0.35. Note: The link opens as a new roof on an existing home, judged against Approved Document L’s 0.15 (2021 edition, Table 4.2), with U-Monkey’s tapered option on (a one-way fall, BS EN ISO 6946:2017 Annex E; tapered boards over a flat base board, so no air-gap correction) and the deck below as flat layers. The condensation check works on one section through the roof, so it is not run on the tapered scheme: the condensation button below opens the thinnest section instead, as a flat 100mm board. Condensation risk with tapered insulation: The thinnest section is the worst case, so that is the one to check. Entered as a flat 100mm board under the membrane — the button below opens it that way — the pre-filled scheme’s outlet gives a conditional pass in U-Monkey’s BS EN ISO 13788 check: about 4 g/m² of seasonal condensation under the membrane in February that evaporates in summer. That link is the outlet section on its own, so the U-value and verdict the app shows with it (0.22, a fail) are the section’s, not the scheme’s 0.147. BS 6229’s 0.35 limit exists for surface condensation at the low points — outlets, gutters and sumps — which is why the thinnest point matters as much as the average. --- # U-value of a mansard roof URL: https://energycount.co.uk/tools/u-value-calculator/mansard-roof/ Typical U-value: 0.15 W/m²K — 150mm PIR between 150mm rafters + 52.5mm insulated plasterboard, slate on battens A mansard’s steep face, with 150mm of PIR (λ 0.022) between 47 × 150mm rafters at 400mm centres and 52.5mm of insulated plasterboard under them, has a U-value of about 0.15 W/m²K (0.146). That meets both values it might be held to under Approved Document L for a loft conversion — 0.15 for a new roof and 0.18 for a new wall in an existing home — and which one applies isn’t settled: energy-modelling conventions treat a pitch of 70° or more as a wall, while Approved Document L doesn’t say, so ask building control. Framed as a stud wall instead, at BR 443’s 15% timber-frame default, the same build-up is 0.152 and needs a 62.5mm board for the roof value (0.141). 100mm between the rafters and a 62.5mm board gives 0.16: enough for a wall, not for a roof. The slates count for nothing: the space behind them is well ventilated, so BR 443 (§4.7.1) leaves them out. Build-up, outside to inside: - Natural slate on battens and counter-battens (a well-ventilated space: disregarded) - Breather membrane - 150mm PIR (λ 0.022) between 47 × 150mm rafters at 400mm centres (12.8% timber) - 52.5mm insulated plasterboard (40mm PIR + 12.5mm board), joints taped | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm PIR between 150mm rafters (50mm gap outside it) + 62.5mm insulated plasterboard | ≈ 0.16 | Meets 0.18 (wall), misses 0.15 (roof); the gap beside the PIR’s foil face counts as a low-emissivity cavity | | 100mm between + 52.5mm board | ≈ 0.17 | Meets 0.18 only | | 100mm between + 72.5mm board | ≈ 0.15 | Meets both | | 120mm between + 52.5mm board | ≈ 0.16 | | | 150mm between (full) + 37.5mm board | ≈ 0.16 | Misses 0.15 (0.165) | | 150mm between + 52.5mm board | ≈ 0.15 | As pre-filled: meets both (0.146) | | 150mm between + 62.5mm board | ≈ 0.14 | | | 150mm between, plain plasterboard | ≈ 0.22 | One layer: the rafters bridge it, and air-gap level 1 | | Rafters at 600mm (8.8%): 100mm between + 62.5mm board | ≈ 0.15 | Misses 0.15 (0.152) | | Framed as a stud wall (BR 443’s timber-frame default, 15%): 150mm + 52.5mm board | ≈ 0.15 | Misses 0.15 (0.152); meets 0.18 | | Framed as a stud wall, 15%: 150mm + 62.5mm board | ≈ 0.14 | Meets both (0.141) | | 150mm + 52.5mm, taken as a pitched roof (heat flow upwards) | ≈ 0.15 | 0.148 against 0.146: BR 443 takes heat flow as horizontal above 60° | What changes the number: Two things set a mansard apart from a pitched roof. The heat flow: BR 443 takes heat flowing within 30° of horizontal as horizontal (§4.1) — any slope steeper than 60° — so the inside surface takes the wall value, 0.13, not the roof’s 0.10 — calculated as a pitched roof the pre-filled build-up reads 0.148 rather than 0.146. And the slates: the space behind slates or tiles on battens is well ventilated (BR 443 §4.7.1), so the slates, the battens and that space are disregarded and the outside surface resistance becomes 0.13 (§4.7.3). After that it is the insulation and the timber. BR 443 has no default for rafters, so their share is width ÷ spacing + 0.01 (§4.5): 12.8% at 400mm centres, 8.8% at 600mm. A mansard framed as a stud wall — sole and head plates, noggings, trimmers round the dormers — takes BR 443’s timber-frame default of 15% instead (§4.4.1), which puts the pre-filled build-up at 0.152. A continuous layer under the timbers breaks that bridge and, as a second layer, takes the air-gap correction to level 0. Planning sets the angle: Camden’s guidance describes the lower slope as “usually 60-70°”, and Kensington and Chelsea’s order for Redcliffe Road sets 72°. Q: Is a mansard a roof or a wall for Building Regulations? A: For energy, Approved Document L doesn’t draw the line. The National Calculation Methodology’s modelling guide for Scotland counts any part of a roof pitched at 70° or more as a wall, as the 2008 guide for England and Wales did, and RdSAP treats a mansard storey with slopes of at least 70° as a storey of its own; practice varies, so meeting the roof value, 0.15, avoids the argument. For fire it is defined: regulation 2(6) of the Building Regulations 2010 counts a roof pitched at more than 70° beside a space people use as part of the external wall. On a relevant building — one with a storey at least 18m above ground containing dwellings, an institution or rooms for residential purposes — regulation 7(2) then requires materials of class A2-s1, d0 or A1 there, which rules out PIR and a timber frame alike (membranes and some other components are exempt); and on a residential building with a storey over 11m, Approved Document B asks for insulation of class A2-s1, d0 or better in the external wall, or a BS 8414 test to BR 135. A slope of exactly 70° is a wall for energy modelling but not an external wall for regulation 7. Note: Calculated as a wall at 70°: heat flow within 30° of horizontal counts as horizontal (BR 443 §4.1), with the slate and its batten space disregarded (§4.7.1, §4.7.3). The timbers are rafters, at width ÷ spacing + 0.01 (§4.5); two rows show a stud-framed mansard at BR 443’s 15%. The insulated plasterboard is entered as its PIR core and a 12.5mm board. The link opens as a new element in an existing home, judged against the wall value, 0.18 (Approved Document L 2021, Table 4.2); the roof value for the same work is 0.15. Condensation risk in a mansard: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled mansard, with each assumed vapour value tested across its range: the breather membrane outside is vapour-open, and the insulated plasterboard’s foil and taped joints hold vapour back on the warm side. The places to watch are the junctions — the box gutter behind the parapet, the floor and the dormer cheeks — where the insulation is easily thinner than on the plane. --- # U-value of a built-up (twin-skin) metal roof URL: https://energycount.co.uk/tools/u-value-calculator/built-up-metal-roof/ Typical U-value: 0.17 W/m²K — 260mm glass mineral wool (λ 0.040) between a sealed steel liner and a profiled steel sheet, rails and brackets by SCI P312 (by SCI P312, with rails on the purlins every 1.8m and a bracket every 1.2m along them; 0.16 counting neither, which BR 443 does not allow) A built-up (twin-skin) metal roof — a 0.4mm steel liner sealed as the air and vapour control layer, a rail-and-bracket spacer system with rails on the purlins every 1.8m and a bracket every 1.2m along them, 260mm of glass mineral wool cladding roll (λ 0.040) filling the void, and a 0.7mm profiled steel outer sheet — has a U-value of about 0.17 W/m²K. That meets the 0.18 Approved Document L Volume 2 sets for a flat roof, and 280mm (0.16) meets the 0.16 it sets for a pitched one. The rails and brackets are part of the number: BR 443 says metal cladding “needs to take account of … any metallic components within the insulation”, and by SCI P312, which it gives as the alternative to a numerical calculation for rails and brackets, they add about 0.01 here — enough that 240mm (0.182) just misses 0.18. Build-up, outside to inside: - 0.7mm profiled steel outer sheet - 260mm glass mineral wool cladding roll (Knauf Insulation FactoryClad Roll 40, λ 0.040) filling the void, crossed by the spacer system: 40 × 40mm rails of 1.2mm steel on the purlin lines every 1.8m, and brackets on thermal break pads every 1.2m along them (0.46 per m², 60mm² each) - 0.4mm steel liner sheet, laps sealed — the air and vapour control layer | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 200mm, rails every 1.8m, brackets every 1.2m | ≈ 0.22 | Fails 0.18 | | 240mm | ≈ 0.18 | Just misses 0.18 (0.182); Tata Steel’s 3-D figure for its own system at 240mm is 0.18 | | 260mm | ≈ 0.17 | As pre-filled (0.169) | | 280mm | ≈ 0.16 | Meets the 2021 pitched-roof 0.16 (0.158) | | 300mm | ≈ 0.15 | 0.149 | | 260mm, 0.8 brackets per m² | ≈ 0.18 | 0.176: SCI P312’s default density | | 260mm, rails every 1.2m and 0.8 brackets per m² (P312’s defaults) | ≈ 0.18 | 0.177 | | 220mm FactoryClad Roll 35 (λ 0.035) | ≈ 0.18 | 0.176: 40mm less depth than Roll 40 for 0.18 | | 240mm FactoryClad Roll 35 (140 + 100mm rolls, entered as one layer) | ≈ 0.16 | 0.163 | | 260mm, rails and brackets not counted | ≈ 0.16 | Not allowed: BR 443 §4.9 (0.160) | What changes the number: Twin-skin metal cladding is beyond the plain BS EN ISO 6946 sum because steel crosses the insulation. BR 443 (2019) §4.9: “The U-value of metal-clad walls and roofs needs to take account of joints between panels and any metallic components within the insulation, including through fixings”; its roof table asks for numerical analysis “to allow for the effect [of] metal elements (such as spacer systems)”, with “the method in SCI P312 … as an alternative for rail-and-bracket systems”, as §4.9.1 says too. SCI P312 (Steel Construction Institute, 2002) is what U-Monkey uses when Metal cladding is set to Built-up. It splits the quilt into the rail zone, bridged by the rail steel and by the air the rail squeezes out of the quilt (“twice the width of the rail”), and the rest, crossed only by the brackets, each costing ΔU = 1.6·λ·A·n·(R_i/R_ub)²/d_i on its cross-section A and number per m² n; the two resistance limits are weighted by p = 0.8·R_min/R_max + 0.44 − 0.1·w/40 − 0.2·600/s − 0.04·d/100 (P312’s equation 3, based on the one for light steel framing), and the brackets must sit on thermal break pads. The method is for rail-and-bracket spacers: Z-spacers and standing-seam clips or halters need the system maker’s figures. U-Monkey reproduces P312’s worked example (U 0.255) and lands close to UK makers’ own 3-D figures at their spacings: Tata Steel’s Trisobuild R (computer modelled to BS EN ISO 10211, steelwork at 1.8m, brackets at 1.2m) gives 0.21 at 200mm, 0.18 at 240mm and 0.15 at 280mm against U-Monkey’s 0.22, 0.18 and 0.16; Euroclad’s Elite 1 (brackets every 1.167m on bars 1.5m apart) lists 0.19 at 240mm and 0.16 at 280mm. So the depth and λ of the quilt come first — a 0.035 roll meets 0.18 at 220mm, 40mm less than a 0.040 one — then the bracket density, which the purlin spacing and the wind load set (P312’s method covers rails at least 1m apart and up to 1.3 brackets per m²). Fill the void without over-compressing the quilt: BR 443 §4.9.2 corrects for shallow liner ribs that press it (no deeper than 25mm, at least 250mm apart), and otherwise says to “measure the insulation thickness at its thinnest point”. Approved Document L Volume 2 (2021) sets 0.18 for a flat roof and 0.16 for a pitched one, and its Table 4.1 gives no dividing pitch; U-Monkey treats the low pitches profiled metal roofs are laid to (Tata Steel’s minimum for Trisobuild is 4°) as flat, so check which your building control body applies. The 2026 edition, which takes effect on 24 March 2027 (24 September 2027 for higher-risk building work, with transitional arrangements), sets one 0.18 for every roof. For a new building the test is the whole-building calculation — the BER and BPER against the TER and TPER — where the NCM notional building takes 0.15 for a side-lit building’s roof and 0.18 for a top-lit one’s. Over-cladding an old roof is renovation, with 0.18 the improved value (Table 4.2). Q: How many brackets per m² should I enter? A: One over the rail spacing times the bracket spacing along the rail, both in metres: rails on purlins 1.8m apart with a bracket every 1.2m is 1 ÷ (1.8 × 1.2) = 0.46 per m², the case pre-filled here. SCI P312’s default is 0.8, and its method covers up to 1.3; average over the whole roof, as edges and corners carry more for wind uplift. On this roof each extra 0.1 bracket per m² adds about 0.002 W/m²K, so the count matters less than the depth of the quilt. Enter the bracket’s cross-section at its thickest part (P312’s example is 50 × 1.2mm, 60mm²). In U-Monkey, set Metal cladding to Built-up and type them in. Note: The pre-filled link opens in the England · non-domestic regime with Metal cladding set to Built-up (SCI P312): rails 40 × 40mm of 1.2mm steel, one thickness crossing the quilt, every 1.8m; 0.46 brackets per m² of 60mm² (P312’s example bracket, 50 × 1.2mm), steel at λ 60 as P312 takes it; no liner ribs pressing the quilt. The air round the rails takes BS EN ISO 6946’s value for upward heat flow (0.16), where P312’s worked example, a wall, used 0.215. Heat flows upward (Rsi 0.10); the quilt is at BR 443’s air-gap level 1. Condensation risk in a built-up metal roof: U-Monkey’s free BS EN ISO 13788 check gives the pre-filled roof a conditional pass: no interstitial condensation in any month while the steel liner works as a sealed air and vapour control layer, but condensate builds up year on year if its laps, fixings and penetrations are left unsealed — the check tests the liner unsealed as well as sealed. With steel on both faces nothing dries out through the outer sheet, so the liner is what keeps the quilt dry. A perforated (acoustic) liner lets vapour through. With no vapour control layer the same roof collects about 7 kg/m² a year in the check, and Knauf’s guidance is plain: “Where a perforated liner sheet is used then a separate vapour control layer should always be installed.” Which one depends on the humidity class and the liner: Kalzip, for one, recommends its foil VCL (43,000 MN·s/g, about sd 8,600m) under a perforated liner in dwellings of low occupancy, and its BBA certificate asks for a hygrothermal assessment in humidity classes 4 and 5 (sports halls, kitchens, pools). U-Monkey’s check takes the outer sheet as vapour-tight, so it asks a lot of that layer: a polythene VCL on the perforated liner still leaves 52 g/m² a year, and a foil one clears it. Where the check and a system maker’s guidance differ, ask the maker for the condensation assessment behind its recommendation, and for a high-humidity use raise the humidity class and run the check again. --- # U-value of an insulated composite roof panel URL: https://energycount.co.uk/tools/u-value-calculator/composite-panel-roof/ Typical U-value: 0.16 W/m²K — Kingspan QuadCore KS1000RW, 115mm PIR core, at its declared U-value (the maker’s declared value, joints and fixings included) An insulated composite roof panel with a 115mm PIR core — Kingspan’s QuadCore KS1000RW trapezoidal panel, λ 0.018 — has a declared U-value of 0.16 W/m²K. That meets both roof limits in Approved Document L Volume 2 (2021): 0.18 for a flat roof, which the 100mm panel meets at its declared 0.18, and 0.16 for a pitched one; the 2026 edition, from 24 March 2027, sets 0.18 for every roof. The thickness is the core at the troughs — the 115mm panel is 146mm over the crowns — and the U-value is the maker’s: BR 443 needs a numerical analysis of the joints, and a layer-by-layer sum at the core thickness reads low: 0.176 for the 100mm core (declared 0.18) and 0.42 for the 40mm one (declared 0.47), the thinner the panel the more the joints count. Build-up, outside to inside: - Kingspan QuadCore KS1000RW roof panel: trapezoidal steel outer face, 115mm QuadCore PIR core at the troughs (λ 0.018; 146mm over the crowns), steel liner face, 1000mm cover width - Entered as one layer at its declared U-value, 0.16 W/m²K (KS1000RW data sheet, eighth issue, 3 July 2023): R = 1/0.16 − (0.10 + 0.04) = 6.11 m²K/W | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | KS1000RW, 80mm core | 0.23 | Fails 0.18 | | KS1000RW, 91mm core | 0.20 | | | KS1000RW, 100mm core | 0.18 | Meets the flat-roof 0.18 at its declared value | | KS1000RW, 115mm core | 0.16 | As pre-filled: meets the 2021 pitched-roof 0.16 | | KS1000RW, 120mm core | 0.15 | Meets the side-lit notional roof, 0.15 | | KS1000RW, 150mm core | 0.12 | | | Tata Steel Trisomet, 120mm core | 0.16 | Datasheet (2025); 100mm: 0.20 | | Joris Ide JI Roof 1000, 120mm PIR | 0.18 | Stated basis “EN 14509:2013”; 100mm: 0.21 | | Stone wool core: Joris Ide JI Vulcasteel Roof, 240mm | 0.18 | A2-s1,d0 in all but two of its coatings; 200mm: 0.21 | | Layer by layer: steel, 100mm core at λ 0.018, steel | ≈ 0.18 | Reads low even here (0.176; air gaps level 0, as a foamed core has none) | | Layer by layer at 40mm | ≈ 0.42 | Reads low: declared 0.47 | What changes the number: As for a wall panel, BR 443 (2019) says a metal-faced composite roof panel cannot be summed layer by layer: its roof table asks for numerical analysis “to take account of joints between panels, if it is thinner at joints or if metal fully or partly penetrates insulation, and to take account of profiled sheets”. Panel makers declare U-values to BS EN 14509 — whose Annex A.10 adds the longitudinal joint, and in its detailed method the effect of the profiled faces, which its simplified method ignores — and many model them to BS EN ISO 10211; Kingspan says its KS1000RW values are “calculated using the method required by the Building Regulations Part L2”, without saying which. A layer-by-layer sum at the core thickness leaves the joints and through-fixings out and reads low: 0.176 for a 100mm core declared at 0.18, and 0.42 for a 40mm one declared at 0.47 — on a thin panel the joints count for more. Compare panels on their declared values by core thickness: at 120mm KS1000RW (λ 0.018) declares 0.15, Tata Steel’s Trisomet 0.16 and Joris Ide’s JI Roof 1000 0.18. A stone wool core (λ about 0.040–0.045), for A2-s1,d0 fire performance, needs about twice the depth — Joris Ide’s Vulcasteel roof panel reaches 0.18 at 240mm. Approved Document L Volume 2 (2021) sets 0.18 for a flat roof and 0.16 for a pitched one, and its Table 4.1 gives no dividing pitch; U-Monkey treats a low-pitched panel roof as flat, so check which your building control body applies. The 2026 edition, which takes effect on 24 March 2027 (24 September 2027 for higher-risk building work, with transitional arrangements), has one 0.18 for every roof. For a new building the test is the whole-building calculation — the BER and BPER against the TER and TPER — with the NCM notional roof at 0.15 for a side-lit building and 0.18 for a top-lit one. Rooflights are a separate element (2.2 in Table 4.1), and over-roofing an old roof is renovation, with 0.18 the improved value (Table 4.2). Q: Is a roof panel’s U-value quoted at the core or the overall thickness? A: At the core thickness, measured at the troughs: Kingspan’s KS1000RW table runs from a 40mm core (71mm overall) to 150mm (181mm), the crowns adding 31mm, and Tata Steel’s Trisomet table is by core thickness too. Use the declared U for your core thickness: a layer-by-layer sum at the core thickness leaves the joints out and reads low (0.176 for a 100mm core declared at 0.18, 0.42 against 0.47 at 40mm). In U-Monkey, pick the maker’s panel and its core thickness and the declared U fills itself in (Kingspan’s KS1000RW, Tata Steel’s Trisomet and Joris Ide’s roof panels are in the materials); for any other panel, add “Insulated composite panel (the maker’s declared U-value)” and type the declared U for your core thickness. Note: The pre-filled link opens in the England · non-domestic regime with the panel as one layer, “Kingspan QuadCore KS1000RW roof panel (115mm core)”, at its declared U-value: U-Monkey takes the panel’s resistance from the U you give it, R = 1/U − (Rsi + Rse), here 1/0.16 − 0.14 = 6.11 m²K/W. Pick another core thickness in its row and U-Monkey fills in Kingspan’s declared U for it; Tata Steel’s Trisomet and Joris Ide’s panels are in the library too, and for any other maker’s panel add the generic one and type its declared U. Declared values are rounded to two places, and the result is no more precise than that. Condensation in composite roof panels: U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in the panel: with steel on both faces nothing diffuses in. What it cannot see is the joints, where vapour and air get through if anywhere; the side-lap seal belongs on the liner (warm) side, with the outer seals for weather. --- # U-value of an insulated concrete ground floor URL: https://energycount.co.uk/tools/u-value-calculator/insulated-concrete-floor/ Typical U-value: 0.17 W/m²K — 100mm PIR over slab, P/A 0.6 (30m / 50m2) The current new-build standard — 150mm PIR under a 75mm screed — gives a U-value of about 0.12 W/m²K on a typical semi footprint, while the older 100mm build-up shown below comes out nearer 0.17 on the same floor. The honest answer always depends on the floor’s perimeter-to-area ratio, because ground floors are calculated to BS EN ISO 13370, where the ground itself provides much of the resistance. U-Monkey does the real 13370 calculation; most free calculators don’t. Build-up, outside to inside: - (Ground resistance via ISO 13370 — P/A dependent) - Concrete slab - 100mm PIR - 75mm sand/cement screed | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 150mm PIR, typical semi (P/A ≈ 0.6) | ≈ 0.12 | Current new-build standard | | 100mm PIR, same floor | ≈ 0.17 | Previous standard spec | | 75mm PIR, same floor | ≈ 0.22 | Older spec / extensions | What changes the number: Perimeter/area ratio is the hidden variable: a big square floor loses proportionally less than a narrow extension. That is why two "identical" floor build-ups get different U-values — and why a tool that ignores ISO 13370 gives the wrong answer. Enter your real perimeter and area in U-Monkey. Q: Why does my floor U-value change with the room size? A: Because ground-floor heat loss is dominated by the edges. BS EN ISO 13370 captures this through the perimeter-to-area ratio — the same build-up in a narrow extension performs worse than in a wide house. U-Monkey asks for P and A and calculates it properly. Note: The pre-filled link opens with U-Monkey’s default 30m perimeter / 50m² area (P/A 0.6) — edit both to your floor. --- # U-value of an insulated suspended timber floor URL: https://energycount.co.uk/tools/u-value-calculator/suspended-timber-floor-insulation/ Typical U-value: 0.30 W/m²K — 100mm quilt between joists, P/A 0.6 (30m / 50m2) Insulating a suspended timber floor with 100mm mineral wool between the joists brings the floor deck itself to roughly 0.39 W/m²K (joist bridging included) — but the floor’s U-value is meaningfully better once the ventilated void and ground resistance are counted to BS EN ISO 13370: about 0.30 on a typical semi footprint (perimeter 30m, area 50m²), and 0.26–0.32 across most house footprints. U-Monkey runs the full suspended-floor method, which is why its answer differs from simple calculators — and is right. Build-up, outside to inside: - (Ventilated void + ground via ISO 13370) - 100–150mm quilt between joists (10.8% timber, BR 443 §4.5.3) - Floorboards / chipboard deck | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm quilt between joists | deck ≈ 0.39 → floor ≈ 0.30 | The standard retrofit | | 150mm quilt between deeper joists (netted) | deck ≈ 0.28 → floor ≈ 0.23 | Worth it if access allows | What changes the number: Support the quilt tight to the boards (netting or battens) — sag creates air paths that wreck the number. Keep the airbricks clear: the void must stay ventilated to protect the joists, and the 13370 method accounts for that ventilation honestly. Q: Why do different calculators give different answers for suspended floors? A: Simple tools treat the floor like an external wall and stop at the element value. The proper method (BS EN ISO 13370) adds the ventilated void and the ground, which improves the effective U-value. U-Monkey implements the full method — the differing answers are the other tools’ shortcut. Note: The pre-filled link opens with default void and exposure settings — edit perimeter, area and void height to your floor. --- # U-value of a beam and block floor with PIR URL: https://energycount.co.uk/tools/u-value-calculator/beam-and-block-floor/ Typical U-value: 0.17 W/m²K — 100mm PIR + 75mm screed, P/A 0.6 (30m / 50m2) The current new-build standard — 150mm PIR under a 75mm screed — gives a U-value of about 0.12 W/m²K calculated to BS EN ISO 13370 §9.2 on a typical dwelling footprint, which clears the 0.13 notional. The older 100mm build-up shown below comes out nearer 0.17 on the same floor: still inside the 0.18 limiting value, but no longer notional-standard. It is the default suspended floor of volume housebuilding. Build-up, outside to inside: - (Ventilated void + ground via ISO 13370) - Beam & block deck - 100mm PIR - 75mm sand/cement screed | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 150mm PIR + 75 screed, typical semi (P/A ≈ 0.6) | ≈ 0.12 | Current new-build standard | | 100mm PIR, same floor | ≈ 0.17 | Previous standard spec | | 75mm PIR, same floor | ≈ 0.21 | Older spec / extensions | What changes the number: As with all ground floors, perimeter-to-area does the quiet work — check the same spec on your actual footprint. Edge insulation and the wall-floor junction detail (a ΨMonkey job) decide how much of the paper performance you keep. Q: Does the insulation go above or below a beam and block deck? A: Above, almost always — rigid boards over the deck, screed or chipboard over the boards. It keeps the insulation continuous and the calculation clean. Model it in U-Monkey with your real perimeter and area for the ISO 13370 figure. Note: The pre-filled link opens with U-Monkey’s default footprint (P/A 0.6) — edit to your floor. --- # U-value of a ground floor with insulation below the slab URL: https://energycount.co.uk/tools/u-value-calculator/ground-floor-insulation-below-slab/ Typical U-value: 0.13 W/m²K — 150mm PIR below the slab, P/A 0.6 (30m / 50m2) A ground-bearing concrete slab cast on 150mm of PIR achieves a U-value of about 0.13 W/m²K on a typical semi footprint (perimeter 30m, area 50m²), meeting the 0.13 notional value; 100mm of PIR gives about 0.18, inside the limiting value. With EPS, often used below slabs, the same 0.13 takes about 250mm. Above or below the slab makes almost no difference to the U-value, because BR 443 and BS EN ISO 13370 leave the dense slab’s own resistance out — its conductivity is close to the ground’s. U-Monkey does the full 13370 calculation from your perimeter and area. Build-up, outside to inside: - (Ground, hardcore, blinding and DPM: not counted; ground resistance via ISO 13370 — P/A dependent) - 100–150mm PIR (or 200–250mm EPS) below the slab - 100mm ground-bearing concrete slab (its resistance not counted — BR 443) | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Uninsulated slab, same floor | ≈ 0.98 | The starting point | | 100mm PIR below the slab | ≈ 0.18 | Inside the 0.18 limiting value (0.176) | | 150mm PIR below the slab, typical semi (P/A ≈ 0.6) | ≈ 0.13 | Meets the 0.13 notional value (0.126) | | 200mm EPS below the slab | ≈ 0.16 | Passes the limit; short of the notional | | 250mm EPS below the slab | ≈ 0.13 | Just meets the 0.13 notional (0.1299) | What changes the number: Perimeter-to-area ratio is the hidden variable, as on every ground floor: the same 150mm of PIR gives about 0.11 on a 100m² floor with 30m of exposed perimeter, but 0.131 — just over the notional — on a 40m² floor with 40m of it. Below a slab the boards carry the slab and everything on it, so they need the compressive strength for the load and a flat, blinded bed, and the slab edge needs its own insulation where it meets the wall — a junction for ΨMonkey. Keeping the slab above the insulation puts its mass inside the heated envelope, so the floor warms and cools more slowly than one with a screed laid over the boards. Q: Is it better to put floor insulation above or below the slab? A: For the U-value it hardly matters: BS EN ISO 13370 leaves the dense slab out of the floor’s resistance, so 150mm of PIR comes out at about 0.13 either way on the same footprint (0.125 above the slab under a 65mm screed, 0.126 below it), the small difference being the screed. The choice is practical. Below the slab, the boards must take the load and go down early in the build; above it, they need a screed or a floating deck over them, which adds height but keeps the slab’s mass outside the heated zone. Model your actual floor in U-Monkey with its real perimeter and area. Note: The pre-filled link opens with U-Monkey’s default 30m perimeter / 50m² area (P/A 0.6) — edit both to your floor. --- # U-value of a floor over a garage URL: https://energycount.co.uk/tools/u-value-calculator/floor-over-garage/ Typical U-value: 0.14 W/m²K — 200mm quilt between joists + 50mm PIR under, VCL (with the garage taken as outside air, as for any exposed floor) A timber floor over a garage with 200mm of mineral wool quilt between the joists and 50mm of PIR fixed under them achieves a U-value of about 0.14 W/m²K, inside the 0.18 limiting value for a new floor; 70mm of PIR takes it to 0.13, the notional value. Quilt between the joists on its own gets only to about 0.22, because the joists bridge it through its full depth and nothing covers them. The figures take the garage as outside air — right for an open passage or car port, and the cautious case for an enclosed garage. Counting an integral garage the way SAP does, as an extra resistance (Ru 0.35), takes the pre-filled floor from 0.145 to about 0.138; U-Monkey applies it when you pick the garage under “On the other side”. Build-up, outside to inside: - 15mm fire-rated plasterboard ceiling to the garage (modelled as high-density plasterboard) - 50–70mm PIR fixed under the joists (continuous) - 200mm mineral wool quilt between 200mm joists (~11% timber) - Polythene VCL (warm side, under the deck) - 22mm chipboard floor deck | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Uninsulated: plasterboard ceiling, empty joists | ≈ 1.45 | The starting point | | 200mm quilt between the joists only | ≈ 0.22 | Misses the 0.18 for a new floor; meets the 0.25 for an upgraded existing one | | 200mm quilt + 50mm PIR under the joists | ≈ 0.14 | Inside the 0.18 limiting value; short of the 0.13 notional | | 200mm quilt + 70mm PIR under the joists | ≈ 0.13 | Meets the 0.13 notional value (0.128) | | 150mm quilt + 50mm PIR under (150mm joists) | ≈ 0.17 | Shallower joists still clear 0.18 with the PIR layer | | Pre-filled floor over an integral garage, fully integral (Ru 0.35) | ≈ 0.14 | 0.138; 0.140 with Table 3.1’s one-wall-and-floor row (Ru 0.25) | | 200mm quilt only, over an integral garage (Ru 0.35) | ≈ 0.21 | Still misses 0.18: 0.206 | What changes the number: The joists are the weak point. Quilt between them is bridged by timber through its full depth, so a continuous layer under the joists does far more per millimetre than extra depth between them. That layer is often foil-faced PIR, which is vapour-tight and sits on the cold side of the quilt, so the vapour control layer belongs on the warm side, under the floor deck: without it, U-Monkey’s free condensation check (at its default settings) puts around 185 g/m² of condensation on the face of the PIR over the winter; with it, the peak drops to about 6 g/m² that dries out each summer. The garage ceiling also has to provide the fire separation Part B asks for between a garage and the house, so use the board and fixings the fire design specifies. Q: Does the garage count as outside air? A: Not in a SAP calculation, if the garage is integral. SAP 10.2 (section 3.3.1) adjusts the U-value of the elements between a dwelling and an integral garage with an extra thermal resistance, Ru, for the garage: 0.35 m²K/W where it shares a side wall, an end wall and the floor with the house and sits outside its insulation, 0.25 where it shares one wall and the floor (Table 3.1), applied as U = 1/(1/Uo + Ru). Pick the garage under “On the other side” and U-Monkey makes the same adjustment: the pre-filled floor goes from 0.145 to about 0.138. A floor over an open passage, undercroft or car port is over outside air, so there the exposed-floor figure stands — and it is the cautious figure in any case. Note: Calculated as an exposed floor to BS EN ISO 6946, with the garage taken as outside air; the Ru rows apply SAP 10.2’s formula (3), U = 1/(1/Uo + Ru), with Ru from Table 3.1 (the same in SAP 10.3). BR 443 applies no air-gap correction to floors, because convection is suppressed when heat flows downwards. --- # U-value of an uninsulated concrete ground floor URL: https://energycount.co.uk/tools/u-value-calculator/uninsulated-solid-floor/ Typical U-value: 0.98 W/m²K — Concrete slab on the ground, P/A 0.6 (30m / 50m2) An uninsulated concrete floor on the ground has a U-value set by the floor’s shape, not its thickness: about 0.98 W/m²K for a typical house floor with 30m of exposed perimeter around 50m² (P/A 0.6), 0.63 for a large, square floor (P/A 0.3) and about 1.2 for a small one such as an extension (P/A 0.9). BR 443 leaves the slab’s own small resistance out, so the ground does almost all the work (BS EN ISO 13370). Most are worse than the 0.70 at which Approved Document L counts a floor as one to upgrade; a large floor can be better than it. Build-up, outside to inside: - (Ground resistance via BS EN ISO 13370 — P/A dependent) - 150mm concrete slab on hardcore (its resistance not counted — BR 443) | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Large, square floor (P/A 0.3) | ≈ 0.63 | Better than the 0.70 threshold | | P/A 0.5 | ≈ 0.88 | | | Typical house floor (P/A 0.6) | ≈ 0.98 | As pre-filled: 30m perimeter, 50m² | | P/A 0.75 | ≈ 1.12 | | | Small floor or extension (P/A 0.9) | ≈ 1.23 | | | P/A 0.6 with a 50mm screed | ≈ 0.94 | The screed counts; the slab does not | | P/A 0.6 with 50mm PIR and 22mm chipboard over the slab | ≈ 0.28 | See the insulating over a concrete floor page | What changes the number: Heat leaves a ground floor mostly through its edges, so the ratio of exposed perimeter to area decides the result: a semi or terrace, with the party walls not counted in the perimeter, does better than a detached house of the same area. The slab’s thickness hardly matters, which is why BR 443 leaves dense concrete out. Insulation over the slab (if there is headroom at the doors) or edge insulation are the ways to improve it; U-Monkey’s edge-insulation option works the second to BS EN ISO 13370 Annex D. Q: Why does a bigger floor have a lower U-value? A: Because the ground under the middle of a large floor is warm, deep and a long way from the outside air, so little heat escapes there; most of the loss is near the edges. BS EN ISO 13370 captures that through the characteristic dimension B′ = A / (½P): the bigger B′, the lower the U-value. That is why an uninsulated floor can be 0.63 W/m²K in a large house and 1.2 in a small extension. Enter your own perimeter and area in U-Monkey. Note: The pre-filled link opens with U-Monkey’s default 30m perimeter / 50m² area (P/A 0.6), as an existing floor judged against Approved Document L’s 0.70 threshold (2021 edition, Table 4.3) — edit the perimeter and area to your floor. --- # U-value of an uninsulated suspended timber floor URL: https://energycount.co.uk/tools/u-value-calculator/uninsulated-suspended-timber-floor/ Typical U-value: 0.81 W/m²K — Floorboards over a ventilated void, P/A 0.6 (30m / 50m2) A suspended timber floor with bare floorboards over a ventilated void has a U-value of about 0.81 W/m²K for a typical 30m perimeter / 50m² floor (P/A 0.6), calculated the full BS EN ISO 13370 way with the void, its ventilation and the ground beneath. Larger floors do better (0.58 at P/A 0.3), smaller ones worse (0.94 at P/A 0.9). 100mm of mineral wool between the joists brings the typical floor to about 0.30. The U-value leaves out draughts through the boards, which are a separate loss. Build-up, outside to inside: - (Ventilated void + ground via BS EN ISO 13370) - 22mm timber floorboards on the joists | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Large floor (P/A 0.3) | ≈ 0.58 | Better than the 0.70 threshold | | Typical floor (P/A 0.6) | ≈ 0.81 | As pre-filled | | P/A 0.75 | ≈ 0.88 | | | Small floor (P/A 0.9) | ≈ 0.94 | | | + 100mm mineral wool between the joists (P/A 0.6) | ≈ 0.30 | See the insulated suspended timber floor page | What changes the number: The boards are a small part of the resistance; the ventilated void and the ground under it provide most of it, which is why the floor’s perimeter-to-area ratio and the void’s ventilation openings move the result. Keep the airbricks clear whatever you do: the void’s ventilation protects the joists. Insulating between the joists from above or below, with a breathable membrane and the boards sealed, is the standard retrofit, and it deals with the draughts as well. Q: Is an uninsulated suspended floor worse than a solid one? A: For the same floor, a little better on paper: about 0.81 W/m²K against 0.98 for an uninsulated slab at P/A 0.6, because the ventilated void adds resistance. In practice it often feels worse, because air leaks through the gaps between the boards and under the skirting, and that is not in the U-value. Sealing the boards matters as much as the insulation. Note: The pre-filled link opens with U-Monkey’s default void and exposure settings and 30m / 50m² (P/A 0.6), as an existing floor judged against Approved Document L’s 0.70 threshold (2021 edition, Table 4.3) — edit the perimeter, area and void height to your floor. --- # Underfloor heating on a ground floor: insulation and U-value URL: https://energycount.co.uk/tools/u-value-calculator/underfloor-heating-floor-insulation/ Typical U-value: 0.17 W/m²K — 100mm PIR under a 65mm heated screed, P/A 0.6 (30m / 50m2) A ground floor with wet underfloor heating — pipes in a 65mm screed on 100mm of PIR over the slab — has a U-value of about 0.17 W/m²K for a typical 30m perimeter / 50m² floor, inside the 0.18 limiting value for a new floor; 125mm gives about 0.15, and 150mm about 0.13, the new-build notional figure. BS EN 1264-4 also sets a minimum for the insulation under the pipes, a thermal resistance of 1.25 m²K/W over the ground (about 28mm of PIR), so for a UK ground floor it is the U-value that sets the thickness. Build-up, outside to inside: - (Ground resistance via BS EN ISO 13370 — P/A dependent) - Concrete slab (not counted — BR 443) - 100mm PIR - 65mm sand/cement screed with the heating pipes | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 50mm PIR | ≈ 0.29 | Meets BS EN 1264-4, not Part L | | 75mm PIR | ≈ 0.22 | | | 100mm PIR | ≈ 0.17 | As pre-filled: meets 0.18 | | 125mm PIR | ≈ 0.15 | | | 150mm PIR | ≈ 0.13 | Meets the 0.13 notional (0.125) | | 150mm EPS (λ 0.038) | ≈ 0.20 | | | 100mm PIR, small extension floor (P/A 0.9) | ≈ 0.18 | Misses 0.18: 0.183 | What changes the number: The insulation under the screed does it all; the screed and the floor finish barely register in the U-value but matter to the heating, because everything above the pipes slows the heat into the room. Run an edge strip of insulation round the screed so it does not lose heat sideways into the walls, and remember that a heated floor runs warmer than the room, so it loses a little more than its U-value at room temperature suggests. On a small extension the perimeter-to-area ratio can tip 100mm over the limit, though Approved Document L lets an extension’s floor be calculated with the perimeter and area of the whole enlarged home (Table 4.2, note 5); enter your own floor. Q: How much insulation do I need under underfloor heating? A: Two rules apply and the stricter wins. BS EN 1264-4 asks for a thermal resistance under the pipes of at least 0.75 m²K/W over a heated room, 1.25 over an unheated room or the ground, and up to 2.00 over outside air in a cold climate; 1.25 is about 28mm of PIR. A ground floor in a new home or extension also needs a U-value of 0.18 W/m²K or better under Approved Document L, which for a typical floor takes about 100mm of PIR. So on the ground, the U-value decides. Note: BS EN 1264-4 Table 1 (as manufacturers reproduce it): 0.75 m²K/W over a heated room; 1.25 over an unheated or intermittently heated room, the ground, or outside air down to 0°C; 1.50 down to −5°C; 2.00 down to −15°C. The pre-filled link opens at U-Monkey’s default 30m / 50m² (P/A 0.6). --- # U-value of insulation laid over an existing concrete floor URL: https://energycount.co.uk/tools/u-value-calculator/insulation-over-concrete-floor/ Typical U-value: 0.28 W/m²K — 50mm PIR + 22mm chipboard over the slab, P/A 0.6 (30m / 50m2) Laying 50mm of PIR and 22mm of chipboard over an existing uninsulated slab takes a typical house floor (P/A 0.6) from about 0.98 to 0.28 W/m²K; 25mm gives about 0.41 and 75mm about 0.21. Approved Document L’s figure for a renovated floor is 0.25, which takes 60mm of PIR here (0.2496). Its Table 4.3 allows a lesser standard where the extra height would cause significant problems with adjoining floor levels, which is usually what limits this job. Build-up, outside to inside: - (Ground resistance via BS EN ISO 13370 — P/A dependent) - Existing concrete slab (not counted — BR 443) - 50mm PIR - 22mm chipboard flooring, floating | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Uninsulated slab | ≈ 0.98 | See the uninsulated floor page | | 25mm PIR + 22mm chipboard | ≈ 0.41 | | | 40mm PIR + 22mm chipboard | ≈ 0.32 | | | 50mm PIR + 22mm chipboard | ≈ 0.28 | As pre-filled | | 60mm PIR + 22mm chipboard | ≈ 0.25 | Meets 0.25 (0.2496) | | 75mm PIR + 22mm chipboard | ≈ 0.21 | | | 25mm / 50mm PIR, large floor (P/A 0.3) | ≈ 0.33 / 0.24 | | What changes the number: Height is the constraint: every millimetre comes off the room and has to be found at doors, stairs, kitchen units and skirtings, which is why thin, high-performance boards are used here. The floor’s perimeter-to-area ratio moves the result as much as the board does, so enter your own. Check the old slab has a damp-proof membrane; if it has none, lay one under the insulation. With a floating chipboard floor the joints want gluing and an expansion gap at the walls. Q: How thin can floor insulation over a slab be? A: As thin as the floor levels force, but each step costs: 25mm of PIR takes a typical uninsulated floor from about 0.98 to 0.41 W/m²K, 50mm to 0.28 and 60mm to 0.25, the figure Approved Document L asks of a renovated floor. Where that height would cause significant problems with adjoining floor levels, its Table 4.3 (note 8) allows a lesser standard. Products with a lower λ, such as phenolic or vacuum panels, get closer in less height. Note: The pre-filled link opens at U-Monkey’s default 30m / 50m² (P/A 0.6), as a retained floor being upgraded, judged against Approved Document L’s 0.25 (2021 edition, Table 4.3). --- # U-value of a garage conversion floor URL: https://energycount.co.uk/tools/u-value-calculator/garage-conversion-floor/ Typical U-value: 0.17 W/m²K — 100mm PIR + 22mm chipboard over the garage slab, P/A 0.6 A garage floor usually sits below the house floor, and that step is room for insulation: 100mm of PIR and 22mm of chipboard over the existing slab gives a U-value of about 0.17 W/m²K at P/A 0.6, inside the 0.25 Approved Document L asks when a garage’s floor is upgraded in a conversion, and the 0.18 for a new floor. 75mm gives about 0.21; 50mm gives about 0.28, which misses 0.25. An uninsulated garage slab is about 0.98. Build-up, outside to inside: - (Ground resistance via BS EN ISO 13370 — P/A dependent) - Existing 100mm garage slab (not counted — BR 443), with a damp-proof membrane over it if it has none - 100mm PIR - 22mm chipboard flooring | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Uninsulated garage slab | ≈ 0.98 | | | 50mm PIR + 22mm chipboard | ≈ 0.28 | Misses 0.25 | | 75mm PIR + 22mm chipboard | ≈ 0.21 | | | 100mm PIR + 22mm chipboard | ≈ 0.17 | As pre-filled: meets 0.25 and 0.18 | | 120mm PIR + 22mm chipboard | ≈ 0.15 | | | 100mm PIR, 2.6 × 5.5m garage exposed on three sides (P/A 0.75) | ≈ 0.18 | Meets 0.18 (0.176) | What changes the number: The thickness you can fit is usually the step down from the house, less the chipboard. A single garage of about 2.6 × 5.5m with its front and one side exposed is close to the P/A 0.6 used here, and one exposed on three sides nearer 0.75; for the floor of a conversion Approved Document L treats it as a retained element. Garage slabs often have no damp-proof membrane, so lay one, and keep the new floor above the outside ground with the old door opening built up and damp-proofed. Q: What floor U-value does a garage conversion need? A: In England, Approved Document L treats the existing garage floor as a retained element (paragraph 4.12(b)): at about 0.98 W/m²K uninsulated it is worse than the 0.70 threshold, so it should be upgraded to 0.25 (Table 4.3), with a lesser standard allowed where floor levels make that impractical (note 8). A floor replaced completely is a new element at 0.18 (Table 4.2). 100mm of PIR meets both on a typical garage floor. Note: The pre-filled link opens at U-Monkey’s default 30m / 50m² (P/A 0.6) — a single garage with its front and one side exposed is close to that — as a retained floor being upgraded, judged against Approved Document L’s 0.25 (2021 edition, Table 4.3). --- # U-value of a basement floor (heated basement) URL: https://energycount.co.uk/tools/u-value-calculator/basement-floor/ Typical U-value: 0.16 W/m²K — 150mm XPS under a 150mm slab, 2.5m deep, P/A 0.6 A heated basement floor with 150mm of XPS under a 150mm concrete slab and a screed, 2.5m below ground in a typical house footprint (30m perimeter, 50m², P/A 0.6), has a U-value of about 0.16 W/m²K. With 100mm of XPS it is about 0.21, and uninsulated about 0.57 — against about 0.92 for the same slab and screed at ground level. Kingspan’s basement tables for 100mm of its GreenGuard XPS (2.5m deep, 300mm walls, sand or gravel) give 0.18 / 0.20 / 0.21 at P/A 0.3 / 0.5 / 0.8; U-Monkey gives 0.18 / 0.20 / 0.22. Build-up, outside to inside: - (The ground and the depth: allowed for by BS EN ISO 13370) - 150mm XPS insulation under the slab (λ 0.033) - 150mm reinforced concrete slab (its resistance not counted — BR 443) - 65mm sand/cement screed | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Uninsulated slab | ≈ 0.57 | | | 100mm XPS, P/A 0.3 | ≈ 0.18 | Kingspan’s table: 0.18 | | 100mm XPS, P/A 0.5 | ≈ 0.20 | Kingspan: 0.20 | | 100mm XPS, P/A 0.8 | ≈ 0.22 | Kingspan: 0.21 | | 100mm XPS, P/A 0.6 | ≈ 0.21 | | | 150mm XPS, P/A 0.6 | ≈ 0.16 | As pre-filled | | 200mm XPS, P/A 0.6 | ≈ 0.13 | Meets the 0.13 notional value | | 150mm XPS, P/A 0.6, 2.0m deep | ≈ 0.16 | 0.161 | | 150mm XPS, P/A 0.6, 3.0m deep | ≈ 0.15 | | What changes the number: BS EN ISO 13370 treats the ground below and around a basement as part of the floor: the bigger the floor for its perimeter (lower P/A) and the deeper it sits, the lower the U-value, so a basement floor reaches a given figure with less insulation than one at ground level. The dense slab’s own resistance is left out, as BR 443 does for ground floors; the screed above it counts. XPS goes under the slab because it takes the load and tolerates the wet; PIR can go above the slab under the screed instead, on the dry side of the waterproofing, with much the same result (100mm of PIR there gives about 0.16). The floor and the walls are calculated together, so the walls’ figure depends on this floor’s insulation as well. Q: How much insulation does a basement floor need? A: Less than a floor at ground level, because the ground and the depth help: in a typical house footprint 2.5m down, 100mm of XPS gives about 0.21 W/m²K and 150mm about 0.16, against the 0.18 Approved Document L sets as the limiting value for a floor in a new dwelling (Table 4.1) and for a new floor in an existing one (Table 4.2). The notional dwelling’s 0.13 takes about 200mm here. Note: Calculated to BS EN ISO 13370 for a heated basement: perimeter 30m, floor area 50m² (P/A 0.6), floor 2.5m below ground, walls 300mm thick above ground, soil λ 2.0. Kingspan’s figures are from its GreenGuard basements brochure (2020), which assumes the same depth and wall thickness on sand or gravel, with the λ 0.034 the board was declared at then. XPS at λ 0.033 (Kingspan GreenGuard GG300 in U-Monkey’s library). --- # U-value of an extension floor (and the whole-house perimeter rule) URL: https://energycount.co.uk/tools/u-value-calculator/extension-floor/ Typical U-value: 0.18 W/m²K — 100mm PIR over the slab, the extension’s own P/A (13m / 20m²) A solid floor in a 4m × 5m rear extension — 100mm of PIR over the slab under a 65mm screed — has a U-value of about 0.18 W/m²K (0.177) on the extension’s own exposed perimeter and area (13m, 20m²), inside the 0.18 Approved Document L sets for a new floor in an existing home. Approved Document L also lets the floor of an extension be calculated with the exposed perimeter and floor area of the whole enlarged dwelling (Table 4.2, note 5): for a semi-detached house that takes the same floor to about 0.17, and lets 90mm of PIR pass (0.179) where on the extension alone it misses (0.192). Build-up, outside to inside: - (Ground, hardcore and DPM: not counted; the ground’s resistance via BS EN ISO 13370 from the perimeter and area) - 100mm concrete slab (its resistance not counted — BR 443) - 100mm PIR insulation - 65mm sand/cement screed | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 75mm PIR, extension alone | ≈ 0.22 | | | 75mm PIR, whole enlarged house | ≈ 0.20 | | | 90mm PIR, extension alone | ≈ 0.19 | Misses 0.18: 0.192 | | 90mm PIR, whole enlarged house | ≈ 0.18 | Meets: 0.179 | | 100mm PIR, extension alone | ≈ 0.18 | As pre-filled: 0.177 | | 100mm PIR, whole enlarged house | ≈ 0.17 | | | 120mm PIR, extension alone | ≈ 0.15 | | What changes the number: A ground floor’s U-value depends on its shape as much as its build-up: BS EN ISO 13370 works from the exposed perimeter (P) and the area (A), and a small extension has a lot of edge for its size (P/A 0.65 here). Approved Document L lets you use the whole enlarged dwelling’s perimeter and area instead (note 5 to Table 4.2, and note 7 to Table 4.3 for a retained floor), which for this example — a 50m² semi with 22m of exposed wall, plus the extension — gives P/A 0.43 and lowers the U-value by about 0.01. Count only walls that bound the heated space: the wall between the extension and the house is not exposed perimeter, and nor is a party wall. Where the new floor has to line up with an existing one, note 4 to Table 4.2 allows a lesser standard if 0.18 would cause significant problems with adjoining floor levels. Q: Can I use the whole house’s perimeter and area for the extension floor? A: Yes. Approved Document L says the U-value of the floor of an extension may be calculated using the exposed perimeter and floor area of the whole enlarged dwelling (2021 edition, Table 4.2 note 5; the 2026 edition keeps it as Table 3.2 note 5). A bigger floor with relatively less edge gives a lower U-value, so it usually helps — here 90mm of PIR meets 0.18 on the whole house (0.179) but not on the extension alone (0.192). Enter the whole house’s figures in U-Monkey’s perimeter and area boxes to check it. Note: Calculated to BS EN ISO 13370: the pre-filled link opens with the extension’s own exposed perimeter (13m: 4m + 5m + 4m) and area (20m²); the whole-house rows use 30m and 70m². It opens as a new element in an existing home, judged against Approved Document L’s 0.18 (2021 edition, Table 4.2). --- # U-value of an insulated raft foundation (EPS raft) URL: https://energycount.co.uk/tools/u-value-calculator/insulated-raft-foundation/ Typical U-value: 0.11 W/m²K — 300mm EPS100 under a 150mm raft slab, P/A 0.6 An insulated raft foundation — a 150mm reinforced concrete raft cast on 300mm of EPS100 (λ 0.036) — has a U-value of about 0.11 W/m²K on a typical house footprint (perimeter 30m, area 50m², P/A 0.6), inside the 0.13 notional value. That matches the KORE insulated foundation’s NSAI Agrément certificate (20/0424), which gives 0.105 for the same build-up at P/A 0.6 and 0.076–0.103 for P/A 0.1–0.5: U-Monkey’s BS EN ISO 13370 calculation gives 0.106 and agrees to within about 0.001 across the table. 200mm of EPS gives about 0.15 and 400mm about 0.08. Build-up, outside to inside: - (The ground: allowed for by BS EN ISO 13370 from the perimeter and area) - 300mm EPS100 insulation (λ 0.036), with EPS edge profiles round the raft - 150mm reinforced concrete raft slab (its resistance not counted — BR 443) | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 300mm EPS100, P/A 0.1 | ≈ 0.08 | NSAI 20/0424: 0.076 | | 300mm EPS100, P/A 0.2 | ≈ 0.09 | NSAI: 0.091 | | 300mm EPS100, P/A 0.3 | ≈ 0.10 | NSAI: 0.097 | | 300mm EPS100, P/A 0.4 | ≈ 0.10 | NSAI: 0.101 | | 300mm EPS100, P/A 0.5 | ≈ 0.10 | NSAI: 0.103 | | 300mm EPS100, P/A 0.6 | ≈ 0.11 | As pre-filled; NSAI: 0.105 | | 200mm EPS100, P/A 0.6 | ≈ 0.15 | | | 250mm EPS100, P/A 0.6 | ≈ 0.12 | | | 350mm EPS100, P/A 0.6 | ≈ 0.09 | | | 400mm EPS100, P/A 0.6 | ≈ 0.08 | | | 300mm graphite EPS (λ 0.031), P/A 0.6 | ≈ 0.09 | | What changes the number: For the U-value a raft is simply a slab on insulation: BR 443 counts raft construction as slab-on-ground and leaves the dense slab out, so the EPS and the footprint (P/A) decide it. White EPS100 at λ 0.036 needs about 300mm for 0.11; graphite EPS (λ 0.031) gets there with about 250mm. What a raft does better than insulation under an ordinary slab is the edge: the insulation runs under the thickened edge and turns up round the perimeter in EPS edge profiles, so the wall-to-floor junction loses far less heat — a gain that shows in the junction’s psi-value rather than the floor’s U-value. Q: Is an insulated raft better than insulation under an ordinary slab? A: For the floor’s U-value, no: 300mm of the same EPS under an ordinary ground-bearing slab gives the same 0.11, because BS EN ISO 13370 sees only the insulation and the footprint. The difference is at the edge. In a raft the insulation is continuous under the slab and up its perimeter, while a conventional wall often sits on a foundation that bypasses the floor insulation, so the wall-to-floor junction of a raft loses much less heat. That shows in the junction’s psi-value, which the free ΨMonkey calculator works out. Note: Calculated to BS EN ISO 13370: perimeter 30m, area 50m², walls 300mm, soil λ 2.0. EPS100 at λ 0.036 (KORE Floor EPS100 white in U-Monkey’s library); the comparison figures are Table 2 of NSAI Agrément certificate 20/0424, for 300mm of EPS100 on soil of λ 2.0. --- # U-value of a limecrete floor (insulated with foamed glass) URL: https://energycount.co.uk/tools/u-value-calculator/limecrete-floor/ Typical U-value: 0.33 W/m²K — 150mm foamed glass under a limecrete slab, P/A 0.6 (30m / 50m²) A limecrete floor on 150mm of foamed glass aggregate (Ty-Mawr’s Glapor, λ 0.078) has a U-value of about 0.33 W/m²K for a typical floor, with 30m of exposed perimeter to 50m² of area; 200mm gives about 0.27, still short of the 0.25 Approved Document L sets for an upgraded floor, which takes about 225mm (0.2496). The uninsulated solid floor it replaces is about 0.98. Foamed glass is a weaker insulant than PIR or EPS, so these floors are deep, but it carries the slab, doesn’t draw up ground water, and lets the floor be laid without a damp-proof membrane — which is why it is used in old buildings with solid walls. Build-up, outside to inside: - 150mm foamed glass aggregate (Glapor, λ 0.078), compacted - 100mm limecrete slab (not counted in the floor’s resistance: BR 443) - (Finish — flags, tiles or boards on a lime screed — not included) | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Before: an uninsulated solid floor | ≈ 0.98 | | | 100mm foamed glass | ≈ 0.42 | | | 150mm foamed glass | ≈ 0.33 | As pre-filled; Ty-Mawr’s minimum | | 200mm foamed glass | ≈ 0.27 | | | 250mm foamed glass | ≈ 0.23 | Meets the 0.25 for an upgraded floor (225mm just does) | | 150mm MISAPOR foam glass gravel (λ 0.09) | ≈ 0.36 | U-Monkey’s library value | | 150mm at a design λ of 0.105 (MISAPOR, German approval) | ≈ 0.39 | | | 150mm, a larger floor (P/A 0.35) | ≈ 0.28 | | | 150mm on clay (ground λ 1.5) | ≈ 0.30 | | What changes the number: Three things decide the figure: the depth of foamed glass, its λ, and the floor’s shape. Ty-Mawr quotes 0.078 for Glapor without saying on what basis; MISAPOR’s European Technical Assessment (ETA-13/0549) gives 0.080 for its 10/75 grade and 0.093 for 10/50 with a moisture factor of 1.2 for use under a slab, and its German approval gives the 10/75 a design value of 0.105. If Ty-Mawr’s figure is a dry one, the same moisture factor (λ 0.094) takes 150mm to about 0.37; at MISAPOR’s 0.105 design value it would be about 0.39. Like any ground floor, a small or narrow one loses more through its edges: the same 150mm gives about 0.28 on a larger floor (P/A 0.35), and clay, which conducts less than the sand or gravel U-Monkey assumes, helps a little (0.30). Ty-Mawr’s minimum system is 250mm deep — 150mm of Glapor and 100mm of limecrete — before the finish, so an old floor usually has to be dug out, and that needs care: the government’s solid-floor guide warns that digging down next to shallow foundations could undermine the wall, and Historic England calls the excavation the principal disadvantage of these systems. Digging out and relaying a whole floor may also count as replacing it rather than renovating it, which in England brings in 0.18 (Approved Document L, paragraph 4.8 and Table 4.2, with a lesser standard where adjoining floor levels make it impractical, note 4) — about 350mm of Glapor — so agree with building control which applies. Q: Does a limecrete floor need a damp-proof membrane? A: Not in the systems built for it. Ty-Mawr’s Sublime floor leaves the membrane out and relies on the foamed glass beneath the slab, which doesn’t draw water up, so the floor can let ground moisture evaporate. That is the point in an old building: Historic England’s guidance on insulating solid floors notes that a membrane laid across a floor can displace ground moisture to the base of the walls, where it can cause rising damp instead. U-Monkey’s condensation check doesn’t cover ground floors — moisture from the ground is a matter of the floor’s damp design, not interstitial condensation — so take the system maker’s and a conservation specialist’s advice on the details. Note: BS EN ISO 13370 at U-Monkey’s default floor (30m of exposed perimeter, 50m², P/A 0.6), ground λ 2.0 (sand or gravel), 300mm walls. The limecrete slab is entered as dense concrete, which BR 443 leaves out of a ground floor’s resistance because it conducts much like the ground, so its exact λ makes no difference. The pre-filled link opens as an upgraded existing floor, judged against Approved Document L’s 0.25 (2021 edition, Table 4.3). --- # U-value of a garden room (garden office) floor URL: https://energycount.co.uk/tools/u-value-calculator/garden-room-floor/ Typical U-value: 0.25 W/m²K — 100mm PIR between 125mm joists, open underneath A garden room floor of 125mm joists with 100mm of PIR between them, a 9mm OSB soffit underneath and an 18mm chipboard deck has a U-value of about 0.25 W/m²K, taken as an exposed floor with outside air underneath; 75mm of PIR gives about 0.30 and 50mm about 0.39. Filling the joists gains little (125mm: 0.23), because the joists bridge the boards; 25mm more PIR across the joists, under the deck, takes 100mm between them to about 0.19. Build-up, outside to inside: - (Open, ventilated space under the room: taken as outside air) - 9mm OSB soffit under the joists - 100mm PIR between 125mm joists (10.8% timber, BR 443) - 25mm air space over the boards (bridged by the joists) - 18mm chipboard deck | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 50mm PIR between the joists | ≈ 0.39 | | | 75mm PIR | ≈ 0.30 | | | 100mm PIR | ≈ 0.25 | As pre-filled | | 125mm PIR, filling the joists | ≈ 0.23 | | | 100mm PIR between + 25mm PIR over the joists | ≈ 0.19 | | What changes the number: The joists bridge the boards through their full depth — 10.8% of the floor, BR 443’s figure for floor joists — so a layer across them does more than extra depth between them: 25mm of PIR over the joists takes 0.25 to 0.19, while filling the last 25mm between them only reaches 0.23. What is under the floor decides which calculation applies. On legs, ground screws or a frame with the sides open, the air under the room is effectively outside air, so U-Monkey’s exposed-floor element is the right one, as here. Keep the soffit board: it stops wind washing through the insulation from below. U-Monkey’s condensation check finds no condensation in any month in any of these floors. Q: Is a garden room floor an exposed floor or a suspended floor? A: It depends on what is under it. With the sides open — on legs, ground screws or a frame, with air moving freely under the room — treat it as an exposed floor, as here: the air below counts as outside air. If a skirting or a low wall encloses the space and it is ventilated only through small openings, it is a suspended floor over a void, which BS EN ISO 13370 calculates from the floor’s size, the void and its ventilation; that comes out a little better — about 0.21 for this floor under a 3 × 4m room at U-Monkey’s default void settings — and U-Monkey has a suspended floor element for it. A garden room on a concrete slab is a ground floor instead. Note: Joists at BR 443’s 10.8% for floors, bridging the PIR and the air space above it. Calculated as an exposed floor to BS EN ISO 6946, with outside air below. In England and Wales, detached (stand-alone) garden rooms of under 50m² that are not dwellings are outside the energy efficiency requirements (Building Regulations 2010, regulation 21(3)(d)), so no U-value is required; the figures are for comparing floors. --- # U-value of a solid floor with edge insulation URL: https://energycount.co.uk/tools/u-value-calculator/slab-edge-insulation/ Typical U-value: 0.68 W/m²K — Uninsulated slab, 100mm XPS edge insulation 0.5m deep, P/A 0.6 An uninsulated concrete ground floor of a typical size, with 30m of exposed perimeter to 50m² of area, has a U-value of about 0.94 W/m²K. Insulating just its edge — 100mm of XPS (λ 0.034) taken 0.5m down the outside of the foundation — takes it to about 0.68, and 1m down to about 0.56: a cut of about 27% to 40% without lifting the floor. It can’t reach the 0.25 Approved Document L sets for an upgraded floor, because the middle of the floor still loses heat into the ground. Laid flat instead, a horizontal strip needs twice the width to match vertical insulation of a given depth. Build-up, outside to inside: - 100mm dense concrete slab (not counted in the floor’s resistance: BR 443) - 50mm sand/cement screed - (Edge insulation, set in the app’s ground-floor panel: 100mm XPS, λ 0.034, 0.5m down the outside of the foundation — BS EN ISO 13370 Annex D) | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | No edge insulation | ≈ 0.94 | | | Vertical, 0.5m deep | ≈ 0.68 | As pre-filled | | Vertical, 1.0m deep | ≈ 0.56 | | | Horizontal, 1.0m wide | ≈ 0.68 | The same as vertical 0.5m | | Horizontal, 0.5m wide | ≈ 0.78 | | | Vertical 0.5m, 50mm thick | ≈ 0.72 | | | Vertical 1.0m, 150mm thick | ≈ 0.53 | | | A smaller, more exposed floor (P/A 0.8), no edge insulation | ≈ 1.10 | | | The same floor, vertical 1.0m deep | ≈ 0.60 | A cut of nearly half | | An insulated slab (75mm PIR over it) + vertical 0.5m | ≈ 0.20 | 0.22 without the edge insulation | What changes the number: Much of a solid floor’s heat leaves near its edges, so insulating the perimeter does more than its area suggests. BS EN ISO 13370 Annex D works the effect out from the edge insulation’s depth (or width) and its thermal resistance, and BR 443 §9 says to calculate the floor without it first and apply it as a correction, counting only the more effective of the two where there is both vertical and horizontal insulation. Depth matters more than thickness: going from 0.5m to 1m deep takes 0.68 to 0.56, while 150mm instead of 100mm at 1m only reaches 0.53. Small, exposed floors gain most, because more of their heat goes out at the edge: on a floor with a perimeter-to-area ratio of 0.8, 1m of vertical insulation cuts the U-value by nearly half. On a floor already insulated over the slab it adds little (0.22 to 0.20). Digging alongside old walls needs care — the government’s solid-floor guide warns that excavating near shallow foundations could undermine the wall — and it notes that external wall insulation can be brought down below floor level, which treats the same junction. Q: Is edge insulation enough to meet the Building Regulations for a floor? A: Not if the floor itself is being renovated. In England, Approved Document L asks for 0.25 W/m²K when an existing floor is renovated — a new layer added to it or one replaced (Building Regulations 2010, regulation 2) — and edge insulation on an uninsulated slab takes a typical floor only to about 0.53 even 1m deep with 150mm of board, because the middle of the floor still loses heat into the ground. Insulation dug in round the outside of the walls leaves the floor itself unchanged, so check with building control whether it counts as renovating it at all. It is worth doing where the floor can’t be lifted — a cut of a quarter to two-fifths on a typical house, more on a small floor — and where the 0.25 does apply, the guidance allows a lesser standard if meeting it would cause significant problems with adjoining floor levels (Table 4.3, note 8), subject to the 15-year payback test (paragraph 4.13). The government’s solid-floor guide also recommends considering an insulated perimeter upstand, to warm the edge of the floor where its surface is coldest. Note: BS EN ISO 13370 at U-Monkey’s default floor (30m of exposed perimeter, 50m², P/A 0.6), ground λ 2.0, 300mm walls, with the edge insulation applied as Annex D’s correction (BR 443 §9). The concrete slab is excluded from the floor’s resistance as BR 443 directs; the screed counts, which is why the bare floor here is 0.94 where the limecrete page’s, with no screed, is 0.98. The pre-filled link opens as an upgraded existing floor with the edge insulation set in the ground-floor panel (Edge insulation: vertical, down the foundation). --- # U-value of a floor over an unheated cellar URL: https://energycount.co.uk/tools/u-value-calculator/floor-over-cellar/ Typical U-value: 0.21 W/m²K — 140mm mineral wool between the joists, plasterboard ceiling, mid-terrace cellar counted (with the cellar below counted to BS EN ISO 13370 §7.4 at its default ventilation; 0.23 for a draughty cellar, 0.26 taken as outside air) A suspended timber floor over an unheated cellar — 140mm of Rockwool Flexi between 175mm joists, a breathable membrane under it and a plasterboard ceiling to the cellar — has a U-value of about 0.26 W/m²K taken as if it faced outside air, and about 0.21 with the cellar counted the way BS EN ISO 13370 §7.4 counts an unheated basement: through its floor and walls to the ground, its walls above ground, and its ventilation. That is for a mid-terrace front cellar of 18m² with 4m of wall to the outside, 1.8m deep, at the standard’s default of 0.3 air changes an hour, the least cautious choice; at the 3 an hour SAP and BR 443 assume for an unheated space of unknown airtightness it is 0.23. The cellar matters most before the floor is insulated: bare boards over it read 2.64 taken as outside air, but 0.70–1.2 with the cellar counted, depending mostly on how draughty it is. Build-up, outside to inside: - (The unheated cellar below: 18m², 4m of exposed wall, 1.8m deep, the floor 0.5m above ground, 0.3 air changes an hour — BS EN ISO 13370 §7.4) - 12.5mm plasterboard ceiling to the cellar - Windtight vapour-open membrane stapled to the joists - 35mm air space between the joists - 140mm Rockwool Flexi slab between 175mm joists at 400mm centres (10.8% timber, BR 443’s figure for floors), tight to the boards - 22mm softwood floorboards | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Taken as outside air (RdSAP’s convention, and the cautious case) | ≈ 0.26 | RdSAP treats a semi-exposed floor as fully exposed | | The cellar counted, 0.3 air changes an hour (the default) | ≈ 0.21 | As pre-filled | | The cellar counted, 1 air change an hour (small vents) | ≈ 0.22 | | | The cellar counted, 3 air changes an hour (open vents, a coal hole) | ≈ 0.23 | SAP’s default for an unheated space of unknown airtightness | | Before: bare boards on joists, no ceiling, as exposed | ≈ 2.64 | | | Before: bare boards, the cellar counted | ≈ 0.70 | 0.704; 0.89 at 1 air change an hour, 1.21 at 3 | | Uninsulated, plasterboard ceiling under the joists, the cellar counted | ≈ 0.58 | 1.43 taken as outside air | | 100mm mineral wool quilt between the joists, the cellar counted | ≈ 0.26 | Misses the 0.25 for a renovated floor (0.263) | | 160mm wood-fibre batts (STEICOflex 036), the cellar counted | ≈ 0.19 | | | 140mm slab + 25mm PIR under the joists, the cellar counted | ≈ 0.16 | | | End terrace: 8.5m of exposed cellar wall, the cellar counted | ≈ 0.22 | | | A whole-house cellar, 50m² with 30m of exposed wall (P/A 0.6), the cellar counted | ≈ 0.23 | | What changes the number: There are two ways to count the cellar that give an answer, and they differ. BS EN ISO 13370 §7.4 treats an unheated basement as a space that loses heat through its floor and walls to the ground, through its walls above ground to outside air, and by ventilation: 1/U = 1/Uf + A/(A·Ubf + z·P·Ubw + h·P·Uw + 0.33·n·V), where Uf is the floor above with an inside surface resistance on both faces — U-Monkey’s “unheated cellar” option, which needs the cellar’s area A, exposed perimeter P, depth z, the height of the floor above ground h and its air change rate n. SAP 10.2’s general formula for an unheated space (formula 4) leaves ground floors out, so it can’t represent a cellar, and SAP points to BS EN ISO 13370 for basements; RdSAP treats the floor as exposed — the cautious figure. The cellar’s numbers then matter: P counts only the cellar wall with outside ground or air behind it — U-Monkey’s reading leaves out party walls to a neighbour’s cellar and the wall to the ground under the rest of the house — so an end-terrace cellar (8.5m) does worse than a mid-terrace one (4m); and the air change rate runs from the standard’s default of 0.3, the least cautious figure, to the 3 an hour SAP and BR 443 take when an unheated space’s airtightness isn’t known (SAP Table 3.5 gives 0.5 for well-sealed spaces, 1 with small vents, 10 with many openings). A floor over a cellar is a thermal element — it separates the house from an unconditioned space (regulation 2(3)) — so insulating more than half of it brings in Approved Document L’s 0.25 for a renovated floor (Table 4.3), with a lesser standard where meeting it would cause problems with adjoining floor levels (note 8) and the general 0.7 backstop (paragraph 4.13). Heat the cellar or open it to the house and it becomes part of the dwelling: then the basement floor and cellar wall pages apply. Q: Why does a floor over a cellar have three different U-values? A: Because the cellar can be counted two ways, and the second depends on how draughty the cellar is. Taken as outside air — RdSAP’s convention and the cautious one — the pre-filled floor is 0.26 W/m²K. Counted as BS EN ISO 13370 §7.4 counts an unheated basement, with its losses to the ground and its ventilation, it is 0.21 for a typical mid-terrace front cellar at the standard’s default ventilation, rising to 0.23 at the 3 air changes an hour SAP assumes when it isn’t known. SAP’s own formula for unheated spaces leaves the ground out and can’t be used for a cellar. The difference is biggest before insulating: bare boards read 2.64 as exposed and 0.70–1.2 with the cellar counted — which matters where a floor kept in a conversion is judged against the 0.70 threshold (Approved Document L paragraph 4.12(b)). Note: The cellar is counted with U-Monkey’s “unheated cellar” option under “On the other side”: BS EN ISO 13370 §7.4 as the Passive House Institute and the Home Energy Model’s reference code set it out, with 0.3 air changes an hour and the cellar’s volume A(h + z) by default, and the floor above taken with the inside surface resistance 0.17 on both faces, as BR 443 §9.2 does for a suspended deck. The cellar here: 330mm brick walls (R 0.43), an uninsulated floor, sand or gravel (λ 2.0). The pre-filled link opens as an upgraded floor with the cellar counted; choose “Outside air” for the exposed figure. Condensation and damp over a cellar: U-Monkey’s free BS EN ISO 13788 check keeps outside air beneath the floor — colder than any cellar, so the cautious case — and gives the pre-filled floor a conditional pass: no interstitial condensation in any month, as long as the floorboards’ vapour resistance factor is at least μ 30 (μ 50 used, an assumed value for softwood). The membrane under the insulation should be windtight but vapour-open, so the floor can dry downwards. Insulating the cellar’s ceiling also makes the cellar colder, so its air will be damper and the joists’ lower edges and their bearings sit on the cold side of the insulation: keep the cellar’s vents and gratings open, deal with any damp in it first, and lag pipes running under the insulated floor, as Ecological Building Systems’ guide advises. --- # U-value of an exposed floor over a passage, undercroft or overhang URL: https://energycount.co.uk/tools/u-value-calculator/exposed-floor-overhang/ Typical U-value: 0.15 W/m²K — 200mm mineral wool between the joists + 50mm PIR under them, cement-particle soffit A timber floor with outside air beneath it — a bedroom over an archway or passage, a room over a car port, a cantilevered or jettied floor — with 200mm of mineral wool between the joists, 50mm of PIR fixed under them and a 12mm cement-particle soffit board, has a U-value of about 0.15 W/m²K: inside the 0.18 limiting value for a new floor, short of the 0.13 notional value, which 70mm of PIR reaches (0.13). Wool between the joists alone gives only 0.22, because the joists bridge it through its full depth. The soffit board itself is worth almost nothing; what it changes is the surface resistance — fixed on battens with a ventilated gap above it, the board and the gap drop out of the calculation and the outside surface takes the inside value instead (0.14). Build-up, outside to inside: - 12mm cement-particle soffit board (Versapanel A2, λ 0.55) - 50mm PIR fixed continuously under the joists - 200mm mineral wool quilt between 200mm joists at 400mm centres (10.8% timber) - Polythene vapour control layer under the deck (the warm side) - 22mm chipboard floor deck | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Uninsulated: soffit board, empty 200mm joists, chipboard | ≈ 1.54 | | | 200mm mineral wool between the joists only | ≈ 0.22 | Misses 0.18; meets the 0.25 for a renovated floor | | 150mm mineral wool + 50mm PIR under the joists | ≈ 0.17 | Meets 0.18 (0.172) | | 200mm mineral wool + 50mm PIR | ≈ 0.15 | As pre-filled | | 200mm mineral wool + 70mm PIR | ≈ 0.13 | Meets the 0.13 notional (0.128) | | Soffit on battens with a ventilated gap above it | ≈ 0.14 | Soffit and gap disregarded; outside surface 0.17 | | Natural: 200mm wood-fibre batts + 60mm wood-fibre soffit board | ≈ 0.16 | STEICOflex 036 between, STEICO protect dry M under | What changes the number: An exposed floor is calculated like any other element with outside air on one side, with BR 443’s surface resistances for heat flowing downwards — 0.17 inside, 0.04 outside — and no air-gap correction, which BR 443 applies to walls and roofs “but not to floors (because convection is suppressed when the heat flow direction is downwards)”. The joists set the first limit: at 400mm centres they are 10.8% of the floor (BR 443 §4.5.3), so 200mm of wool between them gives only 0.22, and a continuous layer under them does more per millimetre than extra depth between them — the same lesson as the floor over a garage, which this is the outdoor version of. The soffit then decides the outside surface: a board fixed straight to the joists or insulation keeps 0.04, while a board on battens with a ventilated gap above it is disregarded with the gap, and the outside surface resistance becomes the inside value (BS EN ISO 6946 §6.9.4) — slightly better, and the gap lets the soffit drain and dry. Under a concrete floor — flats over an undercroft car park — the insulation goes on the soffit instead, and its fixings matter: Unilin’s table for its SR019/MA soffit board under 200mm of concrete reads 0.25 at 75mm and 0.16 at 120mm, with 12 fixings per board. Approved Document L sets 0.18 for a new floor (Tables 4.1 and 4.2), the notional dwelling 0.13, and 0.25 for a renovated one (Table 4.3). The junction where the exposed floor meets the wall is a thermal bridge of its own — model it in ΨMonkey. Q: Is a floor over a passage or archway an exposed floor? A: Yes: with outside air beneath it, it is an exposed floor, calculated with an outside surface resistance of 0.04 (or 0.17 behind a ventilated soffit) and judged against Approved Document L’s floor values — 0.18 for a new floor, 0.25 for a renovated one. It differs from a floor over an enclosed integral garage, where SAP adds the garage’s resistance (Ru); over an open passage, archway or car port nothing is added, which is why its figures here are close to the floor over a garage taken as outside air. Note: Calculated as an exposed floor to BS EN ISO 6946 with outside air below (Rse 0.04); the ventilated-soffit row puts the board on battens over a well-ventilated gap, with a breather membrane under the PIR, so the board and gap are disregarded and Rse is 0.17. The natural row uses STEICOflex 036 between the joists and 60mm of STEICO protect dry M (λ 0.040) as the soffit, with no PIR or polythene. Joists at 10.8% (BR 443 §4.5.3). Condensation risk in an exposed floor: With the polythene vapour control layer under the deck, U-Monkey’s free BS EN ISO 13788 check finds a little seasonal condensation (a peak of 6.6 g/m² from November) that dries out fully in summer, and it stays acceptable with every assumed vapour value tested across its range — a pass. Leave the polythene out and the peak rises to about 190 g/m², and the result depends on the chipboard’s vapour resistance: the foil-faced PIR under the joists is far more vapour-tight than anything above it, so the floor needs its vapour control on the warm side, under the deck. The natural version, with wood-fibre boards as the soffit and no PIR, passes with no condensation in any month and no polythene at all, because every layer below the deck is vapour-open. --- # U-value of a battened timber floor over a concrete slab URL: https://energycount.co.uk/tools/u-value-calculator/timber-floor-on-slab/ Typical U-value: 0.34 W/m²K — 50mm PIR between 47mm battens on a concrete slab, chipboard, P/A 0.6 A timber floor laid on battens over an existing concrete slab, with 50mm of PIR between 47mm battens at 600mm centres and 22mm of chipboard over them, has a U-value of about 0.34 W/m²K on a typical floor of 50m² with 30m of exposed perimeter (P/A 0.6). The battens cost about 0.06: the same 50mm laid continuously under a floating deck gives 0.28. The 0.25 Approved Document L asks of a renovated floor takes about 85mm of PIR between battens, or 50mm between the battens with 25mm more laid over them and a plywood deck (0.24). BR 443 counts a battened layer with a combined calculation inside the floor before working out the ground (§9.1), which U-Monkey applies. Build-up, outside to inside: - Existing concrete slab, with a damp-proof membrane over it (the slab is not counted, BR 443) - 50mm PIR between 47 × 50mm treated battens at 600mm centres (9.4% timber with cross-pieces) - 22mm chipboard flooring | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Uninsulated: empty 50mm batten void, chipboard | ≈ 0.70 | The bare slab: about 0.98 | | 25mm PIR between the battens | ≈ 0.48 | | | 50mm PIR between the battens | ≈ 0.34 | As pre-filled | | 75mm PIR between 75mm battens | ≈ 0.27 | | | 100mm PIR between 100mm battens | ≈ 0.22 | Meets the 0.25 for a renovated floor | | 50mm PIR laid continuous under a floating deck (no battens) | ≈ 0.28 | What the battens cost: about 0.06 | | 50mm PIR between battens + 25mm PIR over them, 18mm plywood | ≈ 0.24 | Meets 0.25 | | Battens at 400mm centres (13.3%), 50mm PIR | ≈ 0.36 | | | 100mm mineral wool slab (λ 0.038) between 100mm battens | ≈ 0.29 | | | 100mm wood-fibre batts (STEICOflex 036) | ≈ 0.28 | | | 50mm PIR, a large floor (P/A 0.3) | ≈ 0.28 | 30m perimeter, 100m² | | 50mm PIR, a small floor (P/A 0.9) | ≈ 0.37 | 27m perimeter, 30m² | What changes the number: The battens are a bridged layer in a floor that is otherwise a ground calculation, and BR 443 (2019) §9.1 says how to count them: “If a floor does have a bridged layer”, use “the method of BS EN ISO 6946 with Rsi = 0.17 m²K/W (downwards heat flow) and Rse = 0”, subtract Rsi “so as not to count Rsi twice”, then put the result into BS EN ISO 13370 “allowing for the effect of the ground”. That is what U-Monkey does with a bridged layer in a ground floor; taking the layer’s parallel resistance instead would read about 0.01 worse. BR 443 gives no figure for floor battens, so the fraction here follows its general rule — timber width over spacing — for 47mm battens at 600mm centres with a cross-piece every 3m: 9.4%, or 13.3% at 400mm centres. The battens then cost about 0.06 against the same insulation laid continuously, and a thin continuous layer over them (25mm under a plywood deck) recovers about half of it: 50mm between plus 25mm over reads 0.24, against 0.27 for 75mm between the battens and 0.21 for 75mm laid continuously. The floor’s shape does the rest, as on any ground floor: the same 50mm gives 0.28 on a large floor (P/A 0.3) and 0.37 on a small one (P/A 0.9); the ground under it matters too — U-Monkey’s default is sand or gravel (λ 2.0), and BR 443 notes the UK’s ground is most commonly clay (λ 1.5). Approved Document L asks 0.25 of a renovated floor (Table 4.3) and, for a floor kept in a conversion, 0.25 where it is worse than 0.70 (paragraph 4.12(b)), with a lesser standard where meeting it would cause problems with adjoining floor levels (note 8) — a real limit when a battened floor raises the room. Lay the battens, treated, on a damp-proof membrane over the slab; in a traditional building with no damp-proof course, Historic England warns that sealing a solid floor can push ground moisture to the base of the walls. Q: How much do battens reduce the effect of floor insulation? A: About 0.06 W/m²K at 50mm: 50mm of PIR between 47mm battens at 600mm centres gives 0.34 on a typical floor, against 0.28 for the same 50mm laid continuously under a floating deck. Closer battens (400mm centres) cost more, 0.36. A thin continuous layer over the battens covers the timber: 50mm between plus 25mm over them, under a plywood deck, reaches 0.24 — inside the 0.25 Approved Document L asks of a renovated floor. Note: The battens are entered as a bridge through the insulation at 9.4% timber — 47mm battens at 600mm centres with a 47mm cross-piece every 3m, BR 443 §4.5’s width-over-spacing rule — because BR 443 gives no figure for floor battens. U-Monkey’s default floor: 30m of exposed perimeter around 50m² (P/A 0.6), 300mm walls, sand or gravel (λ 2.0); set your own. The pre-filled link opens as an upgraded floor. --- # U-value of a suspended timber floor insulated with wood fibre or sheep’s wool URL: https://energycount.co.uk/tools/u-value-calculator/natural-fibre-suspended-floor/ Typical U-value: 0.29 W/m²K — 100mm wood-fibre batts between the joists, membranes, ventilated void, P/A 0.6 Insulating a suspended timber floor with 100mm of wood-fibre batts (STEICOflex 036) between the joists, held in a windtight vapour-open membrane with an airtight layer under the boards, takes a typical floor (30m of exposed perimeter around 50m², P/A 0.6) from about 0.81 W/m²K to 0.29; 125mm reaches the 0.25 Approved Document L asks of a renovated floor, and 150mm gives 0.22. Sheep’s wool and hemp batts (λ 0.039) do about 0.01 worse at the same depth. One measured case is worth knowing: in a UCL study of a London terrace, 100mm of wood fibre took the floor from a measured 1.04 to 0.36 — a 65% cut, a little more than the standard model’s 0.57 to 0.23 — and the authors put what was left above the model’s 0.23 mainly down to the fit: batts that didn’t fill the space, chamfered edges and gaps, so fit it tightly. Build-up, outside to inside: - Ventilated void below (U-Monkey’s defaults: 225mm void, vents 1,500mm² per metre of wall) - Windtight vapour-open membrane stapled under the joists as a “basket” - 100mm STEICOflex 036 wood-fibre batts between the joists (10.8% timber) - Airtight membrane over the joists, taped (not counted) - 22mm timber floorboards | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100mm wood-fibre batts (STEICOflex 036) | ≈ 0.29 | As pre-filled | | 125mm | ≈ 0.25 | Meets the 0.25 for a renovated floor (0.249) | | 150mm | ≈ 0.22 | | | 175mm | ≈ 0.20 | | | 200mm | ≈ 0.18 | | | 100mm sheep’s wool (Thermafleece CosyWool, λ 0.039) | ≈ 0.30 | Thermafleece’s own table: 0.31 | | 150mm sheep’s wool (CosyWool) | ≈ 0.23 | Thermafleece: 0.24 | | 150mm sheep’s wool (Thermafleece UltraWool, λ 0.035) | ≈ 0.22 | Thermafleece: 0.22 | | 140mm hemp batts (IndiTherm, λ 0.039) | ≈ 0.24 | | | 100mm wood fibre + 22mm wood-fibre board under the joists | ≈ 0.25 | The board covers the joists (0.252) | | 150mm wood fibre + 22mm wood-fibre board under the joists | ≈ 0.20 | | | The UCL field-study floor (its published inputs), uninsulated | ≈ 0.57 | The paper’s model: 0.57; measured on site: 1.04 | | The UCL floor, 100mm wood fibre at the paper’s λ 0.038, joists counted | ≈ 0.26 | | | The UCL floor, 100mm wood fibre, joists left out (as the paper modelled it) | ≈ 0.23 | The paper’s model: 0.23; measured: 0.36 | What changes the number: Depth and λ come first: each 25mm of wood fibre between the joists is worth about 0.02–0.04 on a typical floor, and sheep’s wool or hemp at λ 0.039 does about 0.01 worse than wood fibre at 0.036 for the same depth; a 22mm wood-fibre board under the joists covers the timber and is worth about as much as another 25mm between them. The floor’s shape and its void then move it, as on any suspended floor — the perimeter, the area and the ventilation openings all go into BS EN ISO 13370. The membranes do the rest, and they are where the measured case teaches most: in Pelsmakers and Elwell’s UCL study (Energy and Buildings, 2017) of a pre-1919 London terrace, 100mm of wood fibre held in a tightly stretched, stapled breather membrane cut the floor’s measured U-value from 1.04 to 0.36 — 65%, a little more than the 60% the BS EN ISO 13370 model predicted (0.57 to 0.23, the joists left out). The real floor lost more heat than the model said before and after; the authors suggest the uninsulated difference may come from the model’s inputs, the wall–floor junction it leaves out or its steady-state assumptions, and what was left after insulating mainly to the fit — ill-fitting batts, chamfered edges and gaps — hence their conclusion that “good installation quality is important to maximise heat-flow reductions.” U-Monkey reproduces the model on the paper’s published inputs (0.57; 0.23 without the joists, 0.26 with them). Historic England’s guidance on suspended floors favours natural fibres in older buildings, “allowing both air and moisture vapour to pass through slowly”, says the most reliable installations include some form of restraint below the insulation, and notes that a membrane with low vapour resistance below the joists “can usefully enhance the effectiveness of the insulation by reducing air movement through the floor”; it quotes 0.22, the 2010 figure for a new floor; a renovated floor needed 0.25 then and still does (Approved Document L, Table 4.3, with a lesser standard where floor levels prevent it, note 8), and a new floor now needs 0.18. Keep the void ventilated — Thermafleece asks for at least 150mm of ventilated space below the insulation — and every air brick clear. Q: Does underfloor insulation work as well as the calculations say? A: In proportion, yes; in absolute terms, only as well as it is fitted. In a UCL field study of a London terrace, 100mm of wood-fibre insulation between the joists cut the measured heat loss through a suspended timber floor by 65% — from 1.04 to 0.36 W/m²K — slightly more than the standard model’s 60% (0.57 to 0.23). But the real floor lost more heat than the model said both before and after, and the authors tied what remained after insulating mainly to the fit: batts that didn’t fill the space between the joists, chamfered edges and gaps. U-Monkey gives 0.26 for the same insulated floor with its joists counted. Fit the batts tightly to the joists and the boards, restrain them with a stretched membrane below, and seal an airtight layer under the boards. Note: Batts at their declared λ: STEICOflex 036 0.036; Thermafleece CosyWool 0.039 and UltraWool 0.035; IndiNature IndiTherm 0.039; STEICO universal 0.048 under the joists. Joists at BR 443’s 10.8% for floors. U-Monkey’s default floor: 30m of exposed perimeter around 50m² (P/A 0.6), 225mm void, 1,500mm² of vents per metre, sand or gravel (λ 2.0). The UCL rows use the published inputs of Pelsmakers & Elwell (2017) — perimeter 9.1m, area 27.7m², clay (λ 1.5), 220mm walls, the floor 0.17m above ground, void walls U 1.7, 2,400mm² of vents per metre, 19mm boards — as U-Monkey’s validation case for that floor does. The membranes carry no thermal resistance and are not entered as layers. The pre-filled link opens as an upgraded floor. --- # U-value of a concrete floor over a car park URL: https://energycount.co.uk/tools/u-value-calculator/floor-over-car-park/ Typical U-value: 0.17 W/m²K — 110mm phenolic soffit board under a 200mm concrete slab, stainless fixings counted A 200mm concrete slab over a car park, with a 65mm screed and 110mm of phenolic soffit board (λ 0.019) fixed to its underside, has a U-value of about 0.17 W/m²K with the board’s stainless screws counted — meeting the 0.18 Approved Document L sets for the floor of a new home, and for a new floor in an existing one. 100mm gives 0.19 and misses. After the thickness, the screws matter most: twelve stainless ones per board add about 0.01, carbon-steel ones 0.03. Uninsulated, the same slab is about 2.8. Non-combustible stone wool (λ 0.034), which some specifications ask for under flats, needs about 180mm before its fixings are counted. Build-up, outside to inside: - 110mm phenolic soffit board (Kingspan Kooltherm K110, λ 0.019; foil on the exposed face), screwed at Unilin’s 12 per 2.4 × 1.2m board (4.17 per m²), taken as stainless - 200mm reinforced concrete slab - 65mm sand/cement screed | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | Uninsulated: 200mm slab + 65mm screed | ≈ 2.83 | | | 75mm phenolic soffit board (λ 0.019), stainless fixings | ≈ 0.25 | Meets 0.25, the value for an upgraded floor | | 80mm | ≈ 0.23 | | | 90mm | ≈ 0.21 | | | 100mm | ≈ 0.19 | Misses 0.18 | | 110mm | ≈ 0.17 | As pre-filled: meets 0.18 | | 120mm | ≈ 0.16 | | | 150mm (Unilin SR019/MA to order, or two layers; the same λ) | ≈ 0.13 | Meets the notional home’s 0.13 | | 100mm, fixings left out | ≈ 0.18 | The stainless screws add 0.011 | | 100mm, carbon-steel fixings | ≈ 0.21 | Three times the correction: 0.031 | | 100mm under a 250mm slab | ≈ 0.19 | The slab barely matters | | Stone wool soffit slab (λ 0.034, A1), 180mm, fixings left out | ≈ 0.18 | Meets 0.18 before its fixings | | Stone wool, 220mm, fixings left out | ≈ 0.15 | | | 100mm phenolic over an unheated basement instead (3 air changes an hour) | ≈ 0.18 | 0.19 taken as outside air | What changes the number: The board, and the screws through it. Every fixing that passes through the insulation is a small thermal bridge, corrected by BS EN ISO 6946 Annex F (BR 443 §4.8.3): at twelve 5.2mm screws per 2.4 × 1.2m board — Unilin’s pattern for its SR019/MA soffit board, taken here as stainless — they add about 0.01, and carbon steel three times that, so ask for stainless or for a system with a certified figure. Unilin’s published U-values for SR019/MA under a 200mm slab (0.25, 0.23, 0.19 and 0.16 for 75, 80, 100 and 120mm) come out here, with the 65mm screed, only with those screws counted. The slab and screed barely matter: 250mm of concrete instead of 200mm improves the U by 0.001. A floor losing heat downwards has an inside surface resistance of 0.17 and an outside one of 0.04 (BR 443 §4.1), and BR 443 adds no air-gap correction to floors (§4.8.1), because convection is suppressed when heat flows downwards. What a U-value can’t show are the beams, columns and wall junctions the boards stop at: they are thermal bridges for the SAP or SBEM calculation, and the bare concrete around them is where condensation and mould appear first. Q: Does insulation under a car park have to be non-combustible? A: Not by regulation 7(2) on its own wording: that applies to the external walls and specified attachments (balconies, solar shading, solar panels) of relevant buildings — those with a storey at least 18m above ground containing dwellings, an institution or rooms for residential purposes — and regulation 2(6)’s definition of an external wall does not mention a soffit. Inside a car park the soffit is a ceiling lining: Approved Document B’s tables of linings (Volume 1 Table 4.1, and Volume 2 Table 6.1 for a car park that isn’t part of a home) ask for C-s3, d2 in rooms, garages included, and B-s3, d2 in circulation spaces. Phenolic soffit boards are classed B-s2, d0 (Kingspan K110, foil side out), enough for either, or C-s2, d0 (Unilin SR019/MA), enough for a room but not a circulation space. Volume 2 also has a section of its own on car parks (Section 11), so ask building control how it treats soffit insulation in yours. Some project specifications ask for A2-s1, d0 or better anyway, which means stone wool — ROCKWOOL’s Soffit Slab is A1 — at nearly twice the thickness, and a 2026 consultation on Approved Document B proposed reviewing the scope of the combustibles ban, so check the current guidance and your specification. Note: Phenolic soffit board at λ 0.019 (Kingspan Kooltherm K110; Unilin SR019/MA is the same) with 5.2mm screws through the full board at Unilin’s 12 per 2.4 × 1.2m board, taken as stainless — U-Monkey’s “Soffit board screws” preset (BS EN ISO 6946 Annex F); Kingspan’s own 2019 datasheet asked for 11 to 15 per board, with thermally broken fixings. Kingspan’s older K110 table used λ 0.018 and so reads about 0.01 better than today’s board. Stone wool: ROCKWOOL Soffit Slab (λ 0.034, A1), fixings left out — enter your system’s. The basement row uses U-Monkey’s unheated-cellar option (BS EN ISO 13370 §7.4) for a 300m² basement 2.8m deep, at BR 443’s default of 3 air changes an hour. Condensation risk under a car park: With the insulation below the slab, the concrete stays warm and the inside surface passes U-Monkey’s mould check comfortably (fRsi 0.96); the uninsulated slab is the opposite — its surface fails the mould check (fRsi 0.42), and condensation collects within the floor in winter. Interstitial condensation needs more care than a screening check gives it here. U-Monkey’s BS EN ISO 13788 check spreads a foil-faced board’s vapour resistance through its thickness, and on that basis finds none in any month for the pre-filled floor. But Kingspan’s K110 has its foil on the car-park face only, with glass tissue against the concrete, so its vapour-tight layer is on the cold side, where vapour moving down from the rooms above can condense behind it — and the check says so when the board is in a build-up. Entered as made, with a vapour control layer under the core for the foil and the core at μ 50, the same check shows condensate building up behind the foil year on year, some 80 to 110 g/m² of it still there at the end of the year, depending on the value taken for the foil. With foil on both faces, as on Unilin’s SR019/MA, it shows about 10 g/m² in spring that dries out over the summer. Ask the maker for a condensation risk analysis for your slab, floor finish and humidity class, and on a new slab let the concrete dry before the boards close it in. --- # U-value of a warehouse floor slab URL: https://energycount.co.uk/tools/u-value-calculator/warehouse-floor/ Typical U-value: 0.16 W/m²K — 100 × 50m ground-bearing slab, no insulation under it, a 1m strip of 50mm PIR round the edge, on sand or gravel (for a 100 × 50m floor on sand or gravel with a 1m strip of edge insulation; 0.19 with none, 0.15 on clay) A warehouse floor of 100 × 50m — a 200mm ground-bearing concrete slab with no insulation under it — has a U-value of about 0.19 W/m²K on sand or gravel and 0.15 on clay, because a big floor loses heat mostly round its edge: its perimeter-to-area ratio is only 0.06. That is just over the 0.18 Approved Document L Volume 2 sets for a floor on sand or gravel, and a 1m strip of 50mm PIR round the edge under the slab takes it to 0.16 (on clay, 0.13), with no insulation under the rest. BR 443 says it plainly: “Very large floors can have low U-values without all-over insulation.” A smaller shed needs more: 60 × 40m, uninsulated, reads 0.24. Build-up, outside to inside: - 200mm power-floated reinforced concrete slab (not counted in R_f: BR 443 leaves out dense slabs) - Polythene DPM - No insulation under the slab except a 1m wide strip of 50mm PIR (λ 0.022) round the perimeter — horizontal edge insulation, BS EN ISO 13370 Annex D - Ground: sand or gravel (λ 2.0). Floor 100 × 50m: exposed perimeter 300m, area 5,000m², walls 300mm thick | Variant | U-value (W/m²K) | Note | | --- | --- | --- | | 100 × 50m (P/A 0.06), no insulation, sand or gravel | ≈ 0.19 | Misses 0.18 (0.189) | | Same, on clay (λ 1.5) | ≈ 0.15 | Meets 0.18 with no insulation (0.146) | | Same, sand or gravel, 1m strip of 50mm PIR at the edge | ≈ 0.16 | As pre-filled (0.163) | | Same, on clay, with the 1m edge strip | ≈ 0.13 | 0.125 | | Same, sand or gravel, 50mm PIR 1m down the foundation (vertical) | ≈ 0.15 | 0.151: vertical edge insulation does a little more | | 25mm PIR under the whole slab, sand or gravel | ≈ 0.13 | 0.133 | | 50mm PIR under the whole slab | ≈ 0.11 | 0.111 | | 60 × 40m (P/A 0.083), no insulation, sand or gravel | ≈ 0.24 | Fails 0.18 | | 60 × 40m with the 1m edge strip | ≈ 0.21 | Still fails: a floor this size needs insulation under the slab | | 60 × 40m, 25mm PIR all over | ≈ 0.17 | 0.167 | | 120 × 60m (P/A 0.05), no insulation, sand or gravel | ≈ 0.16 | Meets 0.18 with no insulation (0.163) | | 150 × 80m (P/A 0.038), no insulation | ≈ 0.13 | 0.132 | | 200 × 100m (P/A 0.03), no insulation | ≈ 0.11 | 0.108 | What changes the number: A floor on the ground is calculated by BS EN ISO 13370, not layer by layer, because most of its heat goes down into the ground and back out at the edge: the bigger the floor, the further its middle is from the edge and the less it loses. The standard works from the characteristic dimension B′ = A/(½P) — 33m for this 100 × 50m floor, about 3m for a small house — and BR 443 (2019) §9 sums it up: “Very large floors can have low U-values without all-over insulation”, and the floor U-value “decreases as the ratio of perimeter to area decreases”. The perimeter is every exposed wall, including walls to unheated spaces; walls to heated neighbours are left out. The ground matters as much as the size: BR 443 gives clay or silt λ 1.5, sand or gravel 2.0 and rock 3.5 W/m·K and says “the ground type in the UK is most commonly clay”, which takes this floor from 0.19 to 0.15. U-Monkey starts on sand or gravel, the cautious choice, so set yours from the site investigation. Edge insulation is a correction to the floor’s U-value (BS EN ISO 13370 Annex D), horizontal under the slab edge or vertical down the foundation, and only the more effective of the two counts; BR 443’s 2006 edition adds that edge insulation “will usually be needed to avoid thermal bridging at the perimeter” — the floor-to-wall junction’s own loss is a Ψ-value, not part of the floor U-value. Approved Document L Volume 2 sets 0.18 for a floor in its 2021 and 2026 editions. For a new building the test is the whole-building calculation (the BER and BPER against the TER and TPER): the NCM notional building gives a side-lit building’s floor 0.15 and a top-lit one’s 0.22, but where the floor’s own uninsulated value is below 0.15 “this lower value must be used in the Notional building” (NCM Modelling Guide 2021, paragraph 36) — so where a big shed’s uninsulated floor is already under 0.15, the notional floor is no better and the floor’s U-value costs nothing against the target — though the uninsulated slab edge’s thermal bridge still counts in the actual building. BR 443 adds that, as a floor’s U-value falls with its perimeter-to-area ratio, “the U-value can be obtained for the smallest intended building”: worth doing where a shed may be split into smaller units. Insulation under a racked warehouse slab has to carry the racking: specify its compressive strength for the loads. Q: Does a warehouse floor need insulation? A: Often only at the edge, sometimes not at all. On sand or gravel an uninsulated slab meets Approved Document L’s 0.18 once the floor is about 120 × 60m (0.16), and a 100 × 50m floor gets there with a 1m strip of 50mm PIR round the edge (0.16); on clay, a 100 × 50m slab is 0.15 with nothing. A 60 × 40m unit needs insulation under the slab: 25mm of PIR all over gives 0.17. Calculate your own floor in U-Monkey with its real perimeter, area and ground — and the slab edge still needs detailing against thermal bridging. Note: The pre-filled link opens in the England · non-domestic regime with the floor’s geometry set — exposed perimeter 300m, area 5,000m², wall thickness 300mm, sand or gravel (λ 2.0) — and edge insulation horizontal, 1.0m wide, 50mm at λ 0.022. The concrete slab is not counted in R_f, as BR 443 directs for dense slabs, and finishes are ignored. The U-value is for the floor as a whole; the junctions round it are separate. ## U-VALUE CALCULATORS COMPARED --- # How U-Monkey compares with other U-value calculators URL: https://energycount.co.uk/tools/u-value-calculator/compare/ Updated: 2026-10-13 Energycount makes U-Monkey. The comparison describes kinds of U-value calculator rather than naming products, from what their makers publish (read 10 to 12 October 2026); prices are published list prices, given as ranges, and change. ## U-Monkey (Energycount (this site)) - Price: Free, PDF report included. Monkey Pro £10 a month + VAT puts your branding on reports and adds project tools; a condensation report PDF is £25 + VAT, or included in Pro Unlimited at £49 a month + VAT - Access: Web, on any device; no sign-up; works offline once loaded - Condensation check: Yes, free on screen: a monthly BS EN ISO 13788 (Glaser) screening at 21 UK climate stations, picked from your postcode, with the fRsi surface-mould check - Materials: 494 entries: 59 generic materials and 435 named UK products from 114 makers, each with its source and the date it was read - Good at: Shows every step of the working; neutral across makers; England (2021 and 2026), Wales, Scotland, Northern Ireland and Ireland targets; light steel frame (Digest 465), ground floors and basements (BS EN ISO 13370), tapered and inverted roofs; thermal mass (BS EN ISO 13786); RdSAP 10 "before" defaults for an upgrade; 108 worked build-ups that open pre-filled; published validation cases - Limits: A screening, not a BS EN 15026 hygrothermal simulation; no shared team workspace; not a manufacturer-backed calculation ## Insulation manufacturers’ calculators (Made by the insulation makers, for their own products) - Price: Free - Access: Web; several need an account to use them or to save a calculation - Condensation check: Most offer one for their own products, some taking the climate from your postcode - Materials: The maker’s own range, often as pre-set wall, roof and floor constructions you adjust - Good at: The maker’s own data for its own products, with its technical service behind the figure; often the product’s certificates and declaration of performance alongside - Limits: One brand’s products, so a design not yet settled on one maker can’t be compared across makers ## Subscription web calculators (Independent software firms) - Price: Typically a free tier capped at a few calculations a month, then a monthly subscription - Access: Web; a first look without signing up, an account to keep working - Condensation check: Not in every one: at least one leading tool says it is still to come - Materials: A generic materials list - Good at: Newer ideas, such as reading an uploaded drawing or specification into layers for you to confirm; team seats on the dearer plans - Limits: Calculations are metered, even on paid plans; reports may not carry your own branding ## Desktop U-value software (Long-established building-physics software houses) - Price: A paid licence: from about £160 a year, or a one-off fee of several hundred pounds or more plus an annual charge (+ VAT) - Access: Installed software, mostly for Windows - Condensation check: Included - Materials: Large databases, generic and branded, and libraries of pre-calculated constructions - Good at: Long track records; light steel frame, basements, tapered and inverted roofs, heat capacity; helpline support with the licence - Limits: Paid, usually Windows-only, licensed per computer or per site; updates can be infrequent ## Calculators inside SAP software (Part of an energy-assessment software subscription) - Price: Included in, or added to, a SAP software subscription - Access: Online, through the software provider’s account - Condensation check: Not always offered - Materials: Products from many manufacturers, plus construction templates - Good at: Sits beside the SAP calculation it feeds; some include a thermal mass (kappa) calculator - Limits: Needs the SAP software subscription ## Calculators built for other countries (Mostly from continental Europe) - Price: Often free for non-commercial use, with paid plans for professional work - Access: Web; an account to save - Condensation check: Yes, to that country’s own standard - Materials: Thousands of entries, mostly that country’s products - Good at: Some offer 2D heat-flow models, life-cycle carbon and summer heat performance - Limits: Built around another country’s limits, conventions and products, not UK Building Regulations or BR 443 ## Simple free estimators (Single-page web tools) - Price: Free - Access: Web; no sign-up - Condensation check: Usually none, or a separate simplified tool - Materials: A handful of generic materials - Good at: A quick, rough figure - Limits: One bridged layer at most, and no report ## REFERENCE — THE COMPLIANCE FIGURES ON ONE DATED PAGE --- # Building regulations reference URL: https://energycount.co.uk/reference/ Updated: 2026-09-04 The figures UK energy assessors look up every week, each with its source document and the date it was last checked. England unless stated. Cite as: Energycount, Building regulations reference, energycount.co.uk/reference/, reviewed 4 September 2026. ## Part L U-values (W/m²K) Source: Approved Document L Volume 1, 2021 edition incorporating 2023 amendments — Table 1.1 (notional dwelling), Table 4.1 (limiting values, new dwellings), Table 4.2 (new elements in existing dwellings), Table 4.3 (retained elements); Approved Document L 2026 for the Future Homes Standard notional dwelling. Checked 4 September 2026. | Element | Notional — Part L 2021 | Notional — FHS from 24 March 2027 (SAP 10.3 and HEM) | Limiting, new dwelling (unchanged in 2027) | New element in an existing dwelling | Retained element — upgrade if worse than → to | | --- | --- | --- | --- | --- | --- | | External wall | 0.18 | 0.18 | 0.26 | 0.18 | 0.70 → 0.55 (cavity fill) / 0.30 (internal or external insulation) | | Floor | 0.13 | 0.13 | 0.18 | 0.18 | 0.70 → 0.25 | | Roof | 0.11 | 0.11 | 0.16 | 0.15 | 0.35 → 0.16 | | Party wall | 0 | 0 | 0.20 | — | — | | Windows | 1.2 | 1.2 | 1.6 | 1.4 (or WER band B) | — | | Doors | 1.0 | 1.0 | 1.6 | 1.4 (or DSER band B/C) | — | | Rooflights | 1.7 | 1.7 | 2.2 | 2.2 | — | | Air permeability, m³/(h·m²) at 50 Pa | 5.0 | 4.0 | 8.0 | — | — | The notional dwelling sets the target a design is measured against; the limiting values are the worst any element may be. The Future Homes Standard notional keeps the 2021 fabric and tightens air permeability to 4.0; the Home Energy Model version of it differs only in how the notional heat pump is modelled, not in fabric. Explained at /guides/u-values/ and /guides/future-homes-standard/; calculate a build-up free at /tools/u-value-calculator/; extensions: the 25% glazing checker at /services/sap-calculations/extensions/. ## Part L editions and transitional dates Source: The Building Regulations etc. (Amendment) (England) Regulations 2021 and 2026; Future Homes and Buildings Standards Building Circular 01/2026; Welsh and Scottish government publications. Checked 4 September 2026. | Date | What | | --- | --- | | 15 June 2022 | Approved Document L 2021 in force (England); every new dwelling air-tested individually from this date | | 15 June 2023 | Part L 2013 transitional window closed — the 2013 edition is fully revoked | | 23 November 2022 | Wales: Approved Document L 2022 edition in force | | 1 February 2023 | Scotland: revised Section 6 (Energy) standards in force | | 24 March 2026 | SAP 10.3 approved (Notice of Approval) as the methodology for the Future Homes Standard; Approved Document L 2026 published. Work notified before 24 March 2027 may continue in SAP 10.2 | | 24 March 2027 | Approved Document L 2026 (Future Homes Standard) in force for non-higher-risk work; last day to give a building notice, initial notice or full plans under Part L 2021 | | 24 September 2027 | Future Homes Standard in force for higher-risk buildings; a valid Gateway 2 application before this date completes under Part L 2021 | | 24 March 2028 | Transitional deadline: each building notified before 24 March 2027 must have commenced — foundations and the ground-floor structure complete — to keep Part L 2021 | Transitional protection is per building, not per site. Which edition a scheme falls under: the checker at /guides/building-regulations-part-l/#part-l-checker. Explained at /guides/building-regulations-part-l/ and /guides/future-homes-standard/. ## SAP: the carbon and primary energy factors, and the notional dwelling Source: SAP 10.2 Table 12 and BRE technical paper S10TP-15 (Part L 2021 work); SAP 10.3 specification (12-06-2026 version), Table 12 and Appendix R Table R1 (the Future Homes Standard); Notice of Approval, 24 March 2026 — SAP 10.3 is approved from that date, and buildings notified before 24 March 2027 and started before 24 March 2028 may continue in the earlier version; Approved Document L Volume 1 2021, Table 1.1 for the Part L 2021 notional dwelling. Checked 4 September 2026. ### Fuel factors (Table 12) | Fuel factor | SAP 10.2 — Part L 2021 work | SAP 10.3 — Future Homes Standard | Earlier | | --- | --- | --- | --- | | Grid electricity — CO₂ | 0.136 kgCO₂e/kWh (annual average of monthly factors) | 0.086 kgCO₂e/kWh (forward-looking; one annual figure, the same for every tariff) | 0.233 in SAP 10.0; 0.519 in SAP 2012 | | Mains gas — CO₂ | 0.210 kgCO₂e/kWh | 0.214 kgCO₂e/kWh | 0.216 in SAP 2012 | | Grid electricity — primary energy | 1.501 | 1.969 | 3.07 in SAP 2012 | | Mains gas — primary energy | 1.130 | 1.12 | 1.22 in SAP 2012 | ### The notional dwelling the targets come from | Item | Part L 2021 notional (AD L Table 1.1) | Future Homes Standard notional (SAP 10.3 Appendix R) | | --- | --- | --- | | Heating | Gas boiler, 89.5% efficient, 55 °C flow temperature | Heat pump at a SCOP of 2.5, with dMEV ventilation | | PV | kWp = 40% of ground-floor area (unheated spaces included) ÷ 6.5 — roughly 4 kWp on an average house; flats: 40% of the flat’s floor area ÷ (6.5 × storeys in the block) | Same as the actual building’s PV, up to kWp = 40% of ground-floor area (unheated spaces included) ÷ 4.5; flats: 40% of the flat’s floor area ÷ storeys ÷ 4.5. PV is separately a requirement in its own right (L3) from 24 March 2027 | | Air permeability | 5.0 m³/(h·m²) | 4.0 m³/(h·m²) | | Waste water heat recovery | Assumed on every shower — leave it out and the target gets harder | Every shower, except in single-storey dwellings | | Fixed lighting | 80 lumens per circuit-watt (the actual dwelling must reach 75) | 120 lumens per circuit-watt | Part L 2021 work is assessed in SAP 10.2. SAP 10.3, approved on 24 March 2026, is the Future Homes Standard methodology: its electricity factors move both ways — carbon down to 0.086, primary energy up to 1.969 — and SAP 10.3 uses the annual figures directly rather than monthly ones. The four tests a new dwelling passes at once — DPER ≤ TPER, DER ≤ TER, DFEE ≤ TFEE and the limiting values — are explained at /guides/building-regulations-part-l/, /guides/der-ter/ and /guides/fabric-energy-efficiency/. SAP 10 changes: /guides/sap-10-whats-new/. ## Air tightness (Part L) Source: Approved Document L Volume 1 2021, Regulation 43 and Table 4.1; Approved Document L 2026; Volume 2 for non-domestic. Checked 4 September 2026. | Item | Value | | --- | --- | | Measured quantity | q50 — air permeability, m³ of air per hour per m² of envelope at 50 Pa. Not the same as n50 (air changes per hour, used by Passivhaus); there is no valid conversion between them | | Limiting value (worst permitted) | 8.0 m³/(h·m²) — dwellings and non-domestic buildings alike | | Notional dwelling — Part L 2021 | 5.0 m³/(h·m²) | | Notional dwelling — Future Homes Standard (from 24 March 2027) | 4.0 m³/(h·m²) | | Who is tested | Every new dwelling, individually, since 15 June 2022 — sample testing has gone; Wales the same under its 2022 edition | | Who may test | A tester registered with a scheme the regulation recognises — ATTMA or Elmhurst (Regulation 43); the result goes into the as-built SAP | If the design SAP assumed 5 and the test comes back at 7, the as-built SAP uses 7 and the shortfall must be made up elsewhere. Service and pre-test checklist: /services/air-tightness-testing/. ## Part O overheating — the simplified method tables Source: Approved Document O, 2021 edition (in force 15 June 2022): Tables 1.1–1.4, paragraphs 1.3–1.9 and 3.2–3.3. Glazing is the transparent area, frames excluded; limits are read for the façade with the largest glazed area. High-risk locations are the London postcode districts in Appendix C; everywhere else is moderate risk. CIBSE TM59 (2017) for the dynamic-modelling criteria and weather file. Checked 4 September 2026. ### Maximum glazing area — cross-ventilated (Table 1.1) | Largest glazed façade | High risk: % of floor area | High risk: most glazed room | Moderate risk: % of floor area | Moderate risk: most glazed room | | --- | --- | --- | --- | --- | | North | 15% | 37% | 18% | 37% | | East | 18% | 37% | 18% | 37% | | South | 15% | 22% | 15% | 30% | | West | 18% | 37% | 11% | 22% | ### Maximum glazing area — not cross-ventilated (Table 1.2) | Largest glazed façade | High risk: % of floor area | High risk: most glazed room | Moderate risk: % of floor area | Moderate risk: most glazed room | | --- | --- | --- | --- | --- | | North | 15% | 26% | 18% | 26% | | East | 11% | 18% | 18% | 26% | | South | 11% | 11% | 15% | 15% | | West | 11% | 18% | 11% | 11% | ### Minimum free area for removing heat (Tables 1.3 and 1.4) | Situation | High risk | Moderate risk | | --- | --- | --- | | Cross-ventilated — whole dwelling | greater of 6% of floor area or 70% of glazing area | greater of 9% of floor area or 55% of glazing area | | Cross-ventilated — each bedroom | 13% of the room's floor area | 4% of the room's floor area | | Not cross-ventilated — whole dwelling | greater of 10% of floor area or 95% of glazing area | greater of 12% of floor area or 80% of glazing area | | Not cross-ventilated — each bedroom | 13% of the room's floor area | 4% of the room's floor area | ### The other Part O numbers | Item | Value | | --- | --- | | Shading in high-risk locations | Glazing facing between north-east and north-west via south: external shutters, g-value ≤ 0.4 with light transmittance ≥ 0.7, or (south only) an overhang cutting the sun off at 50° altitude | | Night-time noise that defeats open windows | 40 dB LAeq averaged 11pm–7am, or 55 dB LAFmax more than 10 times a night, in a bedroom with the windows open → dynamic modelling | | TM59 (2017) pass criteria | Living rooms, kitchens, bedrooms: ≤ 3% of occupied hours May–September more than 1 K over the adaptive threshold; bedrooms 10pm–7am: ≤ 1% of annual hours (32 h) above 26 °C | | Weather file (TM59) | CIBSE DSY1, 2020s, high emissions, 50th percentile | | Applies to | New residential buildings only — not extensions, not material change of use. Wales: own AD O from 23 November 2022. Scotland: Standard 3.28 from 1 December 2022 | Explained at /guides/building-regulations-part-o/; assessments (simplified method or TM59) at /services/part-o-overheating/; TM59:2026 changes at /news/cibse-tm59-2026-overheating-update/. ## Part G water efficiency Source: Building Regulation 36 and Approved Document G; The Water Efficiency Calculator for New Dwellings (DCLG, 2009), Table 1 — the same constants WaterMonkey uses. Checked 4 September 2026. | Standard | Litres per person per day | When | | --- | --- | --- | | Regulation 36(2) — every new dwelling | 125 | The default for Building Control | | Optional requirement (Regulation 36(3)) | 110 | Where the local plan applies it — most water-stressed areas; the August 2026 NPPF keeps this in place | | London Plan | 105 excluding external use | The same target as 110 with the 5 l/p/d external allowance | | Proposed (Defra consultation, closed 16 December 2025) | 105, with 100 as the optional standard | Government response awaited; implementation intended during 2026 with a six-month transition | ### Table 1 use factors | Fitting | Unit | Use factor | Fixed use (l/p/d) | Effect of one unit on the final figure | | --- | --- | --- | --- | --- | | Shower (bath also present) | Flow rate, litres/minute | 4.37 | 0.00 | 3.98 l/p/d | | Shower only (no bath) | Flow rate, litres/minute | 5.60 | 0.00 | 5.10 l/p/d | | WC, dual flush — full flush | Full flush volume, litres | 1.46 | 0.00 | 1.33 l/p/d | | WC, dual flush — part flush | Part flush volume, litres | 2.96 | 0.00 | 2.69 l/p/d | | WC, single flush | Flush volume, litres | 4.42 | 0.00 | 4.02 l/p/d | | Taps, excluding kitchen and utility | Flow rate, litres/minute | 1.58 | 1.58 | 1.44 l/p/d | | Kitchen and utility sink taps | Flow rate, litres/minute | 0.44 | 10.36 | 0.40 l/p/d | | Bath (shower also present) | Capacity to overflow, litres | 0.11 | 0.00 | 0.10 l/p/d | | Bath only (no shower) | Capacity to overflow, litres | 0.50 | 0.00 | 0.46 l/p/d | | Washing machine | Litres per kg of dry load | 2.1 | 0.00 | 1.91 l/p/d | | Dishwasher | Litres per place setting | 3.6 | 0.00 | 3.28 l/p/d | | Waste disposal unit, if fitted | Present or not | 1 | 3.08 | 0.91 l/p/d | Consumption = Σ(capacity × use factor + fixed use) × 0.91 normalisation, plus 5 l/p/d external use. Consumption = Σ(capacity × use factor + fixed use) × 0.91 normalisation, plus 5 l/p/d external use. Why 105-excluding-external equals 110: /guides/110-litres-water-planning-condition/. Machine-readable factors: /data/. Calculate at /tools/part-g-water-calculator/; explained at /guides/water-calculation-building-control/ and /guides/125-litres-per-person-per-day/. ## Part E sound insulation Source: Approved Document E, Table 0.1a (dwellings) and 0.1b (rooms for residential purposes); Regulation 41 pre-completion testing. Checked 4 September 2026. | Separating element | Airborne, DnT,w + Ctr (minimum) | Impact, L'nT,w (maximum) | | --- | --- | --- | | New-build houses and flats — walls | 45 dB | — | | New-build houses and flats — floors | 45 dB | 62 dB | | Houses and flats formed by material change of use — walls | 43 dB | — | | Houses and flats formed by material change of use — floors | 43 dB | 64 dB | | Rooms for residential purposes (student halls, hotels, hostels), purpose-built — walls | 43 dB | — | | Rooms for residential purposes, purpose-built — floors | 45 dB | 62 dB | | Rooms for residential purposes, change of use — walls | 43 dB | — | | Rooms for residential purposes, change of use — floors | 43 dB | 64 dB | Higher is better for airborne, lower for impact. The conversion values are 2 dB easier because a retained structure cannot always be brought to new-build performance. Pre-completion testing is the default route; registered Robust Details are the alternative for new-build houses and flats. Service: /services/sound-testing/. ## Soakaways — BRE Digest 365 in numbers Source: BRE Digest 365 Soakaway design (2016 edition); Approved Document H. Checked 4 September 2026. | Item | Value | | --- | --- | | Infiltration test | Trial pit 0.3–1 m wide and 1–3 m long, dug to the depth the soakaway will reach; fill to ≥ 75% effective depth; time the fall 75% → 25%; three fills on the same or consecutive days; design on the slowest | | Infiltration rate | f = V(75–25) ÷ (a(p50) × t(75–25)), in m/s — below about 1×10⁻⁶ m/s (heavy clay) a soakaway is rarely practical | | Design storm | 1-in-10-year return period, checked across every duration; the critical duration governs | | Climate-change allowance | An uplift on design rainfall; 40% is what most local authorities ask for on new drainage | | Storage available | Volume × void ratio — 30–40% for a gravel-filled pit, 90–95% for crates | | Half-drain check | Full to half-full within 24 hours: t(s50) = 0.5 S ÷ (a(s50) × f) | | Separation | At least 5 m from any building (Approved Document H) | Not the septic-tank drainage-field test: that one uses a 300 mm square hole and gives a Vp in seconds per millimetre (a drainage field needs Vp 12–100) — the difference is explained at /tools/soakaway-calculator/percolation-test/. Calculate at /tools/soakaway-calculator/; the method at /tools/soakaway-calculator/bre-digest-365/. ## Junction Ψ-values (W/m·K) — SAP’s default and the notional dwelling Source: SAP 10.3 specification (12-06-2026) Table K1 — the default Ψ a junction gets when no calculated value is available (with no junction information at all SAP uses y = 0.2 W/m²K) — and Table R2, the Ψ the notional (target) dwelling assumes. Checked 4 September 2026. Energycount does not publish a calculated Ψ per junction as a lookup table. A Ψ-value is only valid for the detail actually drawn — block density, insulation thickness, lintel type and the position of the floor insulation all move it — so the correct figure has to be modelled per junction, to BS EN ISO 10211. That is free to do at https://energycount.co.uk/tools/psi-calculator/app/?junction= (ΨMonkey; Building Control PDF £25 +VAT a junction). Worked examples showing how far the same junction moves between wall build-ups are at https://energycount.co.uk/tools/psi-calculator/psi-values/, and the full Table K1 vs Table R2 comparison for all 44 references is at https://energycount.co.uk/reference/sap-junctions/. | Junction | SAP default (Table K1) | Notional (Table R2) | Calculate your own | | --- | --- | --- | --- | | E1 Lintel — steel (window head) | 1.000 | 0.050 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E1 | | E2 Lintel — insulated, no base plate (window head) | 1.000 | 0.050 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E2 | | E3 Cill (window sill) | 0.100 | 0.050 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E3 | | E4 Jamb (window reveal) | 0.100 | 0.050 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E4 | | E5 Ground floor (suspended beam & block) | 0.320 | 0.160 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E5 | | E5s Ground floor (solid ground-bearing slab) | 0.320 | 0.160 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E5s | | E6 Intermediate floor (within dwelling) | 0.140 | 0.000 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E6 | | E10 Eaves (insulation at ceiling level) | 0.120 | 0.060 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E10 | | E11 Eaves (insulation at rafter level) | 0.150 | 0.040 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E11 | | E16 Corner (normal) | 0.180 | 0.090 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E16 | | E18 Party wall / external wall | 0.240 | 0.060 | https://energycount.co.uk/tools/psi-calculator/app/?junction=E18 | ## Nutrient neutrality — the 27 Natural England catchments Source: Natural England’s nutrient neutrality advice and catchment calculators (NECR459 methodology, 20% precautionary buffer). Checked 4 September 2026. | Catchment | Designated site | Nutrient | Region | Page | | --- | --- | --- | --- | --- | | Poole Harbour | Poole Harbour SPA / Ramsar | Nitrogen | Dorset | https://energycount.co.uk/tools/nutrient-neutrality-calculator/poole-harbour/ | | Solent | Solent (incl. Chichester & Portsmouth Harbours) | Nitrogen | Hampshire, West Sussex & Isle of Wight | https://energycount.co.uk/tools/nutrient-neutrality-calculator/solent/ | | Tees (Teesmouth) | Teesmouth & Cleveland Coast SPA / Ramsar | Nitrogen | Teesside, Durham & North Yorkshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/tees/ | | Lindisfarne | Lindisfarne SPA / Ramsar | Nitrogen | Northumberland | https://energycount.co.uk/tools/nutrient-neutrality-calculator/lindisfarne/ | | Stodmarsh | Stodmarsh SAC / SPA / Ramsar | Nitrogen and phosphorus | Kent | https://energycount.co.uk/tools/nutrient-neutrality-calculator/stodmarsh/ | | The Broads | The Broads SAC | Nitrogen and phosphorus | Norfolk & Suffolk | https://energycount.co.uk/tools/nutrient-neutrality-calculator/the-broads/ | | River Wensum | River Wensum SAC | Nitrogen and phosphorus | Norfolk | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-wensum/ | | Chesil & the Fleet | Chesil & the Fleet SAC / SPA / Ramsar | Nitrogen and phosphorus | Dorset | https://energycount.co.uk/tools/nutrient-neutrality-calculator/chesil-and-the-fleet/ | | Hornsea Mere | Hornsea Mere SPA | Nitrogen and phosphorus | East Yorkshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/hornsea-mere/ | | Oak Mere | Oak Mere SAC | Nitrogen and phosphorus | Cheshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/oak-mere/ | | Rostherne Mere | Rostherne Mere Ramsar | Nitrogen and phosphorus | Cheshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/rostherne-mere/ | | West Midlands Mosses | West Midlands Mosses SAC | Nitrogen and phosphorus | Shropshire, Staffordshire & Cheshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/west-midlands-mosses/ | | River Clun | River Clun SAC | Nitrogen and phosphorus | Shropshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-clun/ | | River Avon (Christchurch) | River Avon SAC (→ Christchurch Harbour) | Phosphorus | Wiltshire, Hampshire & Dorset | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-avon/ | | Somerset Levels & Moors | Somerset Levels & Moors SPA / Ramsar | Phosphorus | Somerset | https://energycount.co.uk/tools/nutrient-neutrality-calculator/somerset-levels/ | | River Itchen | River Itchen SAC | Nitrogen and phosphorus | Hampshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-itchen/ | | River Axe | River Axe SAC | Phosphorus | Somerset, Devon & Dorset | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-axe/ | | River Camel | River Camel SAC | Phosphorus | Cornwall | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-camel/ | | Derwent & Bassenthwaite | River Derwent & Bassenthwaite Lake SAC | Phosphorus | Cumbria | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-derwent-bassenthwaite/ | | River Eden | River Eden SAC | Phosphorus | Cumbria | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-eden/ | | River Kent | River Kent SAC | Phosphorus | Cumbria | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-kent/ | | River Lambourn | River Lambourn SAC | Phosphorus | Berkshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-lambourn/ | | River Lugg | River Lugg SAC (River Wye system) | Phosphorus | Herefordshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-lugg/ | | River Mease | River Mease SAC | Phosphorus | Leicestershire, Derbyshire & Staffordshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/river-mease/ | | Esthwaite Water | Esthwaite Water Ramsar | Phosphorus | Cumbria | https://energycount.co.uk/tools/nutrient-neutrality-calculator/esthwaite-water/ | | Peak District Dales | Peak District Dales SAC | Phosphorus | Derbyshire & Staffordshire | https://energycount.co.uk/tools/nutrient-neutrality-calculator/peak-district-dales/ | | Roman Walls Loughs | Roman Walls Loughs SAC | Phosphorus | Northumberland | https://energycount.co.uk/tools/nutrient-neutrality-calculator/roman-walls-loughs/ | Calculate a nutrient budget free at https://energycount.co.uk/tools/nutrient-neutrality-calculator/ (planning-ready PDF £79 +VAT). Machine-readable list: https://energycount.co.uk/data/. ## Regs calendar Source: the instruments, circulars and consultations linked in each row. Checked 4 September 2026. | Date | What | More | | --- | --- | --- | | 24 March 2026 | SAP 10.3 becomes the sole approved SAP methodology; Approved Documents L, F and the 2026 suite published | https://energycount.co.uk/guides/future-homes-standard/ | | July 2026 | CIBSE TM59:2026 published — Approved Document O still cites the 2017 edition | https://energycount.co.uk/news/cibse-tm59-2026-overheating-update/ | | 6 August 2026 | RdSAP Conventions v12.2 apply | https://energycount.co.uk/news/rdsap-conventions-v12-2-6-august-2026/ | | 6 August 2026 | Biodiversity Net Gain: 0.2 ha small-site exemption and other changes, for applications from this date | https://energycount.co.uk/news/biodiversity-net-gain-exemptions-6-august-2026/ | | 27 August 2026 | GPDO Amendment Order 2026 — rewritten permitted development rights for domestic and plug-in solar | https://energycount.co.uk/news/gpdo-solar-permitted-development-27-august-2026/ | | 30 September 2026 | Second staircase requirement (Approved Document B 2026 amendments) in force for residential buildings 18 m and above | https://www.gov.uk/government/publications/fire-safety-approved-document-b | | 1 October 2026 | Building Safety Levy starts — collected via Building Control, completion certificate withheld until paid | https://energycount.co.uk/news/building-safety-levy-1-october-2026/ | | 5 October 2026 | Consultation closes: planning powers for mayors in England | https://energycount.co.uk/news/mayoral-planning-powers-consultation-october-2026/ | | 16 October 2026 | Consultation closes: Scotland’s Passivhaus-equivalent standard, stage 2 (implementation now autumn 2029) | https://energycount.co.uk/news/scotland-passivhaus-equivalent-stage-2-consultation/ | | 20 October 2026 | Consultation closes: national s106 templates for 10–49 home sites | https://energycount.co.uk/news/standard-s106-templates-medium-sites-consultation/ | | By end of 2026 | Mandatory water efficiency labelling for water-using products due | https://energycount.co.uk/news/mandatory-water-efficiency-labelling-2026/ | | 24 March 2027 | Future Homes Standard (Approved Document L 2026) and Approved Document F 2026 in force for non-higher-risk work; last day to notify under Part L 2021 | https://energycount.co.uk/guides/building-regulations-part-l/ | | 24 September 2027 | Future Homes Standard in force for higher-risk buildings (Gateway 2 applications before this date complete under Part L 2021) | https://energycount.co.uk/guides/future-homes-standard/ | | 24 March 2028 | Transitional deadline: buildings notified before 24 March 2027 must have commenced (foundations and ground-floor structure) to keep Part L 2021 | https://energycount.co.uk/news/future-homes-standard-transitional-deadline-commencement/ | | 30 April 2028 | Scotland: new-style EPCs required for marketing and letting (an existing EPC under ten years old is accepted for a first let until 30 April 2029) | https://energycount.co.uk/news/scottish-epc-reform-delayed-april-2028/ | ## OPEN DATASETS — REFERENCE TABLES PUBLISHED IN FULL Six datasets, each free, each with the source document named and dated, each downloadable as CSV and JSON from https://energycount.co.uk/data/. Cite as: Energycount, , . --- # RdSAP 10 age bands and default U-values URL: https://energycount.co.uk/reference/rdsap-age-bands/ Source: RdSAP10 Specification, BRE, 7 June 2026 (V4.0) (https://bregroup.com/documents/d/bre-group/rdsap-10-specification-2026-06-07). Every U-value table in the specification reproduced in full with attribution - Tables 1, 3, 4, 6-26; the specification is the authority for a lodged assessment. Note: Energycount does not carry out RdSAP assessments or EPCs on existing homes — this is published as a reference. Table 1, age bands by country. An age band fixes the default U-value RdSAP assigns to an element that cannot be evidenced. Isle of Man uses the England and Wales bands. "not applicable" is the specification’s own wording. | Band | England | Wales | Scotland | Northern Ireland | Park home (UK) | | --- | --- | --- | --- | --- | --- | | A | before 1900 | before 1900 | before 1919 | before 1919 | - | | B | 1900-1929 | 1900-1929 | 1919-1929 | 1919-1929 | - | | C | 1930-1949 | 1930-1949 | 1930-1949 | 1930-1949 | - | | D | 1950-1966 | 1950-1966 | 1950-1964 | 1950-1973 | - | | E | 1967-1975 | 1967-1975 | 1965-1975 | 1974-1977 | - | | F | 1976-1982 | 1976-1982 | 1976-1983 | 1978-1985 | before 1983 | | G | 1983-1990 | 1983-1990 | 1984-1991 | 1986-1991 | 1983-1995 | | H | 1991-1995 | 1991-1995 | 1992-1998 | 1992-1999 | not applicable | | I | 1996-2002 | 1996-2002 | 1999-2002 | 2000-2006 | 1996-2005 | | J | 2003-2006 | 2003-2006 | 2003-2007 | not applicable | not applicable | | K | 2007-2011 | 2007-2011 | 2008-2011 | 2007-2013 | 2006 onwards | | L | 2012-2022 | 2012-2022 | 2012 - 2023 | 2014 -2022 | not applicable | | M | 2023 onwards | 2023 onwards | 2024 onwards | 2023 onwards | not applicable | Tables 6-10, wall U-values: 145 wall-type rows across England, Wales, Scotland, Northern Ireland and the Isle of Man, each with a value for all 13 bands — 1,885 cells. Stone and solid brick in the early bands carry a cross-reference to sections 5.6, 5.7 and 5.8 rather than a default, because RdSAP calculates those from the measured wall thickness. Table 18 gives assumed roof U-values by band for seven roof types. Tables 15, 16, 17, 19, 20 and 21 give the remaining element defaults: party walls; roofs and rooms in roof indexed by MEASURED loft insulation thickness (which override the age-band table when a thickness is known); the assumed construction and insulation that feed the ISO 13370 ground-floor calculation; exposed and semi-exposed floors; and the whole-dwelling thermal bridging y-factor. Full tables: https://energycount.co.uk/data/rdsap-wall-u-values.csv (and .json), https://energycount.co.uk/data/rdsap-age-bands.csv, https://energycount.co.uk/data/rdsap-roof-u-values.csv, https://energycount.co.uk/data/rdsap-element-defaults.csv, https://energycount.co.uk/data/rdsap-lookup-tables.csv --- # Thermal conductivity (lambda) library URL: https://energycount.co.uk/reference/thermal-conductivity/ 494 declared thermal conductivity values in W/m·K: 59 generic construction materials and 435 named UK products. Every row carries the document it was read from, what kind of source that is (a maker’s declaration, a BBA certificate, a retailer listing, a paywalled standard) and the date it was retrieved. Rows whose only source is a paywalled standard are flagged as such rather than passed off as confirmed. The generic materials, in full: | Material | Category | lambda (W/m·K) | Source type | | --- | --- | --- | --- | | Aircrete block (standard) | masonry | 0.15 | paywalled standard — unconfirmed | | Aircrete block (thermal grade) | masonry | 0.11 | paywalled standard — unconfirmed | | Aluminium (typical alloy) | metal | 160 | confirmed | | Anhydrite / calcium sulphate flow screed | screed | 1.8 | generic | | Bituminous felt / membrane | membrane | 0.23 | paywalled standard — unconfirmed | | Bituminous felt underlay (type 1F) | membrane | 0.23 | paywalled standard — unconfirmed | | Blown mineral wool (cavity fill) | insulation | 0.04 | generic | | Breather membrane | membrane | 0.4 | paywalled standard — unconfirmed | | Brickwork (inner leaf) | masonry | 0.56 | confirmed | | Brickwork (outer leaf) | masonry | 0.77 | confirmed | | Brickwork (protected, behind EWI or cladding) | masonry | 0.56 | confirmed | | Cast concrete (dense) | concrete | 1.4 | paywalled standard — unconfirmed | | Cellulose (blown) | insulation | 0.039 | generic | | Cement particle board | board | 0.23 | paywalled standard — unconfirmed | | Cement/sand render (external) | finish | 0.87 | paywalled standard — unconfirmed | | Clay roof tile | covering | 0.84 | paywalled standard — unconfirmed | | Concrete block (dense aggregate) | masonry | 1.13 | paywalled standard — unconfirmed | | Concrete block (medium dense) | masonry | 0.51 | paywalled standard — unconfirmed | | Concrete roof tile | covering | 1.1 | paywalled standard — unconfirmed | | Engineering brick (class A/B) | masonry | 1.17 | paywalled standard — unconfirmed | | EPS (expanded polystyrene) | insulation | 0.038 | paywalled standard — unconfirmed | | EPS (grey / graphite) | insulation | 0.031 | generic | | EPS bead, bonded (cavity fill) | insulation | 0.04 | generic | | EPS bead, graphite (cavity fill) | insulation | 0.033 | generic | | Fibre cement cladding | cladding | 0.35 | paywalled standard — unconfirmed | | Glass wool quilt | insulation | 0.04 | generic | | Gypsum plaster (dense) | finish | 0.5 | paywalled standard — unconfirmed | | Gypsum plaster (lightweight) | finish | 0.16 | paywalled standard — unconfirmed | | Hardwood timber | timber | 0.18 | confirmed | | Lightweight/aircrete screed | screed | 0.24 | paywalled standard — unconfirmed | | Lime plaster | finish | 0.7 | paywalled standard — unconfirmed | | Limestone | masonry | 1.4 | confirmed | | Mild / galvanised steel | metal | 50 | confirmed | | Mineral wool quilt | insulation | 0.04 | generic | | Mineral wool slab | insulation | 0.038 | generic | | Mortar (inner leaf) | masonry | 0.88 | confirmed | | Mortar (outer leaf) | masonry | 0.94 | confirmed | | Natural slate | covering | 2 | paywalled standard — unconfirmed | | OSB (oriented strand board) | board | 0.13 | confirmed | | Particleboard / fibreboard | board | 0.14 | confirmed | | Phenolic board (generic) | insulation | 0.02 | generic | | PIR board (generic) | insulation | 0.022 | generic | | Plaster dabs | finish | 0.43 | confirmed | | Plasterboard (high density, ≤900 kg/m³) | board | 0.25 | confirmed | | Plasterboard (standard, ≤700 kg/m³) | board | 0.21 | confirmed | | Plasterboard, foil-backed (vapour check) | board | 0.21 | confirmed | | Plywood | board | 0.13 | confirmed | | Polythene VCL / vapour barrier | membrane | 0.33 | paywalled standard — unconfirmed | | Prefabricated timber frame panel | timber | 0.12 | confirmed | | Reconstituted (artificial) stone | masonry | 1.3 | confirmed | | Reinforced concrete (1% steel) | concrete | 2.3 | confirmed | | Sand/cement screed (1800 kg/m³) | screed | 1.15 | confirmed | | Sandstone / granite | masonry | 2.3 | confirmed | | Sheep wool | insulation | 0.039 | generic | | Softwood timber (joists/frame) | timber | 0.13 | confirmed | | Stainless steel | metal | 17 | confirmed | | Timber cladding (softwood) | cladding | 0.13 | confirmed | | Wood fibre board | insulation | 0.04 | generic | | XPS (extruded polystyrene) | insulation | 0.034 | paywalled standard — unconfirmed | All 494 rows including the named products: https://energycount.co.uk/data/thermal-conductivity.csv (and .json) --- # UK Government GHG conversion factors 2026 — building and energy subset URL: https://energycount.co.uk/reference/carbon-factors/ 913 of the 8,740 published rows: the fuels, electricity, heat, water, waste, materials and refrigerants a building project uses, without the vehicles and freight. IMPORTANT: these are NOT the SAP emission factors. SAP has its own, fixed for the life of a SAP version and used to demonstrate Part L compliance; these are updated annually by DESNZ and Defra and are used for SECR, energy statements and whole-life carbon. Mixing them is a common and expensive error. | Item | kg CO2e per unit | Unit | | --- | --- | --- | | Electricity: UK — generation | 0.13096 | kWh | | Electricity: UK — transmission and distribution | 0.01299 | kWh | | Natural gas (gross CV) | 0.18231 | kWh | | Water supply | 0.1913 | m³ | Contains public sector information licensed under the Open Government Licence v3.0. All 913 factors: https://energycount.co.uk/data/carbon-factors.csv (and .json) --- # Wastewater treatment works permit register URL: https://energycount.co.uk/reference/wastewater-permits/ 675 treatment works across 22 nutrient neutrality catchments, with their permitted nutrient concentrations — current, post-2025 and post-2030. The works’ permitted concentration is what converts a scheme’s waste-water flow into a nutrient load, so it is the number a nutrient budget turns on, and where a permit tightens in April 2030 the mitigation has to cover the higher of the two figures. Source: Natural England catchment calculators. Natural England’s calculators remain the statutory tool and a permit can be varied at any time. Full register: https://energycount.co.uk/data/wwtw-permits.csv (and .json) --- # Water stressed areas and the 110 l/p/d optional standard URL: https://energycount.co.uk/reference/water-stressed-areas/ The Environment Agency’s classification of all 23 water company areas in England, 15 of which are classified as seriously water stressed. The point it settles: the tighter 110 litres per person per day standard under Building Regulation 36(3) is engaged by a condition on the planning permission, not by a Local Plan policy and not by the postcode. Water stress makes the condition likely; the decision notice is what makes it binding. There is no register of councils, because a council is not what triggers it. ## GUIDES --- # Asked for a water efficiency certificate? Here is what actually exists URL: https://energycount.co.uk/guides/water-calculation-building-control/ Updated: 2026-10-08 ## Is there such a thing as a water efficiency certificate? Not in the Building Regulations, no. Nothing in them requires or defines a *water efficiency certificate*, and no body is appointed to issue one. It is a piece of trade shorthand that has stuck — and because it has stuck, it is what most people search for and what most services sell. What the law actually requires is a **notice**. Regulation 37 of the Building Regulations 2010 says the person carrying out the work must give the building control authority a notice specifying: - **which requirement applies** — in England the 125 litres per person per day of regulation 36(2)(a), or the optional 110 l/p/d of regulation 36(2)(b); in Wales 110 l/p/d for a new build or 125 for a conversion, under the Welsh version of regulation 36; and - **the potential consumption of wholesome water** per person per day for the completed dwelling. Two numbers, in other words: the target and the result. Everything else — the fittings schedule, the arithmetic, the letterhead — exists to show that the second number is right. So when somebody asks for a certificate, they are asking for the document that carries the calculation behind that notice. Nobody is being loose or misleading by calling it one; it just helps to know that the thing with legal force is the notice, and that no particular format is mandated for what sits behind it. One wrinkle worth knowing: the notice goes to the **building control authority**, not the local authority. That wording changed on 1 October 2023 with the Building Safety Act reforms, which also added the higher-risk-building route below. A lot of guidance still in circulation — ours included, until recently — carries the pre-2023 text. ## What the document has to contain There is no prescribed form, which is why submissions vary so much. What every building control body is looking for is the same short set of facts: - **The dwelling it applies to** — plot or house type, and which plots that type covers. - **The fittings schedule** — each fitting, the figure entered for it, and its unit: flow rates in litres per minute, flush volumes and bath capacity in litres, white goods in litres per kilogram and litres per place setting. - **The calculated figure** in litres per person per day, after the normalisation factor and the fixed external-use allowance have been applied. - **The target it is measured against** — 125, or 110 where planning says so (in Wales, 110 for a new build) — and a clear pass. - **The date and who produced it.** That is the whole document. On a site with several house types you need one per type, and one type covers every identical plot. ## Which dwellings need one New dwellings in England and Wales: new builds, barn conversions, flats formed by splitting an existing building. An extension to an existing dwelling does not need one. Most non-domestic buildings do not either — Part G handles those separately. If it is a dwelling and it did not exist before, assume you need the calculation. Sensible modern fittings usually pass comfortably — see [how the 125 l/p/d figure works](/guides/125-litres-per-person-per-day/), or [110 where a planning condition applies](/guides/110-litres-water-planning-condition/). In Wales the requirement is 110 l/p/d for a new build and 125 for a conversion. ## When it is due Regulation 37 sets the backstop: the notice must reach the building control authority **no later than five days after the work has been completed**. Where the dwelling is within a higher-risk building — at least 18 metres tall or at least seven storeys, and containing at least two residential units — the notice must instead accompany the application for a completion certificate. Almost nobody should be working to either deadline, because the calculation depends on sanitaryware that gets specified and ordered months earlier. Most building control bodies ask for it with the plans or at first fix, and the sensible time to run it is **before the bathroom order goes in** — when a shower head is still a decision rather than a delivery. ## How the calculation actually works Each fitting is scored as **capacity × use factor + fixed use**. The use factors are published in Table 1 of the methodology and never change. The lines are added up, the total is multiplied by a **0.91 normalisation factor**, and a fixed **5 litres per person per day** of external use is added on the end. An ordinary specification — a 6/4 dual-flush WC, 6 l/min basin taps, a 180 litre bath with a 10 l/min shower over it, an 8 l/min kitchen tap and unspecified white goods — lands at **123.9 l/p/d**. Against a 125 target that is a pass with **1.1 litres to spare**. That margin is the thing worth understanding. A perfectly normal specification passes by barely a litre, so a late change — a bigger bath, a pumped shower, a waste disposal unit dropped into the kitchen design — can quietly turn a compliant scheme into a non-compliant one. The full arithmetic is set out in our [no-spreadsheet walkthrough](/tools/part-g-water-calculator/water-efficiency-calculator-spreadsheet/), fitting by fitting. ## Why calculations get sent back In rough order of how often it happens: 1. **The figures don't match the fittings.** A flow rate quoted from a brochure for a different pressure, or a spec that changed after the calculation was run. 2. **Multiple fittings averaged flat.** Where several showers or taps of different flow rates are fitted, the methodology takes the higher of the arithmetic average and **70% of the highest-flow fitting** — so one powerful master shower cannot be averaged away by a modest en-suite. 3. **The two adjustments missing.** Leaving out the 0.91 normalisation or the 5 l/p/d external allowance changes the answer by more than most specification changes do. 4. **The wrong target.** A planning condition required 110 and the calculation was run against 125. Check the decision notice, not the assumption — and if you want to know whether your area is likely to impose it, the [Environment Agency's water stressed areas classification](/reference/water-stressed-areas/) lists all 23 water company areas. 5. **No plot schedule.** A calculation for "House Type B" with nothing saying which plots are House Type B. ## What it costs Online services and assessors advertise roughly **£20 to £95** for a single dwelling. The headline price is rarely the number that matters, though — what matters is **what one purchase covers**. Some are priced per dwelling, in which case a 40-plot site of one house type is forty purchases; others per dwelling type, where the same site is one. Check which before you buy. On a repeated house type it is the difference between one fee and forty, for identical arithmetic. ## Do it yourself, free Our [**WaterMonkey water efficiency calculator**](/tools/part-g-water-calculator/) runs the full official methodology in your browser. Enter your fittings, watch the PASS/FAIL update live, and adjust until you're under target — all free. When you're happy, the Building Control-ready PDF is **£25 +VAT**, and that covers **every identical plot** on your site, not one plot. You only pay when it's right. What you'll need to hand: the flow rates (litres/minute) of your taps and showers, bath capacity to overflow, WC flush volumes, and your dishwasher and washing machine figures if they're being specified. All of it is on the product spec sheets or the retailer's listing. If you want to understand one line before you change it, start with [shower flow rate](/tools/part-g-water-calculator/shower-flow-rate/) — it's usually the biggest number on the page. ## Rather hand it off? If you'd prefer it done for you — or your case is complicated by rainwater harvesting, greywater reuse or a tight planning condition — our [Part G service](/services/part-g-water-calculations/) starts from a simple email with your sanitaryware list. Either way, the calculation shouldn't hold up your sign-off. --- # 125 litres per person per day, explained URL: https://energycount.co.uk/guides/125-litres-per-person-per-day/ Updated: 2026-10-08 ## Where the number comes from In England, Part G of the Building Regulations caps a new dwelling's *calculated* water use at **125 litres per person per day** (Wales sets 110 for a new build — see below). Nobody meters your actual usage — the figure comes from the official methodology, which multiplies each fitting's consumption by a standardised "use factor" (how often a typical person uses it daily) and adds a fixed 5 litres for outdoor use. So a 9 litres/minute shower, a 180-litre bath, a 4/2.6-litre dual-flush WC and so on each contribute their share, and the shares must total ≤125. (Some areas use the tighter [optional 110 l/p/d standard](/guides/110-litres-water-planning-condition/) — check your planning conditions.) ## Which standard applies to you There are three numbers in circulation in England, and only one of them applies to your project today. Wales works differently — see the last point. - **125 l/p/d** — the Building Regulations baseline for every new dwelling in England. This is your target unless something says otherwise. - **110 l/p/d** — the optional technical standard. It only applies where a **Local Plan policy or planning condition** imposes it, which is common across much of the south and east of England and standard practice in London. It is a planning route into a Building Regulations requirement, so the trigger is your decision notice, not your postcode — though the [water stressed areas classification](/reference/water-stressed-areas/) tells you which 15 of the 23 water company areas make it likely. - **105 l/p/d** — **proposed, not in force.** Defra has consulted on cutting the baseline from 125 to 105 and the optional standard from 110 to 100. The consultation closed in December 2025 and the government response is still awaited; implementation is intended during 2026 with a six-month transitional period. We're tracking it in [Part G is heading for 105 litres](/news/part-g-105-litres-water-efficiency/). - **Wales: 110 l/p/d for a new build.** The Welsh version of regulation 36 sets **110 l/p/d** where a dwelling is erected — the requirement itself, not an optional standard — and **125 l/p/d** where it is formed by a material change of use, such as a barn conversion. No planning condition is involved, and a specification drawn up for an English site will not always pass there. One thing not to assume: that a single national number is coming. The [August 2026 NPPF kept local water efficiency standards on the table](/news/nppf-2026-local-water-efficiency-standards/), including the possibility of something stricter than 110 in areas of serious water stress. For now, read the decision notice. ## Where the litres actually go The big three are **showers, WCs and baths** — together usually about two-thirds of the total. Taps and white goods matter less than people expect. Here is what an entirely ordinary specification looks like: a 6/4 dual-flush WC, 6 l/min basin taps, a 180 litre bath with a 10 l/min shower over it, an 8 l/min kitchen tap, and unspecified white goods. - Shower — **43.70** l/p/d before the final adjustments - WC, full and part flush together — **20.60** - Bath — **19.80** - Washing machine (default figure) — **17.16** - Kitchen taps — **13.88** - Basin taps — **11.06** - Dishwasher (default figure) — **4.50** That totals 130.70. Multiply by the 0.91 normalisation factor and add the fixed 5 litres of external use, and the answer is **123.9 l/p/d** — a pass, with 1.1 litres to spare. **That margin is the point of this page.** A normal specification passes by about a litre. Which is why a late change to the sanitaryware can turn a compliant scheme into a non-compliant one, and why running the numbers before the bathroom order goes in is worth ten minutes of anybody's time. ## Practical rules of thumb - **The shower is the lever.** Every litre per minute is worth roughly 4 l/p/d on the final figure. Dropping from 12 to 9 l/min usually fixes a fail on its own. See [shower flow rate](/tools/part-g-water-calculator/shower-flow-rate/). - **The part flush is worth twice the full flush.** The methodology assumes two part flushes for every full one, so a litre off the part flush is worth 2.69 l/p/d against 1.33 for the full. And a dual-flush WC beats a 6 litre single flush by about 5.4 l/p/d for no design compromise at all — see [WC flush volume](/tools/part-g-water-calculator/wc-flush-volume/). - **Basin taps repay more than kitchen taps.** A litre per minute off the basins is worth 1.44 l/p/d; the same litre at the kitchen sink is worth 0.40, because most of the kitchen line is a fixed allowance you can't design out. See [tap flow rates](/tools/part-g-water-calculator/tap-flow-rate/). - **A big bath is rarely fatal — unless there's no shower.** With a shower present the bath factor is only 0.11, so 10 litres of capacity is worth 1 l/p/d. Take the shower away and the factor jumps to 0.50, and the bath alone can eat two thirds of the allowance. See [bath capacity](/tools/part-g-water-calculator/bath-capacity/). - **Unspecified white goods cost you.** Leave them out and the methodology charges defaults of 8.17 l/kg and 1.25 l/place setting — about 19.7 l/p/d between them. Real specified appliances usually beat that. See [washing machines and dishwashers](/tools/part-g-water-calculator/washing-machine-and-dishwasher/). - **A waste disposal unit is a flat 2.80 l/p/d penalty** with nothing to offset it. Worth knowing before one appears late in the kitchen design. - **Offsetting is allowed.** Splash out on the shower and claw it back on WCs and taps. Rainwater or greywater systems are credited too, though they're rarely needed just to hit 125. ## See your own numbers The fastest way to understand it is to try it: our free [**WaterMonkey water efficiency calculator**](/tools/part-g-water-calculator/) shows a live breakdown of where your litres go and a PASS/FAIL that updates as you change fittings. Experiment freely; the [Building Control-ready report](/guides/water-calculation-building-control/) is £25 +VAT when you're done. Or [we'll do it for you](/services/part-g-water-calculations/). --- # The 110 litres per person per day planning condition URL: https://energycount.co.uk/guides/110-litres-water-planning-condition/ Updated: 2026-10-08 Not sure whether the condition is on your permission at all? [How the optional requirement is actually triggered](/reference/water-stressed-areas/) walks through the decision notice, and carries the Environment Agency's water stressed areas classification — the evidence councils rely on when they set a 110 policy. ## Why your condition says 110, not 125 In England, the Building Regulations' default water limit for new dwellings is [125 litres per person per day](/guides/125-litres-per-person-per-day/), but councils in water-stressed areas — much of the south and east of England — can require the **optional standard of 110 l/p/d** through a planning condition. If your decision notice mentions water efficiency, regulation 36(2)(b), or "110 litres", this is what it means. Building Control will then expect your [water calculation](/guides/water-calculation-building-control/) to demonstrate 110, not 125. **Building in Wales?** None of the condition business applies there. In Wales, 110 l/p/d is already the Building Regulations requirement for every newly built dwelling (125 for one formed by a conversion), whatever the decision notice says. It is worth being clear about the mechanism, because it catches people out: the tighter number arrives through **planning**, but it is discharged through **Building Regulations**. Nothing in Approved Document G changes automatically because of where you are building. The trigger is the condition on your permission, not your postcode — so read the decision notice rather than assuming. ## If you're building in London, your policy says 105 London Plan **Policy SI 5** requires residential development to achieve mains water consumption of **105 litres or less per person per day, excluding an allowance of up to five litres for external water consumption**. That looks like a third standard. It isn't. The Part G calculation adds a **fixed 5 l/p/d of external use at the very end**, after the normalisation factor. So: - **105 l/p/d excluding external use** (how London writes it), and - **110 l/p/d including external use** (how the Building Regulations optional standard writes it) are the same target, measured the same way. Run your calculation against 110 in the normal way and a London scheme is satisfied. The only thing to watch is that your submission makes clear which convention the figure on the front page uses, because a "105" and a "110" sitting in the same planning file have caused more than one unnecessary round of correspondence. The proposed national cut to **105 l/p/d** currently under consideration by Defra is a different thing again — that one is 105 *including* external use, and it isn't in force. See [Part G is heading for 105 litres](/news/part-g-105-litres-water-efficiency/) for where that stands. ## Is 110 hard to hit? Harder than 125 — the 15-litre haircut removes most of the slack — but very achievable with deliberate fitting choices rather than exotic kit. Start from the ordinary specification that lands at **123.9 l/p/d**: a 6/4 dual-flush WC, 6 l/min basin taps, a 180 litre bath with a 10 l/min shower, an 8 l/min kitchen tap and unspecified white goods. You need about **14 litres**. Here is one route: - **Shower 10 → 8 l/min** — saves **7.95 l/p/d**. The single biggest lever in the calculation. - **WC 6/4 → 4.5/3** — saves **4.69 l/p/d**. Most of it comes from the part flush, which the methodology weights twice as heavily as the full flush. - **Basin taps 6 → 5 l/min** — saves **1.44 l/p/d**. That is about **14 l/p/d** of savings. Run properly through the methodology the dwelling lands at **109.9 l/p/d** — a pass at 110, from three unremarkable specification decisions and no change to the layout. Other things that help, roughly in order of value: - **Specify the white goods.** Left blank, the calculation charges defaults of 8.17 l/kg and 1.25 l/place setting. Real appliances usually beat both — worth around 5 l/p/d where they're genuinely part of the handover. - **Aerated basin taps at 4 l/min** rather than 5, if the pressure suits. - **A standard-size bath** rather than a statement tub — though with a shower present this is worth only about 1 l/p/d per 10 litres of capacity, so it's a smaller lever than people assume. - **Don't fit a waste disposal unit.** It's a flat 2.80 l/p/d penalty with nothing to offset it. Where a client insists on the big shower or the big bath, the offsets have to come from everywhere else — or from **rainwater or greywater harvesting**, which the methodology credits and which sometimes turns an impossible spec into a pass. Both are capped at the demand they can actually displace, and the same WC demand can't be claimed twice across two systems. ## Check before you order sanitaryware The expensive mistake is buying fittings first and calculating second. Run your intended spec through our free [**WaterMonkey calculator**](/tools/part-g-water-calculator/) — set the target to 110, watch the live PASS/FAIL, and adjust before anything's ordered. There's a [110 l/p/d version of the calculator](/tools/part-g-water-calculator/110-litres/) already set to the tighter target, and fitting-by-fitting explainers for [showers](/tools/part-g-water-calculator/shower-flow-rate/), [WCs](/tools/part-g-water-calculator/wc-flush-volume/) and [taps](/tools/part-g-water-calculator/tap-flow-rate/) if you want to understand a line before you change it. The Building Control-ready report is £25 +VAT when you need it. Tricky condition wording or a marginal spec? [Send it to us](/services/part-g-water-calculations/) and we'll sort it. --- # Asked for a condensation risk analysis? Here is what it has to show URL: https://energycount.co.uk/guides/condensation-risk-analysis-building-control/ Updated: 2026-09-27 ## What building control is actually asking for Requirement C2 of the Building Regulations in England and Wales deals with resistance to moisture. It says the walls, floors and roof of a building "shall adequately protect the building and people who use the building from harmful effects caused by" four things, and one of them is **interstitial and surface condensation**. Interstitial condensation is the kind that forms *inside* a construction, between its layers, where warm indoor air working its way out meets a layer colder than its dew point — wetting insulation, corroding fixings and rotting timber where nobody can see it. Surface condensation is the kind you can see, and the mould that follows it. Approved Document C, the government guidance on meeting C2, points to two standards for condensation: **BS 5250** and **BS EN ISO 13788**. It still cites their 2002 editions, because the Approved Document itself was last fully revised in 2004; BS 5250 has since moved on to the 2021 edition, now called *Management of moisture in buildings*. A condensation risk analysis is the calculation that shows a build-up meets C2 on condensation — in practice, BS EN ISO 13788 run on your build-up and read against BS 5250. In Scotland the equivalent is standard 3.15 of the building standards technical handbooks: no threat to the building, or to the health of its occupants, from moisture caused by surface or interstitial condensation. ## When they ask for one Not for every build-up. BS 5250 sets out arrangements that experience shows are safe — a warm flat roof with its vapour control layer under the insulation, a cold loft with a sealed ceiling and ventilated eaves — and the 2021 edition suggests that for the simplest, lowest-risk constructions a condensation risk analysis may not be necessary. The request comes when a design steps outside those arrangements. For example: - **A flat roof with a twist** — a cold deck, or a hybrid with insulation both above and below the deck. Split the insulation of a flat roof and leave the vapour control layer on the deck, and U-Monkey's check puts condensation under the deck every winter; the [flat roof insulation thickness](/tools/u-value-calculator/flat-roof-insulation-thickness/) page runs exactly that case. - **Insulation on the inside of a solid wall**, which turns the old masonry cold. - **Insulation along the rafters**, where the underlay and the vapour control layer decide whether moisture can get out. - **An unfamiliar layer order**, or a membrane whose vapour resistance nobody on the job actually knows. Warranty providers ask for them too, often on the same build-ups. ## What the report has to show No form is prescribed, but whoever checks it will be looking for the same things: - **The build-up**, outside to inside: every layer with its thickness, its thermal conductivity and its vapour resistance (a μ-value, or an sd in metres), and where each vapour figure came from. Published vapour data varies widely for membranes, foil facings and OSB, so a declared value from the product's own datasheet carries more weight than a generic table. - **The conditions**: the external climate used — monthly data for a named weather station near the site, not a default — the internal temperature and humidity class for the building's use, and any allowance for a colder-than-average year. - **The results, month by month**: which interfaces condense, how much accumulates, and when it evaporates again. - **A conclusion in the standard's terms**: no condensation; condensation that dries out completely over the summer; or condensation that accumulates year on year. - **The surface check**: the temperature factor fRsi, which for a dwelling should be at least 0.75 if surface condensation and mould are to be avoided. - **The element and project it applies to, the date, and who produced it.** That is the whole document, and it covers one build-up: a wall and a roof are two analyses. ## Reading the result A **pass with no condensation** is the clean answer. **Seasonal condensation** — some forms in winter but all of it evaporates by the end of summer — is normally acceptable, provided the amount is too small to harm what it forms on. Timber is the thing to watch there, because decay becomes a risk as its moisture content approaches about 20%. **Condensation that accumulates year on year** is a fail, however small the first winter's amount looks. Two things are worth reading beyond the verdict. The first is the margin: a pass at 99% of saturation is a different animal from one at 80%, because a small change to a layer or a product can tip it over. The second is the vapour data: if the result hinges on an assumed figure for a membrane or a board, get the declared value and run it again. ## What a calculation can't prove BS EN ISO 13788 is a screening method: monthly, steady-state, one-dimensional, and diffusion only. It does not model air leaking through gaps, rain, solar gain, moisture built into new construction, or moisture stored in and released by materials. For most build-ups it is still the standard way to show that C2 is met, but it runs out in two places. **Cold roofs.** BS 5250:2021 says that the calculation methods — BS EN ISO 13788 and the more advanced BS EN 15026 alike — do not account for moisture carried on moving air, so they are not suitable for cold flat roofs or cold pitched roofs, and it relies on prescriptive rules instead: a sealed air and vapour control layer on the warm side, a ventilated gap and cross-ventilation. For a cold flat roof that means at least 50mm of air above the insulation and ventilation equivalent to a continuous 25mm opening on each side. BS 6229:2018, the flat roofing code of practice, notes that cold flat roofs are not now recommended, and BS 5250:2021 says not to use one where the void spans more than 5m, can't be ventilated at both ends, or can't have a sealed air and vapour control layer on the warm side. The [cold flat roof](/tools/u-value-calculator/cold-flat-roof/) page shows what that construction does to the U-value, and what the check can and can't tell you about it. **Solid walls and heritage buildings.** Insulating a solid wall on the inside changes how the masonry wets and dries — rain, stored moisture and all — which a diffusion-only calculation cannot see. For those constructions, and other high-risk ones, a full hygrothermal simulation to BS EN 15026 is the right next step. ## When it is due No regulation sets a date. The analysis is evidence that the design meets C2, so it belongs with the plans, or at the latest before the element it covers is built. The sensible time to run it is earlier still — before the insulation, membranes and boards are ordered — because the fix is almost always a change of product or layer order, which is cheap on paper and expensive on a finished roof. ## If the check fails The fix is rarely more insulation. It is usually the order of the layers: 1. **Put a sealed air and vapour control layer on the warm side** of the insulation, continuous at laps, joints and penetrations. 2. **Keep the outer layers more open to vapour than the inner ones**, so moisture that gets in can get out. 3. **In a flat roof, keep all the insulation above the deck** — a [warm deck](/tools/u-value-calculator/warm-flat-roof/) — rather than splitting it around the deck. In an [inverted roof](/tools/u-value-calculator/inverted-flat-roof/) the waterproofing itself sits on the warm side and does that job. 4. **In a cold roof, fix the ventilation first**, because no calculation will prove it for you. 5. **Replace assumed vapour figures with declared ones**, and run it again. ## What it costs - **Manufacturers' technical services** will often run U-value and condensation calculations free for build-ups that use their products. They model their own products, so check the rest of your build-up is in there too. - **Independent consultants** usually price per element. One specialist advertises £250–£350 per element for a Glaser assessment, typically in one to three working days, and from about £500 per element for a WUFI (BS EN 15026) simulation. - **U-Monkey** runs the BS EN ISO 13788 check free on every build-up. The written report is **£25 +VAT** (£30 inc VAT) per build-up, or included on Monkey Pro Unlimited. Whichever route you take, check what one purchase covers. A condensation risk analysis covers one build-up, so a job with a wall, a roof and a floor to evidence is three. ## Do it yourself, free Build the element in [U-Monkey's free condensation risk calculator](/tools/u-value-calculator/condensation-risk-calculator/) and the check runs as you go, alongside the U-value: the verdict, the worst interface, the month-by-month balance and the surface mould check, free and without signing up. Pick the weather station nearest the site and set the building's use before you trust the answer. What you'll need to hand: the build-up from your drawings, and for each product its declared thermal conductivity and vapour resistance — both are on the datasheet, the declaration of performance or the product's certificate. If building control needs it in the file, the written report is £25 +VAT: every layer, every month, every interface and a clear conclusion, with a report ID they can [verify online](/verify/). [See an example report (PDF)](/tools/u-value-calculator/app/example-condensation-report.pdf) first. Or start from a worked example, each opening pre-filled: a [warm flat roof](/tools/u-value-calculator/warm-flat-roof/), a [cold flat roof](/tools/u-value-calculator/cold-flat-roof/), an [inverted flat roof](/tools/u-value-calculator/inverted-flat-roof/), [insulation between the rafters](/tools/u-value-calculator/pitched-roof-rafter-insulation/), [loft insulation](/tools/u-value-calculator/loft-insulation-270mm/), a [timber frame wall](/tools/u-value-calculator/timber-frame-wall/) or a [solid wall insulated on the inside](/tools/u-value-calculator/solid-brick-wall-internal-insulation/). --- # Asked for a soakaway test and design? Here is what building control needs URL: https://energycount.co.uk/guides/soakaway-building-control/ Updated: 2026-09-25 ## What building control is actually asking for Requirement H3 of the Building Regulations covers rainwater from roofs and paved areas, and it sets an order of priority for where that water goes: 1. **an adequate soakaway or other infiltration system;** or, where that is not reasonably practicable, 2. **a watercourse;** or, where that is not reasonably practicable, 3. **a sewer.** So when an extension or a new house adds roof and paving, the first question building control asks is whether the water can go into the ground on site. Approved Document H, the guidance on meeting H3, adds the thing the soakaway must not do — the water has to be distributed so that it does not damage the foundations of the building or any adjacent structure — and then covers where a soakaway can go and how big it has to be. That is what "a soakaway design" means in a building control letter: evidence that the soakaway is **in an acceptable place** and **big enough for your ground**. The first is a plan. The second is a test and a calculation. ## Where it can go Approved Document H says an infiltration device should not be built: - **within 5 m of a building or road**, or in areas of unstable land; - in ground where **the water table reaches the bottom of it** at any time of the year; - so close to **other soakaways, drainage fields or drainage mounds** that the ground's overall soakage capacity is exceeded; - where **contamination in the run-off** could pollute a groundwater source or resource. It sets no distance from the boundary. Some councils do: one building control team asks for 5 m from buildings and boundaries, another for 2.5 m from a boundary. Check your building control body's own guidance before you mark a position on the plan, because on a small plot the location is settled before the size is. ## The test Everything in the design rests on one measured number: how fast the ground at the soakaway's depth takes water away, the **infiltration rate**. Two tests exist, and it is worth knowing which one you are being asked for. **The BRE Digest 365 soakage test** is the one planning conditions and lead local flood authorities usually name, and building control bodies commonly ask for it too. Dig a trial pit where the soakaway will go, to the depth it will reach, **0.3 to 1 m wide and 1 to 3 m long**. Fill it to at least 75% of its effective depth and time the fall **from 75% to 25%**. Do it **three times**, on the same day or consecutive days, so the ground around the pit is saturated. Design on the slowest result — some authorities say so in terms, and tell you not to average. [The percolation test calculator](/tools/soakaway-calculator/percolation-test/) turns the pit size and times into the rate, and has a stopwatch for the pit. **The Approved Document H percolation test** uses a 300 mm square hole and a timed 150 mm fall, giving a percolation value, Vp, in seconds per millimetre. It is the test for a septic tank drainage field, but Approved Document H also allows it for small soakaways and gives a formula to convert Vp to an infiltration rate. If that is the test your building control body accepts, it is less digging; if the request says BRE 365, it is the trial pit. **Mind the season.** Ground that has been dry for months drinks the first fills and flatters the result, and a soakaway sized on a summer test can struggle in a wet winter. Some authorities ask for testing in winter or spring for exactly that reason. We have written about [testing in drought conditions](/news/bre-365-soakaway-testing-drought-conditions/) and how to show what conditions a test was done in. Photograph the pit with a tape in it and write down the dimensions and every time. Some building control bodies have their own test record sheet, and ask for it back once the test is done. ## The design The soakaway has to hold whatever rain arrives faster than the ground can take it. BRE Digest 365 sizes it for a **1-in-10-year storm** — the return period Approved Document H asks for — checked across a range of storm durations, from a few minutes to a day, because a short cloudburst and a long steady rain load it differently. For each duration, the storage needed is the inflow from the drained area less the outflow into the ground, and the worst case governs. The soakaway must then **half-empty within 24 hours**, so it is ready for the next storm. Most authorities now add a **climate-change allowance** to the rainfall, commonly 40%. What it is built from matters as much as its size. A rubble-filled pit stores only the voids between the stones, 30 to 40% of its volume; modular crates store 90 to 95%. [BRE Digest 365 explained](/tools/soakaway-calculator/bre-digest-365/) sets out every step with a worked example, and [what size soakaway do I need?](/tools/soakaway-calculator/what-size-soakaway-do-i-need/) puts typical numbers on it. Approved Document H has a shortcut for the smallest cases: a soakaway serving **25 m² or less** may be designed on an assumed 10 mm of rain in 5 minutes. For larger areas it points to BRE Digest 365 or BS EN 752-4. It still cites the 1991 edition of the digest; the current one is the 2016 edition, and that is the one to work to. ## What the design has to show There is no prescribed form. Whoever checks it will be looking for: - **Where the soakaway goes** — a plan showing it clear of buildings, roads and anything else your authority sets a distance from. - **The test** — the pit's length, width and effective depth, the fall times, and the date it was done. - **The infiltration rate** and how it was derived. - **The area draining to it** — roofs and paving, in square metres. - **The design rainfall** — the site's rainfall figures, the return period and the climate-change allowance. - **The storm table** — each duration checked, and the one that governs. - **The construction and its dimensions** — rubble, crates, concrete rings or a trench, with its void ratio. - **The half-drain time**, inside 24 hours. That is what a [SoakMonkey report](/tools/soakaway-calculator/app/example-soakaway-report.pdf) sets out, with a report ID building control can [verify online](/verify/). ## When it is due With the plans on a full plans application, or before the drainage goes in on a building notice — building control inspects drainage before it is covered, and a soakaway is hard to resize once it is buried. The test itself belongs even earlier: before the crates are ordered, and ideally before the design is fixed, because slow ground can move the soakaway, change its construction or rule it out. ## If the ground won't take it A failed test is not the end of the job; it moves you down the Part H list. The next option is a **watercourse**, where one can be reached; after that, a **sewer** — which means applying to the water company to connect, and a surface water sewer where there is one. Below an infiltration rate of roughly 1×10⁻⁶ m/s — heavy clay, typically — a soakaway usually cannot half-empty within 24 hours at any size, and the answer is **attenuation**: storage that releases water at a controlled rate to a watercourse or sewer. BRE Digest 365 does not size that. Keep the failed test and put it in the submission, because it is the evidence that a soakaway was not reasonably practicable. ## Planning, SuDS and Wales For a single extension, building control is usually the only one asking. For new homes, planning often asks as well: - **The NPPF published in August 2026** expects development that could affect drainage to include sustainable drainage designed to Defra's **National Standards for Sustainable Drainage Systems**, proportionate to the scheme, with lead local flood authority advice on major development — ten or more homes, or 0.5 ha. We covered [the SuDS policy](/news/nppf-2026-suds-national-standards-policy-f8/) and [the SuDS statement on validation lists](/news/nppf-2026-dm2-validation-lists-suds-statement/) when it was published. - **The National Standards** ask infiltration features to half-empty within 24 hours for storms up to the 1-in-30-year event, and say that building regulations continue to apply within the curtilage of each property. - **In Wales**, Schedule 3 of the Flood and Water Management Act 2010 has been in force since 7 January 2019: development with a construction area of 100 m² or more needs approval from the local SuDS Approving Body before work starts. The infiltration test is the same whoever asks for it, so one good test can serve both. ## What it costs - **A calculation from your own test results**: one Greater Manchester firm lists £180 for a BRE 365 calculation alone. - **Testing and design together**: the same firm prices a day's testing at £600, before any digger or water bowser; one consultancy says its fees for the smallest single house exceed £1,200 + VAT. - **SoakMonkey**: free to size on screen, as often as you like. The Building Control-ready report is **£25 + VAT** (£30 inc VAT). The test is the part you can do yourself with a digger, a hose or a bowser, a tape and a watch. The design is arithmetic. ## Do it yourself, free [SoakMonkey](/tools/soakaway-calculator/) runs the BRE Digest 365 method in your browser: enter the pit size and the three fall times, the area draining to the soakaway and the site's location, and it works out the infiltration rate, runs the storm analysis with the climate-change allowance, and sizes a rubble pit, crates, concrete rings or a trench, with the half-drain check. It flags ground that is too slow before anything is dug. On site, it installs on your phone and times the test for you. What you'll need to hand: the trial pit's length, width and effective depth; the three fall times; and the roof and paving areas draining to the soakaway, from your drawings. For an extension, [a soakaway for an extension](/tools/soakaway-calculator/soakaway-for-an-extension/) walks through the usual case. If you'd rather hand the design over entirely, [ask us](/contact/). --- # Asked for heat loss calculations for your extension? Here is what they mean URL: https://energycount.co.uk/guides/over-glazed-extension-calculations/ Updated: 2026-09-25 ## What building control is asking for When building control asks for "heat loss calculations" on an extension, it is almost always about the glazing. Approved Document L, the guidance behind Part L of the Building Regulations, caps the total area of **windows, roof windows, rooflights and doors** in an extension at **25% of its floor area**, plus the area of any existing windows and doors the extension removes or covers over. Inside that allowance an extension complies on the U-values of its parts alone, and there is nothing to calculate. Over it, the extension can still comply, but somebody has to show that it does. Approved Document L gives two ways: - **An area-weighted U-value calculation** (paragraph 10.9): the extension as designed is shown to lose no more heat through its fabric than the same extension built to the 25% limit. - **A whole-dwelling SAP calculation** (paragraph 10.10): the house with your extension is shown to perform no worse than the house with a compliant, notional extension. Building control bodies and their guidance notes call these different things — heat loss calculations, compensatory calculations, excess glazing calculations, "an area-weighted U-value calculation or SAP" — but they are all asking for one of the two. CNC Building Control puts the test plainly: you may exceed 25% if you "demonstrate the increased glazed area does not increase the energy loss above that of a 25% or less glazed building". What it is **not** is a room-by-room heat loss calculation to BS EN 12831, the kind a heating engineer runs to size radiators or a heat pump. That answers a different question — how many watts each room needs on a cold day — and it is not one of the Part L routes. If you are fitting a heat pump as well, you may need both; they are separate jobs. ## First, check you are actually over A surprising number of "over-glazed" extensions are not, once the allowance is worked out properly. Two things decide it. **Measure openings as openings.** The area that counts is the structural opening — frame included, not just the glass — for every window, door, roof window, rooflight and lantern in the extension's walls and roof. **Claim the openings you cover over.** Any existing window or door that no longer exists, or is no longer exposed, because of the extension adds its area to the allowance. Take a 24 m² rear extension: 25% of it is **6.0 m²**, but if it swallows a 1.8 m² back door and builds over a 1.2 m² kitchen window, the allowance is **9.0 m²**. Mark those openings on the existing drawings, because building control will want to see where the extra came from. [The covered-openings allowance](/tools/extension-glazing-calculator/covered-openings-allowance/) works through it. If the extension is inside the allowance after that, the answer to building control is the arithmetic itself — a schedule of openings against the allowance — not a heat loss calculation. ## Route one: the area-weighted U-value calculation This is the usual answer, and the cheaper one. Every element of the extension — the walls net of openings, the roof, the floor, and each window, door and rooflight — is multiplied by its area, the results are added up, and the total is divided by the total area: > **area-weighted U = (U₁A₁ + U₂A₂ + …) ÷ (A₁ + A₂ + …)** The same sum is then done for a **notional extension**: the same size and shape as yours, with its openings cut back to the allowance and the area removed given back to wall or roof, and every element at the limiting U-values in Table 4.2 of Approved Document L — walls 0.18, floor 0.18, roof 0.15, windows and doors 1.4, rooflights 2.2 W/m²K. If your extension's figure is no higher than the notional one, it complies. Because glazing and fabric that beat the limiting values both count, this passes more often than people expect. GlazeMonkey's worked example is a 24 m² extension with over 60% of its floor area in openings — bifolds, a side window and two rooflights — and it passes, because the glazing is at 1.0 and 1.2 W/m²K and the walls, roof and floor beat their limits. Put the same extension in with glazing at the limiting values and it fails. [The area-weighted method](/tools/extension-glazing-calculator/area-weighted-u-value/) sets out that example line by line. One rule does not bend: every new element must still meet its own limiting U-value. The trade is between area and performance across the whole extension, not a licence for one poor element carried by good ones. ## Route two: the whole-dwelling SAP calculation When the area-weighted comparison cannot be made to pass, the SAP route brings the existing house into the sum. The dwelling is modelled twice: once with your extension as designed, and once with a notional extension of the same size and shape, its openings at the allowance, its door area the same as yours and the rest counted as windows. The first must not exceed the second on three measures — primary energy, carbon emissions and fabric energy efficiency. Where the existing house's construction is unknown, standard values from SAP's Appendix S are used. Its advantage is that improvements to the existing house count in your favour: replacement windows, a topped-up loft, a better heating system. It is more work and it costs more, and it is the route that gets heavily glazed designs through. [SAP calculations for over-glazed extensions](/services/sap-calculations/extensions/) is work we do every week. ## What the document has to show No form is prescribed. For the area-weighted route, whoever checks it will be looking for: - **The extension it applies to** — the address, and the drawings it was worked from. - **The allowance** — the extension's floor area, 25% of it, and each covered-over opening with its size. - **The proposed extension, element by element** — the area and U-value of the walls, roof, floor and every opening, and where each U-value came from: the manufacturer's declared whole-window value or energy rating for the glazing, and a BS EN ISO 6946 calculation for the walls, roof and floor. - **The notional extension**, built as above, totalling the same area as the proposed one. - **The two area-weighted U-values**, and a clear pass. - **Confirmation that every new element meets its own limiting value.** - **The date, and who produced it.** A SAP-route submission carries the same identification, the results for both cases on each of the three measures, and the assessor's details. ## Why they get sent back 1. **Glass measured instead of openings.** The frame counts. 2. **Covered openings missed** — or claimed with nothing on the existing drawings to show them. 3. **A rooflight treated as a roof window, or the other way round.** A roof window in the plane of the roof is a window at 1.4; a rooflight on an upstand takes 2.2. Swap them and the notional extension is wrong. [Rooflights and roof windows](/tools/extension-glazing-calculator/rooflights-and-roof-windows/) sets out which is which. 4. **A notional extension that shrinks.** The glazing cut back to the allowance has to reappear as wall or roof. Delete it instead and the two sides no longer describe the same building. 5. **Centre-pane glazing figures.** The value that counts is the whole window, frame included, which is always worse than the glass alone. 6. **Assumed fabric U-values.** "Insulated to current regs" is not a U-value. [U-Monkey](/tools/u-value-calculator/) calculates the real one for each build-up, free. ## When it is due No regulation sets a date. On a full plans application building control will want it before approving the plans; on a building notice, before it can sign the extension off. The time it is most useful is earlier than either: before the glazing is ordered, while a rooflight is still a decision rather than a delivery and a better glazing specification is a line on a quote rather than a refit. ## What it costs - **An area-weighted calculation** from a consultancy runs to roughly **£95–£300**; SAPgen lists £125. - **A whole-dwelling SAP** is advertised from about **£75 + VAT** at the cheapest, with other providers starting at £275 or more, because the existing house has to be modelled as well as the extension. - **GlazeMonkey** runs the area-weighted check free, as many times as you like, and charges **£35 + VAT** (£42 inc VAT) for the Building Control PDF. Whichever you choose, ask what a revision costs. The glazing changes more than anything else on an extension's drawings, and every change is a re-run. ## Do it yourself, free [GlazeMonkey](/tools/extension-glazing-calculator/) does the whole area-weighted route in your browser: the allowance, the covered-openings credit, the notional extension built the way Approved Document L describes it, and the comparison, live as you type. If you are short, it tells you what the glazing would need to be to pass. It is free to use; the Building Control-ready PDF is £35 + VAT, with a report ID building control can [verify online](/verify/). [See an example report (PDF)](/tools/extension-glazing-calculator/app/example-extension-report.pdf) first. What you'll need to hand: the extension's floor area; the width and height of every window, door and rooflight at structural opening size; the same for any existing openings the extension covers over; and a U-value for each element — the whole-window figure from the glazing supplier, and the wall, roof and floor build-ups from your drawings, which [U-Monkey](/tools/u-value-calculator/) turns into U-values if you don't have them. If it still will not pass, two options are left: the [whole-dwelling SAP](/services/sap-calculations/extensions/), or less glass — which is sometimes a single rooflight. The rest of an extension's Part L, from the U-values to conservatories, is in [building regulations for an extension](/guides/extension-building-regulations/). ## Wales, Scotland and the 2027 changes **Wales** runs the same idea with different numbers: windows and doors limited to 25% of the extension's internal floor area plus the openings it does away with, an area-weighted or SAP route as the alternatives, and tighter limiting U-values for new roofs (0.13) and floors (0.15). **Scotland** limits windows, doors and rooflights in an extension to 25% of its floor area plus any openings built over, and allows more where a "compensatory approach" shows the extra glazing causes no additional heat loss — or where a whole-dwelling SAP passes. **England's** Approved Document L 2026 takes effect on **24 March 2027**. It keeps the 25% rule (renumbered as paragraph 10.8), both alternative routes and the same limiting U-values for new elements. An extension with its building notice or full plans application in **before 24 March 2027** stays under the 2021 edition, provided the work has **commenced by 24 March 2028** — and for an extension on new foundations, [commenced has a legal meaning](/news/future-homes-standard-transitional-deadline-commencement/): its foundations and ground-floor structure complete, not the first spade in the ground. --- # Planning application stuck on nutrient neutrality? Here is what the council needs URL: https://energycount.co.uk/guides/nutrient-neutrality-planning-permission/ Updated: 2026-09-25 ## Why the council can't decide it yet Some rivers, estuaries and wetlands in England are protected habitats that are already carrying more nitrogen or phosphorus than they can cope with. New homes add to that load — mostly through their waste water, which reaches the water after treatment — and Natural England has advised the planning authorities in 27 of these catchments that development there should be **nutrient neutral**: adding nothing, once mitigation is counted. The advice matters because of the **Conservation of Habitats and Species Regulations 2017**. Where a development is likely to have a significant effect on a protected site, the planning authority must carry out an **appropriate assessment**, consulting Natural England, and may grant permission only once it has ascertained that the development will not adversely affect the site's integrity. More nutrients reaching a site that is already failing is a significant effect. Until the council has evidence that your scheme's load is zero, or mitigated to zero, it cannot reach that conclusion — and the application waits. That is why a scheme with everything else agreed can sit undetermined for months. It is a legal test, not a local policy, and there is no discretion to waive it. ## Is your scheme caught? Two questions settle it. **Where does it drain?** The catchment is decided by where the site's water ends up, and foul and surface water can go different ways — a site outside a catchment on the map can still be in it through the treatment works its sewage goes to. [The catchment map](/tools/nutrient-neutrality-calculator/map/) shows all 27, coloured by the nutrient that applies, and [our permit register](/reference/wastewater-permits/) has the consented concentrations for 675 treatment works across 22 of them. **Does it add overnight stays?** New dwellings, conversions, houses in multiple occupation, holiday lets, care homes, student accommodation, hotels and campsites all do. The trigger is a net increase, so an extension to an existing home is generally not caught unless it creates a separate dwelling. Natural England's advice also applies at reserved matters and to prior approval applications, so an outline consent does not get a scheme past it. Non-residential development is caught where it adds waste water — a new commercial kitchen, for instance. ## What the council needs **1. A nutrient budget.** The kilograms of nitrogen or phosphorus a year the scheme adds, worked through Natural England's methodology and the calculator for the catchment. The waste water term comes from the number of dwellings, an occupancy rate (2.4 people per dwelling by default), daily water use (usually the 110 litres per person a day optional standard, plus Natural England's 10-litre allowance) and the permitted discharge concentration at the treatment works — or the performance of a private treatment plant. The change in land use is added, any sustainable drainage designed to remove nutrients is credited, and the net figure is multiplied by **1.2**, Natural England's 20% precautionary buffer. The [nutrient neutrality assessment](/services/nutrient-neutrality-assessment/) page goes through each stage. **2. A mitigation proposal.** If the budget is above zero, how you will bring it to zero: nutrient credits from a scheme in the same catchment, measures on your own land, or a treatment plant that removes more than the works would. One credit is normally one kilogram a year, secured in perpetuity. **3. Evidence that the mitigation is secured.** Not a promise — a credit certificate or allocation, a legal agreement, or a condition the council is satisfied can be met. Some councils also want the budget wrapped in a short statement to inform their Habitats Regulations Assessment: the same facts, set out for the ecologist. ## What the ecologist checks The council's ecologist is not re-deriving the methodology; they are checking you applied it to the right facts: - **The right catchment and the right nutrient.** Some catchments are nitrogen, some phosphorus, a few both — and the wrong nutrient is the commonest mistake in a do-it-yourself submission. - **The right treatment works and permit**, including whether the permit tightens in 2030. - **Occupancy and water use.** A lower figure than the defaults needs a reason, such as a water efficiency condition or one-bedroom flats the authority accepts a lower occupancy for. - **The current land use**, with evidence where it matters. A greenfield site on fertilised arable land can come out with a large negative land-use term; the authority will want to see that the land really was farmed that way. - **The buffer**, applied once and to the net figure. - **The current version of the calculator.** Natural England's own credit scheme rejects applications worked on an out-of-date one. ## Securing the mitigation **Credits** are the usual route for small schemes. Each catchment has its own providers, and prices vary enormously; [the nutrient credits calculator](/tools/nutrient-neutrality-calculator/nutrient-credits-calculator/) turns a budget into the number of credits and, where a scheme publishes a price, the likely cost. How they are secured depends on the scheme. Natural England's own scheme — currently selling credits in Poole Harbour and the Tees — asks for your planning application reference before it allocates credits, gives you a provisional certificate for the council once a deposit is paid, and takes the balance after permission, when a final certificate goes to the council to show the condition has been met. Other providers sell outright, or the council ties the purchase into a **section 106 agreement**. Schemes of ten to 49 homes may soon use [national section 106 templates](/news/standard-s106-templates-medium-sites-consultation/). Whatever the mechanism, the council has to be able to rely on it before it decides. **The 2030 split.** Where the treatment works has a permit that tightens on 1 April 2030 under the Levelling-up and Regeneration Act 2023, the council can take the upgrade as certain. The budget then has two figures, and the permanent mitigation need only cover the lower, post-2030 one; the difference until 2030 can be covered with cheaper temporary credits where a scheme sells them. **On-site measures** suit larger sites: taking fertilised land out of production within the red line, a wetland, or sustainable drainage designed and evidenced for nutrient removal. They need to be secured for as long as the development's effect lasts, which in practice means a legal agreement. ## The levy route The Planning and Infrastructure Act 2025 created a second route. Natural England can make an **Environmental Delivery Plan** for a catchment, and a developer can then pay a **nature restoration levy** instead of finding its own mitigation. The process is set out in the [levy rulebook](/news/nature-restoration-levy-guidance-parliament-approval/): request to use the plan, receive a commitment certificate, submit it with the planning application, and pay before work starts. The Nature Restoration Levy Regulations 2026 have been in force since **11 September 2026**. The first plan — the [draft Norfolk Environmental Delivery Plan](/news/first-environmental-delivery-plan-norfolk-consultation/), covering the River Wensum and the Bure and Yare Broads catchments at a proposed **£2,675 per home** — went out to consultation on 17 September 2026 and is **not in force**. Until a plan is in force for your catchment, and your site is inside it, the budget and the mitigation are still the only route. ## When to do what **Before you submit.** Run the budget as soon as the number of dwellings and the drainage route are known — it can change the scheme, and some councils' validation lists already ask for it in the affected catchments. Line up the mitigation at the same time: credit schemes have limited capacity, and some will not allocate until the application has a reference. **While it is being decided.** Expect questions on the land use, the occupancy and the treatment works. Answering with evidence rather than argument is what keeps it to weeks rather than months. **After permission.** Complete the credit purchase or discharge the condition before the trigger it sets — usually before work starts or before anyone moves in. ## What it costs - **The assessment.** Consultants' nutrient assessments routinely run into the hundreds of pounds for a small scheme. NutrientMonkey works the budget out free on screen; the planning-ready report is **£79 + VAT** (£94.80 inc VAT). - **The mitigation.** Published credit prices run from about **£1,800–£3,300 per kilogram of nitrogen** a year in Poole Harbour, the Tees, Stodmarsh and the Solent to **£28,000 per kilogram of phosphorus** in Cumbria and as much as **£82,000** in the Broads and the Wensum. Where no price is published, providers quote. The budget is also the place to save money. The default occupancy and water use are conservative for small flats and for schemes with a 110-litre condition, a fertilised greenfield site earns a negative land-use term, and a better treatment plant cuts the waste water load directly. Each kilogram argued away is a credit you do not buy. ## Do the budget yourself, free [NutrientMonkey](/tools/nutrient-neutrality-calculator/) runs Natural England's methodology for all 27 catchments in your browser: pick the catchment and the nutrient locks itself, enter the dwellings, the drainage route and the treatment works, add the land use and any sustainable drainage, and the budget updates as you type — with the pre- and post-2030 figures where a permit tightens. It is free to use; the planning-ready report, with every line of the arithmetic and a report ID the ecologist can [verify online](/verify/), is £79 + VAT. [See an example report (PDF)](/tools/nutrient-neutrality-calculator/app/example-nutrient-report.pdf) first. Your figures should reproduce in Natural England's own calculator for the catchment, which remains the reference. What you'll need to hand: the site address and its area; the number and type of dwellings; where the foul drainage goes — the treatment works, or a private plant; the current use of the land; and any surface water proposals. Where the case is harder than the calculator — a private treatment plant, a phased scheme across the 2030 change, a land-use history the authority may dispute, or an authority that wants a consultant's signed assessment — our [nutrient neutrality assessment service](/services/nutrient-neutrality-assessment/) prepares it. --- # What is Thermal Bridging? URL: https://energycount.co.uk/guides/thermal-bridging/ Updated: 2026-09-20 > **Quick answer:** a thermal bridge is a localised weak point — usually a junction like wall-to-floor or a window reveal — where heat escapes faster than through the surrounding fabric. Each junction's extra loss is measured as a **Ψ-value (psi value), in W/m·K**, calculated to BS EN ISO 10211. At modern insulation levels junctions can carry 20–30% of a dwelling's fabric heat loss. ## Thermal bridges: the weak points in the fabric A thermal bridge is a localised path through the building envelope where heat escapes more readily than through the surrounding fabric — typically at **junctions**: wall-to-floor, wall-to-roof, around windows and doors, at corners. Even with superb u-values everywhere else, poorly detailed junctions can account for 20–30% of a dwelling's fabric heat loss. There are two kinds. **Geometric bridges** — like an external corner, which simply has more exposed surface area than the flat wall. And **constructional bridges** — where a high-conductivity material (a concrete slab edge, a steel lintel, an uninsulated cavity closer) pierces the insulation line and carries heat straight through.
Section through a wall-to-floor junction showing the perimeter heat-loss path around the slab edge
Wall-to-floor: heat escapes around the slab edge.
Section through an eaves junction where the wall meets the roof, showing the thermal bridge
Eaves: where the wall meets the roof.
Plan through a window jamb junction showing heat loss around the window reveal
Window jamb: around the opening.
Plan through an external corner junction showing the thermal bridge at the corner
External corner: heat loss at the corner.
## Ψ-values (psi values) Each junction type has a **linear thermal transmittance**, or Ψ-value, measured in W/m·K — the heat loss per metre of junction per degree of temperature difference. In SAP and HEM, every junction length is multiplied by its Ψ-value and summed into the heat-loss calculation. A typical well-detailed junction sits around **0.02–0.08 W/m·K**; a poor one can be **0.15–0.30**. Because the lengths run for metres (a wall-to-floor junction runs the whole perimeter), small differences multiply quickly — which is why a SAP calculation with default Ψ-values can need noticeably more insulation or PV than one with calculated values. The number itself — its units, what a good value looks like junction by junction, and SAP's default and notional figures from Tables K1 and R2 — has its own page: [what is a psi value?](/guides/psi-values/) ### The maths in SAP (why a few hundredths matter) In SAP, junction heat loss is simply **length × Ψ-value**. A detached house has roughly 50 m of wall-to-floor junction around its perimeter. At a calculated 0.05 W/m·K that's 50 × 0.05 = **2.5 W/K**. With SAP's default for that junction — 0.32 W/m·K in [Table K1](/reference/sap-junctions/) — it's 50 × 0.32 = **16 W/K**: a 13.5 W/K difference that must be made up in better walls, more insulation or bigger PV. Across all your junctions, that gap is often the difference between a comfortable pass and a design rework. It's also why "measure the junction lengths, not just the wall area" is the first thing an assessor checks. ### What a good detail looks like The golden rules for every junction: **keep the insulation line continuous**, **avoid bridging it with high-conductivity material**, and **close cavities with insulated closers**. Concretely: - **Wall-to-floor** — edge insulation under the slab that meets the wall insulation; a DPC can't bridge the line. - **Window jamb/sill/lintel** — an insulated cavity closer with the frame set against the insulation line; a metal lintel with no thermal break is a classic failure. - **Eaves and gables** — loft or rafter insulation meeting the wall insulation without a gap. - **Corners** — nothing to fix structurally, but they're geometric bridges you can't remove — they just need to be *included* in the calculation, not forgotten. - **Party walls, balconies, exposed floors** — any element that penetrates the envelope needs a thought-through detail; balconies are the notorious one. Each junction type has a page showing how to model it in ΨMonkey — from [wall-to-floor](/tools/psi-calculator/wall-to-floor/) and [window jambs](/tools/psi-calculator/window-jamb/) to [balconies](/tools/psi-calculator/balcony/) and [exposed floors](/tools/psi-calculator/exposed-floor/). ## Which junctions cost you most SAP / Approved Document L labels each junction with an "E-number" reference, and they're all modelled in our free [ΨMonkey psi calculator](/tools/psi-calculator/): - **[Wall-to-floor (E5)](/tools/psi-calculator/wall-to-floor/)** — runs the whole heated perimeter; usually the biggest single contributor. - **[Eaves (E9/E10)](/tools/psi-calculator/eaves/)** and **[gable (E11/E12)](/tools/psi-calculator/gable/)** — where the wall meets the roof. - **[Window jamb (E4)](/tools/psi-calculator/window-jamb/)**, **[sill (E3)](/tools/psi-calculator/window-sill/)** and **[lintel (E1/E2)](/tools/psi-calculator/lintel/)** — around every opening; jamb and sill lengths add up fast on glazed elevations. - **[External corner (E13)](/tools/psi-calculator/external-corner/)** and **[inverted corner (E14)](/tools/psi-calculator/inverted-corner/)** — geometric bridges. - **[Intermediate floor (E6)](/tools/psi-calculator/intermediate-floor/)** and **[party wall (E15)](/tools/psi-calculator/party-wall/)** — for flats, terraces and semi-detached homes. - **[Balcony (E8/E24)](/tools/psi-calculator/balcony/)** and **[exposed floor (E20/E21)](/tools/psi-calculator/exposed-floor/)** — balconies and floors over voids are classic constructional bridges. ## Your three options in SAP 1. **Default values** — punitive, and they make passing harder and more expensive. 2. **Accredited/standard details** — better, if your details exactly match the published ones ([more on ACDs](/guides/accredited-construction-details/)). Note that SAP 10 removed Accredited Construction Details over accuracy concerns, leaving manufacturer schemes or calculated values. 3. **Calculated Ψ-values** — modelled to BS EN ISO 10211. Almost always the best result, often the cheapest route to compliance. ## Calculate your own Ψ-values Our free in-browser tool, [**ΨMonkey**](/tools/psi-calculator/), models junctions with a finite-difference solver to BS EN ISO 10211 and gives you the Ψ-value and f_Rsi temperature factor. Use it to test your details — PDF reports for Building Control are £25 +VAT a junction, one junction per report. The process: 1. **Model the junction** — paint the build-up (walls, floors, roof, insulation lines, cavities, DPC) on the grid. 2. **Solve** — ΨMonkey runs the 2-D heat-flow calculation and returns the Ψ-value and the coldest-surface temperature factor. 3. **Iterate** — move the insulation line, change the closer, compare details until the number is good. 4. **Export** — the branded PDF carries the parameters, the result and a unique Report ID that [Building Control can verify](/verify/). 5. **Hand to your SAP assessor** — calculated values slot straight into the [SAP calculation](/services/sap-calculations/) as an alternative to defaults. ## Condensation risk: f_Rsi Thermal bridges aren't just about heat loss. A cold internal surface at a junction invites condensation and mould. The **temperature factor f_Rsi** (0 to 1) measures how cold the worst internal surface gets; Building Regulations require f_Rsi ≥ 0.75 for dwellings. ΨMonkey reports it automatically — and the same junction that leaks heat is usually the one that risks mould, so fixing the detail fixes both. That covers the inside surface at the junction. Condensation inside the wall or roof between the junctions — interstitial condensation — is a separate check, made on the build-up itself to BS EN ISO 13788: U-Monkey's free [condensation risk calculator](/tools/u-value-calculator/condensation-risk-calculator/) runs it alongside the U-value, with the same f_Rsi check on the flat part of the wall or roof. ## The bottom line Get the [U-values](/guides/u-values/) right, then get the junctions right — calculated Ψ-values are frequently the cheapest single route to a [SAP 10](/guides/sap-10-whats-new/) pass, and they de-risk your [compliance pack](/services/sap-calculations/compliance-pack/). Model yours free with [ΨMonkey](/tools/psi-calculator/) — no subscription, no sign-up. --- # New build compliance: what you need at each stage URL: https://energycount.co.uk/guides/new-build-compliance-timeline/ Updated: 2026-09-23 > **Quick answer:** A new dwelling in England needs, in order: **planning**, which may ask for an energy statement, a nutrient neutrality assessment and the 110-litre water standard; **before work starts**, the design-stage SAP and BREL report with the Part O overheating assessment; **during the build**, photographs of the insulation and a water calculation before the sanitaryware is ordered; **before completion**, an air test on every dwelling and sound tests on attached ones; and **within five days of completion**, the as-built SAP, the completion BREL, the EPC and the Part G notice. The completion certificate waits for all of them. ## The whole sequence on one page | Stage | What's needed | What goes wrong if it's late | |---|---|---| | **1. Feasibility** | A quick check of the site's catchment, water stress and local energy policy | A planning requirement you didn't price | | **2. Planning application** | [Energy statement](/services/energy-statements/) where local policy asks; [nutrient neutrality assessment](/services/nutrient-neutrality-assessment/) in an affected catchment | Application stalled or refused | | **3. Pre-commencement conditions** | Evidence to discharge the energy, water and nutrient conditions | Work that starts is unauthorised | | **4. Building Regs application** | [Design-stage SAP](/services/sap-calculations/new-builds/), [Part O overheating](/services/part-o-overheating/), U-values and junction details | A specification that can't pass | | **5. Before work starts** | Design-stage BREL notified by the day before; Robust Details registration if you're using it; a [PEA](/guides/predicted-energy-assessment/) if you're selling off-plan | A notice breach, or no route out of sound testing | | **6. Construction** | [Photographic evidence](/guides/part-l-photographic-evidence/) of every insulation detail; [Part G water calculation](/services/part-g-water-calculations/) before the bathroom order; SAP re-run on any specification change | Plots that can't be signed off | | **7. Pre-completion** | [Air test](/services/air-tightness-testing/) on every dwelling; [sound tests](/services/sound-testing/) on attached ones | A failed test with the decorators in | | **8. Within 5 days of completion** | As-built SAP, completion BREL with the photos, the [EPC](/services/sap-calculations/epc/), the Part G notice, the home user guide | No completion certificate, so no sale | The rest of this page takes each stage in turn. ## 1. Feasibility: before anyone draws it properly Three things about the site can each add a report and a cost. It's much better to know them before the land price or the layout is fixed. - **Is it in a nutrient neutrality catchment?** If it is, the planning authority can't grant permission until a nutrient budget shows the scheme adds no nitrogen or phosphorus to the protected water, and that usually means buying credits. The [NutrientMonkey calculator](/tools/nutrient-neutrality-calculator/) gives you the budget free. - **Will planning impose the 110-litre standard?** The tighter **110 litres per person per day** water standard, instead of the default 125, is triggered by a condition on the planning permission, and councils in water-stressed areas commonly impose it. The [water-stressed areas reference](/reference/water-stressed-areas/) explains how to tell. - **What does the local plan say about energy?** Many councils want a percentage cut in energy or carbon from renewables. A growing number set energy use intensity and space heating targets that standard SAP can't demonstrate and need [PHPP modelling](/guides/phpp-energy-statements/). ## 2. The planning application Planning deals with local policy, not Building Regulations. Where local policy calls for it, you'll need an **energy statement** showing how the scheme meets the council's renewables or carbon target. In an affected catchment you'll need a **nutrient neutrality assessment**, prepared to Natural England's methodology. In London, the London Plan adds [its own carbon target](/services/sap-calculations/london/) on top of Part L. The energy statement is modelled in SAP, so doing it properly now gives you a head start on the design-stage calculation later. ## 3. Discharging the pre-commencement conditions Energy and water requirements are often attached to the permission as **conditions**, and some of them are **pre-commencement**: they must be discharged before any work starts. Read the decision notice carefully. "Prior to commencement" means exactly that, and work begun without discharging it is unauthorised. Typical ones are a detailed energy statement, confirmation of the 110-litre water standard, and, in a nutrient catchment, proof that the mitigation credits are secured. ## 4. The Building Regulations application This is where most of the calculation work happens, and the cheapest point to change the answer. - **Design-stage SAP.** The dwelling has to beat its target rates for carbon and primary energy and its fabric energy efficiency target, and meet the limiting values. The [Part L guide](/guides/building-regulations-part-l/) explains the four tests. A design that fails now can be fixed by changing the specification on paper. - **Part O overheating.** Every new home needs either the simplified method or a TM59 model. The [Part O guide](/guides/building-regulations-part-o/) shows which. Glazing is the usual problem, and it's far easier to reduce before the windows are ordered. - **The fabric and the junctions.** Wall, floor and roof [U-values](/tools/u-value-calculator/) and the [junction psi-values](/tools/psi-calculator/) are what the SAP runs on. Calculated junctions instead of default ones are often the cheapest way to close a SAP gap. ## 5. Before work starts The regulations fix one deadline here. The target rates, the as-designed rates and the list of specifications the calculation depends on must be **notified to Building Control by the day before work begins**, on the design-stage **BREL report**. Two other things belong here: - **Robust Details.** If you're building attached homes and want to avoid pre-completion sound testing, the plots must be registered with Robust Details Ltd **before work starts**, and the separating walls and floors built exactly to the registered detail. Details are on the [sound testing page](/services/sound-testing/). - **A predicted energy assessment** if the homes will be marketed off-plan. It comes from the design-stage SAP. ## 6. During construction This is the stage people think needs nothing from the assessor, and it's where most completion problems start. - **Photographs.** Every dwelling needs its own set of photographs of thirteen insulation, airtightness and services details, taken before each one is covered. The [photographic evidence guide](/guides/part-l-photographic-evidence/) has the list and a printable plot checklist. - **The water calculation.** Do it before the bathroom and kitchen fittings are ordered. Many specifications pass by only a litre or two, so a larger bath or a higher-flow shower can tip it over. [WaterMonkey](/tools/part-g-water-calculator/) checks a fittings schedule free. - **Specification changes.** A different window, heating system or insulation product usually saves money for someone. Every change has to be reported at the as-built stage and the SAP re-run, and some turn a pass into a fail. Ask before the substitution is ordered, not after it's fitted. ## 7. Pre-completion testing - **Air tightness.** Every new dwelling is tested once the envelope is complete and sealed. The result goes into the as-built SAP, so a test worse than the design assumption can fail a plot that passed on paper. The [pre-test checklist](/air-tightness-pre-test-checklist.pdf) covers the usual leaks. - **Sound.** Attached homes need airborne and, where there are separating floors, impact sound tests, unless they're registered under Robust Details. Tests happen once the rooms on both sides are finished. [How many tests you need](/services/sound-testing/how-many-sound-tests-do-i-need/) depends on the number of plots and their construction types. Book both early. Each test needs the building in a particular state, and a failed test has to be remediated and retested before completion. ## 8. Completion: the five-day window Within **five days of completion**: - The **as-built SAP** results and any specification changes go to Building Control on the **completion-stage BREL report**, signed by the assessor and the builder, with the **photographic evidence**. - The **EPC** is lodged from the as-built SAP and given to the owner. - The **Part G water notice** goes to Building Control under Regulation 37. The [water calculation guide](/guides/water-calculation-building-control/) explains what it must say. - The **home user guide** goes to the occupier. Building Control can't issue the completion certificate without these, and without the certificate the sale can't complete. Five days is the legal backstop, not a target. Most of what goes in the completion pack can be prepared as the build progresses, so the window is used for signatures rather than chasing. ## Commercial buildings follow the same shape A building other than a dwelling runs on the same pattern with different paperwork. [SBEM](/services/sbem-calculations/) replaces SAP, and the **BRUKL report** replaces the BREL: one before work starts, one within five days of completion. The non-domestic EPC is lodged from the as-built model, and the air tightness limit is the same. Planning can still ask for an energy statement or BREEAM, depending on the council. ## Conversions and extensions are shorter - A **conversion** that creates a new dwelling needs SAP, the Part G water calculation and an EPC, but not the new-build target rates, the photographs or Part O. See [SAP for conversions](/services/sap-calculations/conversions/). - An **extension** usually needs no SAP at all, provided it meets the limiting U-values and its glazing stays within 25% of the floor area. [GlazeMonkey](/tools/extension-glazing-calculator/) checks the 25% rule, and the [extension guide](/guides/extension-building-regulations/) covers what to do when it fails. ## The bottom line Nearly every late-stage compliance problem is an earlier stage that got skipped. The fixes that are free on a drawing cost scaffolding and plaster once the house is up. If you tell us where your project has got to, we'll tell you what it still needs. The [New Build Compliance Pack](/services/sap-calculations/compliance-pack/) covers the SAP, EPC, water, air and sound testing as one job with one set of assumptions. --- # What is a psi value? URL: https://energycount.co.uk/guides/psi-values/ Updated: 2026-09-24 > **Quick answer:** a **psi value (Ψ-value)** is the linear thermal transmittance of a junction, in **W/m·K** — the heat that leaks along the line where two elements meet, per metre of junction, per degree of temperature difference, beyond what the elements' U-values already count. SAP multiplies each Ψ by the junction's length and adds the results to the fabric heat loss. Leave a junction uncalculated and SAP uses a **default** from Table K1 (1.00 for a lintel, 0.32 for a ground floor); the **notional dwelling** you are measured against assumes far lower values (0.05 and 0.16). A **calculated Ψ-value**, modelled to BS EN ISO 10211, is usually the cheapest way to close that gap. ## Psi value meaning: heat loss along a line Heat leaves a building through areas and along lines. Through the flat middle of a wall the loss is set by its **[U-value](/guides/u-values/)**, in watts per square metre per kelvin. But where that wall meets a floor, a roof, a window or another wall, the insulation is interrupted, the geometry changes and heat finds a shorter path out. That extra loss, concentrated along the junction, is the **thermal bridge** — and its size is the **psi value**, written Ψ, in watts per metre of junction per kelvin (W/m·K). The definition matters: Ψ is the loss **over and above** what the adjoining elements' U-values already explain. A junction with Ψ = 0.16 W/m·K and a length of 40 m adds 6.4 W/K to the dwelling's heat loss coefficient — the same as another 35 m² of wall at 0.18. Across a house, junctions typically add **10 to 30 per cent** to the fabric heat loss, which is why [thermal bridging](/guides/thermal-bridging/) is treated as a separate line in SAP rather than folded into the U-values. SAP also uses a summary figure, the **y-value** (W/m²K): the sum of every Ψ × length, divided by the total area of exposed elements. It is a convenient single number, and a trap — the y-value of a dwelling with no junction information at all is fixed at 0.20, roughly three times what a well-detailed house achieves. The [y-value calculator](/tools/psi-calculator/y-value-calculator/) works it out from your junction lengths, at SAP's defaults, at the notional values and at your own. ## What a good psi value looks like Two figures sit behind every junction in a SAP 10.3 calculation, and both are published in the specification. **Table K1** is the default: the value SAP gives a junction when nothing better is available. **Table R2** is what the **notional dwelling** — the target your design is measured against — assumes for the same junction. The gap between them is the penalty for not calculating. The junctions below are the ones that come up most. The complete set — all 44 E, P and R references with both values and the gap between them — is on the [SAP junction reference](/reference/sap-junctions/). | Junction | SAP default (Table K1) | Notional dwelling (Table R2) | | --- | --- | --- | | E1 Steel lintel with perforated base plate | 1.00 | 0.05 | | E2 Other lintels | 1.00 | 0.05 | | E3 Sill | 0.10 | 0.05 | | E4 Jamb | 0.10 | 0.05 | | E5 Ground floor (normal) | 0.32 | 0.16 | | E6 Intermediate floor within a dwelling | 0.14 | 0.00 | | E7 Party floor between dwellings | 0.28 | 0.07 | | E10 Eaves, insulation at ceiling level | 0.12 | 0.06 | | E11 Eaves, insulation at rafter level | 0.15 | 0.04 | | E14 Flat roof | 0.16 | 0.08 | | E16 Corner (normal) | 0.18 | 0.09 | | E17 Corner (inverted) | 0.00 | −0.09 | | E18 Party wall between dwellings | 0.24 | 0.06 | | P1 Ground floor at a party wall | 0.32 | 0.08 | | R1 / R2 / R3 Roof window head / sill / jamb | 0.24 | 0.08 / 0.06 / 0.08 | W/m·K, from the SAP 10.3 specification (12 June 2026 version), Appendix K and Appendix R. The default and notional figures for the common junctions, set against ΨMonkey's calculated values for eight named wall build-ups, are on the [building regulations reference](/reference/#thermal-bridging); all 38 ΨMonkey templates carry both figures in the [Ψ-value library](/tools/psi-calculator/psi-values/). Read across a row and the point of calculating is obvious. A steel lintel left at its default costs twenty times what the notional dwelling assumes; a modelled lintel with a thermal break lands somewhere between 0.05 and 0.3 depending on the product. A ground floor at 0.32 against a target of 0.16 is a common single reason a design misses its [fabric energy efficiency](/guides/fabric-energy-efficiency/) target. Sills and jambs are small numbers but there are a lot of metres of them. "Good", then, is relative to the notional: a **calculated value at or below the Table R2 figure** is beating the target at that junction. Typical calculated results for a well-detailed masonry or timber-frame wall are 0.02–0.05 for sills and jambs, 0.03–0.10 for eaves, corners and intermediate floors, 0.10–0.20 for a ground floor, and anywhere from 0.05 to 0.30 for a lintel depending on whether it is thermally broken. ## How SAP uses psi values Appendix K of SAP gives four ways to feed junctions into a calculation, in order of preference. **Details from a government-approved source** with independently assessed Ψ-values. **Details from a reputable independent database** — the successor to the old Accredited Construction Details, which [SAP 10 withdrew](/guides/accredited-construction-details/). **Ψ-values calculated by a suitably experienced person** to BS EN ISO 10211 and BRE report BR 497. And, failing all three, **the defaults** — Table K1 per junction, or y = 0.20 W/m²K for the whole dwelling when the junctions themselves are unknown. The routes can be mixed within one calculation: a manufacturer's assessed lintel detail, calculated values for the floor and eaves, defaults for a junction nobody has drawn. Every calculated value needs documentary evidence — the model and its result — which is what the Building Control PDF from a calculation tool is for. ## How a psi value is calculated A Ψ-value comes from a **two-dimensional numerical model** of the junction, built and run to **BS EN ISO 10211** with the UK conventions in **BR 497**. The junction is drawn as a cross-section — the wall, the floor slab and its insulation, the cavity closer, the lintel — extended at least a metre into each adjoining element so the edges of the model behave like the flat elements they represent. Each material gets its thermal conductivity — [the values we use, with their sources](/reference/thermal-conductivity/) — the inside and outside surfaces get the standard surface resistances; a temperature difference is applied and the model solves for the heat flow through the whole section. That total, per metre of junction, is the **L2D** value. The elements' own share is already known: U-value × the length of each element in the model. Subtract it and what is left is the junction's extra loss: **Ψ = L2D − Σ (U × l)** A worked example: a ground-floor junction modelled with 1.5 m of wall at U = 0.18 and 1.5 m of floor at U = 0.13 returns L2D = 0.62 W/m·K. The elements account for 0.18 × 1.5 + 0.13 × 1.5 = 0.465, so Ψ = 0.62 − 0.465 = **0.155 W/m·K** — a whisker inside the notional dwelling's 0.16, and half the 0.32 default. The same model gives a second number at no extra cost: the **temperature factor, f_Rsi**: how warm the coldest internal surface at the junction stays, as a fraction of the way from the outside temperature to the inside one (1.0 would be a surface at room temperature, 0 one at outdoor temperature). For dwellings f_Rsi should be at least **0.75** (BRE Information Paper IP 1/06) to keep surface condensation and mould off the junction — a check Building Control can ask for alongside the Ψ-value, and one that a junction can fail even when its Ψ is respectable. Numerical modelling used to mean desktop software and a consultant's day rate; it can now be done in a browser. [ΨMonkey](/tools/psi-calculator/) carries ready-made models for 33 standard SAP junctions, solves them to ISO 10211 against your build-up with Ψ and f_Rsi live on screen, is [validated against the ISO 10211 reference cases](/tools/psi-calculator/validation/), and produces the Building Control PDF for £25 +VAT a junction. ## Psi values for roof windows and rooflights The roof-window junctions — head, sill and jamb, R1 to R3 — are the ones most often left at default, and among the most expensive to leave. The default is **0.24 W/m·K** for each; the notional dwelling assumes **0.08, 0.06 and 0.08**. A dormer with three roof windows can carry 15 m of these junctions, so the defaults cost 3.6 W/K against a notional 1.1 W/K — the difference between passing and failing the fabric target on an otherwise good design. Modelled with a properly insulated upstand the calculated values usually sit close to the notional figures. ## SAP 10.2, SAP 10.3 and the Home Energy Model Nothing about psi values changes between the SAP versions: the same Appendix K routes, the same Table K1 defaults, the same calculation method. What changes is the pressure. The Future Homes Standard's notional dwelling has an air permeability of 4 and a heat pump in place of the gas boiler, so the easy wins elsewhere in the calculation are smaller, and the ten- or twenty-fold gap between a default and a calculated junction is more likely to be the whole margin. The Home Energy Model applies thermal bridges at every half-hourly timestep rather than in a monthly heat-loss sum, but it takes the same Ψ-values as its input. ## The bottom line A psi value is a junction's heat loss per metre. SAP will happily use a punitive default if you let it, and the target dwelling it compares you with does not. Calculating the value — for the junctions with the most metres and the biggest default-to-notional gap first: ground floor, lintels, roof windows, eaves — is the cheapest fabric improvement on most new-build designs, because it changes the number without changing the building. --- # Why two U-value calculators give different answers for the same wall URL: https://energycount.co.uk/guides/why-u-value-calculators-disagree/ Updated: 2026-09-20 Run the same wall through two different online calculators and you will often get two different numbers. Sometimes the gap is trivial. Sometimes it is enough to move a design from passing to failing. Either way it is corrosive: if the answer changes with the website, none of them feel trustworthy. It is not confined to U-values. Discussing thermal-mass tools, contributors on the Green Building Forum found that different calculators produced *"very different results for decrement delay"* for the same construction — and were left unsure which model to believe. (U-Monkey's decrement factor and time lag follow the BS EN ISO 13786 heat-transfer-matrix method with the layer order and surface films stated, and the density and specific heat behind every figure is sourced and shown — so at least you can see *why* two tools differ.) The reassuring part is that the calculators are usually not broken. A U-value is the output of a method with several inputs, and two tools can each be arithmetically correct while disagreeing, because they made different assumptions about those inputs. There are five places the difference nearly always hides. ## 1. The bridging fraction This is the big one, and on a framed wall it will usually account for most of any gap. Studs, joists and rafters conduct far better than the insulation between them, so the calculation has to know what proportion of the area they occupy. On a timber frame wall at 600mm centres with 38mm studs, the timber is not simply 38 ÷ 600 — you have to include noggins, top and bottom plates, lintels and the extra studs at openings, which is why the figure used in practice is typically higher than the naive one. Move that fraction from 0.09 to 0.15 and a 140mm mineral wool wall moves by roughly 0.02 to 0.03 W/m²K. That is the whole margin on a 0.18 target. **How to check:** find where each calculator states its fraction. If one of them never asked you, it assumed one. ## 2. Whether the combined method was used at all Once anything bridges an insulation layer, resistances cannot just be added up. BS EN ISO 6946 requires two separate calculations: - an **upper limit**, treating heat as flowing down parallel paths (through timber, through insulation) that do not interact - a **lower limit**, treating each bridged layer as a single equivalent layer The answer is the average of the two. A tool that adds a single column of resistances is not doing this, and on a bridged element it will read optimistic. **How to check:** a calculator following the method can show you both limits. If it only ever shows one total resistance and your wall has studs in it, that is your answer. ## 3. The ΔU corrections BR 443 adds small allowances to the finished U-value: - for **air gaps** in or around the insulation layer, depending on how well it is installed and whether it is continuous - for **mechanical fixings** that penetrate the insulation They are usually between 0.00 and 0.04 W/m²K. Individually small; collectively the difference between a pass and a fail on a tight target. They always make the number worse, they are easy to leave out, and plenty of free calculators leave them out. **How to check:** look for ΔU as its own line. If it is not shown, assume it was not applied. ## 4. Surface resistances and the direction of heat flow Every element has an internal and external surface resistance, and they depend on which way the heat is going: | Element | Rsi | Rse | |---|---|---| | Wall (horizontal) | 0.13 | 0.04 | | Roof (upward) | 0.10 | 0.04 | | Floor (downward) | 0.17 | 0.04 | Use wall values on a roof and the answer shifts. There is a second trap here too: if the build-up contains a **well-ventilated cavity**, ISO 6946 §6.9.4 requires the cavity and every layer outside it to be disregarded entirely, with Rse replaced by Rsi. A calculator that quietly includes the outer leaf in that situation will give a much better — and wrong — answer. ## 5. The λ values themselves A generic value for "mineral wool" and a manufacturer's declared value for a specific product are not the same number. Declared values are revised. Some products have thickness-dependent λ, so the same board is 0.027 at 50mm and 0.025 at 100mm. And a fair few online calculators carry values with no stated origin at all. **How to check:** every material should show where its figure came from and when it was taken. If a calculator cannot tell you that, its answer cannot be audited — and an unauditable number is a poor thing to attach to a Building Regulations submission. Ours is published: the [thermal conductivity library](/reference/thermal-conductivity/) lists every λ U-Monkey uses, what kind of source it came from and when it was retrieved. ## So which one is right? Work through the five in order. In our experience the difference is found by the second or third on almost every occasion, and it is nearly always a bridging fraction or a missing correction. Then apply the harder test: **which calculator will show you its working?** A tool that prints the method, every layer with its thickness and λ, both resistance limits, the corrections applied and the source of every material can be checked by you, by a Building Control officer, or by whoever inherits the file in five years. A tool that prints a number cannot. That is the difference that matters, and it is why [U-Monkey](/tools/u-value-calculator/) shows the full substitution — `R = d ÷ λ` with the actual figures in place — and puts a verifiable report ID on every report, including the free ones. For the tools themselves side by side — price, sign-up, condensation checks and materials — see [how U-Monkey compares with other U-value calculators](/tools/u-value-calculator/compare/). --- *Sources: discussion of calculator discrepancies and thermal-mass tools on the [Green Building Forum](https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16112); material-matching and vapour-unit difficulties on [BuildHub](https://forum.buildhub.org.uk/topic/8021-online-condensation-analysis/). The calculation requirements described here are from BS EN ISO 6946:2017 and BR 443 (2019).* --- # Building Regulations Part L Explained URL: https://energycount.co.uk/guides/building-regulations-part-l/ Updated: 2026-10-14 > **Quick answer:** Part L is the section of the Building Regulations (England) covering the conservation of fuel and power — the energy performance rules for new and existing buildings. As of September 2026 the edition in force is **Part L 2021**, assessed with **SAP 10.2**; the Future Homes Standard edition, assessed with SAP 10.3, applies to new schemes from **24 March 2027**. A new dwelling complies by passing four SAP tests at once, then proving it as built with photographs, a compliance report and an EPC. ## What Part L is Requirement L1 of Schedule 1 to the Building Regulations 2010 says, in essence, that reasonable provision must be made for the conservation of fuel and power in buildings. **Approved Document L** is the government's guidance on how to meet it, and it is where the numbers live. Since the 2021 edition it comes in two volumes: **Volume 1: Dwellings**, covering new *and* existing homes, and **Volume 2: Buildings other than dwellings**. The old names — L1A for new dwellings, L1B for existing ones, L2A and L2B for other buildings — were retired in 2021, although the paperwork and half the industry still use them. Which volume you are under decides which calculation you need. Volume 1 work is assessed with [SAP](/services/sap-calculations/); Volume 2 work with [SBEM](/services/sbem-calculations/). ## Which edition applies to my project? This is where most of the confusion lives. The picture in England: - **Part L 2013** — introduced the fabric energy efficiency target. **Fully revoked**: its transitional window closed on 15 June 2023. - **Part L 2021** — the edition **in force today**, applying to work from **15 June 2022**. It targets roughly a 31% carbon improvement over 2013 as a stepping stone to net zero. Assessed with **SAP 10.2**: the Notice of Approval of 24 March 2026 brought in SAP 10.3 for the Future Homes Standard but lets buildings notified before 24 March 2027 (and started before 24 March 2028) stay in the earlier version. - **Approved Document L 2026** — the [Future Homes Standard](/guides/future-homes-standard/), published 24 March 2026 and **in force from 24 March 2027** (24 September 2027 for higher-risk buildings). Projects with a building notice or full plans submitted before 24 March 2027 can build to the 2021 standards if work starts on site by **24 March 2028**. If your scheme straddles a date, the question is when it was notified and when it starts, not when it finishes. The transitional protection works **building by building**, not site by site: starting one plot does not protect the rest of the scheme. And "started" has a legal meaning since October 2023 — for a new building it is the point at which the foundations and the structure of the ground floor are complete, not site clearance or the first pour.

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Which edition of Part L applies to your project?

Four questions. You get the edition, the assessment method and the dates that matter for this building — the same logic we run before quoting.

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## Part L for new dwellings: the four tests It is easy to think of Part L as "the carbon test". A new dwelling has to clear **four hurdles at once**: | Test | Metric | What it means | | --- | --- | --- | | Primary energy | DPER ≤ TPER | The headline target in Part L 2021 — total primary energy demand | | Carbon | [DER ≤ TER](/guides/der-ter/) | CO₂ emissions per m² per year | | Fabric efficiency | [DFEE ≤ TFEE](/guides/fabric-energy-efficiency/) | Fabric-only heat demand — cannot be bought back with renewables | | Limiting standards | Backstop values | Every U-value, the air permeability and the fixed services within their limits | The targets come from a **notional dwelling** the same size and shape as yours, built to a reference specification: walls 0.18, floors 0.13, roof 0.11 and windows 1.2 W/m²K; air permeability 5; a gas boiler at 89.5%; waste water heat recovery; and — the one that surprises people — **solar PV sized at 40% of the ground floor area divided by 6.5**. That is why fabric improvements alone rarely get a modern dwelling through without [PV](/guides/solar-pv/) or a [heat pump](/guides/air-source-heat-pumps/). The limiting values are the other half — nothing worse than a wall at 0.26, a floor at 0.18, a roof at 0.16, windows and doors at 1.6, rooflights at 2.2, or an air permeability of 8 — and a dwelling built merely to those will fail the targets by a distance. The full detail is on the [U-values guide](/guides/u-values/). ## Proving it: notices, the BREL report and the photographs The 2021 edition put a process around the calculation, and this is the part people searching for "Part L compliance" usually need. **Before work starts**, the regulations require the target rates, the as-designed rates and a **list of the specifications** the calculation depends on to be notified to Building Control — by the day before work begins — on the design-stage **BREL report** (Building Regulations England Part L compliance report). **Within five days of completion**, the as-constructed rates and any changes to that specification list are notified on the completion-stage BREL, signed by the assessor and the builder. The homeowner gets a copy of each. The completion BREL is accompanied by **photographic evidence** — new in 2021, and the thing most often missing at the end. Every dwelling needs its own set, showing the insulation continuous and properly installed at six stages: the foundations and ground floor; each external wall type; each roof type; each opening type including the reveals and cavity closers; the airtightness details; and the building services, including pipework insulation. They are normally taken by the builder, need to be date-stamped and located, and are reviewed by the SAP assessor against the specification before the report is signed. There is no way to photograph insulation that has been plastered over, so the site team needs to know on day one. The [photographic evidence guide](/guides/part-l-photographic-evidence/) breaks the six stages into thirteen details, says when on the programme to catch each one, and has a free plot checklist to print. Then the **EPC**, produced from the as-built SAP by an accredited on-construction assessor and given to the owner within five days of completion, and the **home user guide** for the occupier. Building Control cannot issue the completion certificate without the EPC. ## The other things Part L 2021 asks of a new dwelling **Air tightness.** [Every new dwelling is tested](/services/air-tightness-testing/) — sample testing ended in 2022 — and the measured result goes into the as-built SAP. **Lighting.** Fixed internal lighting should have lamps of at least **75 lumens per circuit-watt**, with local controls per space; fixed external lighting needs daylight-responsive switch-off, and occupancy control where the efficacy is 75 or below. The notional dwelling assumes 80. **Heating design.** A design flow temperature of 55°C or lower is what the notional dwelling assumes, which is why radiators are getting bigger and why [heat pumps](/guides/air-source-heat-pumps/) fit the standard so naturally. **Thermal bridging.** Junction heat losses are assessed separately from U-values, as Ψ-values, and SAP's default values are punitive — [every Table K1 default alongside the Table R2 notional value](/reference/sap-junctions/) shows how punitive, junction by junction. Calculating your actual junctions — which the free [ΨMonkey psi value calculator](/tools/psi-calculator/) does to BS EN ISO 10211 — is usually the cheapest improvement available; the alternative is to build to [Recognised Construction Details](/guides/accredited-construction-details/), the scheme that replaced Accredited Construction Details under SAP 10. **Overheating is a separate requirement.** Since June 2022 a new dwelling also has to pass [Part O](/guides/building-regulations-part-o/) — glazing limits and free-area rules on the drawings, or TM59 modelling — assessed alongside the SAP, not inside it. **So is condensation.** Requirement C2, in Part C of the regulations, expects walls, floors and roofs to protect the building and the people in it from the harmful effects of interstitial and surface condensation, and the more insulation an element carries, the colder the layers outside it run. Where a build-up steps outside the arrangements BS 5250 treats as safe — insulation along the rafters, a flat roof insulated both above and below the deck — Building Control can ask for a [condensation risk analysis](/tools/u-value-calculator/condensation-risk-calculator/) to BS EN ISO 13788 too, on extensions and conversions as well as new homes. ## Part L for existing dwellings Work to existing homes — [extensions](/services/sap-calculations/extensions/), [conversions](/services/sap-calculations/conversions/), renovations, replacement windows — falls under Volume 1 too, but without the target-rate machinery. New thermal elements must meet the limiting values for new elements in existing dwellings: walls **0.18**, floors **0.18**, roof **0.15**, windows and doors **1.4**, rooflights **2.2**. Retained elements being renovated must be brought from a threshold to an improved value where that is feasible and pays back within 15 years — [RenoMonkey](/tools/thermal-element-calculator/) checks an element against both tests free in your browser, and prints the payback justification Building Control asks for. And every window and door in an existing dwelling has to meet 1.4 on its own — they are excluded from area-weighting. [Building regulations when you renovate](/guides/renovation-building-regulations/) sets out the triggers, the thresholds and the payback test in full. **Extensions** comply on those U-values provided the windows, doors and rooflights stay within **25% of the extension's floor area** plus the area of any existing openings the extension covers ([check yours here](/services/sap-calculations/extensions/#glazing-checker)). Exceed it and there are two routes to showing reasonable provision — an area-weighted U-value calculation, which [GlazeMonkey](/tools/extension-glazing-calculator/) runs free against the notional extension, or a [whole-dwelling SAP](/services/sap-calculations/extensions/) in which improvements to the existing house offset the extra glazing. A **conservatory** escapes Part L only if it is at ground level, no more than 30 m², thermally separated from the house and not on the house's heating system. The whole of the extension side, including the credit for openings the extension covers over, is in [building regulations for an extension](/guides/extension-building-regulations/). **Conversions** — a material change of use creating a dwelling — bring Part L in full, but with no target emission rate; compliance is element by element, and a [SAP](/services/sap-calculations/conversions/) is the usual way to show the trade-offs when a retained stone wall cannot reach the improved value. ## Part L for other buildings Volume 2 does the same job for offices, retail, industrial and everything else that is not a dwelling, with [SBEM](/services/sbem-calculations/) as the calculation, a BRUKL report in place of the BREL, its own limiting standards, and the same design-stage and as-built duties. The commercial EPC is a separate duty on completion. Volume 2's 2026 edition lands on the same dates as Volume 1's. ## What changes under the Future Homes Standard The biggest shake-up to Part L in over a decade lands with **[Approved Document L 2026](/guides/future-homes-standard/)** — retitled "Energy and greenhouse gas emissions". For dwellings: - **In force 24 March 2027**, with the transitional buffer above. - Roughly a **75% cut in carbon emissions** against a 2013 baseline home — effectively the end of new-build gas boilers, with heat pumps the default. - A **new requirement L3** making on-site renewable electricity — solar PV — a legal requirement for new dwellings, rather than something the notional dwelling merely assumed. The [solar PV calculator](/tools/solar-pv-calculator/) works out how much a given ground floor area needs. - A tighter **air permeability** in the notional dwelling. The **limiting U-values do not change**; anyone expecting new backstops will find the same numbers, renumbered. - **SAP 10.3** is the compliance methodology at launch — with new fuel factors: electricity 0.086 kgCO₂e/kWh and a primary energy factor of 1.969, gas 0.214 and 1.12 ([what changed](/news/sap-10-3-what-changed/)); the [Home Energy Model](/guides/future-homes-standard/) follows later and eventually replaces it. Tighter airtightness and a heat-pump baseline make [thermal bridging](/guides/thermal-bridging/) matter *more*, not less. Designing now? The SAP 10 decisions — good fabric, PV, low-carbon heating, [calculated Ψ-values](/tools/psi-calculator/) — are exactly the ones the Future Homes Standard requires. ## Wales and Scotland **Wales** has its own Approved Document L, 2022 edition, in force since **23 November 2022**, with its own tighter carbon target for new homes and its own approval of the SAP methodology; the process — design-stage and as-built calculations, testing of every dwelling, the EPC — is the same shape as England's. **Scotland** has no Part L. The equivalent is **Section 6 (Energy)** of the Building Standards Technical Handbook, with revised standards for building warrants from **1 February 2023**, assessed with SAP 10 under a target emissions rate of its own. ## The bottom line Part L is four tests dressed as one, followed by a paper trail — and the target moves with your dwelling's shape, so the earlier you model it, the cheaper compliance is. Whether you are on Part L 2021 today or planning for the [Future Homes Standard](/guides/future-homes-standard/), the fundamentals are the same: good fabric first, then renewables, and photographs of the insulation before it disappears. Unsure which requirements apply to your project? [Ask us](/contact/) — it is quicker than reading the Approved Documents — or get your [SAP calculations](/services/sap-calculations/) under way. --- # Part L photographic evidence: what to photograph, and when URL: https://energycount.co.uk/guides/part-l-photographic-evidence/ Updated: 2026-09-23 > **Quick answer:** Every new dwelling built under **Part L 2021** needs its own set of photographs, taken during construction, showing the insulation and airtightness were built properly. There are **thirteen details** across six categories: ground floor, walls, roof, openings, airtightness and building services. Each photo must be **plot specific**, carry the **location, date and time**, and be taken **before the next trade covers the detail**. The SAP assessor reviews them before signing the completion BREL report. A plot with gaps in its photographs can't be signed off.
Free download: the plot checklist (PDF)
One page per plot. All thirteen details, when to catch each one, and the rules for every photograph. Print it and keep it in the site office. No sign-up.
Download the checklist ↓
## Why the photographs exist A SAP calculation is only as good as the building it describes. For years the as-built SAP assumed the insulation had gone in as drawn, because nobody could check once it was plastered over. The measured gap between designed and actual performance was large enough that the 2021 edition of **Approved Document L** added a paper trail. The builder photographs the details while they are visible, and the assessor checks them against the specification before signing that the dwelling was built as calculated. The photographs sit alongside the rest of the completion package: the as-built SAP, the completion-stage **BREL report** (Building Regulations England Part L compliance report) signed by the assessor and the builder, the air test result and the **EPC**. The [Part L guide](/guides/building-regulations-part-l/#proving-it-notices-the-brel-report-and-the-photographs) covers how those fit together. ## Who does what - **The builder takes them.** There is no restriction on who holds the camera, but the site team is the only one there at the right moment. - **The SAP assessor reviews them** against the specification the calculation used: insulation type and thickness, the junction details, the heating system and ventilation actually installed. Anything that doesn't match gets raised before the as-built SAP is finalised. - **Building Control receives them** with the completion BREL report. The review is part of the job, not an extra. Our [new-build SAP](/services/sap-calculations/new-builds/) includes it as standard. ## The thirteen details The requirement is organised in six categories. Walls, roofs and openings are **per type**, so a house with a brick-and-block main wall and a timber-framed dormer needs the wall details for both. | Ref | Detail | The photograph must show | Catch it before | |---|---|---|---| | **1a** | Ground floor insulation | Boards laid tight with closed joints, and the thickness | the screed or deck goes down | | **1b** | Perimeter edge insulation | The upstand at the slab edge, continuous all round | the screed goes down | | **1c** | External door thresholds | Insulation under and around the threshold, below the DPC | the frame and screed go in | | **2a** | Wall / ground floor junction | Wall insulation starting low enough to overlap the floor insulation | the floor is finished | | **2b** | Wall insulation and penetrations | Batts or boards tight, no gaps or mortar snots, penetrations sealed | the outer leaf or boarding closes it | | **3a** | Roof insulation at joist level | Both layers, cross-laid, carried into the eaves without compressing | the loft is boarded or handed over | | **3b** | Roof insulation at rafter level | Between and over the rafters; continuity at gables and verges | the plasterboard goes up | | **4a** | Window position | Frame in line with the insulation or cavity closer; jambs, sill and head | the reveals are plastered | | **4b** | Door position | Frame in line with the insulation or cavity closer | the reveals are plastered | | **5** | Airtightness details | Sealing of service penetrations, membrane laps, tapes at frames | dry-lining and boxing-in | | **6a** | Building services: plant | Legible data labels on the boiler or heat pump, cylinder and controls | handover | | **6b** | Building services: pipework | Primary pipework insulated, including runs in voids, and the cylinder | floors and ceilings are closed | | **6c** | Ventilation and renewables | Extract fans, the MVHR unit and insulated ducting, PV or solar thermal | the ceilings close | Not every plot has every detail. A cold-roof bungalow has no rafter-level insulation, and a flat on the second floor has no ground floor. Mark those as not applicable rather than leaving them blank, so the assessor can see the gap is deliberate. Most of the thirteen are **the junctions**, where one element meets another. That's where insulation most often stops short, and those junctions are exactly what the [psi-values](/guides/psi-values/) in your SAP assume are continuous. A photograph of a wall that stops 50 mm short of the floor insulation shows a thermal bridge the calculation has been told doesn't exist. ## What makes a photograph acceptable **It belongs to this plot.** Every dwelling needs its own set, even on a terrace of identical houses. Photographs cannot be shared or reused between plots. **It records where and when.** Turn on location services for the camera and check that the date and time are right before the first photo. Those details tie the photo to a particular plot on a particular day, and they are the first thing checked when a set looks too tidy. **It is named so someone else can find it.** Put the plot and the detail reference in the file name, for example `Plot07_2b_rear.jpg`. A folder of 400 files called `IMG_4471.jpg` has to be sorted by hand at the end, which is when the gaps are found. **It shows the whole detail and the close-up.** A wide shot shows where on the building you are. A close-up shows the joint, the lap or the closer. One photo rarely does both, so take two. **It shows thickness where thickness matters.** A tape measure against the insulation answers the assessor's first question without a phone call. **It is in focus and lit.** Inside a dark cavity or a loft, use the flash. Blurred or dark photos get sent back. ## When on the programme to take them The table above gives the latest moment for each detail. In programme order it runs like this: 1. **Groundworks and slab:** 1a, 1b, 1c. Photograph once the insulation is down, before the screed or deck. 2. **Superstructure:** 2a, 2b, lift by lift for a masonry cavity, or before the sheathing and cladding go on for timber frame. 3. **Roof:** 3b before the plasterboard. 3a once the second layer is down in the loft. 4. **First fix:** 4a, 4b and 5 as the frames go in and the penetrations are sealed, before dry-lining. 5. **Services:** 6b before floors and ceilings close, then 6a and 6c at second fix and commissioning. Brief each trade at the start of its package, not at the end. The groundworker who leaves the slab on a Friday is the only person who can photograph 1a, and nobody will ask them to on Monday. ## The failures we see - **One plot photographed for the whole site.** That's the most common gap, and there's no fixing it after completion. - **The detail is already covered.** The insulation photo taken after the first sheet of plasterboard went on. - **No context.** A close-up of a board edge that could be anywhere on any plot. - **Missing types.** The main wall is photographed but the dormer, bay or rear extension wall is not. - **Services left to the end.** Pipework in the floor void gets boarded over during second fix, before anyone thinks of it. - **Mismatch with the SAP.** The photos show 90 mm of insulation where the SAP assumed 100 mm, or a different window frame. The photos do their job and the as-built SAP has to be re-run. The last one isn't a photography problem, and it's the best reason to send photos to the assessor **as you go**. A mismatch found at first fix is a conversation. Found at completion, it's a failed as-built. ## If you get to the end with gaps The assessor can't sign the completion BREL report for a plot without its evidence. Without the as-built SAP and the [EPC](/services/sap-calculations/epc/), Building Control can't issue the completion certificate. From there the choices are opening up enough of the construction to photograph it, or asking Building Control whether it will accept something else, such as delivery notes, a clerk of works' records or another inspection. That is Building Control's decision, and not one to plan around. ## Which edition applies This guide describes **Approved Document L 2021**, which applies to new dwellings in England today. The Future Homes Standard edition takes effect for new schemes from **24 March 2027**, with transitional arrangements for projects already under way. The [Future Homes Standard guide](/guides/future-homes-standard/) has the dates. Check the edition your plots are being built under with your assessor, and we'll update this page if the requirement changes. ## The bottom line The photographs are cheap to take and very expensive to be without. Print the [checklist](/part-l-photographic-evidence-checklist.pdf), brief every trade, name the files by plot and detail, and send them to your assessor as each stage closes. If you'd like one firm to run the SAP, check the photographs and sign off the BREL, [send us the drawings](/contact/). Our [New Build Compliance Pack](/services/sap-calculations/compliance-pack/) also covers the air tests, water calculation and EPCs. --- # Building Regulations Part O explained URL: https://energycount.co.uk/guides/building-regulations-part-o/ Updated: 2026-10-04 > **Quick answer:** Part O (Overheating) applies to every new residential building in England notified since **15 June 2022** — not to extensions or conversions. A home complies either by the **simplified method** (glazing capped at 11–18% of floor area depending on orientation, location and cross-ventilation, plus a minimum free area of openings) or by **dynamic thermal modelling to CIBSE TM59**, which is what you need when the glass exceeds the tables or the windows cannot be assumed open at night — the 40 dB bedroom-noise rule is the usual trigger. Mechanical cooling comes last, after every passive measure. ## What Part O is For decades the Building Regulations cared about keeping heat in. **Requirement O1**, added to Schedule 1 in 2021 and in force from 15 June 2022, is the first to care about keeping it out. It says that reasonable provision must be made in a new residential building to *limit unwanted solar gains in summer* and to *provide an adequate means to remove heat from the indoor environment*. **Approved Document O** is the government's guidance on how to do that, and it is a short document with two routes through it. The trigger was straightforward: better-insulated, more airtight homes with large areas of glass were overheating, and the 2022 heatwaves made the point. Part O sits alongside [Part L](/guides/building-regulations-part-l/) rather than inside it — a home has to pass both — and it is assessed separately, by an overheating assessment rather than a SAP calculation. ## Who it applies to **New residential buildings only.** New dwellings, and new institutions and other buildings with rooms for residential purposes — care homes, student halls, boarding accommodation — but not hotel rooms. A conservatory built as part of a new home is inside the requirement; one added later is not. Extensions, loft conversions and a **material change of use** — an office turned into flats, a barn into a house — are outside it altogether, however glassy they are. That surprises people, because it is precisely the conversions with floor-to-ceiling glazing that overheat; but the regulation is written around buildings that are *erected*. Part O also does not apply in Wales or Scotland in this form (see below), and it is not retrospective. ## The two routes **The simplified method** (Section 1 of Approved Document O) is a check you can do on the drawings: is the glazing under a limit, and do the windows and other openings give enough free area to purge heat? No modelling, no software, no weather file. If a design meets the limits, that is the evidence. **Dynamic thermal modelling** (Section 2) is a computer model of the dwelling — its fabric, glazing, shading, ventilation and occupancy — run through a design summer to the **CIBSE TM59** methodology, which sets the pass criteria. It costs more and takes longer, but it can pass designs the simplified method cannot, because it credits what the simplified method ignores: thermal mass, external shading, secure night ventilation, the real orientation of every window. The assessor chooses the route, and the choice is usually made for them by three questions: where is the site, which way does the glass face, and can the windows be left open at night? Nothing in Approved Document O says the simplified method has to be done by a specialist. Part 2a of the Appendix B checklist is signed by a person competent to design the home, so on most houses the designer can do their own Part O assessment from the drawings, and a consultant is needed only when the design is heading for the modelling route. ## The simplified method, step by step [O-Monkey](/tools/part-o-overheating-checker/), our free Part O checker, lets you do your own Part O assessment by this route: it runs every step below on your own drawings — the postcode lookup, the tables, Appendix D for every window, the Section 3 rules — and fills in the Appendix B checklist. Here is what it is doing. ### 1. Location Approved Document O splits England in two. A **high-risk location** is one of the urban and suburban **London postcode districts listed in Appendix C**, where night-time temperatures stay high and the urban heat island bites; everywhere else is a **moderate-risk location**. The official FAQ adds that central Manchester also has elevated night temperatures — its minimum standard is the moderate-risk one, but designers are pointed at the high-risk figures for the postcodes it lists. [Check a postcode against Appendix C](/tools/part-o-overheating-checker/high-risk-postcodes/) — all 207 districts are listed there. ### 2. Cross-ventilation A dwelling is cross-ventilated when it has openings on **opposite façades**, so air can be driven through it. A house usually is; a single-aspect flat is not; a corner flat with windows on two adjacent walls does not count either. Cross-ventilation earns more generous glazing limits and lower free-area requirements, because the heat has somewhere to go. ### 3. The glazing limit Find the **largest glazed façade** and its orientation, to the nearest compass point (if it sits exactly between two, use the stricter). Then read the maximum glazing area from Table 1.1 or 1.2 — as a percentage of the **floor area of the dwelling**, with a second, tighter cap on the **most glazed room** as a percentage of that room's floor area. Glazing means the transparent area — glass, not frames. **Table 1.1 — cross-ventilated** (Approved Document O, 2021 edition): | Largest glazed façade | High risk: % of floor area | High risk: most glazed room | Moderate risk: % of floor area | Moderate risk: most glazed room | | --- | --- | --- | --- | --- | | North | 15% | 37% | 18% | 37% | | East | 18% | 37% | 18% | 37% | | South | 15% | 22% | 15% | 30% | | West | 18% | 37% | 11% | 22% | **Table 1.2 — not cross-ventilated:** | Largest glazed façade | High risk: % of floor area | High risk: most glazed room | Moderate risk: % of floor area | Moderate risk: most glazed room | | --- | --- | --- | --- | --- | | North | 15% | 26% | 18% | 26% | | East | 11% | 18% | 18% | 26% | | South | 11% | 11% | 15% | 15% | | West | 11% | 18% | 11% | 11% | Two things jump out. West is the enemy: low afternoon sun arrives when the house is already warm, which is why a west-facing, single-aspect flat is capped at 11% of its floor area. And some high-risk figures are *more* generous than the moderate-risk ones — because in a high-risk location the glazing also has to be shaded (next step), and the tables assume it is. For scale: a 90 m² house with 18% glazing has 16 m² of glass in total, and a 20 m² living room capped at 37% can have 7.4 m² of it. A south-facing rear elevation with bifolds across the full width will usually blow through the room cap before it troubles the whole-dwelling figure. ### 4. Shading (high-risk locations) In a high-risk location, glazing facing anywhere between north-east and north-west through south — in other words everything but a northerly aspect — must be shaded: external shutters that still allow ventilation, glazing with a **g-value no higher than 0.4** and a light transmittance of at least 0.7, or, for south-facing glazing only, an overhang that cuts the sun off entirely once it reaches 50° above the horizon. Internal blinds do not count. ### 5. Free area for removing heat Limiting the gain is half the requirement; the other half is getting rid of what still comes in, and the simplified method does that with **minimum free areas** — the actual open area of windows and other openings when opened as designed (Appendix D shows how to work it out from the opening angle), not the size of the frame. | | High risk | Moderate risk | | --- | --- | --- | | **Cross-ventilated** — total free area, whole dwelling | the greater of 6% of floor area or 70% of the glazing area | the greater of 9% of floor area or 55% of the glazing area | | — bedroom minimum | 13% of the room's floor area | 4% of the room's floor area | | **Not cross-ventilated** — total free area | the greater of 10% of floor area or 95% of the glazing area | the greater of 12% of floor area or 80% of the glazing area | | — bedroom minimum | 13% of the room's floor area | 4% of the room's floor area | Note the trap: the free-area requirement is tied to the *glazing* area, so a design that maximises glass right up to the limit needs a great deal of it to open — and a window that only opens 100 mm on a restrictor provides a small fraction of its frame size as free area. The [equivalent area calculator](/tools/part-o-overheating-checker/equivalent-area-calculator/) works out any window from Appendix D. Two rules in Section 3 cut what a window counts for, and both catch designs out. An **outward-opening window** counts only up to the angle at which its handle is **650 mm from the inside face of the wall** (paragraph 3.9a, with MHCLG's FAQ 15) — so a 600 mm casement set 150 mm back in the wall counts at about 56°, not 90°, and the deeper the reveal the less it counts; [the 650 mm rule, worked out](/tools/part-o-overheating-checker/650mm-handle-rule/). And on an **upper floor**, an opening counts as more than 100 mm only if nobody can fall out — a sill at 1,100 mm or guarding to 1.1 m (paragraph 3.9 and Table 3.1). Inward-opening tilt-and-turn windows are not limited by the first rule, so swapping a casement for one can be the cheapest way to add free area. ### What fails the simplified method The same things every time: a glazed rear elevation facing south or west; single-aspect flats, especially in London; fixed glazing and picture windows that add to the glazing total without adding any free area; and bedrooms with one small opening. And then there is the case where the numbers pass on paper but the assumption behind them does not hold — which is the next section. ## Noise, security and the windows you cannot open The simplified method's free areas assume the windows are **open at night**. Approved Document O says plainly that they will not be if bedroom noise with them open would exceed **40 dB LAeq averaged over 11pm to 7am**, or **55 dB LAFmax more than ten times a night** — a main road, a railway, a pub garden. It says the same where openings cannot be left open securely: ground-floor bedrooms and any window reachable from a flat roof or balcony, and where outdoor air quality means the ventilation strategy has to limit intake under Part F. When any of those apply, the simplified method cannot honestly be used for the affected rooms. The route is dynamic modelling, with the windows in the model behaving as they will in life — closed at night in the noisy bedroom — and the design made to pass anyway, through shading, acoustic ventilators, secure louvres, mechanical ventilation with a summer bypass, or, last of all, cooling. This is where most Part O effort and money goes on urban sites, and it is why a noise survey often arrives on the desk before the overheating assessment does. ## The dynamic route: what TM59 asks for A TM59 assessment builds a thermal model of the dwelling — every room, its glazing and orientation, the fabric and its thermal mass, the shading, the ventilation openings and their real free areas — and simulates a whole year with TM59's occupancy and internal-gain profiles, in the **CIBSE Design Summer Year** weather file for the 2020s, high emissions, 50th percentile. Windows open in the model when the room passes 22 °C and are fully open at 26 °C; at night they open only where the room is above 23 °C at 11pm and the window is above ground floor and not easily accessible. The home passes if it meets **both** criteria: - **Living rooms, kitchens and bedrooms:** the operative temperature exceeds the adaptive comfort threshold by 1 K or more for no more than **3% of occupied hours** from May to September. - **Bedrooms at night:** the operative temperature between 10pm and 7am exceeds **26 °C** for no more than **1% of annual hours** — 32 hours a year. The bedroom criterion is the one that fails, and it fails on west-facing bedrooms with a single window first. The fixes, in the order an assessor reaches for them: cut or shade the glazing, improve the g-value, add secure night ventilation, use cross-ventilation where the plan allows, add thermal mass, add mechanical ventilation with a boost — and only then cooling, because Requirement O1 makes mechanical cooling the last resort and Approved Document O asks the designer to show Building Control that **all practicable passive means** were used first. Cooling that is installed also has to be carried into the [Part L](/services/sap-calculations/new-builds/) calculation, where it costs energy. CIBSE published a substantially revised **TM59 in July 2026** — a passive-first three-stage method, four criteria and a rewritten bedroom test. Approved Document O still cites the 2017 edition, so for Building Control the 2017 criteria remain the test; [what the 2026 rewrite changes](/news/cibse-tm59-2026-overheating-update/) is worth knowing if you are designing now. ## What Building Control gets For either route: the **compliance checklist** in Appendix B of Approved Document O, completed and signed, showing which route was used and the figures. For the modelling route, the **TM59 report** with the contents the methodology specifies. And the owner must be given non-technical information on how the home is meant to stay cool — which windows to open, what the shading is for — because a passive strategy only works if the people in the house know it exists. ## Wales, Scotland and London **Wales** has its own Part O and Approved Document O, in force since 23 November 2022, with the same two routes but a simplified method organised by single- or dual-aspect dwellings rather than by London postcode. **Scotland** has no Part O; overheating is **Standard 3.28** of the Domestic Technical Handbook, in force since 1 December 2022, again with a simple method and a TM59 route. **London** adds a planning-stage layer on top of Part O: the London Plan's cooling hierarchy expects major schemes to demonstrate overheating performance with TM59 modelling at application stage — so a London scheme often models twice, once for planning and once for Building Control. [Our Part O service](/services/part-o-overheating/) covers all three. ## The bottom line Part O is a drawings problem before it is a modelling problem. If the largest glazed façade faces north or east, the dwelling is cross-ventilated, the site is not in a London high-risk postcode and the bedrooms are quiet enough to sleep with the windows open, the simplified method will usually pass and the assessment is quick. Move any one of those — south or west glass, a single-aspect flat, a main road — and you are into TM59, where the cost of the fix rises the later it is found. The cheapest Part O assessment is the one done on the planning drawings. One thing Part O never asks about is thermal mass, though it is what decides how much of the afternoon's heat reaches a bedroom at midnight. If that question is live on your scheme, the [thermal mass calculator](/tools/u-value-calculator/thermal-mass-calculator/) gives the decrement factor and time lag for any build-up to BS EN ISO 13786, free — useful context for a TM59 model, though not a substitute for one. --- # What is PHPP (and when do you actually need it)? URL: https://energycount.co.uk/guides/phpp-energy-statements/ Updated: 2026-06-21 ## What PHPP is **PHPP** — the Passive House Planning Package — is an energy model developed by the Passivhaus Institute. It predicts a building's **space heating demand** and **total energy use** far more reliably than a Part L [SAP calculation](/services/sap-calculations/), because it models the building physics in much greater detail. Post-occupancy studies consistently show PHPP gets close to real-world performance, where SAP can drift. ## When you actually need it for planning Most planning energy conditions are still satisfied with a standard [energy statement](/services/energy-statements/) and a modest renewables contribution. But a wave of authorities have moved to tougher, **energy-based net-zero targets** — typically a space heating demand of around 15–30 kWh/m²/yr and an Energy Use Intensity (EUI) cap of around 35–40 kWh/m²/yr, covering all the energy a building uses. SAP wasn't built to demonstrate those figures, so these councils run their own **energy summary tools** that are populated from PHPP modelling. If your site is in one of these areas, PHPP is effectively the entry ticket: - **Cornwall** — SEC1 Energy Summary Tool (Climate Emergency DPD) - **Bath & North East Somerset** — Energy Summary Tool 2, required for developments of 10+ dwellings - **Central Lincolnshire** — Local Plan Policy S6 - **Greater Cambridge** — Policy CC/NZ ## PHPP for planning is *not* Passivhaus certification This is the bit that trips people up. Using PHPP to **demonstrate a planning energy target** is not the same as **certifying a building to the Passivhaus standard**. Certification is a separate, more demanding process — design scrutiny, airtightness targets, certified-product detailing and sign-off — and it's run by certified Passivhaus designers. You can (and often must) use PHPP modelling to satisfy these planning policies *without* pursuing the Passivhaus badge. If you do want the badge as well, you'll want a certified Passivhaus designer on the team for that side of it. ## How this fits your project The practical upshot: in these areas, [fabric performance](/guides/fabric-energy-efficiency/) and [u-values](/guides/u-values/) matter even more, and the energy statement is a more involved piece of work that's best started early — before the design is fixed. We run the PHPP modelling for these planning energy targets and produce the statement in-house, so the modelling and the planning report come from one place. Got a condition that mentions space heating demand, EUI, or an energy summary tool? [Send it over](/contact/) and we'll tell you exactly what's needed — see our [energy statements service](/services/energy-statements/) for more. --- # SAP 10: What's New in Part L 2021? URL: https://energycount.co.uk/guides/sap-10-whats-new/ Updated: 2026-09-20 ## What SAP actually is SAP (Standard Assessment Procedure) is the government's methodology for rating the energy performance of dwellings — the engine behind Energy Performance Certificates, [Building Regulations Part L](/guides/building-regulations-part-l/) compliance, and most planning conditions. A trained assessor builds a model of your dwelling — fabric, heating, hot water, ventilation, lighting, renewables — and SAP turns it into the numbers that decide whether you pass: the **DER/DPER** (dwelling emission rate) against the **TER/TPER** target ([plain-English guide](/guides/der-ter/)). SAP 10 is the current methodology; this page is about the changes it brought in 2021–22. ### Who needs a SAP assessment You'll hit SAP in four places: **Building Regulations compliance** for new dwellings (Part L 1A) and material works to existing ones (Part L 1B); the **EPC** you must hand over on completion or sale; **planning conditions** on many developments (especially via [energy statements](/services/energy-statements/)); and increasingly **mortgage/valuation and net-zero policies** on new homes. Our [SAP calculations service](/services/sap-calculations/) covers all of them — and the [New Build Compliance Pack](/services/sap-calculations/compliance-pack/) bundles SAP with the [water](/services/part-g-water-calculations/), [acoustics](/services/sound-testing/) and [air-tightness](/services/air-tightness-testing/) evidence most schemes need anyway. ## The headline Part L 2021 went live on 15 December 2021 and has applied to all developments registered after **15 June 2022**. It targets a **31% improvement in CO₂ emissions** over Part L 2013, achieved through better fabric plus renewables and low-carbon energy in the target calculation. The assessment tool is **SAP 10**, successor to SAP 2012. ## A much tougher notional dwelling The notional dwelling that sets your targets (TER/TPER) is far tighter in SAP 10: | Element | Notional u-value (TER) | Maximum u-value allowed (DER) | | --- | --- | --- | | Walls | 0.18 | 0.26 | | Floor | 0.13 | 0.18 | | Roof | 0.11 | 0.16 | | Doors | 1.0 | 1.6 | | Windows | 1.2 | 1.6 | | Fully-glazed doors | 1.2 | 1.6 | | Roof windows | 1.2 | 1.4 | Plus: air test target of 5.0, waste water heat recovery to all showers, an 89.5%-efficient gas boiler, and 80 lm/W lighting. To pass, your [DER and DPER](/guides/der-ter/) must beat the targets — exceed any notional value and you'll need to offset it elsewhere. ## The big one: PV in the notional dwelling SAP 10's notional dwelling assumes PV is installed — for houses, kW of PV = 40% of the ground-floor area ÷ 6.5 (for flats, 40% of the flat's floor area ÷ 6.5 ÷ the number of storeys in the block). That's roughly **4kW for an average dwelling**. In our experience fabric improvements alone won't get most dwellings through SAP 10: the practical routes to a pass are [solar PV](/guides/solar-pv/) or a [heat pump](/guides/heat-pumps/). It's a deliberate policy choice — the target assumes you're generating — and it's the change that surprises most people new to SAP 10. ## Other changes worth knowing **Electricity's carbon factor fell from 0.519 kgCO₂/kWh to 0.136** — below mains gas (0.210) for the first time, reflecting the greener grid. SAP 10.0 (2018) had cut it to 0.233; SAP 10.1 and SAP 10.2 took it to 0.136, applied month by month rather than as one flat figure, and SAP 10.3 — approved on 24 March 2026 as the Future Homes Standard methodology, while Part L 2021 work stays in 10.2 — uses a forward-looking 0.086 against 0.214 for gas, with electricity's primary energy factor rising to 1.969 ([what changed in SAP 10.3](/news/sap-10-3-what-changed/)). All-electric designs, brutal to pass under SAP 2012, now have far more flexibility. It's why heat pumps, [MVHR](/guides/mvhr/) and efficient electric hot water have become so common in new-build specs. **Thermal bridging options changed.** SAP 10 removed Accredited Construction Details over accuracy concerns. That leaves manufacturer schemes — or bespoke calculated Ψ-values for each junction, which is exactly what our [ΨMonkey psi calculator](/tools/psi-calculator/) produces. [The defaults those replace](/reference/sap-junctions/) are tabulated in full. [More on thermal bridging](/guides/thermal-bridging/). **Heating pattern** — now a consistent daily pattern instead of different weekday/weekend assumptions, reducing modelled energy use. Heat network loss factors increased, which matters for London planning carbon targets. **Lighting** — proper lighting design now feeds the calculation (like SBEM), recognising high-efficacy LED products. **Hot water** — shower flow rates are now entered, creating crossover with [Part G water calculations](/services/part-g-water-calculations/). **PV configuration matters** — flats only benefit from PV they're directly connected to; landlord-supply-only arrangements no longer count for every flat. Battery storage and PV diverters can now be included, and MCS overshading data can replace defaults. **Fabric & services beyond the table** — [fabric energy efficiency](/guides/fabric-energy-efficiency/), the [energy hierarchy](/guides/energy-hierarchy/) and [fuel choice in SAP](/guides/fuel-choice-in-sap/) all interact with the target: better fabric shrinks the PV/heat-pump requirement; the right fuel choice changes the whole calculation. ## What this means for you Design for SAP 10 from day one: realistic fabric, PV or heat pump from the start, and calculated junction Ψ-values instead of defaults. Our [SAP team](/services/sap-calculations/) can model your options. ## Where it's going: the Future Homes Standard SAP 10 is the stepping stone to the [**Future Homes Standard**](/guides/future-homes-standard/) — and that's no longer a distant "someday." [Approved Document L 2026](/news/future-homes-standard-adl-2026/) was published on 24 March 2026 and **comes into force on 24 March 2027**, effectively ending new-build gas boilers and making on-site renewables — in practice [solar PV sized to requirement L3](/tools/solar-pv-calculator/) — a requirement, with **SAP 10.3** the compliance methodology at launch (the [Home Energy Model](/news/home-energy-model-launch-delayed/) follows later). If you're designing now, the SAP 10 decisions — fabric, PV, heat pumps, low-carbon heating — are exactly the decisions the FHS will require. The [energy statements](/services/energy-statements/) and [PHPP](/guides/phpp-energy-statements/) worlds are converging on the same place. ## SAP 10 vs SAP 2012 at a glance | Change | SAP 2012 | SAP 10 | | --- | --- | --- | | Electricity carbon factor | 0.519 kgCO₂/kWh | 0.233 in SAP 10.0 → 0.136 in SAP 10.2 (Part L 2021) → 0.086 kgCO₂/kWh in SAP 10.3 (Future Homes Standard) | | PV in the notional dwelling | No | Yes (~4 kW typical) | | Accredited Construction Details | Accepted | Removed — manufacturer schemes or calculated Ψ-values | | Heating pattern | Weekday/weekend profiles | Consistent daily pattern | | Shower flow rates | Not modelled | Entered into the calculation | | Lighting | Default assumptions | Lighting design feeds in (like SBEM) | | WWHR | Optional | Assumed on all showers in the notional dwelling | | Air-tightness target | 5.0 m³/h/m² | 5.0 m³/h/m² | ## Quick facts - Carbon factors: electricity **0.136**, gas **0.210** kgCO₂e/kWh in SAP 10.2 (Part L 2021 work); **0.086** and **0.214** in SAP 10.3 (the Future Homes Standard methodology, approved 24 March 2026), whose primary energy factors are 1.969 for electricity and 1.12 for gas. Do not mix these with the [annual UK Government conversion factors](/reference/carbon-factors/) — those are for reporting, these are for compliance, and a submission built on the wrong set will not reconcile. - Notional PV for a house: 40% of lowest storey area ÷ 6.5. - Air-tightness target in the notional dwelling: 5.0 m³/h/m². - WWHR assumed on all showers in the notional dwelling. - Calculated junction Ψ-values (BS EN ISO 10211) beat SAP defaults — often the cheapest route to a pass. Further reading: [BRE SAP 10](https://bregroup.com/sap/sap10) and [Approved Document L](https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l). --- # What is the Future Homes Standard? URL: https://energycount.co.uk/guides/future-homes-standard/ Updated: 2026-10-02 > **Quick answer:** the Future Homes Standard is England's next set of new-build energy regulations — published 24 March 2026 as Approved Document L 2026, and applying to schemes from **24 March 2027**. New homes must emit **at least 75% less carbon** than 2013-standard homes, in practice meaning heat pumps or heat networks, tighter fabric limits and, for most houses, rooftop solar. As of August 2026, compliance at launch is assessed in **SAP 10.3** (the Home Energy Model comes later). ## What it is The Future Homes and Buildings Standard (FHS) is the next generation of Building Regulations energy requirements for England, and the biggest change to new-build standards since [Part L](/guides/building-regulations-part-l/) was last fundamentally rewritten. It requires new homes to produce **at least 75% less carbon** than a home built to 2013 standards — a step far beyond the 30-31% cut delivered by the [Part L 2021 uplift](/guides/sap-10-whats-new/). The aim is homes that are **zero-carbon-ready**: low-carbon heating (in practice, [heat pumps](/guides/heat-pumps/) or heat networks), high fabric standards and on-site renewable generation, with no reliance on fossil fuels. A zero-carbon-ready home isn't zero-carbon on day one — it's designed so it becomes net-zero as the electricity grid decarbonises, needing no retrofit along the way. ## The timeline The final Approved Documents (new editions of **Approved Document L** Volumes 1 and 2, and **Approved Document F** Volume 1) were published on **24 March 2026**, alongside the government's response to the 2023 consultation and Building Circular 01/2026. | Milestone | Date | | --- | --- | | Approved Documents published | 24 March 2026 | | FHS comes into force (most work) | 24 March 2027 | | Deadline to apply under Part L 2021 | 24 March 2027 | | FHS in force for higher-risk buildings | 24 September 2027 | | Transitional projects must have commenced | 24 March 2028 | A **12-month transition period** protects projects already in the pipeline — more on the detail [below](#transitional-arrangements) — after which the FHS applies in full. ## The four big changes ### 1. Low-carbon heating — the effective end of the gas boiler The FHS doesn't "ban" gas boilers by name. Instead the **75% carbon reduction** target is set at a level no fossil-fuel system can meet — so hybrid boilers and hydrogen-ready boilers won't comply either. In practice that means **air source or ground source [heat pumps](/guides/heat-pumps/)**, or connection to a low-carbon heat network, in the vast majority of new homes. This only affects **new homes**. Existing properties are unaffected: you can still install or replace a gas boiler in an existing house. The government estimates FHS homes will save households up to **£830 a year** versus a typical EPC-C property — though, honestly, savings against a *Part L 2021* new build are more marginal, since heat pump electricity still costs roughly four times as much per unit as gas. ### 2. Fabric — the same U-values, tighter airtightness The FHS keeps a whole-building performance approach, so you can trade off between elements. Two things catch people out. First, **the U-values do not tighten**: the limiting values are identical to Part L 2021 (walls 0.26, floors 0.18, roofs 0.16, windows and doors 1.6), and the SAP 10.3 notional dwelling has the same fabric U-values the 2021 notional already had — what tightens is its air permeability, from 5 to 4. Second, **SAP 10.3 and the Home Energy Model share one notional dwelling** — published as Appendix R of SAP 10.3 and, for HEM, as the dwelling notional buildings document — with only minor differences in implementation: HEM sizes the notional heat pump to the dwelling, where SAP fixes its seasonal COP (SCOP) at 2.5. The fabric is the same either way: | Element | Part L 2021 notional | FHS notional (SAP 10.3 and HEM) | Limiting value (unchanged) | | --- | --- | --- | --- | | External walls | 0.18 | 0.18 | 0.26 | | Ground floor | 0.13 | 0.13 | 0.18 | | Roof | 0.11 | 0.11 | 0.16 | | Windows | 1.2 | 1.2 | 1.6 | | Doors | 1.0 | 1.0 | 1.6 | | Airtightness | 5.0 | 4.0 | 8.0 | U-values in W/m²K; airtightness in m³/(h·m²) @ 50 Pa. The notional dwelling sets the target you have to beat; the limiting value is the backstop no element may be worse than. See our [U-values guide](/guides/u-values/) for what these mean in practice, or test a wall, roof or floor build-up against these figures in the free [U-value calculator](/tools/u-value-calculator/). One thing that **doesn't** change: junction [thermal bridging](/guides/thermal-bridging/) stays central to the fabric calculation. Calculated Ψ-values (see [ΨMonkey](/tools/psi-calculator/)) remain the best alternative to [penal default values](/reference/sap-junctions/) under both SAP 10.3 and HEM — and with a tighter air permeability and a heat-pump baseline, well-detailed junctions matter more, not less. ### 3. Mandatory solar PV — a genuine first This is the headline everyone missed in the earlier drafts. A new **functional requirement L3** makes on-site renewable generation a **legal requirement of Building Regulations** — the first time English regs have mandated renewable generation outright, rather than merely assuming it in the notional dwelling. The rule (AD L1 para 5.73): an array whose annual output at least matches a fixed benchmark — panels of 0.22 kWp/m² over **40% of the dwelling's ground-floor area**, facing south-east to south-west, pitched at 45° and unshaded. In SAP 10.3 that comes to about **70.7 kWh a year per m² of ground floor**, whichever way your roof faces; a south-facing roof gets there with slightly less than the benchmark's kWp, an east–west roof needs around 13% more. Where the roof genuinely cannot take it, an array covering the reasonably practicable roof area is accepted instead. The [solar PV calculator](/tools/solar-pv-calculator/) works out the target and what your own roof needs, and [MHCLG's FAQ on requirement L3](/news/adl-2026-requirement-l3-solar-pv-faq/) settles how north-facing slopes, garages and flats are treated. That 40% *sounds* unchanged from [SAP 10](/guides/sap-10-whats-new/) — but there's a catch. The conversion factor from roof area to installed power has changed from 1/6.5 (about 153 Wp/m²) to **1/4.5 (about 222 Wp/m²)**, reflecting more efficient modern panels. So while the headline number looks the same, the **benchmark array is over 40% bigger** than the one the 2021 notional dwelling assumed: about **0.09 kWp per m² of ground floor** — 3.5 kWp for a 40 m² ground floor, 5.3 kWp for 60 m², 7 kWp for 80 m². Here is [how many panels that is, by floor area and roof direction](/tools/solar-pv-calculator/how-many-solar-panels/). **Exemptions:** requirement L3 does not apply to buildings with a storey 18 metres or more above ground (the "relevant buildings" of regulation 7(4)), and where the whole usable roof cannot produce **720 kWh per year** — for example because of heavy shading or awkward roof geometry — paragraph 5.77 treats it as an exceptional circumstance in which no PV may be required. For flats, tight roofs and the paperwork, see [solar PV for flats](/tools/solar-pv-calculator/flats/), [the reasonably practicable roof area](/tools/solar-pv-calculator/reasonably-practicable-roof-area/) and [the evidence Building Control will want](/tools/solar-pv-calculator/building-control-evidence/); our [solar PV guide](/guides/solar-pv/) covers how it earns its keep in a compliance calculation. ### 4. Ventilation and airtightness — Part F tightens too The notional dwelling's tighter airtightness — 4.0, down from 5.0, with the 8.0 backstop unchanged — means **continuous mechanical ventilation becomes effectively unavoidable** in most FHS homes, whether decentralised mechanical extract ([dMEV](/guides/mvhr/)) or MVHR for the heat-recovery benefit: design below 5 and the updated Approved Document F no longer lists natural ventilation as a route ([more below](#the-notional-dwelling-one-specification-to-beat)). It also sets out how systems are signed off: - Installations are certified either by a **registered competent person** (self-certification) or by the **building control body** (AD F paragraph 4.2). - Air flows from **continuous** extract and supply fans must be measured with a **powered flow hood**; a vane anemometer is still allowed for intermittent extract fans. The device needs a proprietary hood, ±5% accuracy and calibration within the last 12 months (paragraph 4.24). Poorly commissioned ventilation has been a persistent problem in airtight new builds, and these changes are a direct response. They sit alongside completion-stage [air tightness testing](/services/air-tightness-testing/), which becomes more important than ever as targets drop. ## The notional dwelling: one specification to beat There is **one** notional dwelling. The 2023 consultation floated two — a stricter Option 1 and a less demanding Option 2 — and the government's March 2026 response adopted **Option 1's fabric and services**, with solar handled by requirement L3 instead. SAP 10.3 sets it out in Appendix R, Table R1: | Item | FHS notional dwelling (SAP 10.3 Table R1) | | --- | --- | | Heating | Heat pump, seasonal COP (SCOP) 2.5 | | Ventilation | Decentralised mechanical extract (dMEV), SFP 0.15 W/(l/s) | | Air permeability | 4.0 m³/(h·m²) at 50 Pa | | Waste water heat recovery | Every shower, including showers over baths (instantaneous, 50% efficient) — none in single-storey dwellings | | Chimneys and flues | None | | Solar PV | Same kWp as the actual dwelling's array, capped at 0.4 × ground-floor area ÷ 4.5 (houses) | | Fabric | As in the table above | You don't have to build the notional: the targets are calculated from it, and you can trade one element off against another. Two of its lines shape most designs, though. - **Waste water heat recovery.** Leave it out of a dwelling with more than one storey and you start behind the target, so the shortfall has to come from somewhere else — fabric, a more efficient heat pump or more PV. Our [WWHRS guide](/guides/waste-water-heat-recovery/) covers the options. - **Airtightness decides the ventilation.** At 4.0 the notional is "highly airtight" under Approved Document F 2026, which means a design air permeability below 5 (or an as-built result below 3). For highly airtight homes, natural ventilation — background ventilators with intermittent extract fans — is no longer a listed route (Table 1.6), so a design at 4.0 needs continuous mechanical extract, such as dMEV, or MVHR. Design at 5.0 or looser to keep natural ventilation and you carry an airtightness shortfall against the notional that has to be made up elsewhere; and if the finished home then tests below 3, AD F paragraph 1.46 calls for expert advice or continuous extract anyway. More in [Part F 2026: what 'highly airtight' means](/news/part-f-2026-highly-airtight-dwellings/). ## How compliance is assessed: SAP 10.3 now, HEM later Compliance is demonstrated the familiar way — showing your dwelling performs at least as well as a **notional building** of the same size and shape. Approved Document L 2026 retains the three [Part L 2021 metrics](/guides/der-ter/): primary energy rate (DPER), carbon emissions rate (DER) and [fabric energy efficiency](/guides/fabric-energy-efficiency/) (DFEE). No simplification — compliance stays multi-dimensional. At launch, assessment is via **SAP 10.3**, an updated version of the familiar [SAP methodology](/services/sap-calculations/). The government's new **Home Energy Model (HEM)** — a complete replacement for SAP that models performance at half-hourly intervals — was expected to follow around three months after publication, but was **delayed again on 8 June 2026**. As things stand, **SAP 10.3 is the sole approved methodology**, with HEM expected in the coming months, followed by a dual-running period before it eventually replaces SAP entirely. HEM is a bigger shift than the version number suggests: it demands far more data than SAP (early testing suggests a house type takes around 1h40 in HEM versus roughly 20 minutes in SAP), with punitive defaults where data is missing, and it will be delivered through a single central calculation service rather than competing third-party engines. We're tracking every software release closely. ## What it costs The government's impact assessment puts the extra build cost at roughly **£4,350 per dwelling** (weighted average, 2025 prices) — or in the region of **3–8% per home** — driven mainly by the heat pump, solar PV, extra insulation and mechanical ventilation. That's offset by lower running costs and no gas standing charge, and the premium is expected to narrow as heat pump and PV supply chains scale up. Expect a **surge in demand** for both as the deadline approaches, so procurement planning matters. ## Transitional arrangements The 12-month transition runs from **24 March 2027 to 24 March 2028**, and it's stricter than previous uplifts: - Projects with an application submitted **before 24 March 2027** can build to Part L 2021 — **if work commences before 24 March 2028**. - "Commenced" has the [regulation 46A](https://www.legislation.gov.uk/uksi/2010/2214/regulation/46A) meaning (regulation 5(5) of SI 2026/335): for most buildings the foundations, any basement and the structure of the ground floor must be **complete** — digging foundations or laying drains is not enough. More in [what "commenced" actually means](/news/future-homes-standard-transitional-deadline-commencement/). - Arrangements operate on an **individual-building basis**, not site-wide, so different plots on the same site can fall under different regs. - The 2013 transitional provisions (and the related 2021 provision) that still let some long-running sites build to 2010 standards are **revoked** — but a building already relying on them keeps that protection if work on it starts before 24 March 2028 (regulation 10). Any plot not started by then must meet the FHS. ## What to do now Projects gaining planning consent from here on will most likely be built under the FHS — so design for it now: heat pump heating, strong fabric, mechanical ventilation planned from the start, and [solar PV](/guides/solar-pv/) sized to L3. Get the [thermal bridging](/guides/thermal-bridging/) detailed properly while the drawings are live, and don't fix a specification for 2027 starts before you've stress-tested it against the numbers. We're tracking the secondary legislation and software releases closely — [talk to us](/contact/) before you lock in a spec, and we'll tell you where the risks are. --- # What is DER/TER? URL: https://energycount.co.uk/guides/der-ter/ Updated: 2026-10-01 > **Quick answer:** DER is the Dwelling Emission Rate — your design's calculated CO₂ emissions in kgCO₂/m² per year. TER is the Target Emission Rate, calculated for a notional dwelling of the same size and shape. **Pass = DER ≤ TER.** Both come out of the SAP calculation, and under Part L 2021 they sit alongside a primary-energy test (DPER ≤ TPER), a fabric test (DFEE ≤ TFEE) and the limiting values. ## What does DER stand for? DER stands for **Dwelling Emission Rate**: the carbon dioxide emissions of your dwelling as calculated by SAP, in **kgCO₂/m² per year** — the annual emissions from space heating, hot water, fixed lighting, pumps and fans, less anything generated on site, divided by the floor area. It is calculated for the building as designed, and again as built. ## What does TER stand for? TER stands for **Target Emission Rate**: the emission rate of the **notional dwelling** — a version of your dwelling with the same size, shape, orientation and opening areas (capped at 25% of the floor area), built to the reference specification in Approved Document L. Under Part L 2021 that specification is walls at 0.18 W/m²K, floors 0.13, roofs 0.11, windows 1.2, an air permeability of 5, an 89.5% gas boiler, waste water heat recovery on the showers and a PV array sized at 40% of the ground-floor area divided by 6.5; the full set is on the [reference page](/reference/#sap). SAP works the notional dwelling through the same calculation as yours, and its result is your TER. Because the notional dwelling is your dwelling with a standard specification, the TER is different for every design. A compact mid-terrace and a sprawling bungalow of the same floor area get different targets, which is why there is no national pass mark to design to, and why the target only exists once the SAP has been run. ## The carbon pass/fail The design-stage [SAP calculation](/services/sap-calculations/) produces both numbers at once. Building Control needs the design-stage figures before work starts and the as-built figures within five days of completion — regulation 27 of the Building Regulations, with 27A and 27C doing the same for the fabric and primary energy rates — and the as-built calculation is also what the [EPC](/services/sap-calculations/epc/) is lodged from. Under [SAP 10](/guides/sap-10-whats-new/) the notional dwelling has a demanding specification — including the assumed PV array — which is why the TER is much harder to beat than under older versions of SAP: a gas-heated design with no PV of its own will not pass on carbon, whatever its fabric. Under the Future Homes Standard that changes: SAP 10.3's notional dwelling has the same PV as yours, up to a cap (40% of the ground floor area ÷ 4.5 for a house), so below that cap PV no longer buys headroom against the target — and requirement L3 makes the array a requirement in its own right. The [solar PV calculator](/tools/solar-pv-calculator/) works out how much. ## DPER and TPER: the primary energy test Part L 2021 added a parallel pair, the **Dwelling Primary Energy Rate** and **Target Primary Energy Rate**, in kWh of primary energy per m² per year. Primary energy counts what it took to generate and deliver each kilowatt-hour, so a unit of grid electricity is worth 1.501 kWh of primary energy in SAP 10.2 against 1.130 for gas, and the primary energy test is the headline test of the 2021 edition. The two tests pull in different directions for electric heating — a heat pump wins both, a panel heater passes on carbon and fails on primary energy — and SAP 10.3, the Future Homes Standard methodology, moves the electricity factors further apart still: carbon down to 0.086 kgCO₂e/kWh, primary energy up to 1.969. [What changed in SAP 10.3](/news/sap-10-3-what-changed/). ## What moves the DER Plenty: orientation, [fuel choice](/guides/fuel-choice-in-sap/), [u-values](/guides/u-values/), [air tightness](/services/air-tightness-testing/), [thermal bridging](/guides/thermal-bridging/), heating system efficiency and renewables. Energy assessors use the DER/TER gap to show the design team exactly where a dwelling is underperforming — and the cheapest places to claw it back. The fabric side is the part you can test yourself before an assessor runs the full SAP: model element [U-values in the free U-value calculator](/tools/u-value-calculator/) and junction [Ψ-values in the free psi value calculator](/tools/psi-calculator/) to see how much headroom your specification gives the DER — often the junctions are the cheapest place to find it, because SAP's default Ψ-values are often several times a [calculated psi value](/guides/psi-values/) — [the Table K1 versus R2 comparison](/reference/sap-junctions/) puts a number on the gap for each junction. ## The four tests together CO₂ isn't the only criterion. A new dwelling under Part L 2021 passes four tests at once — DPER ≤ TPER, DER ≤ TER, [DFEE ≤ TFEE](/guides/fabric-energy-efficiency/), and the limiting values for U-values, air permeability and services — and missing any one of them is a fail. The fabric test is the one renewables cannot rescue; the carbon and primary energy tests are the ones a change on the heating side moves most. The [Part L guide](/guides/building-regulations-part-l/) walks through all four, and the [Future Homes Standard](/guides/future-homes-standard/) keeps the same three metrics against a heat-pump notional dwelling from 24 March 2027. --- # What is Fabric Energy Efficiency (DFEE/TFEE)? URL: https://energycount.co.uk/guides/fabric-energy-efficiency/ Updated: 2026-09-20 > **Quick answer:** Fabric Energy Efficiency (FEE) is SAP's fabric-only heat demand metric, in kWh/m² per year. **DFEE** (your dwelling's figure) must not exceed **TFEE** (the target — the fabric demand of a notional dwelling the same size and shape as yours, built to the reference fabric). Renewables can't rescue it: only insulation, air tightness and thermal bridging move the number. It is one of the three Part L 2021 metrics, and it stays one under the Future Homes Standard from 24 March 2027. ## What does DFEE stand for? DFEE stands for **Dwelling Fabric Energy Efficiency** — your design's fabric-only heat demand in kWh/m² per year, before any credit for renewables or heating. To pass Part L your DFEE must be equal to or lower than your **TFEE** (Target Fabric Energy Efficiency). ## What does TFEE stand for? TFEE stands for **Target Fabric Energy Efficiency** — the fabric-only heat demand of the notional dwelling, in kWh/m² per year: a dwelling the same size and shape as yours, with the reference fabric specification from Approved Document L, run through SAP under standardised conditions. Your DFEE must be equal to or lower than it. ## Why a fabric target exists Before April 2014, new homes in England & Wales were assessed in Part L against a carbon target only ([DER/TER](/guides/der-ter/)). That skewed designs in odd ways — you could build with inefficient fabric and rescue the CO₂ number with renewables or low-carbon heating. Sensible engineering (and the [energy hierarchy](/guides/energy-hierarchy/)) says reduce heat losses first, then add renewables. So Fabric Energy Efficiency (FEE) was added to Part L in the 2013 edition (effective 6 April 2014). In SAP it appears as **DFEE/TFEE** — Dwelling vs Target Fabric Energy Efficiency, measured in **kWh/m² per year**. The dwelling's DFEE must be equal to or lower than the TFEE of the notional dwelling. ## What drives it Fabric energy efficiency is exactly what it sounds like: the heat-loss performance of the building fabric itself — - [u-values](/guides/u-values/) of walls, floors, roofs and openings - [air tightness](/services/air-tightness-testing/) - [thermal bridging](/guides/thermal-bridging/) at junctions Renewables can't help you here — that's the point. If your DFEE is marginal, the fixes are fabric fixes, and calculated junction Ψ-values (try [ΨMonkey](/tools/psi-calculator/)) are often the cheapest one available, since they improve the number without changing anything on site. [Every K1 default and its R2 notional counterpart](/reference/sap-junctions/) is tabulated if you want to size the opportunity first. ## How the TFEE target is actually set Your target isn't a fixed national number — it's built from **your** dwelling. SAP takes a **notional dwelling** of identical size, shape and orientation, gives it the reference fabric specification from Approved Document L — walls at 0.18 W/m²K, floors 0.13, roofs 0.11, windows 1.2, an air permeability of 5 — and calculates its fabric energy demand under standardised conditions: UK average climate, natural ventilation, a fixed heating pattern, so that only the fabric is being compared. That figure is your target: **TFEE = the notional dwelling's fabric energy efficiency.** Part L 2013 added 15% to the notional figure as a flexibility allowance. Part L 2021 dropped it — the government chose the "full fabric specification" option in its Future Buildings Standard response of December 2021 — so there is no slack in the target beyond the notional dwelling itself. Because the notional building is the same shape as yours, the target automatically reflects how compact your design is. A blocky mid-terrace has far less exposed surface per m² of floor than a sprawling detached bungalow, so its TFEE lands lower — the more exposed fabric a form has, the more heat it can lose, and the target moves with it. This is why two dwellings on the same site can have very different TFEE figures, and why "just copy the plot next door's spec" doesn't always work. ### A worked example Say the notional version of your dwelling comes out at **40 kWh/m²/yr** of fabric demand: that is your **TFEE**. Your job is to design fabric whose **DFEE** lands at or below 40. Get your walls, roof, floor, glazing, air tightness and junctions modelled and your DFEE comes in at, say, 38 — you pass the fabric test with 2 kWh/m²/yr to spare. Come in at 43 and you've failed it, no matter how much PV or how efficient a heat pump you bolt on afterwards. (The figures here are illustrative — real ones depend on dwelling type, size and compactness, so always work from your own SAP model rather than a rule of thumb.) ## How FEE fits with the other SAP targets Fabric energy efficiency is **one of several tests** a new-build SAP has to pass at once, not the whole story. Under [SAP 10 / Part L 2021](/guides/sap-10-whats-new/) a new dwelling must satisfy: - **Primary energy** — DPER must beat TPER (the headline metric in Part L 2021). - **Carbon** — [DER must beat TER](/guides/der-ter/). - **Fabric energy efficiency** — DFEE must beat TFEE (this page). - **Limiting standards** — individual u-values, air tightness and fixed building services must all sit within their backstops. Miss any one of them and the dwelling fails. FEE is the one that can't be bought back with kit — it's a deliberate floor under the fabric, so a home stays efficient even as the grid and its heating system change around it. ## DFEE looking marginal? Where to claw it back Because FEE is fabric-only, the levers are all fabric levers — and some are far cheaper than others: - **Calculated junction Ψ-values.** Usually the cheapest win. Poorly-detailed junctions can be 20–30% of fabric heat loss, and swapping punitive SAP defaults for [calculated Ψ-values](/tools/psi-calculator/) improves the number without changing a thing on site. - **Air tightness.** Tightening the target the build can actually achieve (and prove on test) directly cuts fabric demand. - **U-values.** Better walls, roof and — often overlooked — [glazing](/guides/u-values/) and its frames. Marginal upgrades to insulation thickness or a better window spec move the DFEE, and it is worth knowing by how much before you pay for either: [U-Monkey](/tools/u-value-calculator/) rebuilds any wall, roof or floor to BS EN ISO 6946 and gives you the new figure on screen, free. - **Reduce needless glazing / sort orientation.** Oversized north-facing glazing loses heat with little solar gain in return; the fabric metric notices. An assessor reads the DFEE/TFEE gap to show you exactly which of these is the cheapest route back to a pass — often it's the junctions, not another 25mm of insulation everywhere. ## What the Future Homes Standard keeps Under **[Approved Document L 2026](/news/future-homes-standard-adl-2026/)** — the Future Homes Standard, published 24 March 2026 and **in force 24 March 2027** — the fabric test stays. The 2026 edition keeps the three metrics of 2021 — primary energy, carbon and fabric energy efficiency — and is explicit that the target fabric energy efficiency rate "can be achieved only through fabric energy efficiency, this includes U-values, thermal bridging and airtightness". The notional dwelling's U-values are the same as 2021's, but its air permeability tightens from 5 to 4, and airtightness is part of the fabric calculation, so expect the TFEE to tighten with it. Around it, the notional dwelling gets a heat pump, which means the carbon and primary energy targets are set by a heat-pump home — a far harder pair of tests for anything else to pass. The [Future Homes Standard guide](/guides/future-homes-standard/) has the full notional specification. Fabric matters more under the new standard, not less. As walls, roofs and floors get tighter, [thermal bridging](/guides/thermal-bridging/) becomes a bigger slice of what's left, and fabric performance also emerges as one of the four headline metrics on the [reformed EPC](/news/reformed-epcs-four-metrics-home-energy-model/). ## The bottom line Fabric energy efficiency is the test you can't cheat with renewables — get the [u-values](/guides/u-values/) right, tighten the [air test](/services/air-tightness-testing/), and nail the [junctions](/guides/thermal-bridging/). If a DFEE is coming up marginal, calculated Ψ-values are frequently the cheapest single fix: model yours free with [ΨMonkey](/tools/psi-calculator/), then hand the numbers to your [SAP assessor](/services/sap-calculations/) — no subscription, no sign-up. --- # What is a U-value? URL: https://energycount.co.uk/guides/u-values/ Updated: 2026-10-14 > **Quick answer:** a U-value measures how fast heat passes through a building element, in watts per square metre per degree (W/m²K) — so **lower is better**. New-build walls in England must not exceed 0.26 W/m²K under Part L 2021, and typically need to reach about 0.18 to pass the overall SAP. Those limits do **not** change under the Future Homes Standard in 2027. ## The definition A U-value (thermal transmittance) measures how readily heat passes through a building element — wall, floor, roof, window or door — in **W/m²K**: watts of heat lost per square metre, per degree of temperature difference between inside and out. **Lower is better.** A solid Victorian brick wall might be around 2.0; a modern insulated cavity wall 0.18; a Passivhaus wall 0.10–0.15. Put another way: a 1 m² wall with a U-value of 0.2 W/m²K, with 20°C inside and 0°C outside, loses 0.2 × 20 = **4 watts** through that square metre. Scale that across the whole envelope and you can see why the figure matters so much for both energy bills and the [SAP calculation](/services/sap-calculations/) that proves compliance. If you have come from an American source, **U-factor** is the same thing in Btu/(h·ft²·°F); multiply a U-factor by 5.68 to get W/m²K, or a U-value by 0.176 to go the other way. ## Which U-value do I need? This is the question most people arrive with, and the answer depends on which of three situations you are in. All figures W/m²K, from Approved Document L Volume 1, 2021 edition. | Element | New dwelling — notional (the target) | New dwelling — limiting (never exceed) | New element in an existing dwelling | Retained element — upgrade if worse than → to | | --- | --- | --- | --- | --- | | External wall | 0.18 | 0.26 | 0.18 | 0.70 → 0.55 (cavity fill) / 0.30 (internal or external insulation) | | Floor | 0.13 | 0.18 | 0.18 | 0.70 → 0.25 | | Roof | 0.11 | 0.16 | 0.15 | 0.35 → 0.16 | | Party wall | 0 | 0.20 | — | — | | Windows | 1.2 | 1.6 | 1.4 (or Window Energy Rating band B) | — | | Doors | 1.0 | 1.6 | 1.4 (or Doorset Energy Rating band B/C) | — | | Rooflights | 1.7 | 2.2 | 2.2 | — | Three things to take from the table. **New build:** the limiting column is a backstop, not a target — a dwelling built to 0.26 walls passes that row and fails the SAP by a distance, because the notional dwelling it is measured against has walls at 0.18 and solar panels on the roof. Build between the two columns and the shortfall has to be made up elsewhere in the calculation. **Existing dwellings:** the "new element" column is what an extension wall, a new roof or a replacement window has to hit, and it is tighter than the new-build backstop. **Renovation:** a retained element worse than the threshold has to be improved to the second figure where that is technically and functionally feasible and pays back within 15 years — the clause that keeps solid-wall houses insurable and barn conversions possible. The last two columns have a guide each: [extensions](/guides/extension-building-regulations/) and [renovation](/guides/renovation-building-regulations/). The [Part L guide](/guides/building-regulations-part-l/) explains which edition applies to your project; [U-Monkey](/tools/u-value-calculator/) checks a build-up against every column live. ## Area-weighting — and the window trap The limiting values for walls, floors and roofs apply to the **area-weighted average** of all elements of that type: multiply each section's U-value by its area, add them up, divide by the total area. A short run of weaker wall can be carried by better wall elsewhere, and that same arithmetic is one of the two routes to compliance for an [over-glazed extension](/services/sap-calculations/extensions/). The exception catches people every week. In an existing dwelling, **windows, doors, roof windows and rooflights are excluded from area-weighting** — every single one has to meet 1.4 on its own. You cannot average a poor window against a good one. Where a building's character genuinely requires the original windows, the Approved Document accepts a centre-pane U-value of 1.2 or better, or single glazing with low-emissivity secondary glazing behind it. ## How a U-value is actually calculated U-values are calculated from the **build-up** of the element — not measured on a wall. The calculation, to **BS EN ISO 6946**, adds up the thermal resistance of every layer: 1. **Each material layer** contributes its thickness (m) divided by its thermal conductivity λ (W/m·K) — its *R-value*. The lower the λ, the better the insulator. 2. **Surface resistances** — the still-air films on the inside and outside faces. Inside: 0.13 m²K/W for a wall, 0.10 for a roof (heat flowing up), 0.17 for a floor (heat flowing down). Outside: 0.04 for all of them. 3. **Corrections** — for repeating thermal bridges like timber studs or mortar joints, air gaps between layers, and mechanical fixings that pierce the insulation. The total resistance RT = Rsi + R1 + R2 + … + Rse, and the U-value is simply **U = 1 / RT** — the [U-value to R-value converter](/tools/u-value-calculator/u-value-to-r-value-converter/) does that arithmetic either way, with the surface resistances built in. Ground floors use **BS EN ISO 13370** instead, because heat loss through the ground depends on the floor's perimeter-to-area ratio — the same slab gets a different U-value in a different floor plan. And the whole thing follows the conventions of **BR 443** (third edition, 2019) so that one assessor's software agrees with another's: default bridging fractions, air-gap corrections, and — critically — which conductivity values you are allowed to use. That is exactly the method our free [**U-Monkey calculator**](/tools/u-value-calculator/) uses — build up your element from real UK products and it computes the U-value, the [Part L](/guides/building-regulations-part-l/) and [Future Homes Standard](/guides/future-homes-standard/) checks, a condensation screening, and the thermal mass — decrement factor, time lag and areal heat capacity to ISO 13786 — with the PDF report free. [How to use U-Monkey](/tools/u-value-calculator/how-to-use/) walks through a build-up layer by layer, bridging fractions and cavities included. ### A worked example Take a modern insulated cavity wall, outside → inside: | Layer | Thickness (mm) | λ (W/m·K) | R (m²K/W) | | --- | --- | --- | --- | | External surface (Rse) | — | — | 0.04 | | Brick outer leaf | 102.5 | 0.77 | 0.13 | | Full-fill mineral wool | 100 | 0.035 | 2.86 | | Aircrete block inner leaf | 100 | 0.11 | 0.91 | | Plasterboard | 12.5 | 0.25 | 0.05 | | Internal surface (Rsi) | — | — | 0.13 | Total resistance RT = 0.04 + 0.13 + 2.86 + 0.91 + 0.05 + 0.13 = **4.12**, so U = 1 / 4.12 ≈ **0.24 W/m²K** — before corrections for mortar joints and wall ties, which push it a touch higher; the [full-fill cavity wall page](/tools/u-value-calculator/cavity-wall-full-fill/) shows the real-world figures. Change the insulation to 150 mm and U drops to roughly 0.18; switch to PIR (λ 0.022) and it is better still. That sensitivity is why the build-up — not the wall "type" — is what your SAP assessor needs. Hand-summing like this only works for a homogeneous element. Anything with timber studs, wall ties, metal fixings or a ground floor needs the bridging and correction methods — which is what the software is for. ### R-value vs U-value They are often confused. **R-value** is a property of a single layer or assembly (m²K/W) — resistance, higher is better. **U-value** is the overall element performance including surface films (W/m²K) — transmittance, lower is better. You cannot sensibly compare one with the other; Part L and SAP work in U-values. ## Materials do not have U-values — they have conductivities People search for "the U-value of wood" or "the U-value of steel". A material has a **thermal conductivity, λ**, in W/m·K; it only acquires a U-value once it is a layer of a certain thickness in an element with surfaces. The conductivities that matter, from the BR 443 conventions and the manufacturers' declared values: | Material | λ (W/m·K) | Note | | --- | --- | --- | | Brickwork, outer leaf | 0.77 | BR 443 default (inner leaf 0.56; a solid wall behind external insulation or cladding takes 0.56, §7; U-Monkey takes a cavity wall's outer leaf behind external insulation as protected too, by §7's diaphragm-wall row) | | Softwood timber | 0.13 | BR 443 default (hardwood 0.18) | | Mild steel | 50 | BR 443 default — a stud or a lintel is a thermal bridge, not an insulator | | Reinforced concrete | 2.30 | BR 443 default | | Plasterboard | 0.21–0.25 | BR 443 default, by density | | Unventilated cavity, ≥25 mm | R = 0.18 m²K/W | A resistance, not a conductivity; it does not improve with a wider cavity | | Dense concrete block | ~1.1–1.3 | Typical — use the manufacturer's declared value | | Aircrete block | ~0.11–0.19 | Typical — use the manufacturer's declared value | | Mineral wool | ~0.032–0.044 | Typical — use the declared value | | EPS | ~0.030–0.038 | Typical — use the declared value | | PIR | ~0.022 | Typical — use the declared value | The distinction in the right-hand column matters. BR 443 tabulates defaults for masonry, timber and metals but **not for blocks or insulation** — for those it requires the manufacturer's *declared* λ, which is a certified figure a little more pessimistic than the lab value. A calculation built on a generic insulation figure instead of the declared one is a common reason for a U-value being rejected, and why the same build-up can give different answers in different software. U-Monkey uses declared values throughout, and every λ it works with — 418 figures across 53 generic materials and 365 named products from 108 manufacturers, each carrying the document it was read from and the date — is listed in the [thermal conductivity library](/reference/thermal-conductivity/). ## Windows: whole-window, not centre-pane Part L uses the **whole-window U-value** — glass, frame and spacer together — not the centre-pane figure a glazing brochure leads with. A unit with a centre-pane value of 1.0 can sit in a window whose whole-window value is 1.4 to 1.6, so the glass-only figure is not a compliance figure. In an existing dwelling the Approved Document accepts a **Window Energy Rating** as an alternative to a U-value — band B or better for windows — which can reward a window that admits useful solar gain in a way the U-value alone cannot. ## Passivhaus U-values Passivhaus does not actually set a maximum U-value for walls, roofs or floors. The standard is performance-based — a **space heating demand of 15 kWh/m² a year** or less, an **airtightness of 0.6 air changes per hour** at 50 Pa, and a primary energy limit — and the fabric is whatever it takes to get there, which in the UK's cool-temperate climate typically means opaque elements at **0.10 to 0.15 W/m²K**. Windows are certified as components: a whole-window U-value of **0.80** or better and **0.85 installed**, with glazing at 0.70 or better. That is about half the Part L limit for a new dwelling, and it is where triple glazing stops being optional. ## Measuring a U-value in an existing wall When the build-up is unknown — a rendered wall that might or might not have a cavity, a stone wall of uncertain thickness — the U-value can be **measured in situ** with a heat flux plate to **BS ISO 9869-1**. A sensor on the inside face and temperature probes inside and out log for a minimum of **72 hours** in the heating season, and the test ends only when the result has stopped moving — within 5% over the last 24 hours. Heavy masonry walls often need considerably longer; lightweight walls are analysed on night-time data only, to exclude the sun. It is the honest answer to "what is my wall actually doing?" for a retrofit design or a suspected workmanship problem — but BRE's guidance is that it is not suitable for producing an individual dwelling's EPC. An assessment of an existing home takes a different route entirely: where the build-up cannot be evidenced, RdSAP assigns a default from the dwelling's age band, and [all of those defaults are tabulated here](/reference/rdsap-age-bands/). ## Typical U-values — a reality check | Element | Typical U-value (W/m²K) | | --- | --- | | Solid brick wall (uninsulated, pre-1900) | ~2.0–2.2 | | Uninsulated cavity wall (1930s–60s) | ~1.5–1.7 | | Insulated cavity wall, full-fill (100–150 mm) | ~0.20–0.27 | | Timber frame wall, 140 mm studs | ~0.30 | | Timber frame wall with an insulated lining | ~0.17–0.19 | | Loft insulation at 270 mm | ~0.15–0.17 | | Rafter-level pitched roof | ~0.17–0.19 | | Insulated concrete ground floor | ~0.12–0.17 | | Suspended timber floor, insulated | ~0.23–0.30 | | Single glazing | ~5.8 | | Double glazing (low-e, argon) | ~1.2–1.6 | | Triple glazing | ~0.8–1.0 | There is a page of typical values and editable build-ups for the most common constructions — [cavity walls](/tools/u-value-calculator/cavity-wall-full-fill/), [solid walls](/tools/u-value-calculator/solid-brick-wall-internal-insulation/) (including the [one-and-a-half-brick, 327.5mm solid wall](/tools/u-value-calculator/one-and-a-half-brick-wall/)), [timber-frame walls](/tools/u-value-calculator/timber-frame-wall/), [pitched roofs](/tools/u-value-calculator/pitched-roof-rafter-insulation/), [flat roofs](/tools/u-value-calculator/warm-flat-roof/) (including [how much flat-roof insulation you need](/tools/u-value-calculator/flat-roof-insulation-thickness/)), [loft insulation](/tools/u-value-calculator/loft-insulation-270mm/), [concrete ground floors](/tools/u-value-calculator/insulated-concrete-floor/), [suspended timber floors](/tools/u-value-calculator/suspended-timber-floor-insulation/) and [beam & block floors](/tools/u-value-calculator/beam-and-block-floor/). ## How to improve a U-value Almost always in this order: 1. **Add insulation** — the single biggest lever. Where it goes (cavity fill, external wall insulation, internal wall insulation, loft, floor overlay) depends on the element and the building. 2. **Insulate the whole element, not part of it** — a thermal bridge at the edge undoes good work elsewhere (see below). 3. **Upgrade the windows** — low-e glass, argon fill and warm-edge spacers take a typical double-glazed unit from ~1.6 to ~1.2. 4. **Don't chase the last 0.02** — beyond ~0.15, each extra millimetre of insulation buys very little; the [energy hierarchy](/guides/energy-hierarchy/) and [fabric energy efficiency](/guides/fabric-energy-efficiency/) guides put this in perspective. ## What changes in 2027 — and what does not Approved Document L 2026, the Future Homes Standard, takes effect on **24 March 2027**. The limiting U-values **do not change**: walls 0.26, floors 0.18, roofs 0.16, windows and doors 1.6, rooflights 2.2 — the same numbers, renumbered into new tables — and the notional dwelling's fabric values are unchanged too. What tightens is the notional dwelling's **air permeability**, the switch to a **heat-pump** baseline, and a new legal requirement for **on-site renewables**. If you arrived here expecting a new set of U-value limits for 2027, there isn't one; the fabric you design to Part L 2021 is the fabric the Future Homes Standard expects. ## U-values aren't the whole story An element's U-value covers its flat area — but heat also escapes where elements meet. Those junction losses are measured separately as Ψ-values, and at modern U-value levels they can be 20–30% of total fabric heat loss. See [thermal bridging explained](/guides/thermal-bridging/), calculate junction values with the free [psi value calculator](/tools/psi-calculator/), or build to [Accredited and Recognised Construction Details](/guides/accredited-construction-details/) and use their published values. The [Table K1 defaults](/reference/sap-junctions/) are what you are measured against if you do neither. A U-value also says nothing about moisture. Insulation changes the temperature of every layer in an element, and warm, moist air working its way out of the house can condense where it meets a layer colder than its dew point — inside the construction, where nobody sees it. Requirement C2 of the Building Regulations covers that interstitial condensation, and where a build-up steps outside the arrangements BS 5250 treats as safe — insulation along the rafters, a flat roof insulated both above and below the deck, an unfamiliar layer order — building control can ask for a [condensation risk analysis](/tools/u-value-calculator/condensation-risk-calculator/) to BS EN ISO 13788 as well as the U-value. U-Monkey runs that check free on the same build-up. U-values also feed everything downstream: [SAP](/services/sap-calculations/) and [SBEM](/services/sbem-calculations/) calculations, [energy statements](/services/energy-statements/), [Part O](/services/part-o-overheating/) overheating checks (better fabric often means more careful glazing design) and [Part G](/services/part-g-water-calculations/) hot-water demand. Get the fabric right and the rest of the compliance picture gets easier. --- # Accredited Construction Details (ACDs) and Recognised Construction Details (RCDs) explained URL: https://energycount.co.uk/guides/accredited-construction-details/ Updated: 2026-09-19 ## What ACDs were for Accredited Construction Details are a government-published set of junction specifications — wall-to-floor, eaves, openings and so on — each with a pre-approved Ψ-value. Build exactly to the published detail, and your SAP assessor could apply that improved value instead of the punitive default. They were the easiest way to soften [thermal bridging](/guides/thermal-bridging/) losses without commissioning calculations. ## Why SAP 10 removed them The ACD set aged badly: the details no longer reflect modern constructions, and there were long-standing concerns about the accuracy of the published values. [SAP 10](/guides/sap-10-whats-new/) removed the option to use them entirely. ## Recognised Construction Details (RCDs) — the current scheme Recognised Construction Details are what replaced the ACD shortcut for masonry. They are a **free database of independently assessed junction details** — aggregate block and aircrete, at standard cavity widths — each published with a **psi value and an f-value** (the condensation-risk factor), and certified for use in demonstrating Part L compliance. The portal is run by Building Alliance CIC; the junctions were commissioned by the Concrete Block Association and the Aircrete Products Association and modelled independently. The deal is the same as it was with ACDs: build the junction *exactly* as drawn and the assessor can enter the RCD psi value in SAP instead of the default. Deviate — a different cavity closer, insulation stopped short, a lintel the detail didn't assume — and the value no longer applies. They cover the standard junction set for masonry housing; they do not cover timber frame, SIPs, steel frame or anything non-standard, which is where a calculated value comes in. LABC's **Registered Construction Details** are the other scheme you will meet, aimed at smaller builders. Both are valid sources where the match is exact. ## The junction set — E-numbers and what each one is Both the old ACD set and the schemes that replaced it are organised around the same reference junctions, and SAP carries a default Ψ-value for each. Most specs name them by E-number and assume everyone knows which is which. Here is the set people actually model, what each one is, and where it tends to cost you — or see the [full SAP junction reference](/reference/sap-junctions/) for all 44 references with their Table K1 default and Table R2 notional Ψ-values: | Ref | Junction | What it is | | --- | --- | --- | | E1 / E2 | [Lintel](/tools/psi-calculator/lintel/) | Head of an opening. Steel lintels bridge hard; the default is punitive and a calculated value usually beats it comfortably. | | E3 | [Window sill](/tools/psi-calculator/window-sill/) | Below the opening, where the cavity closer meets the frame. | | E4 | [Window jamb (reveal)](/tools/psi-calculator/window-jamb/) | The vertical return at each side of an opening — the longest opening junction by total length on most dwellings. | | E5 / E19 / E22 | [Wall-to-floor](/tools/psi-calculator/wall-to-floor/) | Ground floor perimeter. Depends heavily on whether the floor insulation meets the cavity insulation. | | E6 / E7 | [Intermediate floor](/tools/psi-calculator/intermediate-floor/) | Where a first floor is built into or hung off the wall. | | E8 / E9 / E23 | [Balcony](/tools/psi-calculator/balcony/) | Anything penetrating the insulation line to carry a load outside it. The worst bridges on most apartment schemes. | | E10 / E11 / E24 | [Eaves](/tools/psi-calculator/eaves/) | Wall to roof at the eaves, insulated at ceiling or at rafter. | | E12 / E13 | [Gable](/tools/psi-calculator/gable/) | Wall to roof at the gable, and the ceiling junction behind it. | | E16 | [External corner](/tools/psi-calculator/external-corner/) | A normal corner. Small Ψ, long total length. | | E17 | [Inverted corner](/tools/psi-calculator/inverted-corner/) | A corner turning the other way — routinely gives a *negative* Ψ-value, which is correct and which SAP accepts. | | E18 / E25 / P1–P8 | [Party wall](/tools/psi-calculator/party-wall/) | Separating wall junctions, including the party wall itself where it is not fully filled and sealed. | | E20 / E21 | [Exposed floor](/tools/psi-calculator/exposed-floor/) | A floor over an unheated space or open air — over a passage, a car port, an overhang. | Neither the retired ACD set nor the current schemes cover every construction, and none of them cover yours exactly unless you drew it that way on purpose. If you want to see what these junctions actually come out at across different wall build-ups — rather than a single generic figure — we publish [our own solved Ψ-value library](/tools/psi-calculator/psi-values/): every standard junction template solved against every wall type in the calculator, with the figures regenerated whenever the app changes. ## Your options now 1. **Scheme details — RCDs, LABC Registered Details, manufacturer schemes** — independently assessed junctions with published Ψ-values, free or near-free, valid only if your construction matches the drawing exactly. Masonry is well covered; other systems mostly are not. 2. **Calculated Ψ-values to BS EN ISO 10211** — bespoke values for your actual details. Almost always gives the best result, and it's what our free [ΨMonkey psi value calculator](/tools/psi-calculator/) produces, with Building Control-ready reports at £25 +VAT a junction. 3. **Default values** — always available, always punitive. Avoid if the SAP is anywhere near marginal. As-built [EPCs](/services/sap-calculations/epc/) need the junction evidence documented, so decide your thermal bridging strategy at design stage, not at sign-off. --- # What is the Energy Hierarchy? URL: https://energycount.co.uk/guides/energy-hierarchy/ Updated: 2026-09-20 ## The principle The energy hierarchy ranks the ways to cut a building's carbon footprint, in order of preference: 1. **Reduce energy demand** — fabric first: insulation ([u-values](/guides/u-values/)), [air tightness](/services/air-tightness-testing/), minimised [thermal bridging](/guides/thermal-bridging/), good form and orientation. 2. **Use energy efficiently** — efficient heating systems and controls, [MVHR](/guides/mvhr/), LED lighting, [waste water heat recovery](/guides/waste-water-heat-recovery/). 3. **Supply from low-carbon and renewable sources** — [heat pumps](/guides/heat-pumps/), [solar PV](/guides/solar-pv/), [solar hot water](/guides/solar-hot-water/). London plans phrase it as "be lean, be clean, be green" — and many Local Planning Authorities structure their [energy statement](/services/energy-statements/) requirements around exactly this logic, expecting demand reduction to be demonstrated before renewables are counted. ## Why the order matters A kilowatt-hour you never need is cheaper than one you generate. Fabric measures are permanent, maintenance-free and invisible to occupants; renewables degrade, need replacing, and underperform on poorly designed buildings. Part L embeds the same thinking through the [fabric energy efficiency target](/guides/fabric-energy-efficiency/), which renewables can't satisfy. It's also why we push [calculated junction Ψ-values](/tools/psi-calculator/) so hard: they're a step-1 improvement that costs nothing on site, and [the K1 defaults you are improving on](/reference/sap-junctions/) are published so you can see the headroom before committing. The same goes for the build-ups themselves — [U-Monkey](/tools/u-value-calculator/) will tell you what a wall, roof or floor actually achieves while the specification is still a drawing and step 1 is still cheap. --- # Why is Fuel Choice Important in SAP? URL: https://energycount.co.uk/guides/fuel-choice-in-sap/ Updated: 2026-09-20 ## Every fuel carries a carbon factor SAP converts every kWh your dwelling uses into CO₂ and primary energy using fuel-specific factors. Your target ([TER](/guides/der-ter/)) is set by a notional dwelling using mains gas — so fuels with worse factors than gas start at a disadvantage that must be clawed back elsewhere. ## The SAP 10 shake-up Under SAP 2012, grid electricity carried a carbon factor 2.4× that of mains gas — making all-electric new builds notoriously hard to pass. [SAP 10](/guides/sap-10-whats-new/) cut electricity's factor from 0.519 to 0.233 kgCO₂/kWh, and SAP 10.2 — the version Part L 2021 is assessed with — took it to 0.136, below gas (0.210) for the first time; SAP 10.3, the Future Homes Standard methodology, goes to 0.086 against 0.214 for gas ([what changed](/news/sap-10-3-what-changed/)). Direct electric heating is now viable in the calculation — and a [heat pump](/guides/heat-pumps/), delivering 3+ units of heat per unit of electricity, comfortably beats gas on carbon. One caution: these are SAP’s own factors, fixed for the life of a SAP version. The DESNZ and Defra factors used in energy statements and SECR reporting are updated annually and are different numbers for a different job — [the building-relevant set is here](/reference/carbon-factors/). ## Practical implications **Mains gas** remains the baseline the targets assume — but with the [Future Homes Standard](/guides/future-homes-standard/) phasing fossil-fuel heating out of new builds, it's a shrinking option. **Heat pumps** are now usually the strongest choice in SAP terms. **Direct electric** (panel heaters, electric boilers) passes more easily than before but high running costs still show in the calculation. **LPG and oil** carry worse economics and carbon, and typically need fabric or renewable compensation. One caveat worth holding on to: the electricity factor SAP uses is a single annual average, and the real grid swings a long way either side of it through the day — [GridMonkey](/tools/grid-mix/) shows what Britain is generating right now if you want to see the spread behind the number. Choosing fuel early — before the [design-stage SAP](/services/sap-calculations/) — avoids expensive redesign. If you're undecided, we can model the options side by side. [Ask us](/contact/). --- # What is a Predicted Energy Assessment (PEA)? URL: https://energycount.co.uk/guides/predicted-energy-assessment/ Updated: 2026-06-11 ## The off-plan EPC A Predicted Energy Assessment (PEA) shows the predicted energy rating of a dwelling that hasn't been built yet. It's produced from the design-stage [SAP calculation](/services/sap-calculations/) and is required when a new dwelling is marketed for sale or rent off-plan — viewers are entitled to see an energy rating even though the final [EPC](/services/sap-calculations/epc/) can't yet exist. ## PEA vs EPC The PEA presents the same A–G energy band as an EPC but is based on the proposed specification, carries no certificate number, and isn't lodged on the national register. Once the dwelling is complete, the as-built SAP calculation produces the official EPC, which supersedes the PEA — and which Building Control needs before sign-off. If we're already doing your design-stage SAP, a PEA is a quick add-on — just ask when sales or lettings marketing is planned. [Get a quote](/contact/) or call 01202 623236. --- # Building regulations for an extension: the energy side, start to finish URL: https://energycount.co.uk/guides/extension-building-regulations/ Updated: 2026-09-25 > **Quick answer:** an extension's glazing must not exceed **25% of its floor area**, *plus* the area of any existing windows and doors the extension covers over — and that second part is routinely forgotten. Stay inside the allowance and the extension complies on its **U-values alone**: walls 0.18, floors 0.18, roof 0.15, windows and doors 1.4, rooflights 2.2 W/m²K. Go over it and you need an area-weighted calculation, a whole-dwelling SAP, or less glass. ## Where Part L fits among the other regs An extension picks up most of the building regulations at once. Structure, fire escape, stairs, glazing safety, ventilation, drainage and electrical work all apply, and your building control body will look at all of them. This guide is about **Part L**, the conservation of fuel and power — the part that decides how much glazing you can have and how well the new walls, roof and floor have to perform. It is also the part that most often produces the sentence *"Building Control have asked for a calculation"*, usually because of the glazing. None of what follows is about planning permission, which is a separate consent with separate rules. ## The number everything hangs on Approved Document L caps the area of **windows, roof windows, rooflights and doors** in an extension at **25% of the extension's floor area**. That much is widely known. What is much less widely claimed is the rest of the sentence: you may **add the area of any existing windows and doors that the extension covers over or removes**. The logic is heat loss, not aesthetics — a back door that now opens into the new room has stopped losing heat to the outside, so the envelope has not gained as much glazing as the raw numbers suggest. So the allowance is: > **25% × extension floor area + the area of covered-over openings** Take a 24 m² rear extension. The base allowance is 25% × 24 = **6.0 m²** — one modest set of bifolds and you are finished. But if the extension swallows an existing back door of 1.8 m² and builds over a kitchen window of 1.2 m², that is another 3.0 m², and the allowance becomes **9.0 m²**: half as much again, and often the difference between needing a calculation and not. Measure covered openings at structural opening size, mark them on the existing elevations and keep the drawing — your building control body will want to see where the extra came from. [The covered-openings allowance](/tools/extension-glazing-calculator/covered-openings-allowance/) works through it in detail. ## What every new element has to hit Whether or not you are inside the glazing allowance, each new element of the extension has a limiting U-value it must not exceed. These are the Table 4.2 values for new elements in an existing dwelling — tighter than the backstops that apply to a whole new house, which surprises people. | New element | Limiting U-value (W/m²K) | | --- | --- | | External wall | 0.18 | | Floor | 0.18 | | Roof | 0.15 | | Windows and doors | 1.4 | | Roof windows (in the plane of the roof) | 1.4 | | Rooflights (on an upstand) | 2.2 | Two traps live in that table. **Windows and doors are not area-weighted** in an existing dwelling — each one has to meet 1.4 on its own, so a single heritage-style unit cannot be carried by better ones elsewhere. And the **last two rows are a genuine distinction**, not a synonym: a roof window sitting in the plane of the roof is treated as a window and assessed vertically at 1.4, while a rooflight on an upstand is assessed horizontally at 2.2. Getting it the wrong way round can turn a pass into a fail on paper. [Rooflights and roof windows](/tools/extension-glazing-calculator/rooflights-and-roof-windows/) sets out which is which, and [what U-values an extension needs](/tools/extension-glazing-calculator/extension-u-values/) has the full table with the exceptions. If you need to know what a particular build-up actually achieves rather than what you hope it does, [U-Monkey](/tools/u-value-calculator/) calculates it to BS EN ISO 6946, free. ## Over the allowance? Three routes Bifolds across the back, a lantern over the kitchen and a run of rooflights will clear 25% without much effort. That is not a failure — it just means the simple route is closed and one of three others has to be used. Building control often asks for it as "heat loss calculations" — [here is what they mean, and what to send](/guides/over-glazed-extension-calculations/). **1. The area-weighted U-value calculation.** The usual answer, and the cheapest. You build a *notional* extension of the same size and shape, with its openings capped at the allowance and the excess area returned to wall or roof at the limiting U-values, then show that your design performs no worse overall. Because better-than-limiting glazing and fabric both count, a well-specified extension frequently passes with more glass than the 25% rule alone would allow. [The area-weighted method](/tools/extension-glazing-calculator/area-weighted-u-value/) has the formula and a worked example. **2. A whole-dwelling SAP calculation.** House plus extension assessed together against the same house notionally extended. This route is often easier to pass than expected, because improvements you are making to the existing house count in your favour. It is more work and more cost, and it is the route to take when the glazing is not negotiable. That is a job we do every week — see [SAP calculations for over-glazed extensions](/services/sap-calculations/extensions/). **3. Less glass.** Worth saying out loud. Moving from the allowance being exceeded by a little to not exceeded at all is sometimes one rooflight. Which of the three you need is decided by the arithmetic, and the arithmetic is free: [GlazeMonkey](/tools/extension-glazing-calculator/) does the allowance, the covered-openings credit and the full area-weighted comparison on screen, and tells you what the glazing would have to be if you are short. ## Conservatories, porches and the things that are not either A conservatory or porch escapes the energy efficiency requirements entirely — but only while **every** condition holds at once: at ground level, no more than **30 m²**, substantially glazed (at least 75% of the roof and 50% of the external walls), glazing compliant with Part K, thermally separated from the house by the retained external wall, door or window, and unheated, or heated with its own independent controls. Break one and it is an extension. Take the back wall out to open it into the kitchen and the thermal separation has gone; run a radiator off the house system and the heating condition has gone. An **orangery or garden room** that is not substantially glazed was never a conservatory in the first place — it is an extension, 25% rule and all. [The conservatory exemption](/tools/extension-glazing-calculator/conservatory-building-regulations/) tests each condition in turn. ## The part that catches people: work to the existing house An extension rarely stops at the new bit. Take the roof off to tie in, re-render the flank wall, dry-line the room behind, bring a garage into the heated envelope — and you are into a **different** set of Part L rules, the ones that govern existing elements. Renovating more than half of a wall, roof or floor triggers an upgrade duty with its own thresholds, and a retained element that joins the heated envelope is triggered whatever proportion you touch. Those rules are set out in [building regulations when you renovate](/guides/renovation-building-regulations/). The practical point here is that they are assessed alongside the extension, not instead of it — and both halves belong in one calculation. ## What changes on 24 March 2027 Approved Document L Volume 1, **2021 edition incorporating the 2023 amendments**, is the edition behind every figure on this page. The **2026 edition**, published alongside the [Future Homes Standard](/guides/future-homes-standard/), takes effect in England on **24 March 2027**. Which one your extension falls under is decided by two dates, not one. If the building notice or full plans application goes in **before 24 March 2027**, the 2021 edition applies — provided the work has **commenced by 24 March 2028**. For an extension on new foundations, commenced has a legal meaning: the foundations and the ground-floor structure complete, not the first spade in the ground ([the commencement test in full](/news/future-homes-standard-transitional-deadline-commencement/)). Miss either date and the 2026 edition applies. For an extension the change matters less than the dates suggest. The 2026 edition keeps the 25% rule, the area-weighted and SAP routes and the same limiting U-values for new elements; mostly it renumbers them, and the SAP route moves to SAP 10.3. ## The bottom line Count the glazing, then count the openings you are covering over, and compare the two totals against a quarter of the floor area. Inside it, specify to the Table 4.2 values and there is nothing to submit. Outside it, the area-weighted route is the cheap answer and passes more often than its reputation suggests — and the only way to know which side of the line you are on is to do the arithmetic before the glazing is ordered, not after it arrives. --- # Building regulations when you renovate: the upgrades you may not expect URL: https://energycount.co.uk/guides/renovation-building-regulations/ Updated: 2026-09-25 > **Quick answer:** renovate more than **50%** of a wall, roof or floor — or more than **25%** of the whole envelope — and if that element is worse than its **Table 4.3 threshold** it should be upgraded to the improved value: roof **0.16**, cavity wall **0.55**, wall with internal or external insulation **0.30**, floor **0.25** W/m²K. If that is not feasible or will not pay back within **15 years**, upgrade to the best value that does, and record why. Not sure what the existing element achieves? [RdSAP’s age-band defaults](/reference/rdsap-age-bands/) are the assessor’s fallback and a reasonable first estimate. ## Why this catches people The rule is not aimed at renovation as an activity. It exists because the moment somebody has a roof covering off or a scaffold up, the cost of adding insulation collapses — and if the opportunity is missed, the next one is thirty years away. So Part L attaches a duty to the moment rather than to the intention. The practical consequence is that the duty usually arrives as a surprise, mid-job, from a building control officer, at the point where it is most expensive to satisfy. Everything below is cheap on a drawing and dear on a scaffold. ## What counts as renovating a thermal element Renovation, in Part L's sense, means **providing a new thermal layer, or replacing an existing one**. In practice that is: - stripping a roof and re-covering it - applying new external render or cladding - dry-lining a wall internally - filling a cavity - re-screeding a floor It is **not** redecorating, repointing, or patching a small area. The distinction is whether you are re-forming the element, not whether you are spending money on it. Then the size test. The duty bites when you renovate **more than 50% of the surface area of that individual element** — one elevation, one roof slope, one floor, assessed on its own rather than averaged across the house. A second test catches larger projects: work affecting **more than 25% of the total building envelope** brings every element in the envelope into scope. [Renovating thermal elements](/tools/thermal-element-calculator/renovating-thermal-elements/) works through the triggers in detail, and [re-roofing](/tools/thermal-element-calculator/re-roofing-building-regulations/) covers the most common case on its own. ## The values, and how to read them | Existing element | Upgrade if worse than | Improve to | | --- | --- | --- | | Roof | 0.35 | 0.16 | | Wall — cavity insulation | 0.70 | 0.55 | | Wall — internal or external insulation | 0.70 | 0.30 | | Floor | 0.70 | 0.25 | All figures W/m²K, from Approved Document L Volume 1, 2021 edition, Table 4.3. Read the first column as a **test, not a target**. An element already better than its threshold is left alone — a roof at 0.25 W/m²K being re-covered has no upgrade duty attached to it, because it is already better than 0.35. It is only when the existing element is worse than the threshold that the second column becomes the aim. And whatever happens, an element should not be left **worse than 0.70 W/m²K**, and never worse than it started. Knowing which side of the threshold an existing element sits on means calculating it rather than guessing, which is what [U-Monkey](/tools/u-value-calculator/) is for — and the same tool will tell you what a proposed upgrade actually achieves. ## The 15-year payback escape This is the clause that makes solid-walled and stone houses workable, and it is also the clause most often waved at building control without a number behind it. If reaching the improved value is not **technically** feasible (the wall cannot carry it), not **functionally** feasible (the room becomes unusable, or the detail creates a problem), or not **economically** feasible — a simple payback longer than **15 years** — then the duty changes. It becomes: upgrade to the best value that *is* feasible, and record the reasoning. The economic test is arithmetic: > annual saving = (existing U − target U) × area × degree-days × 24 ÷ 1000 ÷ system efficiency × fuel price > > simple payback = cost ÷ annual saving Every input is arguable, and that is the point — degree-days, pence per kWh and boiler or heat-pump efficiency all move the answer, and mains gas gives a very different result from electricity. Building control may reasonably ask for your figures rather than accept a default. What they should not accept is the conclusion without the working. [RenoMonkey](/tools/thermal-element-calculator/) runs the calculation, lets you change all three inputs, and prints the justification. ## Three cases that are not renovation **Replacing an element outright.** A wall taken down and rebuilt, or a new floor, is a **new** element, not a renovated one. It takes the Table 4.2 limiting values — wall 0.18, roof 0.15, floor 0.18 W/m²K — and must be no worse than what it replaced. The 50% trigger and the payback escape do not enter into it. **Retained elements joining the heated envelope.** A garage conversion, a loft conversion, or any change of energy status brings walls, floors and roofs into the heated envelope that were previously outside it. These are treated as renovated elements and go to the Table 4.3 improved values **whatever proportion is being worked on** — the 50% trigger does not apply, and this is the case people most often assume they have escaped. [Garage and loft conversions](/tools/thermal-element-calculator/garage-conversion-building-regulations/) covers it. **Controlled fittings.** Replacement windows and doors take **1.4 W/m²K** or the equivalent energy rating, each on its own — they are not area-weighted in an existing dwelling. Where a building's character genuinely requires the original windows, Approved Document L accepts a centre-pane U-value of 1.2 or better, or single glazing with low-emissivity secondary glazing, as a recorded case. [Replacement windows and doors](/tools/thermal-element-calculator/replacement-windows-u-value/) has the detail. ## When there is an extension too Plenty of jobs are both — an extension across the back and a re-roof over the rest, or a garage conversion that also gains a new side wall. The two halves are assessed against different tables, but they belong in **one** submission rather than two, and the extension side has its own rules. [Building regulations for an extension](/guides/extension-building-regulations/) sets those out; if the work is a conversion of a building to a dwelling rather than an alteration to one, [SAP calculations for conversions](/services/sap-calculations/conversions/) is the route. ## What changes on 24 March 2027 Every figure on this page is from Approved Document L Volume 1, **2021 edition incorporating the 2023 amendments**. The **2026 edition**, published alongside the [Future Homes Standard](/guides/future-homes-standard/), takes effect in England on **24 March 2027**. Renovation work with a building notice or full plans application in **before 24 March 2027** stays under the 2021 edition, provided it has **commenced by 24 March 2028** — and commenced has [a legal meaning](/news/future-homes-standard-transitional-deadline-commencement/) that is stricter than starting. Work that does not go to building control before it starts, such as a window replacement self-certified under a competent person scheme, has no notice to rely on for that protection. A job planned across the boundary is worth pinning down early. ## The bottom line Three questions, in order. Am I re-forming more than half of this element? Is it currently worse than its threshold? And if so, does the improvement pay back within fifteen years? Only if the first two are yes does the third one matter — and the third is a calculation, not an opinion. Work all three out before the scaffold goes up and the answer is cheap; work them out after the covering is back on and it is not. --- # Renewable Technologies: An Overview URL: https://energycount.co.uk/guides/renewable-technologies/ Updated: 2026-06-11 ## The headliners **[Solar PV](/guides/solar-pv/)** — generates electricity, directly offsets the dwelling's energy and carbon figures, and is assumed in the [SAP 10 notional dwelling](/guides/sap-10-whats-new/). For most new builds it's the simplest route to a pass. For new homes under the Future Homes Standard, the [solar PV calculator](/tools/solar-pv-calculator/) works out how much requirement L3 asks for. **[Heat pumps](/guides/heat-pumps/)** — [air source](/guides/air-source-heat-pumps/) or [ground source](/guides/ground-source-heat-pumps/), delivering 3–4 units of heat per unit of electricity. The default heating choice under the [Future Homes Standard](/guides/future-homes-standard/). **[Solar hot water](/guides/solar-hot-water/)** — solar thermal panels feeding the cylinder; a useful contributor, though PV has largely displaced it on cost. ## The supporting cast **[MVHR](/guides/mvhr/)** — mechanical ventilation with heat recovery; shines in airtight dwellings. **[Waste water heat recovery](/guides/waste-water-heat-recovery/)** — recovers shower drain heat, cheap, and assumed in the SAP 10 notional dwelling. **[Flue gas heat recovery](/guides/flue-gas-heat-recovery/)** and **[weather compensation](/guides/weather-compensation/)** — boiler-side efficiency improvements worth real SAP points for small money. ## Choosing for your project The right mix depends on the planning condition wording, the fabric, the fuel and the budget — and the [energy hierarchy](/guides/energy-hierarchy/) says fix fabric first. We model options side by side as part of [SAP work](/services/sap-calculations/) and [energy statements](/services/energy-statements/), so you buy the technology that earns its keep. [Ask us](/contact/). --- # Solar PV (Photovoltaic Panels) URL: https://energycount.co.uk/guides/solar-pv/ Updated: 2026-09-29 ## Why PV dominates new-build compliance Solar photovoltaic panels generate electricity from daylight, directly reducing a dwelling's modelled energy demand and CO₂ emissions. Under Part L 2021, the [SAP 10 notional dwelling](/guides/sap-10-whats-new/) includes an assumed PV array — roughly **4kW for an average house** — so a dwelling without PV starts behind its target and must make the difference up elsewhere. In practice, PV (or a [heat pump](/guides/heat-pumps/)) is how most SAP 10 passes are achieved. PV is also the most common answer to [planning conditions](/services/energy-statements/) requiring a percentage CO₂ or energy demand reduction from renewables. ## From 2027: a requirement, not just a SAP lever Under Approved Document L 2026 — the [Future Homes Standard](/guides/future-homes-standard/), for work notified from 24 March 2027 — PV stops being optional. Requirement L3 asks for an array whose annual output matches a fixed benchmark: panels of 0.22 kWp/m² over 40% of the ground floor area, facing south-east to south-west, pitched at 45° and unshaded. In SAP 10.3 that is about **70.7 kWh a year per m² of ground floor**. A south-facing roof reaches it with slightly less than the benchmark's kWp; an east–west roof needs around 13% more. Where the roof genuinely cannot take it, the reasonably practicable roof area applies instead, down to a floor of 720 kWh a year. The [solar PV calculator](/tools/solar-pv-calculator/) works out the target and what your roof needs, and [MHCLG's FAQ on requirement L3](/news/adl-2026-requirement-l3-solar-pv-faq/) is explained here. In more depth: [how many panels by ground floor area](/tools/solar-pv-calculator/how-many-solar-panels/), [solar PV for flats](/tools/solar-pv-calculator/flats/), [the reasonably practicable roof area](/tools/solar-pv-calculator/reasonably-practicable-roof-area/) and [the evidence Building Control asks for](/tools/solar-pv-calculator/building-control-evidence/). The SAP side changes with it. The SAP 10.3 notional dwelling's PV is the same as your array, up to 40% of the ground floor area ÷ 4.5 — so below that cap, PV no longer buys headroom against the target. Requirement L3 is what makes you fit it. ## What affects the SAP contribution Array size (kWp), orientation and pitch (south-facing ~35° is ideal; east/west loses ~15–20%), and overshading. SAP 10 allows MCS overshading data instead of defaults, and now recognises **battery storage** and **PV diverters** (which dump surplus generation into the hot water cylinder). For flats, only dwellings directly connected to an array get the benefit — landlord-supply arrangements no longer count for everyone. ## Practical notes Panels are typically roof-mounted on-roof or integrated (in-roof trays look cleaner on new builds and avoid raised fixings). No moving parts, 25-year+ panel warranties, minimal maintenance. Tell us early if PV is in the design — sizing it precisely against the [SAP calculation](/services/sap-calculations/) avoids paying for panels you don't need. --- # Heat Pumps: An Introduction URL: https://energycount.co.uk/guides/heat-pumps/ Updated: 2026-09-04 ## How they work A heat pump moves heat rather than generating it: refrigerant absorbs low-grade heat from outside air or the ground, a compressor raises its temperature, and a heat exchanger delivers it to your heating and hot water. The efficiency measure is the **SCOP** (seasonal coefficient of performance) — typically 3–4, meaning 3–4 kWh of heat per kWh of electricity. No flue, no fossil fuel, no combustion. ## In the SAP calculation With [SAP 10's electricity carbon factor](/guides/sap-10-whats-new/) down at 0.136 kgCO₂/kWh in SAP 10.2 (0.086 in SAP 10.3, the Future Homes Standard methodology), a heat pump at SCOP 3+ beats a gas boiler on carbon by a wide margin — usually the single biggest lever available for a [SAP pass](/services/sap-calculations/). And with the [Future Homes Standard](/guides/future-homes-standard/) removing fossil-fuel heating from new homes from 2027, heat pumps are becoming the default rather than the alternative. They suit low-temperature systems best: underfloor heating or generously sized radiators, in a well-insulated, airtight dwelling — which new builds are by definition. ## Air source or ground source? **[ASHP](/guides/air-source-heat-pumps/)** — an external unit extracting heat from air. Cheaper, simpler, fits almost any plot; SCOP slightly lower and drops in the coldest weather. **[GSHP](/guides/ground-source-heat-pumps/)** — extracts heat from the ground via buried loops or boreholes. Higher SCOP and steadier year-round, but significantly higher install cost and needs land or drilling. Best on larger plots, larger houses, or shared-loop developments. We model both options in SAP as standard — [ask us](/contact/) which earns its cost on your project. --- # ASHP — air source heat pumps explained URL: https://energycount.co.uk/guides/air-source-heat-pumps/ Updated: 2026-09-20 > **Quick answer:** an air source heat pump (**ASHP**) heats a home by extracting heat from outside air — working even below freezing — and upgrading it with a compressor. A modern unit is rated at **3.5–4.5 units of heat per unit of electricity** in the lab and returns a median of about **2.8 in monitored UK homes**, against roughly 0.9 for a gas boiler's best. Grant of **£7,500** (£9,000 off-grid from oil or LPG), no planning permission on most houses since May 2025, and the default heating in new-build [SAP](/services/sap-calculations/) and under the [Future Homes Standard](/guides/future-homes-standard/). ## What ASHP means Air source heat pump. It is one of two families of heat pump you will meet — the other, the [ground source heat pump](/guides/ground-source-heat-pumps/) or GSHP, takes its heat from the ground through buried pipes instead. An ASHP takes it from the air, which is why it is a box with a fan on an outside wall rather than a trench in the garden, and why it costs a fraction of a GSHP to install. There are two types. **Air-to-water** heat pumps — the great majority of UK installations — heat the water in a wet central heating system: radiators or underfloor heating, plus a hot water cylinder. **Air-to-air** heat pumps blow warm air straight into the room through indoor units and generally do not provide hot water; they are what a reversible air-conditioning unit is. Everything on this page about SAP, grants and heating design is about air-to-water unless it says otherwise. ## How it works It runs a fridge in reverse, in four steps. A refrigerant at a very low temperature passes through the outdoor unit and **absorbs heat from the air**, evaporating as it does. A **compressor** squeezes the gas, and compressing a gas raises its temperature sharply. The hot gas then gives its heat up to the water in your heating circuit and **condenses** back to a liquid. Finally it passes through an **expansion valve**, drops in pressure and temperature, and goes round again. The trick is that the pump moves heat rather than making it. The electricity runs the compressor and the fan; the heat itself came free from the air. That is why one unit of electricity can deliver three or more units of heat, and why the physics still works when the air is below freezing — there is heat in air at any temperature above about minus 273°C, and modern units run down to about **minus 25°C**. The coldest UK design temperatures are only around minus 2 to minus 5°C. ## Efficiency — the three numbers, and the one that matters You will see three ratios, and they are not the same thing. **COP** — coefficient of performance — is an instantaneous figure at one set of test conditions. **SCOP** — seasonal COP — is a laboratory figure representative of a heating season, calculated to BS EN 14825 and quoted on the product label; current units are typically in the **3.5–4.5** range, higher at low flow temperatures. **SPF** — seasonal performance factor — is what the system actually did in a real house over a real year, and it is the figure that decides your bills. The largest UK field trial, the Electrification of Heat project, monitored 742 heat pumps for nearly two years. The **median SPF for air source was 2.80**, with the middle half of installations between 2.53 and 3.09, falling to about 2.44 on the coldest days. That gap between a label of 4 and a home at 2.8 is mostly design and installation: flow temperature, emitter sizing, controls and commissioning. In a well-insulated, airtight new build with underfloor heating or correctly sized radiators, the real figure sits at the top of that range. ## Running costs against gas Whether an ASHP is cheaper to run than a gas boiler comes down to one ratio: the price of a unit of electricity divided by the price of a unit of gas, set against the heat pump's SPF and the boiler's efficiency. For July to September 2026 the Ofgem price cap puts electricity at **26.11p per kWh** and gas at **7.33p** — a ratio of 3.56. Against a condensing boiler at 85% efficiency, a heat pump needs an SPF of about **3.0** to break even. From October the cap moves to 26.32p and 7.97p, a ratio of 3.30, and break-even drops to about **2.8** — right on the monitored median. On a flat standard tariff, then, a typical installation is roughly a draw with gas and a good one is ahead. A **heat pump time-of-use tariff**, with cheap overnight and off-peak periods, is what moves the sums decisively — and a new build designed around the pump from the start beats the median comfortably. ## The grant The **Boiler Upgrade Scheme** in England and Wales pays **£7,500** towards an air-to-water heat pump, and since 21 July 2026 **£9,000** for homes off the gas grid that are replacing oil or LPG heating — that uplift runs to 31 March 2027. Air-to-air heat pumps qualify for **£2,500**. The scheme has been extended to **2030**, the requirement to have an EPC was removed in 2026, and the earlier insulation conditions went in 2024. The installer must be MCS certified and commission within 120 days of the application. One grant per property. In **Scotland**, Home Energy Scotland offers a **£7,500 grant plus a £7,500 interest-free loan**, with an additional £1,500 on each for rural and island homes — up to £18,000 in all — for a heat pump that provides all the home's heating and hot water. The grant is for existing homes. In a **new build** there is no grant, and the heat pump is simply what the SAP expects — see below. ## Planning and noise For most houses in England an ASHP is **permitted development** — no planning application — and the rules were loosened on **29 May 2025**. The old requirement to keep the unit at least one metre from the boundary has been removed. The size limit for the outdoor unit rose from 0.6 to **1.5 cubic metres** for a house (0.6 for a block of flats). A detached house may have **two** units; any other house or a block of flats, one. The unit may not be used solely for cooling. What stayed: the unit must comply with the **MCS planning standard, MCS 020**, and permitted development still does not cover listed buildings, pitched roofs, flat roofs within a metre of the edge, or — in conservation areas and World Heritage Sites — walls or roofs facing a highway. MCS 020 is where noise comes in. Since September 2025 the test is a calculated sound level of **no more than 37 dB(A)** one metre from the centre of a neighbour's nearest habitable-room window, worked out from the unit's sound power, the distance and any barrier between them. (You may see 42 dB quoted; that was the old way of expressing the same limit and is superseded.) The unit itself typically runs at 40–60 dB up close; distance and siting do the rest, and on most plots a modern unit passes comfortably. ## Designing it properly The difference between a heat pump that returns 2.5 and one that returns 3.5 is nearly all design. The MCS design standard requires a **room-by-room heat loss calculation** to BS EN 12831-1 — not a rule of thumb from the boiler that was there before — at design temperatures for your location, and the pump has to meet **100% of that heat load without help from its backup electric heater**. That calculation needs a U-value for every element it touches, and an assumed one sizes the pump wrong in both directions; [U-Monkey](/tools/u-value-calculator/) builds up the real wall, roof and floor to BS EN ISO 6946, free, so the heat loss starts from the fabric you are actually building. Flow temperature is the lever: the standard steers designers to **55°C or below**, and the lower the better for efficiency, which is why radiators are often upsized and why underfloor heating suits heat pumps so well. A **hot water cylinder** is needed — the standard sizes it at 45 litres per person, counting bedrooms plus one — and the pump must be able to reach 55°C to heat it, with a legionella regime built in. Cylinders need a cupboard; plan for it. ## In the SAP calculation This is where an ASHP earns its place in a new build. SAP 10.2's carbon factor for electricity is **0.136 kgCO₂ per kWh** against **0.210 for mains gas**, and its primary energy factors are 1.50 and 1.13; under SAP 10.3, the Future Homes Standard methodology, the figures are 0.086 and 0.214 for carbon and 1.969 and 1.12 for primary energy — a stronger carbon case, a slightly weaker primary-energy one. Divide either by an SPF of 3 and a heat pump beats a gas boiler on both carbon and primary energy by a wide margin — often enough to carry the [SAP pass](/services/sap-calculations/) on its own, and the usual answer to a [planning renewables condition](/services/energy-statements/) too. If you are writing the energy statement rather than the SAP, note that it usually wants the [annual DESNZ conversion factors](/reference/carbon-factors/) instead of SAP’s. The product has to be in the **PCDB**, the Product Characteristics Database that SAP draws its performance data from; a unit that is not listed gets conservative defaults and a worse result than it deserves, and an MCS certificate does not guarantee a PCDB entry. Tell us the make and model. Under the [Future Homes Standard](/guides/future-homes-standard/), in force from **24 March 2027**, the notional dwelling that new homes are measured against is heated by a heat pump — so the heat pump stops being the ambitious option and becomes the baseline. Gas boilers are not banned by name, but the government expects them to be effectively unviable in new homes. ## ASHP or GSHP? A [ground source heat pump](/guides/ground-source-heat-pumps/) runs a little more efficiently in deep winter, because the ground stays warmer than the air, and it makes no noise outside. It also costs several times as much to install and needs the land for a ground loop or the budget for a borehole. For most new builds the ASHP is the pragmatic choice — and the one the SAP will have assumed. --- # Ground Source Heat Pumps (GSHP) URL: https://energycount.co.uk/guides/ground-source-heat-pumps/ Updated: 2026-06-11 ## How a GSHP works A few metres down, the ground holds a stable temperature all year. A ground source heat pump circulates fluid through buried pipework — **horizontal loops** in trenches (needing significant land) or **vertical boreholes** (compact but costly to drill) — and upgrades that heat for space heating and hot water. Because the ground source doesn't drop with the air temperature, GSHPs deliver a higher, steadier **SCOP (typically 4+)** than air source units, with peak efficiency exactly when heating demand peaks. ## In the SAP calculation Everything said for [heat pumps generally](/guides/heat-pumps/) applies, with a better SCOP: a GSHP is usually the strongest heating entry a [SAP calculation](/services/sap-calculations/) can carry, and a robust answer to demanding [planning conditions](/services/energy-statements/). ## When it makes sense Install cost is the catch — groundworks or drilling put GSHPs well above [ASHPs](/guides/air-source-heat-pumps/). They earn their premium on larger dwellings with bigger heat demands, plots with available land, projects where the external unit of an ASHP is unwelcome, and multi-dwelling schemes using shared ground loops (increasingly popular, and well-treated in SAP). No outdoor fan unit also means silent operation. Weighing GSHP vs ASHP for a project? We'll model both in SAP so the decision is numbers, not guesswork. [Ask us](/contact/). --- # Solar Hot Water (Solar Thermal) URL: https://energycount.co.uk/guides/solar-hot-water/ Updated: 2026-06-11 ## How it works Solar thermal collectors — **flat plate** panels or **evacuated tubes** — absorb solar radiation and transfer the heat via a glycol loop to a coil in a twin-coil hot water cylinder. A well-sized system provides 50–60% of a dwelling's annual hot water, with the boiler or heat pump topping up. Per square metre, solar thermal converts sunlight to useful energy more efficiently than PV. ## In the SAP calculation SAP credits solar hot water against the dwelling's water-heating energy, improving the [DER](/guides/der-ter/) — a useful contribution, though smaller than what a decently sized [PV array](/guides/solar-pv/) achieves, because water heating is only part of a dwelling's energy balance. ## Solar thermal vs PV + diverter The market has largely moved to PV: panel prices fell, and a **PV diverter** (now recognised in [SAP 10](/guides/sap-10-whats-new/)) sends surplus PV generation into the immersion heater — heating water *and* offsetting electricity, with fewer roof penetrations, no glycol maintenance, and full credit in the calculation. Solar thermal still suits projects with high hot water demand and limited roof area. Unsure which suits your project and planning condition? [We'll model both](/contact/). --- # MVHR (Mechanical Ventilation with Heat Recovery) URL: https://energycount.co.uk/guides/mvhr/ Updated: 2026-06-11 ## How MVHR works An MVHR system continuously extracts stale, moist air from kitchens and bathrooms and supplies fresh air to habitable rooms, passing both streams through a heat exchanger. Up to ~90% of the heat in outgoing air transfers to the incoming supply — ventilation without the heat penalty of trickle vents and extract fans. The payoff: constant filtered fresh air, no condensation problems, and lower heating demand. ## The airtightness rule of thumb MVHR only makes sense in airtight dwellings. If the building leaks, uncontrolled infiltration delivers (cold) fresh air regardless and the heat exchanger achieves little — wisdom is that MVHR starts earning below roughly q50 = 3, and the tighter the better. That makes design-stage commitment essential: the [air test](/services/air-tightness-testing/) must deliver what the [SAP calculation](/services/sap-calculations/) assumes. ## In the SAP calculation SAP models MVHR through the system's heat-exchange efficiency and specific fan power, balanced against the dwelling's airtightness. In a tight dwelling it cuts ventilation heat loss substantially and helps the [DER and DFEE](/guides/der-ter/); in a leaky one it can model *worse* than natural ventilation once fan energy is counted. Duct design matters too — short, rigid, insulated runs. Get the SAP assumptions and ventilation design agreed together; we can advise on both. [Ask us](/contact/). --- # WWHR (Waste Water Heat Recovery) URL: https://energycount.co.uk/guides/waste-water-heat-recovery/ Updated: 2026-06-11 ## How WWHR works A waste water heat recovery unit is typically a vertical copper heat exchanger plumbed into the shower waste. Warm grey water spirals down the inside surface while incoming cold mains water passes around it, pre-warming the cold feed to the shower mixer and/or cylinder by recovering **around 40–60%** of the heat that would otherwise go down the drain. No moving parts, no electricity, no maintenance — it just sits in the pipework. Vertical units (through a floor) perform best; horizontal variants exist for ground-floor showers. ## Why SAP 10 made it standard The [SAP 10 notional dwelling assumes WWHR on all showers](/guides/sap-10-whats-new/) — so a dwelling without it starts behind target. For the modest cost of a unit per shower, WWHR is among the cheapest CO₂ and energy savings available in a [SAP calculation](/services/sap-calculations/), particularly in homes where showers dominate hot water use. It also helps [Part G water-energy crossover](/services/part-g-water-calculations/) thinking: shower flow rates now feed both calculations, so fittings choices pay off twice. Specify it at design stage — retrofitting into finished pipework is far harder. [Ask us](/contact/) where it earns most in your SAP. --- # FGHR (Flue Gas Heat Recovery) URL: https://energycount.co.uk/guides/flue-gas-heat-recovery/ Updated: 2026-06-11 ## How FGHR works Even a modern condensing boiler loses usable heat in its flue gases. A flue gas heat recovery device — fitted above or integrated into the boiler — captures part of that heat and uses it to pre-warm incoming water, mainly benefiting **domestic hot water** production. The boiler then needs less gas to reach temperature, typically saving a few percent of annual gas use, most in homes with high hot water demand. It's passive in operation, compact, and best specified with the boiler rather than retrofitted. ## In the SAP calculation SAP recognises FGHR through improved water-heating efficiency, helping the [DER](/guides/der-ter/) for gas-heated dwellings at modest cost. It pairs naturally with [WWHR](/guides/waste-water-heat-recovery/) — one recovers drain heat, the other flue heat. ## The bigger picture FGHR only applies to combustion boilers, so its future is bounded: the [Future Homes Standard](/guides/future-homes-standard/) removes fossil-fuel heating from new homes from 2027, and [heat pumps](/guides/heat-pumps/) have no flue to recover from. For conversion or extension projects keeping gas in the meantime, it remains a worthwhile line in the [SAP](/services/sap-calculations/) toolkit. [Ask us](/contact/) whether it earns its place in yours. --- # What is Weather Compensation? URL: https://energycount.co.uk/guides/weather-compensation/ Updated: 2026-06-11 ## How it works A weather compensator uses an external temperature sensor to modulate the heating flow temperature: bitter outside, hotter radiators; mild outside, cooler ones. Instead of cycling on and off at a fixed high temperature, the system runs longer, lower and steadier — keeping a condensing boiler in its efficient condensing range far more of the time, and keeping homes more evenly comfortable. For [heat pumps](/guides/heat-pumps/), weather compensation is essentially native: lower flow temperatures directly raise the SCOP, which is why almost every heat pump runs on a compensation curve. ## In the SAP calculation Heating **controls** carry real weight in SAP. Weather compensation (and load compensation, its indoor-sensing cousin) improves the modelled efficiency of the heating system, nudging the [DER](/guides/der-ter/) down for very little hardware cost — many boilers only need the sensor and a setting enabled. Modern boiler-interlock, time and zone control requirements still apply alongside. It's the kind of cheap specification line that's easy to forget and annoying to miss when a [SAP calculation](/services/sap-calculations/) is marginal. We'll flag it when it helps. [Ask us](/contact/). --- # NHS Carbon Reduction Plans from April 2027: what suppliers need URL: https://energycount.co.uk/guides/nhs-carbon-reduction-plan/ Updated: 2026-09-30 > **Quick answer:** from **1 April 2027**, NHS procurements above the published threshold ask every bidder for a published **Carbon Reduction Plan (CRP)**. Contracts under **£5 million a year** (including VAT) need a **2024 CRP**: UK operations, scope 1 and 2, and five named scope 3 categories. Contracts of **£5 million a year or more**, and every new framework and dynamic market, need a **2027 NHS CRP**: a global boundary and every relevant scope 3 category. Both commit to **net zero by 2050 or earlier**, are approved by the board and published on your website, and follow the government's PPN 006 template. The check is **pass/fail**: a plan that falls short stops the bid at the first stage. ## What changes on 1 April 2027 NHS England published the new requirements on 9 June 2026. From 1 April 2027 they "replace the previous CRP and net zero commitment (NZC) requirements", and no new procurement will use the Net Zero Commitment. They apply to procurements run under both the Procurement Act 2023 and the Provider Selection Regime. Procurements that started before 1 April 2027 carry on under the old guidance. The biggest change is at the smaller end. Until 31 March 2027, a bid for an NHS contract under £5 million a year needs only a Net Zero Commitment. From 1 April 2027 the same bid needs a full Carbon Reduction Plan, with your emissions in it. | Contract (above the procurement threshold) | Until 31 March 2027 | From 1 April 2027 | | --- | --- | --- | | Under £5 million a year (incl. VAT) | Net Zero Commitment | **2024 CRP** | | £5 million a year or more (incl. VAT) | Carbon Reduction Plan | **2027 NHS CRP** | | New frameworks and dynamic markets | — | **2027 NHS CRP**, whatever the value | ## Which plan does your contract need? **The 2024 CRP** applies to "procurements of lower value (below £5 million per annum including VAT, and above the relevant published procurement thresholds)". Below the published threshold, neither plan is asked for. The threshold depends on the buyer and the type of contract, so check the tender notice. **The 2027 NHS CRP** applies to "procurements of high value (£5 million per annum and above, including VAT)", to all new frameworks "irrespective of the value of the framework contract, where relevant and proportionate", and to all new dynamic markets whatever their value. For frameworks it is applied when the framework itself is tendered, not at each call-off. The owner of a framework or dynamic market can drop to the 2024 CRP where the fuller plan is "not proportionate and relevant". ## What a 2024 CRP must contain - **A net zero commitment:** net zero by 2050 or earlier, for your UK operations as a minimum. - **Your emissions, baseline and current,** for: - **scope 1** — fuel burned in your own buildings and vehicles, and refrigerant leaks; - **scope 2** — the electricity (and any heat) you buy; - **five scope 3 categories**, numbered as the GHG Protocol numbers them: - 4 — upstream transportation and distribution (freight and deliveries you pay for); - 5 — waste generated in operations; - 6 — business travel; - 7 — employee commuting; - 9 — downstream transportation and distribution (transport of what you sell, where you don't pay for it). - **CO2e** covering the seven greenhouse gases of the Kyoto Protocol. - **The environmental management measures in effect** — what you already do to cut emissions. - **Board approval**, or approval by a company director where there is no board. - **Publication:** "clearly signposted and published on the supplier's website". - **Format:** the structure and length of central government's PPN 006 template, "though stylistic differences are acceptable". ## What the 2027 NHS CRP adds The 2027 NHS CRP has the same list with two changes, both substantial: - **A global boundary.** "A global geographical boundary must be applied for the net zero commitment and emissions reporting." A UK subsidiary of an international group reports the group's emissions, not just its own. - **All relevant scope 3.** "Suppliers must report emissions for all Scope 3 categories deemed relevant", and for any category judged not relevant, "an explanation… must be provided". The guidance allows spend-based estimates. There is a first-year allowance: in tenders run in the NHS 2027/28 financial year, gaps in scope 3 data can pass with "an acceptable explanation and plan for building capability". ## How the plan is checked in a tender The plan sits under the **conditions of participation**. NHS England's guidance is plain: "Suppliers whose 2027 NHS CRP or 2024 CRP does not meet the minimum requirements will not be able to proceed to the next stage", and "CRPs will not be scored or compared against each other". In the tender questions you confirm that you are publicly committed to net zero by 2050, and give baseline-year and reporting-year figures for scopes 1, 2 and 3. So a CRP will not win you a contract, but a missing or deficient one loses it before the quality and price are looked at. ## How recent your plan has to be The NHS test is that the reporting period must fall "no more than 18 months prior to the date of the tender notice being published". An older period fails unless there is an "acceptable explanation". On our reading, a plan reporting the year to 31 March 2026 stays in date for tenders published up to about the end of September 2027. Buyers are also told to include "the requirement for annual updates within the terms and conditions", so a contract you win will expect the plan to be refreshed each year. Central government uses different wording (below), so a plan that is in date for one buyer may not be for another. ## Central government contracts: PPN 006 Central government's version is Procurement Policy Note 006, *Taking account of Carbon Reduction Plans in the procurement of major government contracts*. It applies to procurements started on or after 24 February 2025 (PPN 06/21 covers earlier ones), by central government departments, executive agencies and non-departmental public bodies, for contracts with an estimated value "above £5 million per year (including VAT)". Its technical standard asks for: - a commitment to net zero by 2050 for your UK operations; - scope 1 and 2 and the same five scope 3 categories (4, 5, 6, 7 and 9), for sources in the UK; - a footprint that adheres to the Greenhouse Gas Protocol Corporate Accounting and Reporting Standard, "conducted to a reasonable level of assurance", using the UK Government's published conversion factors; - sign-off by a director (or equivalent, or a designated member of an LLP); - review and update "within 6 months of your organisation's financial year-end"; - publication on your UK website, on the [PPN 006 template](https://assets.publishing.service.gov.uk/media/6800e3930b24153af1e7c700/2025-04-16_PPN_006_Carbon_Reduction_Plan_Template.odt). It is also a check, not a score: PPN 006's FAQ says plans "should not be 'scored'". Its recency test differs from the NHS one: the plan must have been published since the tender notice, or in the 12 months before it. ## NHS Supply Chain: Evergreen Level 1 since April 2026 NHS Supply Chain runs its own gate. "From 6 April 2026, all suppliers submitting tenders will be required to have achieved Evergreen Level 1 at the point of tender close." The Evergreen Sustainable Supplier Assessment is hosted on Atamis, and its criteria were updated in April 2026. NHS England describes Level 1 as having "Published a Carbon Reduction Plan aligned to PPN requirements, with a UK net zero commitment and emissions reporting". Level 2 is aligned to the April 2027 requirements (global boundary, all relevant scope 3), and Level 3 adds 2045 targets that are independently validated. For a supplier on NHS Supply Chain frameworks, a published CRP is therefore already the entry ticket, not a 2027 problem. ## After 2027: the rest of the roadmap The NHS Net Zero Supplier Roadmap keeps going after April 2027. Its next step, from **April 2028**, is that "New requirements will be introduced overseeing the provision of carbon footprinting for individual products supplied to the NHS". NHS England's supplier page also lists a 2030 step, under which suppliers "will only be able to qualify for NHS contracts if they can demonstrate their progress through published progress reports and continued carbon emissions reporting"; the June 2026 roadmap document does not repeat it, so treat the detail as unsettled. The direction is clear either way. The plan you write for 2027 is the baseline you will be measured against later, so the numbers in it need to be ones you can reproduce. ## Getting a plan ready: what to gather A CRP reports a 12-month period, usually your financial year. Collecting the data is most of the work; the arithmetic is quick once you have it. | What | Collect | Where it usually comes from | | --- | --- | --- | | Scope 1 — buildings | Gas, oil or LPG used (kWh or litres) | Fuel bills, meter reads | | Scope 1 — vehicles | Fuel for company-owned or leased vehicles (litres, or miles by vehicle type) | Fuel cards, mileage logs | | Scope 1 — refrigerants | Top-ups of air-con or refrigeration gas (kg, by gas) | Maintenance or F-gas records | | Scope 2 | Electricity used at each site (kWh) | Electricity bills | | Category 4 | Freight and deliveries you pay for (tonne-km, or spend) | Haulier and courier invoices | | Category 5 | Waste by stream and disposal route (tonnes) | Waste transfer notes, contractor reports | | Category 6 | Rail, air, taxi, hire car and staff's own cars used for work | Expense claims, travel bookings | | Category 7 | Commuting distance and mode per employee | A short staff survey | | Category 9 | Delivery of your products that customers pay for | Delivery records, estimates | Then: 1. **Pick a baseline year.** Usually the first year you have good data for; the PPN 006 template asks for it ("Baseline year: 20XX"). 2. **Convert to CO2e** with the UK Government's conversion factors. The 2026 set was published on 11 June 2026 and corrected on 31 July 2026 for values in the flat file "initially reported as 0" — use the corrected file. The factors are published under the Open Government Licence. 3. **Write the measures and the commitment.** What you already do, and your net zero year. The PPN 006 template also has a line for a five-year projection. 4. **Get it approved** by the board or a director. 5. **Publish it** on your website where a buyer can find it, and keep your workings. A buyer can ask how a figure was made. ## Watch-outs - **Exactly £5 million.** The NHS says "£5 million per annum and above"; PPN 006 says "above £5 million". A contract valued at exactly £5 million a year is in scope for the 2027 NHS CRP but outside PPN 006. - **Three recency tests.** NHS: a reporting period within 18 months of the tender notice. PPN 006: a plan published since the tender notice or in the 12 months before it. PPN 006's technical standard: an update within 6 months of your financial year-end. - **Targets.** The roadmap talks about suppliers publicly reporting targets, but the NHS minimum requirements set no numeric reduction target beyond the net zero commitment. Don't promise a figure you can't evidence. - **Assurance.** PPN 006 wants the footprint "conducted to a reasonable level of assurance". Keep the bills, surveys and records behind every number; a calculation is only as good as what went into it. ## Free help - The [PPN 006 guidance, technical standard and template](https://www.gov.uk/government/publications/ppn-006-taking-account-of-carbon-reduction-plans-in-the-procurement-of-major-government-contracts). - The [UK Business Climate Hub](https://businessclimatehub.uk/), whose policy content is reviewed and endorsed by DESNZ, and the [SME Climate Hub](https://smeclimatehub.org/) calculators. - NHS England's Sustainable Supplier Accelerator, which the 2027 guidance points SMEs to. ## Sources - NHS England, [2027 NHS Carbon Reduction Plan requirements for the procurement of NHS goods, services and works](https://www.england.nhs.uk/long-read/2027-nhs-carbon-reduction-plan-requirements-for-the-procurement-of-nhs-goods-services-and-works/) (published 9 June 2026). - NHS England, [Carbon Reduction Plan requirements for the procurement of NHS goods, services and works](https://www.england.nhs.uk/long-read/carbon-reduction-plan-requirements-for-the-procurement-of-nhs-goods-services-and-works/) — the rules until 31 March 2027 (version 3, 10 December 2025). - NHS England, [Market sounding findings: net zero supplier requirements from April 2027](https://www.england.nhs.uk/long-read/market-sounding-findings-net-zero-supplier-requirements-from-april-2027/). - NHS England, [Greener NHS: suppliers](https://www.england.nhs.uk/greenernhs/get-involved/suppliers/) and the [Evergreen Sustainable Supplier Assessment](https://www.england.nhs.uk/nhs-commercial/sustainability/evergreen/). - NHS Supply Chain, [Evergreen: what suppliers need to know for April 2026](https://www.supplychain.nhs.uk/news-article/evergreen-sustainable-supplier-assessment-what-suppliers-need-to-know-for-april-2026/) (26 February 2026). - Cabinet Office, [PPN 006: Taking account of Carbon Reduction Plans in the procurement of major government contracts](https://www.gov.uk/government/publications/ppn-006-taking-account-of-carbon-reduction-plans-in-the-procurement-of-major-government-contracts) (17 February 2025; template April 2025; FAQ July 2025). - DESNZ, [Greenhouse gas reporting: conversion factors 2026](https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2026) (11 June 2026, corrected 31 July 2026). ## NEWS & REGULATION WATCH --- # New minimum evidence rules for energy assessors took effect on 1 October URL: https://energycount.co.uk/news/easob-minimum-evidence-requirements-1-october-2026/ Date: 2026-10-02 > **Quick answer:** The Energy Assessor Scheme Operating Board (EASOB) has published minimum evidence requirements for energy assessors in England and Wales, effective **1 October 2026**. For on-construction domestic assessors (OCDEAs) the list includes the software data file, plans, construction specification, U-value calculations, a developer as-built sign-off, the airtightness test certificate, **psi-value documentation** and photographs where Building Regulations require them. The guiding principle is that an auditor must be able to reproduce the EPC from what you hold. ## What has been published EASOB has issued separate minimum evidence documents for each assessor type — DEA, NDEA and OCDEA, with DEC and ACEA documents alongside. They are dated September 2026 and apply from 1 October 2026. At the time of writing the EASOB site shows the DEA document at v1.1 and the OCDEA document at v1.2, so check the live versions before relying on a printed copy. Each document is explicitly a *minimum* list. EASOB notes that further evidence may be needed depending on the property, so treat it as a floor, not a ceiling. ## What OCDEAs need on file For a new-build SAP assessment the OCDEA document lists: - the **software data file** submitted with the EPC lodgement; - **plans** — floor plans, sections and elevations to support geometry, plus a site plan showing orientation and overshadowing; - the **construction specification**; - **U-value calculations** — for solid elements to BR 443 and BS EN ISO 6946; for windows to BS EN ISO 12567 or 10077, or BFRC certificates where applicable; - an **as-built sign-off** — a statement from the developer (or whoever is the client for construction) confirming the building matches the design, and flagging any discrepancies; - the **airtightness test certificate**; - **thermal bridging details** — the psi values used, either from a published scheme or calculated to BR 497; - **photographic evidence** where the relevant Building Regulations version requires it. ## What DEAs need The DEA document asks for floor plans in metric to two decimal places with areas, windows and heat-loss/party walls identified; site notes covering age, construction, heating and the reasoning behind decisions; and photographs of elevations, construction, insulation (with measured thickness), windows, doors, lighting, heating, hot water, ventilation, renewables and meters. Documentary evidence can substitute for a photograph where one cannot reasonably be taken, but it must be property-specific and independently verifiable, in line with RdSAP Convention 9.02. NDEAs have an equivalent list built around annotated zoning drawings and supporting calculations. ## Why it matters for new-build work Two lines in the OCDEA list deserve attention. First, the **as-built sign-off** puts a document from the developer in your file; if you do not already collect one on every plot, start. Second, **psi values must be traceable**: a published-scheme source or a BR 497 calculation, not a number typed into the software. That sits squarely alongside the direction of travel in the Home Energy Model, where [a single blanket thermal bridging number is being removed](/news/ecaas-hem-1-0-0a9-thermal-bridging-breaking-changes/). Junction-level evidence is becoming the norm in both worlds. ## What to do now Check your current file template against the OCDEA list, and chase developers for sign-offs and airtightness certificates before lodging rather than after. Where junction psi values are bespoke, make sure the calculation is on file and can be reproduced. Our guides on [psi values](/guides/psi-values/) and [thermal bridging](/guides/thermal-bridging/) explain what an auditor will expect to see. If you need a BR 497-style junction calculation for the evidence file, [ΨMonkey](/tools/psi-calculator/) models it free on screen and produces a Building Control-ready PDF. *Sources: [EASOB minimum evidence requirements](https://www.easob.co.uk/); [OCDEA Minimum Evidence Requirements (PDF)](https://www.easob.co.uk/OCDEA_Minimum_Evidence_Requirements.pdf); [DEA Minimum Evidence Requirements (PDF)](https://www.easob.co.uk/DEA_Minimum_Evidence_Requirements.pdf); [Energy Trust summary](https://energy-trust.co.uk/what-are-the-new-minimum-evidence-requirements-for-deas-ndeas-and-ocdeas/).* --- # The draft London Plan drops carbon offsetting for absolute energy targets — and the consultation closes on 15 October URL: https://energycount.co.uk/news/draft-london-plan-2026-energy-standards/ Date: 2026-10-01 > **Quick answer:** On **16 July 2026** the Mayor of London published the **draft London Plan** for consultation, and Chapter 5 rewrites how energy performance is assessed on major schemes. The current approach — Policy SI 2's four-stage **"be lean, be clean, be green, be seen"** hierarchy, judged as a percentage improvement on Part L and backed by a **carbon offset payment** for any shortfall — gives way to four new policies, **GHR1 to GHR4**. GHR1 introduces **absolute Energy Use Intensity (EUI) and Space Heating Demand (SHD) targets**, in kWh/m² a year, with both a baseline and a tighter aspirational figure. The draft **does not carry forward the carbon-offsetting route**: offset payments are proposed to go, replaced by a requirement to meet the numbers at source. The **five years of post-occupancy energy reporting** that major schemes already owe under "be seen" carries over — now set against the kWh/m² targets the scheme was approved on. The consultation closes on **15 October 2026**; examination is expected in 2027 and adoption in early 2028. Nothing in the current London Plan changes today — this is a draft — but the direction of travel, and the closing date, are both worth acting on now. ## Why an assessor outside London should still read this Energycount is a national audience, and this is, on its face, a London-only planning document. Two things make it worth ten minutes of anyone's time who works on SAP, DER/TER or energy statements anywhere in England. First, if you [work on London schemes at all](/guides/energy-hierarchy/) — and a lot of national housebuilders do — this changes what the energy statement has to demonstrate before the SAP calculation is even run. Second, the London Plan has form as a bellwether: London led on the energy hierarchy itself, on carbon offset funds, and on requiring overheating assessment years before Part O existed. A Mayor's plan that drops offsetting in favour of absolute, monitored performance targets is a reasonable preview of where national policy — and where more Local Planning Authorities' own [local energy standards](/news/nppf-2026-local-energy-standards-ter-uplift/) — are heading next. ## What actually changes: four policies instead of one The current London Plan runs energy performance mainly through Policy SI 2 and its four-stage hierarchy: be lean (reduce demand), be clean (supply energy efficiently), be green (renewables), be seen (monitor and report). Major schemes are judged on carbon: at least a 35% on-site cut beyond Part L, with any shortfall to zero carbon offset. The draft reorganises energy, heat and overheating into four policies: | Policy | What it covers | | --- | --- | | **GHR1** | Energy efficiency — moves from a percentage-improvement test to absolute Energy Use Intensity and Space Heating Demand targets, with in-use monitoring | | **GHR2** | Low-cost low-carbon heat — every major scheme submits a Low Carbon Heat Appraisal to find the lowest-cost, lowest-carbon heating for the site | | **GHR3** | Whole life-cycle carbon — for referable schemes, links demolition and retention decisions to a whole-life carbon assessment and a pre-redevelopment audit of the existing building | | **GHR4** | Heat risk — overheating and the urban heat island: orientation, glazing, shading, ventilation and layout justified at planning stage, not just a late-stage cooling check | The reduce-supply logic of the old hierarchy hasn't disappeared — GHR1 and GHR2 still separate demand reduction from low-carbon supply — but the single carbon number is gone. Each strand now has its own test, and GHR1's test has changed shape entirely: from "beat Part L by X%" to "hit this many kWh per square metre, and report what the building actually uses." GHR1 also tells London's boroughs not to set their own energy-efficiency or emissions standards in local plans — one set of numbers for the whole city. ## The end of the offset cheque Under the current London Plan, a scheme that cannot reach zero-carbon on site pays a cash-in-lieu contribution into the borough's carbon offset fund for the shortfall, priced per tonne of CO₂. It is a well-worn route, and boroughs have built retrofit and carbon-saving programmes around the money it raises. The draft does not carry that mechanism forward — its supporting text refers plainly to "the removal of carbon offsetting policy" (paragraph 5.21). The direction is source-level performance — fabric, heat source and efficiency measures that actually deliver the kWh/m² figure — rather than a calculated shortfall bought out at the end of the process. For a design team, that removes a fallback that has quietly absorbed difficult sites (tight urban plots, retained façades, poor solar access) for years. Where an offset payment might once have closed the gap between what was achievable and what the policy required, the draft expects the gap not to exist. ## The numbers GHR1's policy box sets the targets out in full, in kWh/m² a year on gross internal area: | Target | Baseline | Aspirational | | --- | --- | --- | | Space Heating Demand — all typologies | 20 | 15 | | Energy Use Intensity — residential | 40 | 35 | | Energy Use Intensity — office | 90 | 70 | | Energy Use Intensity — primary school | 40 | 35 | | Energy Use Intensity — industrial | 40 | 35 | | Energy Use Intensity — hotel | 120 | 80 | | Energy Use Intensity — student accommodation | 65 | 50 | EUI counts **regulated and unregulated** energy — everything the building uses in a year — so it is a wider number than the regulated energy a Part L calculation covers. Uses outside these typologies, and major refurbishments, still have to minimise and report EUI and SHD against industry benchmarks such as LETI and the UK Net Zero Carbon Buildings Standard. These are consultation figures, taken from the GHR1 policy box in the [draft London Plan](https://www.london.gov.uk/programmes-strategies/planning/london-plan/next-london-plan/draft-london-plan-consultation), and they can still change at examination before the Plan is adopted. One independent check worth reading alongside the GLA's own text: [an analysis on BE News](https://benews.co.uk/informed/the-draft-london-plan-introduces-clearer-energy-targets-but-does-it-provide-a-consistent-basis-for-decision-making/) (27 August 2026, by Andy Crowther of Cundall) points out that an office meeting the 90 kWh/m² baseline would beat the CRREM decarbonisation pathway in 2030 but exceed it from 2040 — and would not meet the UK Net Zero Carbon Buildings Standard pathway in any of the years compared. Multi-family residential is the one typology where both London Plan targets stay under both pathways out to 2050. The practical point for anyone advising a client: a single "policy compliant" figure doesn't automatically mean "net-zero-aligned" for the building's whole life, and it's worth being able to explain the difference when a client asks why their scheme clears planning but a lender's ESG due diligence still flags it. ## GHR4: overheating moves earlier The current London Plan already requires overheating assessment ahead of national Part O. The draft's GHR4 (Heat risk), which applies to all major development, goes further by tying overheating, shading and passive design into masterplanning and layout rather than treating it as a check run once the layout is fixed. Applicants have to justify, in the planning application, how the design maximises passive measures — orientation, less unnecessary glazing, external shading, high-albedo materials, openable windows and cross-ventilation — and minimises the energy used for any cooling, allowing for a warmer future climate. We'd treat the detail of which CIBSE methodology (TM59, TM52 or TM49) applies to which building type as one to confirm from the policy text itself once you're working a live scheme against it, rather than from a summary — including ours. ## Five years of reporting — now against a kWh target Post-occupancy reporting isn't new in London: the current Plan's "be seen" stage already requires major developments to report their energy performance to the Mayor for at least five years. GHR1 keeps the duty (Part D) but changes what the reported figure is measured against. Today the planning test is a percentage cut in regulated CO₂ beyond Part L, so a building's metered energy use isn't the number it was approved on. Under GHR1 it is — the target itself is kWh per square metre a year of everything the building uses. That is the performance gap [EPC accuracy research has been quantifying nationally](/news/epc-accuracy-research-performance-gap/) — the difference between a compliant design-stage calculation and what a building actually uses — set directly against the planning permission. A design that scrapes GHR1's baseline on paper and then under-delivers in year one will show up in the Mayor's own data, on a five-year clock. ## What to do before 15 October 1. **If you have live or upcoming London schemes**, read GHR1 to GHR4 in the consultation document itself, not just the summaries — this article included. The figures above are from the GHR1 policy box, but they are consultation figures and may change before adoption. 2. **Model early.** An absolute kWh/m² target with no offset fallback means the [fabric-first, demand-reduction stage of the hierarchy](/guides/energy-hierarchy/) has to work harder before you get to heat source and renewables — there's no longer a cheque waiting at the end for the shortfall. 3. **Treat GHR4 as a masterplanning input**, not a late-stage report. Orientation and massing decisions made before an overheating consultant is appointed are the ones the draft is trying to catch. 4. **Respond to the consultation** if the targets, the loss of offsetting, or the five-year monitoring duty affect a scheme you're working on. Responses are taken through the [GLA's draft London Plan consultation](https://www.london.gov.uk/programmes-strategies/planning/london-plan/next-london-plan/draft-london-plan-consultation) until 5pm on 15 October 2026. 5. **Watch for the ripple effect** even if you never work in London. A Mayor's plan that abandons percentage-improvement targets for absolute, monitored ones is exactly the kind of precedent that turns up in the next round of local plan energy policies elsewhere. Nothing here changes a live Building Regulations submission today. But if you're pricing, designing or writing an energy statement for a London scheme that will still be in the pipeline in 2028, this is the version of the rules it's likely to meet. --- # Half of every planning decision in England is somebody's extension — and the 25% rule is still the surprise URL: https://energycount.co.uk/news/planning-applications-q2-2026-householder-extensions/ Date: 2026-09-25 The quarterly planning statistics are not usually read for what they say about Building Regulations. This set is worth it, because of a single proportion that has held steady for years: about half of all the planning work local authorities in England get through is householder work. ## What the April to June 2026 release says The **Planning applications in England: April to June 2026** statistical release was published by MHCLG on **24 September 2026**. In the quarter, authorities in England: - received **78,700 applications** for planning permission, down 2% on the same quarter a year earlier - **decided 74,500** applications, down 8% - **granted 65,200** decisions, down 8% Of those decisions, **37,700 were on householder developments** — "developments to a residence which require planning permission such as extensions, loft conversions and conservatories", in the release's own words. That is **51% of all decisions**, down from 54% in the quarter ending June 2025, with the volume itself **down 14%** year on year. Authorities **granted 91%** of them and decided **92% within 8 weeks** or the agreed time. Over the **year ending June 2026** the pattern is the same: **145,600 householder decisions, 50% of all decisions**, 90% granted. So the softening is real — householder volumes fell faster than the total this quarter — but the share is not moving much. Half the planning system's caseload, give or take, is someone adding a room to their own house. ## And that undercounts the work These figures only count extensions that needed planning permission. A great many do not: a single-storey rear extension within the permitted development limits needs no application at all, and the same release records that **large householder extensions made up 57% of all prior approval applications reported** under permitted development rights. Every one of those extensions still needs Building Regulations approval. Planning and Building Control are separate consents, decided by different tests, and a granted planning permission carries no implication whatsoever that the scheme meets Part L. The homeowner who has just had a 91%-likely approval land in their inbox has cleared the consent they knew about and not yet met the one they did not. ## The number nobody mentions at the planning stage Approved Document L limits the total area of **windows, roof windows and doors** in an extension to **25% of the extension's floor area** — plus, and this is the part most often missed, **the area of any existing windows and doors the extension covers over or removes**. A back door that now opens into the new room, a kitchen window built over: both add to the allowance. Stay inside it and the extension complies on its element U-values alone, with no calculation to submit. That is the quiet reward for getting the glazing right, and it is why the number matters before the bifolds are ordered rather than after. ## The U-values that come with it For new elements in an existing dwelling, Approved Document L Volume 1 (2021 edition incorporating the 2023 amendments) sets limiting values of **walls 0.18, floors 0.18, roofs 0.15, windows and doors 1.4, and rooflights 2.2 W/m²K**. A roof window in the plane of the roof is a window and takes 1.4, assessed vertically; a rooflight on an upstand takes 2.2, assessed horizontally. A conservatory or porch is outside the energy efficiency requirements only while every condition holds — ground level, no more than **30 m²**, substantially glazed, thermally separated, no heating or heating on its own controls. Break one and it is an extension, with the 25% rule attached. Go over the allowance and there are three routes rather than a refusal, the cheapest of which is usually to show the extension's area-weighted U-value is no worse than a compliant one of the same size and shape. The [full guide to building regulations for an extension](/guides/extension-building-regulations/) works each of those through, along with what happens to the existing house when the work touches it. ## Where this leaves the glazing Modern extensions are designed around glass. A 24 m² rear extension has a 6 m² opening allowance before covered-over openings are added, and a set of bifold doors alone can be 10 m². That is not a failure — it is the ordinary starting point for most schemes, and the reason the area-weighted route exists. **GlazeMonkey** runs the three tests an extension has to pass on the areas you actually have: the opening-area check including the covered-openings credit, the Table 4.2 element checks, and the full area-weighted comparison against the notional extension. It is free to use in the browser with no sign-up, so the answer arrives before the glazing is ordered rather than during a Building Control query. [Try GlazeMonkey →](/tools/extension-glazing-calculator/) The next quarterly release, covering July to September 2026, is due in December. If you have an extension where the glazing looks marginal and you would rather have it settled properly, [get in touch](/contact/). *Sources: [Planning applications in England: April to June 2026 — statistical release, MHCLG, 24 September 2026](https://www.gov.uk/government/statistics/planning-applications-in-england-april-to-june-2026/planning-applications-in-england-april-to-june-2026-statistical-release) · [Planning applications in England: April to June 2026 — technical notes, MHCLG](https://www.gov.uk/government/statistics/planning-applications-in-england-april-to-june-2026/planning-applications-in-england-april-to-june-2026-technical-notes) · [Planning applications statistics collection, GOV.UK](https://www.gov.uk/government/collections/planning-applications-statistics) · [Approved Document L: Conservation of fuel and power, GOV.UK](https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l)* --- # The first Environmental Delivery Plan has landed — and it is a flat £2,675 a home URL: https://energycount.co.uk/news/first-environmental-delivery-plan-norfolk-consultation/ Date: 2026-09-19 > **Quick answer:** On **17 September 2026** Natural England and Defra opened consultation on the **draft Norfolk Environmental Delivery Plan** — the [first EDP of its kind](https://www.gov.uk/government/consultations/nutrient-pollution-draft-norfolk-environmental-delivery-plan-consultation) under the Nature Restoration Fund. It addresses **nutrient pollution** affecting the **Broads SAC (Yare and Bure)**, **Broadland Ramsar** and the **River Wensum SAC**, across three catchments. Instead of a calculated nutrient budget, the draft proposes a **flat levy of £2,675 per house or flat**, indexed to the RICS CIL Index and before VAT. Natural England models capacity for up to **15,780 homes**. Participation would be **voluntary**. The plan is **not in force** — it is a draft, the Secretary of State has not decided it, and its start and end dates are still blank. The consultation closes at **9.29am on 27 October 2026**. Until a made EDP is operative, every site in every nutrient catchment still needs the existing route: calculate the budget, secure the mitigation, evidence it. ## Why this one matters even if you never work in Norfolk We have written about the Nature Restoration Fund three times this year — the [regulations being laid](/news/nature-restoration-fund-nutrient-regulations-laid/), the [levy rulebook and Parliament's approval](/news/nature-restoration-levy-guidance-parliament-approval/) — and each time the honest answer was the same: the framework exists, but nothing you can actually use exists yet. That has now changed in one place. The Norfolk EDP is the first working example of what an EDP looks like on paper, and the structure it sets will almost certainly be the template for the other fifteen nutrient EDPs Natural England notified the Secretary of State about in December 2025. If you want to know what nutrient mitigation is going to look like in the Stour, the Solent, the Tees or the Avon in eighteen months' time, this document is the best available preview. Defra's own framing, in a [blog post published alongside the consultation](https://defraenvironment.blog.gov.uk/2026/09/17/nature-restoration-fund-turning-national-ambition-into-practical-action-in-norfolk/), is that this is "the first opportunity to test, in a real place", an approach meant to replace project-by-project mitigation with pooled, coordinated delivery. Two design choices stand out, and both are departures from how nutrient neutrality works today. ## Departure one: a flat charge, not a calculated budget Nutrient neutrality as practised since 2022 is a calculation. You work out the additional nitrogen or phosphorus load your development produces — occupancy, wastewater treatment works discharge concentration, water consumption, surface water, land use change — and then you buy or build mitigation equal to that load plus a precautionary buffer. The draft Norfolk EDP does not do that. It proposes **£2,675 for each house or flat**, full stop. Annual indexation is linked to the RICS Community Infrastructure Levy Index. The draft developer-process supplement states that levy amounts exclude VAT, charged at 20%, though that appears in illustrative draft paperwork rather than a settled charging schedule. Ten homes would therefore be a base levy of **£26,750** before indexation and VAT. (The charging, eligibility and drainage detail summarised in this article comes from the draft EDP and its supplements; [Planning Geek's breakdown](https://www.planninggeek.co.uk/2026/norfolk-edp/) of the same documents is a useful cross-check. Anyone budgeting against these figures should read the final charging documentation rather than the draft.) For anyone who has spent the last three years assembling credit purchases and agreeing nutrient budgets with a case officer, a flat per-unit charge is a significant simplification. It also means the levy is **indifferent to how efficient your scheme is**. A dwelling designed to 105 litres per person per day pays exactly what a dwelling designed to 125 l/p/d pays. Under a calculated budget, water efficiency reduces your nitrogen and phosphorus load and therefore your mitigation cost; under the EDP levy, it does not. That is worth knowing before anyone in a design team argues that the EDP removes the reason to push water efficiency. It removes the *nutrient* reason. It does not touch **Part G**, the optional 110 l/p/d technical standard where a local plan applies it, Defra's proposed move to a **105 l/p/d baseline**, or any local plan policy on water neutrality. Those obligations are unchanged. ## Departure two: an improvement test, not equivalence The second departure is conceptual. Buying nutrient credits means buying a quantified reduction that offsets your quantified load. The EDP works differently: Natural England proposes a **strategic programme of conservation measures** and the statutory test is whether, by the plan's end date, the positive effects of those measures **materially outweigh** the negative effects of the maximum development the plan covers — assessed for each identified environmental feature. The draft programme includes wetland creation and restoration, land management change, fen management and river restoration. Specific commitments in the draft include around **109 hectares of fen-type wetland creation or restoration** across the Bure and Yare, removal or modification of a river structure alongside roughly **one kilometre of River Wensum restoration**, deer management, and habitat creation for Desmoulin's whorl snail. The draft proposes securing conservation measures for at least **100 years**, with maintenance, monitoring and back-up measures where they underperform. Natural England considers its draft meets the overall improvement test. That assessment is precisely the thing consultees are being invited to scrutinise. ## What the plan actually covers — and what it carves out This is not a county-wide scheme, and "my site is in Norfolk" is not eligibility. | Catchment | Nutrient pressure addressed | | --- | --- | | River Wensum | Phosphorus, for the River Wensum SAC | | Bure Broads and Marshes | Phosphorus and nitrogen | | Yare Broads and Marshes | Phosphorus and nitrogen | **Ant Broads and Marshes, Trinity Broads, and Upper Thurne Broads and Marshes are expressly left out** of this draft, on the basis of limited development coming forward in those catchments. The draft identifies affected areas within **South Norfolk, Broadland, Norwich, Breckland, North Norfolk, King's Lynn and West Norfolk, and the Broads Authority** — but the proportion of each authority's area covered varies a great deal. The development-area map in the draft, not the council name, is the starting point. Three practical traps are worth flagging now: - **The 50-metre buffer is a site test, not a drainage setback.** The draft excludes the protected-site land itself and a 50-metre buffer around the Bure and Yare Broads and Marshes sites and the River Wensum SAC. A development site falling **wholly or partly** within that buffer cannot use the plan at all. That is a different and stricter thing from keeping an outfall 50 metres clear. - **Wastewater catchments extend past surface-water catchments.** Where your foul sewage ends up can bring a site into the mapped area even where surface water drains elsewhere. Both drainage routes need reading against the map together. - **Non-mains foul drainage carries a site-level condition.** The draft proposes that, before occupation, a package treatment plant must be installed and operating at discharge concentrations no higher than **5 mg/l total phosphorus and 27 mg/l total nitrogen**, maintained for the lifetime of the development. That is a demanding specification, and it is a condition the developer carries — the levy does not discharge it. ## The process, and where the risk sits Participation would be voluntary. For an eligible proposal the intended sequence is: 1. Confirm the final EDP is in force and covers the development, its location and the relevant environmental effects. 2. Obtain an indicative levy quote and request to use the scheme. 3. Receive a **commitment certificate** if Natural England accepts the request and has capacity. 4. Submit that certificate with the planning application. 5. After permission, assume liability and receive the liability notice. 6. Pay the levy in full, or the first instalment, **before starting development**. The distinction that matters commercially: **a quote does not reserve capacity — acceptance of the request does.** The commitment also expires after six months if it is not followed by a planning application. Instalments are provided for in the regulations, but the Norfolk supplement's example is expressly illustrative and still contains an unspecified qualifying threshold and an additional charge to be confirmed. It is not a settled offer. A valid commitment under an applicable EDP allows the specified environmental effects to be **disregarded for the relevant Habitats Regulations assessment**. Payment is then secured separately through a statutory condition preventing commencement until the levy, or its first instalment, is paid. Commitment and payment are two different gates. ## Does it really unlock 15,780 homes? **15,780 is a modelled maximum capacity, not a pipeline.** The figure is built from catchment housing need, an assumed **75% delivery rate** and an assumed **80% take-up** of the Nature Restoration Fund route. It is not a schedule of permissions and not a forecast of completions. The plan is proposed to run for ten years, but new requests to pay the levy would **close at the end of year six**, giving Natural England time to deliver conservation measures before the plan ends. The actual start and end dates are blank in the draft, pending the Secretary of State's decision. And because participation is voluntary, 15,780 is not a ceiling on development in the area either. Other acceptable mitigation routes remain available. ## What is law today, and what is still a proposal | Element | Position on 19 September 2026 | | --- | --- | | Planning and Infrastructure Act 2025, Part 3 | In force. Provides the statutory EDP and Nature Restoration Fund framework | | Nature Restoration Levy Regulations 2026 (SI 2026/1020) | Made 10 September 2026, **in force 11 September 2026**. Sets levy procedure, liability, charging and enforcement | | Draft Norfolk EDP | **A draft under consultation.** Scope, conservation programme and the £2,675 charge are proposals, not an operative scheme | | Implementation | Natural England may submit the plan to the Secretary of State after considering responses. It can only be made if the Secretary of State considers it passes the overall improvement test, and must then be published with a start date reached before use | One tidy-up worth noting: some references in the consultation pack and in earlier government guidance still point at the *draft* levy regulations. **SI 2026/1020 is now the operative instrument.** That does not make the Norfolk EDP a live plan. ## What to do this month If you have a stalled site in one of the three catchments, this is a reason to review options — not a reason to release an existing mitigation solution or promise a client a start date. 1. **Check eligibility properly.** Mapped boundary, the 50-metre exclusion, and *both* foul and surface-water drainage routes. A council name is not an answer. 2. **Compare like with like.** Levy plus indexation plus potential VAT plus payment timing, against the cost and programme of the mitigation you can secure today. A flat charge is not automatically cheaper than a calculated budget on an efficient scheme. 3. **List what the EDP would leave unresolved** — other environmental effects, the non-mains drainage condition, BNG, Part G, local plan water policies. 4. **Respond to the consultation** if site-specific eligibility, drainage, charging or delivery concerns apply to you. The [questionnaire and full draft documents](https://consultations.defra.gov.uk/consultation-on-a-new-environmental-delivery-plan) are on Defra's consultation hub, and responses after 9.29am on 27 October will not be counted. 5. **Keep working the existing route** in the meantime. Nothing about nutrient neutrality obligations has changed today. For everyone outside Norfolk, the useful action is simpler: read this draft as the shape of the thing coming to your catchment, and note the two structural changes — a flat charge per dwelling, and an improvement test in place of equivalence. --- # From 31 October, discharging a planning condition can never go to committee URL: https://energycount.co.uk/news/national-scheme-delegation-planning-conditions-officers-31-october-2026/ Date: 2026-09-18 > **Quick answer:** The **Town and Country Planning (Discharge of Local Planning Authority Functions) (England) Regulations 2026** — **SI 2026/854**, made **15 July 2026**, in force **31 October 2026** — set out the national scheme of delegation. **Schedule 1** applications *must* be determined by an officer, and that schedule includes **applications made under a planning condition**, **biodiversity gain plans**, **prior approval applications** under the GPDO, **permission in principle**, and **minor residential applications** — one to nine dwellings on a site under 0.5 hectares. **Schedule 2** applications, which is everything else of any size, go to an officer *by default*; a committee only sees one if the authority's **nominated member and nominated officer both agree** that it raises an issue of economic, social or environmental significance to the local area, or a significant planning matter. Committees are capped at **13 members**. For anyone who submits condition-discharge packs, the practical change is simple: the decision is now always made on the papers by one person. ## What has actually changed Until now, which planning applications went to committee and which were delegated to officers was a matter for each authority's own constitution. The result was roughly 330 different schemes of delegation in England, and a genuinely unpredictable answer to the question "will this go to committee?" Section 54(1) of the **Planning and Infrastructure Act 2025** inserted sections 319ZZC to 319ZZF into the Town and Country Planning Act 1990, giving the Secretary of State power to prescribe that answer nationally. [SI 2026/854](https://www.legislation.gov.uk/uksi/2026/854/made) is the instrument that does it. It was signed by Matthew Pennycook, Minister of State, on 15 July 2026, and it comes into force on **31 October 2026** — a date that [slipped from an originally indicated 30 September](https://www.bdonline.co.uk/news/government-pushes-back-introduction-of-national-scheme-of-delegation/5142536.article) to give authorities time to update their constitutions and train members and officers. The regulations apply in England only. They do not change any of the substantive tests — the development plan, material considerations, the Building Regulations, none of it. They change **who decides**. ## Schedule 1: officer, always Regulation 4 is short and absolute. A relevant local planning authority **must** make arrangements so that any Schedule 1 application is determined by an officer. There is no referral route and no discretion. Schedule 1 contains fifteen entries. The ones most likely to appear in an Energycount reader's inbox: - **Applications made under a planning condition** — article 27(1) of the DMPO 2015. This is condition discharge, and it is the entry that matters most below. - **The submission of a biodiversity gain plan** under paragraph 13(2)(a) of Schedule 7A to the TCPA 1990. - **Prior approval applications** under Schedule 2 to the GPDO 2015, and determinations as to whether prior approval is required. Solar and other permitted development routes sit here. - **Minor residential applications** — defined as development comprising **at least one but not more than nine dwellings** on a site **smaller than 0.5 hectares**, consisting only of dwellings and incidental development. - **Householder applications** and **minor commercial applications**. - **Permission in principle**. - **Reserved matters approval** for any outline permission that is not a "large outline permission". - **Section 96A** non-material amendments. - **Certificates of lawfulness** under sections 191 and 192, and section 26H of the Listed Buildings Act. - **Section 73** applications where the original permission was itself a Schedule 1 permission. - **Section 106A** requests to modify or discharge a planning obligation connected with a Schedule 1 approval. Two carve-outs apply. An application that is *also* of a kind listed in Schedule 2 is handled under regulation 5 instead — the explanatory note gives the example of a minor residential application that the authority considers is connected with a listed building consent application. And an **own-interest application** — one made by the authority, or one of its members or officers, or where the authority otherwise has an interest — is handled under regulation 6, where a referral to committee remains possible at the nominated member and officer's discretion. Neither carve-out reaches ordinary condition discharge. An application under article 27(1) is not an application for planning permission, and Schedule 2 paragraphs 3 and 4 both begin "an application for planning permission". So unless the council is discharging conditions on its own scheme, **a condition-discharge application from 31 October is an officer decision, full stop**. ## "Large outline permission" has a number on it Reserved matters is where the schedules split on scale. Schedule 1 paragraph 13 sends reserved matters for an ordinary outline permission to an officer. Schedule 2 paragraph 8 keeps reserved matters for a **large outline permission** available to committee. The regulations define a large outline permission as an outline planning permission permitting either or both of: - the provision of **500 or more dwellings**; or - **50,000 square metres or more** of floorspace created by the development. That is a high bar. On the overwhelming majority of outline sites — including most strategic allocations — reserved matters will now be determined by an officer. ## Schedule 2: officer by default, committee by agreement Schedule 2 covers full applications for planning permission that are not householder, minor commercial or minor residential; listed building consent and section 19 variations; section 73A retrospective permissions; section 73 applications flowing from a Schedule 2 permission; section 106A modifications connected with a Schedule 2 approval; advertisement consent; and tree preservation order consent. Regulation 5 sets the default: any Schedule 2 application **not referred** to committee must be determined by an officer. A referral requires the authority's **nominated member and nominated officer to agree**, and they may only refer where in their view the application raises: - one or more issues of **economic, social or environmental significance to the local area**; or - one or more **significant planning matters** having regard to the development plan and any other material considerations. Both must have regard to guidance issued by the Secretary of State under section 319ZZE(4). Authorities may nominate different members and officers for different purposes, and may appoint substitutes. There is also a quiet but important provision in regulation 2(2). Where an application falls to an officer under regulation 4, 5(2) or 6(3), the authority **must not make arrangements that limit the officer's discretion as to how to determine that application**. A council cannot delegate the decision and then bind the outcome. Finally, regulation 7 caps any committee determining an application under these regulations at **13 members**, and regulation 8 requires the Secretary of State to review the regulations and publish a report by **31 October 2028**. ## What this means if you produce compliance evidence Most of the commentary on the national scheme of delegation has been about democratic accountability and committee culture. The consequence for technical consultants is more prosaic, and it cuts both ways. **The upside is predictability.** Pre-commencement conditions — drainage, energy, water efficiency, overheating, materials, BNG — have always been vulnerable to being pulled into a committee cycle, usually because a scheme was locally contentious rather than because the condition itself was. That route is now closed. From 31 October, a condition-discharge application goes onto a case officer's list and is determined there. On a scheme where commencement is waiting on a soakaway design or a drainage strategy, removing the possibility of a committee slot removes weeks of uncertainty from the programme. **The downside is that nobody will explain your evidence for you.** A committee report is a summary written by an officer who has already read the file. An officer decision is the file. There is no presentation, no debate, no chance for a consultee to elaborate. Whatever you submitted is what gets weighed. That puts more load on three things: 1. **Self-contained submissions.** The pack has to answer the condition on its own terms, in its own covering note, without requiring the reader to reconstruct the argument from appendices. 2. **Stated assumptions.** Where a calculation depends on a value that could reasonably be challenged — an infiltration rate, a ψ-value, a ventilation rate, an occupancy assumption — say where it came from, in the document, with a date and a source. 3. **Internal consistency.** An officer working through a file alone will notice a number that appears differently in two places long before they will notice a good argument buried on page 40. None of that is new advice. It is simply that from 31 October it is the only mechanism you have. ## Dates worth writing down | Date | What happens | | --- | --- | | 15 July 2026 | SI 2026/854 made | | **31 October 2026** | **Regulations come into force; national scheme of delegation applies** | | 31 October 2028 | Secretary of State must publish a review of the regulations | Applications submitted before 31 October but undetermined on that date will fall to be handled under the new arrangements once they are in force, so there is little value in trying to time a submission around the date. There is value in knowing, for each outstanding condition on your sites, who will be reading it. ## Getting the drainage condition right first time The most common pre-commencement condition on a residential site is surface water drainage, and the most common reason a discharge application stalls is that the infiltration evidence does not stand up on its own — a BRE Digest 365 test with no record of the ground conditions it was taken in, a rate quoted without the three trials, a soakaway sized against a rate the site will never see in February. That is exactly the kind of gap an officer reading alone will find, and now there is no committee to talk it through with. Our **[soakaway calculator](/tools/soakaway-calculator/)** does the [BRE Digest 365](/tools/soakaway-calculator/bre-digest-365/) calculation properly — three-trial infiltration rates, storage volumes, half-drain times — and produces output you can put straight into a condition-discharge pack. If the ground conditions your test was taken in are part of the argument, [this year's soil moisture deficit figures](/news/soil-moisture-deficit-august-2026-recharge-not-started/) are worth quoting alongside it. --- # Approved Document B has been reissued — check which PDF you are working from URL: https://energycount.co.uk/news/approved-document-b-reissued-2026-amendments-incorporated/ Date: 2026-09-17 > **Quick answer:** On **16 September 2026** MHCLG republished **both volumes of Approved Document B**. They are now titled the *2019 edition incorporating 2020, 2022, 2025 and 2026 amendments, collated with 2029 amendments* — meaning the **2026 amendments are now inside the main document**, not sitting in a separate booklet alongside it. GOV.UK's own update note describes the change as introducing "guidance on second stairs and **evacuation lifts** in taller residential buildings". The amendments come into force on **30 September 2026**. If you have a residential scheme at or above 18m, the practical action this week is to confirm which PDF your design team and your building control body are each working from. ## What actually changed on 16 September Nothing in the policy changed. What changed is the document. Since March 2025, the published version of Approved Document B has been the 2019 edition incorporating the 2020, 2022 and 2025 amendments, **collated with** the 2026 and 2029 amendments. "Collated with" meant the future changes were shown for information — visible, but not yet part of the operative guidance. The 2026 changes lived in their own amendment booklet, issued with [MHCLG Circular 04/2024](https://www.gov.uk/government/publications/amendments-to-approved-document-b-fire-safety-circular-and-letter/approved-document-b-fire-safety-new-updates-to-support-enhanced-fire-safety). As of 16 September, the titles read differently: - **Volume 1: Dwellings** — 2019 edition incorporating 2020, 2022, 2025 and 2026 amendments, collated with 2029 amendments. 200 pages. - **Volume 2: Buildings other than dwellings** — same basis. 224 pages. The 2026 amendments have moved from "collated with" to "incorporating". The 2029 amendments — the removal of BS 476 references for fire resistance, in force 2 September 2029 — remain collated for information. [We wrote a fortnight ago](/news/second-staircase-deadline-30-september-2026/) that if you were working from a version of ADB and were not sure which one, that was the question to resolve this month. This is why. The answer has now changed. ## The evacuation lift point Circular 04/2024's summary table describes the content of the 2026 amendment booklet in a single line: *"Second stair provisions in residential buildings over 18m in height."* GOV.UK's update note for 16 September is broader. It says the updates "introduce guidance on second stairs **and evacuation lifts** in taller residential buildings." That is worth reading properly rather than assuming. Disabled evacuation was one of the most heavily argued points in the [consultation response](https://www.gov.uk/government/consultations/sprinklers-in-care-homes-removal-of-national-classes-and-staircases-in-residential-buildings/outcome/consultation-response-amendments-to-approved-document-b-second-staircases) — respondents pressed for the draft BS 9991 approach of evacuation lifts operable in managed and automatic mode, with protected evacuation lift lobbies, to be brought into ADB. If your scheme is at or above 18m and your fire strategy was written against the pre-2026 text, the lift and lobby provisions are the part most likely to have moved underneath you. The detail sits in the reissued volumes. Read the sections rather than the summary. ## The dates that matter, unchanged The 2026 amendments **come into force on 30 September 2026**. The transitional arrangements in Circular 04/2024 are unchanged by this week's republication: **The old guidance still applies** where a building notice or initial notice was given to, or a building control approval application with full plans was made to, the relevant authority **before 30 September 2026**, and the work has started and is sufficiently progressed either before that day or **within 18 months** beginning on that day — so by the end of March 2028. **"Sufficiently progressed"** means, for new construction, that the pouring of concrete for the permanent placement of trench, pad or raft foundations has started, or permanent piling has started. For work to an existing building, that the work has started. For a material change of use, that work to effect the change has started. The base version being amended is the 2019 edition incorporating the 2020, 2022 and 2025 amendments — that is, the document as it stood before Tuesday. ## What to do this week **Fix the version in writing.** For every live scheme at or above 18m, record which edition of ADB the fire strategy was assessed against, and confirm your building control body is holding the same one. A version mismatch discovered at handover is expensive; discovered now, it is an email. **Re-read the lift provisions, not just the stair provisions.** The stair requirement has been trailed since 2024 and most teams have absorbed it. The evacuation lift and lobby guidance has had far less attention. **Confirm submission dates from the building control body.** The 18-month transitional route turns on when the notice or application was actually made and recorded — not the date in your programme. Get it confirmed by the body that holds it. **Check the height measurement if you are near the line.** 18m is a threshold, and schemes cluster around thresholds. How height is measured under ADB is a question to resolve before you rely on being under it. ## Why the document keeps moving Since April 2023 the Building Safety Regulator has had a duty under the Building Safety Act 2022 to keep the safety and standards of all buildings in England under review, which includes advising government on updates to the Approved Documents. In December 2025 BSR published its [continuous review of Approved Document B](https://www.gov.uk/government/collections/continuous-review-of-approved-document-b) collection, alongside ten research projects covering everything from balconies and laminated glass to fire spread over pitched roofs with solar panels. The stated intention is planned changes at regular intervals rather than occasional large rewrites. For anyone maintaining a house type library or a standard fire strategy, that means version control stops being administrative housekeeping and becomes part of the compliance record. ## The short version The policy has not changed. The document has. As of 16 September 2026, Approved Document B volumes 1 and 2 incorporate the 2026 amendments, which take effect on 30 September. Second stairs at 18m are the headline; evacuation lifts are the part worth reading again. If you are relying on the transitional route, the submission date on the building control body's record is the only date that counts. If your scheme sits close to a fabric or overheating threshold as well as a height one, [ΨMonkey](/tools/psi-calculator/) will give you defensible junction-by-junction thermal bridging figures rather than a blanket value — useful when a late change to cores and lobbies pulls the envelope around with it. *Sources: [GOV.UK — Fire safety: Approved Document B (updated 16 September 2026)](https://www.gov.uk/government/publications/fire-safety-approved-document-b) · [GOV.UK — Approved Document B (Fire Safety): new updates to support enhanced fire safety, MHCLG Circular 04/2024](https://www.gov.uk/government/publications/amendments-to-approved-document-b-fire-safety-circular-and-letter/approved-document-b-fire-safety-new-updates-to-support-enhanced-fire-safety) · [GOV.UK — Consultation response: Amendments to Approved Document B, second staircases](https://www.gov.uk/government/consultations/sprinklers-in-care-homes-removal-of-national-classes-and-staircases-in-residential-buildings/outcome/consultation-response-amendments-to-approved-document-b-second-staircases) · [GOV.UK — Continuous review of Approved Document B (Building Safety Regulator)](https://www.gov.uk/government/collections/continuous-review-of-approved-document-b)* --- # England needs 120% of typical rainfall by April to clear the drought URL: https://energycount.co.uk/news/national-drought-group-120-percent-rainfall-april-2027/ Date: 2026-09-17 > **Quick answer:** The **National Drought Group** met on **15 September** and government published the outcome on **16 September 2026**. The headline number is new and it is specific: England needs **120% of typical rainfall over the next seven months** for all areas to be considered out of drought. Almost **three-quarters of England** is still in drought. Under the Environment Agency's **most likely** scenario, **Devon & Cornwall and East Anglia are still in drought at the start of April 2027**. Under the **dry** scenario — 80% of average rainfall, with roughly a **one-in-four** likelihood — **seven areas** are. Buried in the same release is a line that matters more to new-build than the rainfall does: government will "**respond soon**" to the consultation on improving water efficiency in new homes. ## The number, and why it is a hard one Until now, the honest answer to "when does this end?" has been that it depends on the winter. The Environment Agency has now modelled it and put a figure on it. **120% of typical rainfall, September to March**, is what it takes for every area to be out of drought by spring. For scale: between **September 2025 and March 2026**, England received **130% of typical rainfall**, and that is what pulled the country out of last year's drought. So 120% is not fantastical — it happened last winter. But it is a long way above normal, and it has to happen on top of a deficit that is considerably deeper than the one 130% cleared. How the deficit got this deep, in the EA's own figures: - Spring 2026 delivered just **56%** of typical rainfall. - Summer delivered **64%** of expected rainfall. - **July was the driest on record**; **August the joint-hottest**, with **five heatwaves** across the summer. - Demand for water rose by as much as **30%** on the hottest days. Drought, as the release puts it, is only over when rivers, groundwaters and reservoirs are back to normal. Rivers respond to rain in days. Groundwater and reservoirs do not — and both are still receding. ## The three scenarios, by area This is the part to read if you have sites in the south or east. **Most likely** (Met Office three-month outlook and seasonal indications): the north recovers first. Cumbria & Lancashire and Greater Manchester, Merseyside & Cheshire could move to recovery in **October** and be back to normal in **November**. By **December** most areas see a fragile recovery, but four stay in drought — **Devon & Cornwall, East Anglia, Thames Valley and the West Midlands**. By **February** most places are normal. At the **start of spring 2027**, **Devon & Cornwall and East Anglia are still in drought**, entering the traditionally dry months already carrying a deficit. **Average rainfall** (100%): patchy recovery from October. Most areas stay in drought for the rest of 2026. By January, everywhere except East Anglia is in recovery or normal. At the start of April, **East Anglia may still be in drought** and Devon & Cornwall in fragile recovery. **Dry** (80% of average, around a one-in-four likelihood): the four northern areas move to recovery in November and are normal by January. Drought persists across the midlands and south for the rest of 2026. By the **start of April 2027**, **East Anglia, Lincolnshire & Northamptonshire, Thames Valley, Devon & Cornwall, Wessex, Solent & the South Downs and the West Midlands** are still in drought. Overall, the EA puts it at roughly a **one-in-four chance that around half the country is still in drought next spring**, and around a **1% chance of a severe drought** in the coming months. England has not seen a severe drought since **1976**. The Met Office expects the second half of September to turn drier, **particularly in the south, where the rain is needed most**. The developing El Niño raises the chance of a wet, stormy late autumn and early winter, then colder and calmer conditions later in the season. ## What this means for infiltration testing This is the first official, area-by-area projection running through to April 2027, and it is directly useful evidence for a drainage report. Two things follow. **First, the autumn testing window is not a normal one.** We have written repeatedly this summer that a soakage test taken in dry-season conditions reads flatteringly, and that the honest way to handle it is to record the conditions and treat the result as a best case rather than a design case. That argument now has a government projection behind it: in most of the south and east, the ground you test in October will not be the ground the soakaway has to perform in. **Second, you can now name the scenario.** Rather than writing "conditions were dry at the time of testing", a drainage strategy can say which EA scenario the area sits in and what the projected status is at the start of April 2027. A lead local flood authority can check that in one click. It is a far stronger position than an unsupported caveat. The EA also makes a point worth repeating to anyone who assumes a wet autumn solves it: flood water is hard to store for public supply, because it arrives too fast and carries too much sediment and nitrate run-off. Heavy autumn rain can produce surface flooding and still leave groundwater short. ## The Part G line The release lists government actions, and one of them is a commitment rather than a fact: > Continued work on mandatory water efficiency labelling and will respond soon to a consultation on improving water efficiency in new homes. That is the **Part G** reform. The proposal is to cut the water efficiency standard for new homes from **125 to 105 litres per person per day**, with the optional tighter standard also on the table. [Water UK backed it publicly in August](/news/part-g-105-litres-water-efficiency/) and the government response has been outstanding since. "Soon" is not a date. But it is the first time in months that a dated government publication has put the new-homes water efficiency response on a near-term footing, and it appears in a release whose entire premise is that demand reduction has to continue. If you are pricing fittings schedules or writing water calculations for plots that will be built out in 2027, that is the direction to plan around. ## The rest of the package For completeness, the other measures announced or restated: a **£65 million** package for farmers affected by drought, including up to **£15 million** for on-farm reservoirs; **£100 million** to strengthen response capacity for extreme weather and wildfires; **nine new reservoirs by 2050** alongside desalination and water transfers; and a change in the **Planning and Infrastructure Act** allowing non-water-sector companies to build reservoirs that are automatically treated as nationally significant infrastructure projects. The National Drought Group is now meeting **weekly** as part of a heightened incident response. ## The short version England needs 120% of typical rainfall over seven months to clear the drought, and the most likely outcome is that Devon & Cornwall and East Anglia do not clear it. For anyone producing drainage evidence this autumn, the modelling is free, published, area-specific and dated — use it. And a government response on water efficiency in new homes is now flagged as coming soon. [SoakMonkey](/tools/soakaway-calculator/bre-digest-365/) will run BRE Digest 365 from your test results and produce a printable design for the drainage strategy. The judgement it cannot make for you is whether the conditions on test day represent the conditions the soakaway has to survive — and this week the Environment Agency has given you the numbers to answer that in writing. *Sources: [GOV.UK — Government, Environment Agency, and water companies step up efforts to prevent England remaining in drought by next spring (16 September 2026)](https://www.gov.uk/government/news/government-environment-agency-and-water-companies-step-up-efforts-to-prevent-england-remaining-in-drought-by-next-spring) · [GOV.UK — Dry weather and drought in England 2026: summary reports](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports) · [GOV.UK — Water situation reports for England](https://www.gov.uk/government/collections/water-situation-reports-for-england) · [GOV.UK — Drought: how it is managed in England](https://www.gov.uk/government/publications/drought-management-for-england)* --- # August's water situation reports: soil moisture deficit near double average, and no recharge at all URL: https://energycount.co.uk/news/soil-moisture-deficit-august-2026-recharge-not-started/ Date: 2026-09-15 > **Quick answer:** The Environment Agency published its **area monthly water situation reports for August 2026** on **14 September**. These are a different publication from the weekly drought bulletins, and they contain something the weeklies do not: **soil moisture deficit (SMD)**, reported by hydrological area. The headline figures are stark. Across the **Thames area SMD reached almost twice its end-of-August long-term average**, with the Cotswolds East and Cotswolds West areas approximately **double** their respective averages. In **Solent and South Downs** there was **"no effective rainfall across SSD and so no recharge this month"**. **East Anglia** ended August with an SMD of **121mm**, still **notably high for the time of year**. Even the **north-west**, which had near-normal August rainfall at 84% of average, recorded **above-average SMD in every hydrological area but one**. If you are commissioning or reviewing infiltration testing this autumn, SMD is the free, published, third-party number that tells you — and your lead local flood authority — exactly how flattering the ground conditions were on the day. ## What these reports are, and why they are not the weekly bulletin Most people following the drought have been reading the Environment Agency's **weekly** dry weather and drought summaries. Those cover rainfall, river flows, groundwater classifications, reservoir stocks and area drought status. We [wrote about last week's edition](/news/groundwater-exceptionally-low-autumn-recharge-soakaway-testing/), which put two monitored sites into the "exceptionally low for the time of year" class. The **area monthly reports** are a separate series, published for thirteen Environment Agency areas, each with a short HTML summary and a full PDF running to twenty or thirty pages. They cover the same ground, but they add two things the weeklies leave out: - **Soil moisture deficit**, in millimetres, compared against the long-term average for that area and that month - **An explicit statement on recharge** — whether any effective rainfall reached the aquifer For drainage design, those are the two most useful lines in the whole drought record. ## Soil moisture deficit, in one paragraph Soil moisture deficit is the amount of rain, in millimetres, that would have to fall to bring the soil back up to field capacity. A deficit of zero means the soil is saturated and any further rain becomes effective rainfall — it runs off, or it percolates down to the water table. A deficit of 120mm means the first 120mm of rain that falls will be absorbed by the soil profile and go nowhere near the aquifer. That is why SMD matters to a soakaway. A trial pit dug into soil carrying a large deficit is being tested against a sponge that is nowhere near full. Water leaves the pit quickly, partly into the ground beneath and partly sideways into unsaturated soil that would not accept it in February. The measured infiltration rate is real — it just is not the rate the finished system has to work at. ## What August 2026 actually recorded The picture varies by area, and that variation is worth understanding rather than flattening. **Thames.** The area received **28mm of rainfall, 45% of the long-term average**, and more than three-quarters of that fell in the last week of the month. SMDs **continued to increase**, with the area reaching **almost twice its end-of-August LTA** and the Cotswolds East and West areas approximately **double** theirs. The report is blunt about the consequence: **"Effective rainfall was negligible during the month, even in areas that would typically receive some at this time of year."** **Solent and South Downs.** **42mm, 63% of the LTA.** Soils in **all** hydrological areas ended the month drier than average despite meaningful rain after 18 August, and the report states there was **"no effective rainfall across SSD and so no recharge this month."** For context on how unusual the preceding period was: April to August was the **fifth driest on record for the Test Chalk** hydrological area, and March to August the **fourth driest for Sussex Coast**, in records going back to **1871**. **East Anglia.** **41mm, 68% of the LTA**, and the **fourth driest March to August on record** across the area at just **51% of average**. For **North West Norfolk and Wissey** and for **North Norfolk**, that six-month period was the **driest on record**. End-of-August SMD was **121mm** for the area, **6mm to 25mm above the LTA** in most hydrological areas, and **above 160mm in South Essex**. **North-west England.** This is the useful counter-example. August rainfall was classed **normal at 84% of the LTA**, and SMD **decreased** across every hydrological area compared with July. And yet, with the single exception of the Cheshire Rivers Group, **every hydrological area still recorded higher-than-average SMD**, with the highest values — **71mm to 100mm** — across Lancashire and Greater Manchester, Merseyside and Cheshire. A normal month of rain did not undo a dry spring and summer. Reservoir storage tells the same story: down from **66.7% to 50.8%** over the month, against an average of **71.7%**. The pattern across all four is the same. A wetter fortnight moved river flows. It did not refill soils, and it did not start recharge. ## Why "no recharge in August" is not, on its own, alarming — and why it still matters Recharge is not supposed to happen in August. In a normal year, evapotranspiration takes everything the summer delivers, the deficit builds through the warm months, and the aquifer year begins somewhere around October or November once the soil has been rewetted and rain finally gets past it. So the finding is not that recharge failed. It is **where the deficit sits at the point recharge is due to begin**. A soil profile entering October carrying close to double its normal deficit needs that much more rain before the first millimetre reaches the water table. The recharge season does not start on a date. It starts when the soil lets it — and this year the soil will let it later. For a drainage designer, that has a narrow, practical consequence. The gap between the conditions you can test in during September, October and November, and the conditions the system has to perform in, is wider this year than the seasonal-correction rules of thumb assume. ## What to do with the number None of this is an argument against testing. It is an argument for writing down what you tested into. **Quote the SMD in the report.** The area monthly report for the month your test was taken is a free, dated, third-party publication from the regulator. A single line — the area, the month, the SMD in millimetres and its relationship to the long-term average — converts "the test was taken in a dry period" from an assertion into a citation. It costs one sentence. **Say whether recharge had begun.** Where the report states there was no effective rainfall, say so. An LLFA officer reading an infiltration rate measured in a month the Environment Agency has formally recorded as producing no recharge will draw the right conclusion either way. Better that you drew it first. **Keep the design water table separate from the observed one.** This is the point we made [last week](/news/groundwater-exceptionally-low-autumn-recharge-soakaway-testing/) and it has not changed: the water table in your trial pit is close to its annual minimum, and the separation distance applies to the **highest expected** groundwater level, not the one you happened to measure. SMD is evidence about the *rate*; groundwater classification is evidence about the *level*. You need both. **Watch the regional variation rather than assuming the national headline.** North-west England had a normal month of rain and still carries an above-average deficit. Thames had a poor month and carries close to double. The Cheshire Rivers Group is, on this measure, back to normal. "There is a drought" is not a design input. The SMD for your specific hydrological area is. **And remember the rule of three fills cannot rescue you.** [BRE Digest 365 requires three successive fills and the slowest result](/news/bre-365-soakaway-testing-drought-conditions/), which controls for the pit wetting up over the course of a test. It does not control for a soil profile carrying a 120mm deficit before you started, because all three fills happen into the same dry ground. ## The short version The Environment Agency now publishes, every month and for free, the one variable that explains why a drought-season soakage test reads high — and for August 2026 that variable sits near double its average across the Thames area, above average almost everywhere else, and with recharge formally recorded as absent in southern England. Use it. A drainage strategy that cites the soil moisture deficit for the month of the test is considerably harder to argue with than one that does not mention conditions at all. [SoakMonkey](/tools/soakaway-calculator/bre-digest-365/) will take your BRE Digest 365 test results and produce a printable soakaway design for the drainage strategy. The seasonal judgement — what the test would have shown in February, and what you assumed about it — still belongs in your report, in your words, with the evidence attached. *Sources: [Water situation: area monthly reports for England (Environment Agency, updated 14 September 2026)](https://www.gov.uk/government/publications/water-situation-local-area-reports) · [Thames water situation: August 2026 summary](https://www.gov.uk/government/publications/water-situation-local-area-reports/thames-water-situation-august-2026-summary) · [Solent and South Downs water situation report: August 2026 summary](https://www.gov.uk/government/publications/water-situation-local-area-reports/solent-and-south-downs-water-situation-report-august-2026-summary) · [East Anglia water situation: August 2026 summary](https://www.gov.uk/government/publications/water-situation-local-area-reports/east-anglia-water-situation-august-2026-summary) · [North-west England water situation: August 2026 summary](https://www.gov.uk/government/publications/water-situation-local-area-reports/north-west-england-water-situation-august-2026-summary) · [Water situation reports for England (collection)](https://www.gov.uk/government/collections/water-situation-reports-for-england)* --- # Two wet weeks in, and groundwater has fallen to 'exceptionally low' URL: https://energycount.co.uk/news/groundwater-exceptionally-low-autumn-recharge-soakaway-testing/ Date: 2026-09-14 > **Quick answer:** The Environment Agency's report for **4 to 10 September 2026**, published on **11 September**, records a second consecutive wetter week — **14mm in east England to 46mm in the north-west** — and the seasonal groundwater recession continuing "at almost all sites". Two sites are now classed **exceptionally low for the time of year**: **Tilshead** in the Upper Hampshire Avon Chalk, and **Jackaments Bottom** in the oolite limestone of the Cotswolds. **Reservoir storage fell again to 56.9%**, **19.7% below average**, with **12 reservoirs or reservoir groups under half full** and **eight classed exceptionally low**. The **ten areas in drought** stayed in drought. If you are about to commission infiltration testing for a SuDS design, the water table you measure this month is close to the annual minimum — and that is not the number a soakaway has to work at. ## What the report says England had a second wetter week, but not a transformative one. Rainfall ranged from **14mm in east England to 46mm in the north-west**. After the first eight days of the month England had received **33% of the long-term average** for September — **27%** in the north-east, **45%** in the north-west. Rivers responded at **just over half** the monitored sites. Even so, **23 sites (42%)** are classed normal for the time of year, **11 (20%)** below normal and **7 (13%)** notably low. Groundwater did not respond at all. The Environment Agency records that "the seasonal recession of groundwater levels continued at many sites across England, with levels decreasing at almost all sites." Every chalk aquifer site is **normal or lower**. Four sites in the Wessex Downs, Yorkshire Wolds and the North and South Downs are **notably low**. And two have dropped a class further: - **Tilshead**, in the Upper Hampshire Avon Chalk — **exceptionally low for the time of year** - **Jackaments Bottom**, in the oolite limestone of the Cotswolds — **exceptionally low for the time of year** In the north-west, **Skirwith** in the Carlisle Basin and Eden Valley Sandstone is classed below normal. Reservoirs kept falling. Storage for England was **56.9%** in the week ending **8 September** — **19.7% below average** — down from 58% the week before. **Twelve** reservoirs or reservoir groups are now **under half full**, up from eleven. Only **Vyrnwy** and **Draycote** are classed normal for the time of year. **Eight** are exceptionally low: Ardingly, Clatworthy, Clywedog, the Dove Group, Rutland Water, Hanningfield, Haweswater and Thirlmere, and Wimbleball. Area drought status did not move. **Ten areas remain in drought** — East Anglia, Hertfordshire and North London, Thames, Devon Cornwall and Isles of Scilly, West Midlands, Wessex, Kent and South London, Solent and South Downs, East Midlands, and Lincolnshire and Northamptonshire. The remaining four are in prolonged dry weather. **Temporary use bans still cover 30 million people** across ten water companies, and **Wessex Water has moved into drought level 1b**. ## Why "exceptionally low" in September matters more than it sounds Groundwater in England is supposed to be low in September. The recession runs through the summer as evapotranspiration takes everything the rain delivers, and the water table bottoms out somewhere around late September to October before the recharge season begins. A low reading now is normal. The classification is not a reading. It is a **comparison against the historic range for that site at that time of year** — so "exceptionally low for the time of year" means the level is unusually low *even measured against other Septembers*. At Tilshead and Jackaments Bottom, the water table is not just at its seasonal floor. It is below where the seasonal floor has historically sat. That matters for two reasons. **It sets the starting point for recharge.** Aquifers refill from where they are. A chalk or limestone aquifer entering October from an exceptionally low base needs an above-average winter simply to reach a normal spring. The Environment Agency has asked water companies for updated **autumn and winter projections**, to be discussed at the **National Drought Group on 15 September** — which is the meeting worth watching, because it is the first formal read on whether this drought ends in 2027 or carries into it. **And it makes September and October the most misleading months of the year for infiltration testing.** ## The soakaway problem, stated plainly An infiltration test measures how fast water leaves a trial pit. What it does *not* measure is the condition the finished system has to work in. Two things are wrong with a test taken now: **The infiltration rate is flattering.** Dry ground with a deep unsaturated zone takes water quickly. The same pit in February, with the water table up and the soil profile wet, will not. [BRE Digest 365's rule of three fills, taking the slowest](/news/bre-365-soakaway-testing-drought-conditions/), controls for short-term antecedent conditions — but it cannot control for a multi-year drought, because all three fills happen in the same dry week. **The water table is at its annual minimum.** Standard practice — and the expectation behind BRE Digest 365 and the SuDS Manual — is that the base of an infiltration system sits clear of the **highest expected groundwater level**, with **one metre** the separation commonly applied and commonly asked for by lead local flood authorities. A trial pit dug at the seasonal floor of an exceptionally low year will show a water table that is metres deeper than the level the design actually has to respect. Read that depth as the design condition and you can end up with a soakaway base sitting inside the winter saturated zone of a normal year. Neither of these is a reason not to test. They are reasons to be explicit about **when** the test was taken and **what** you assumed about seasonal variation. ## What to do about it **Date-stamp the test and say what the conditions were.** A soakage test report submitted with a planning condition discharge or a Building Regulations application should record the date, the antecedent rainfall and the measured groundwater level. An LLFA officer reading a test dated September 2026 in a catchment that has been in drought since 2025 will want to know you noticed. **Do not take the observed water table as the design water table.** Establish the highest expected groundwater level from a source that is not a single September pit — long-term monitoring data, the British Geological Survey's records, borehole logs from the site investigation, or seasonal monitoring across the winter. Then apply the separation distance to *that* level. **Where the design is marginal, monitor through the winter.** If the scheme only works at the infiltration rate a drought-season test produced, it does not work. Winter monitoring costs a few months. Redesigning surface water strategy after a refusal, or after a system fails in its first wet winter, costs considerably more. **Expect the LLFA to be sceptical.** Lead local flood authorities have spent two summers looking at test results taken in the driest conditions in the record. The ones that scrutinise seasonal correction are not being awkward — they are applying the [national SuDS standards that the 2026 NPPF now points planning policy towards](/news/nppf-2026-suds-national-standards-policy-f8/). **And keep the water efficiency evidence tidy.** The same drought that is depressing the water table is the backdrop to [Part G's 105 litre standard](/news/part-g-105-litres-water-efficiency/) and to local planning policies pushing below it. Temporary use bans covering 30 million people are not an argument that will be made *to* you — but they are the reason the fitting schedule on a new dwelling is being read more carefully than it was three years ago. ## The wider numbers, for context Agriculture has passed peak irrigation, but **549 abstraction licence restrictions** remain, alongside **1,094 voluntary restrictions** cutting abstraction by 50% across the Thames, Wye and Severn catchments. Formal section 57 restrictions continue in parts of East Anglia until **31 October**. Environmental damage is still accumulating: **330 confirmed drought and dry weather incidents** since the start of 2026, against 288 at the same point in 2025. Fish death incidents rose by **19 in a single week** to 70. Blue-green algae continues to affect a **10km stretch of the Lower Thames**, including the Ham and Kingston bathing water. And three drought orders and permits are live: the **River Severn Drought Order** (applied 17 August, appeals hearing 3 September, decision imminent), Southern Water's **River Test** order awaiting a Secretary of State decision, and South East Water's **River Ouse** drought permit application of 28 August. ## The short version A wet fortnight has changed river flows and changed very little else. Groundwater is still falling, two monitored sites have fallen out of the bottom of their historic range for September, and the recharge season is about to start from a worse position than last year's. For anyone producing drainage evidence this autumn, the practical consequence is narrow and specific: **treat an infiltration test taken in the next eight weeks as a best case, not a design case.** [SoakMonkey](/tools/soakaway-calculator/bre-digest-365/) will run BRE Digest 365 from your test results and produce a printable design for the drainage strategy. What it cannot do is tell you whether the water table you measured in September is the one the soakaway has to live with in February — that judgement has to be made, recorded, and defended in the report. *Sources: [Dry weather and drought in England: 4 to 10 September 2026 (Environment Agency, 11 September 2026)](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports/dry-weather-and-drought-in-england-4-to-10-september-2026) · [Dry weather and drought in England 2026: summary reports (collection)](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports) · [Water situation reports for England (Environment Agency)](https://www.gov.uk/government/collections/water-situation-reports-for-england) · [Drought: how it is managed in England (Environment Agency)](https://www.gov.uk/government/publications/drought-management-for-england)* --- # HEM 1.0.0a9 is due this month — and you can no longer hand it one thermal bridging number URL: https://energycount.co.uk/news/ecaas-hem-1-0-0a9-thermal-bridging-breaking-changes/ Date: 2026-09-13 > **Quick answer:** MHCLG's **ECaaS** technical documentation now lists the breaking changes coming in **HEM and FHS version 1.0.0a9**, with the API move from **1.0.0a7** estimated for **late September 2026**. The change most likely to alter how a dwelling is modelled is that **`ThermalBridging` will no longer accept a plain number** — a blanket value in place of junction-by-junction data. Alongside it: **rectangular MVHR ductwork gains `duct_perimeter_mm`**, **party walls gain `u_value_whole_wall`**, **suspended ground floors gain `smart_air_bricks` and `vents_open_during_airtightness_test`**, and the **window input is reworked**. None of this changes what you submit today — SAP 10.3 is still the compliance route — but it tells you what HEM will want when it arrives. ## What ECaaS is, and why its changelog matters **Energy Calculation as a Service (ECaaS)** is the calculator service MHCLG runs to provide the **Home Energy Model**. It is an API: authorised bodies — in practice, the software houses whose tools you already use — build against it, and MHCLG's own documentation states plainly that the service "will become the only valid means to e.g. confirm compliance with Part L of the Building Regulations when making reference to the Future Homes Standard." That is worth sitting with for a moment. Under SAP, approved software houses each implement the published methodology and BRE approves the implementation. Under HEM, there is one engine, run centrally, and every compliance calculation goes through it. Your software becomes the thing that collects the inputs and presents the answer. Which makes the ECaaS changelog the most direct view any assessor has of where HEM is actually going. It is not a consultation or a summary. It is the field-level specification of what the calculation will ask for. The API has been open to authorised bodies since **27 November 2024**. It currently runs **HEM 1.0.0a7** (specification published **27 February 2026**) with the **FHS wrapper 1.0.0a7** (published **9 March 2026**). The documentation states that "work is ongoing to move the API to reflecting 1.0.0a9 of HEM/FHS, **estimated for late September 2026**." It is still flagged `beta`, and MHCLG is explicit that backwards-compatibility breaks will keep happening "as the input specifications for FHS and HEM are finalised." ## The thermal bridging change Buried in the list of breaking changes for 1.0.0a9 is a single line: > removed possibility to provide number as a value for `ThermalBridging` In the current input specification, thermal bridging can be given either as a set of junction objects — each with its Ψ value and length — or as a single number standing in for the whole dwelling. From 1.0.0a9, the single number goes. If you have spent any time with SAP, you will recognise the shape of this. SAP allows a default **y-value** (0.15 W/m²K, or 0.20 for existing dwellings) as an alternative to entering junctions individually, and that default is deliberately punitive: it is a penalty designed to make bespoke Ψ calculations the better commercial choice. Removing the blanket option from HEM goes a step further. It does not make the shortcut expensive. It removes it. Two caveats, because this is a beta specification and it deserves to be read carefully. First, the changelog describes the **input schema**, not the compliance policy — it tells you what the API will accept, and it is possible the FHS wrapper still applies defaults of its own to junctions you declare but cannot evidence. Second, this is a **forthcoming release with an estimated date**, and MHCLG has moved HEM dates before: the launch itself [slipped on 8 June 2026](/news/home-energy-model-launch-delayed/) to "the coming months". But the direction is not ambiguous, and it is the same direction Part L has been travelling since **Accredited Construction Details were withdrawn in 2022**. Junction-by-junction Ψ values, calculated to **BS EN ISO 10211** and reported under **BR 497**, are becoming the only way to describe a thermal envelope. A dwelling with good fabric and no junction data will simply not be describable. ## The other breaking changes worth knowing about The 1.0.0a9 list runs long. These are the ones that touch things an assessor or a technical manager actually specifies. **MVHR ductwork changes shape.** A new `duct_perimeter_mm` field arrives for ductwork with a **rectangular cross section**, and the changelog pairs it with a "new inadmissibility of `internal_diameter_mm` and `external_diameter_mm` fields for ductwork with a circular cross section". The wording there is worth reading twice — the natural engineering reading is that diameter fields stop being accepted for *rectangular* duct, which is the change the new perimeter field implies, and the full release notes should settle it. Either way, the direction is the same: rectangular duct gets described by its perimeter rather than squeezed into a diameter. This lands in the same quarter as [Approved Document F 2026's expanded ductwork evidence expectations](/news/part-f-2026-ductwork-evidence-building-control/), and points the same way — the ventilation system has to be described as installed, not as a nominal system type. Two related fields disappear from `MechanicalVentilation` objects: `design_zone_cooling_covered_by_mech_vent` and `design_zone_heating_covered_by_mech_vent`. **Party walls gain a whole-wall U-value.** A `u_value_whole_wall` field is added for party wall building elements, and `defined_resistance` is removed as a party wall cavity type. Party wall heat loss has been a long-running argument in SAP — fully filled and sealed cavities at 0.0 W/m²K, unfilled at 0.5 — and a whole-wall U-value input suggests HEM wants the constructed reality rather than a category. **Suspended ground floors get more specific.** `shield_fact_location` is removed and replaced by **`smart_air_bricks`** and **`vents_open_during_airtightness_test`**. That second field is an interesting one: it makes the relationship between the underfloor void and the airtightness test an explicit input rather than an assumption, which means what the test engineer did on site now has to be recorded and carried into the model. **Windows are reworked.** `mid_height`, `max_window_open_area` and `free_area_height` are removed from transparent building elements, and the definition of `window_part_list` is "reworked". Given that window opening geometry drives both purge ventilation under Part F and the useful-opening-area calculations that sit behind overheating, this one is worth watching when the release notes land in full. **Smaller removals.** `NumberOfSanitaryAccommodations` goes. `storey_of_dwelling` is removed from the General object. `PreHeatedWaterSource` gains an `init_temp` field. A point-of-use hot water source must now send an `efficiency` value of exactly `1`. The `building_level_distribution_losses` field is no longer required for heat interface units. **Additions on the services side.** New `temp_min_useful` and `HeatSource` fields for **PCM heat battery** inputs, a `power_limit_export` field for electricity energy supplies, and `heat_exchanger_surface_area` and `tank_is_integral` for hot-water-only heat pumps with separate tanks. ## Two caveats MHCLG flags itself Worth reading, because they are candid. **Speed.** "The time taken for successful FHS compliance calculations is currently in the region of **11 seconds**." MHCLG expects to improve this. For now, anyone imagining HEM as a click-and-wait experience like SAP should adjust: a half-hourly simulation over a year is not free, and iterating a specification twenty times to find the compliant combination is a different working day. **Product data.** The API "currently accepts PCDB product references for **boilers and heat pumps only**", with remaining product categories to follow. So the Product Characteristics Database integration that will eventually pull real product data into a calculation is partial. Everything else still needs entering by hand. The documentation also acknowledges that "there are still a number of inaccuracies in both the core HEM implementation and the FHS wrapper", which is exactly what a `beta` designation is for. ## What to actually do about this **Nothing changes for a live job.** SAP 10.3 remains the sole approved methodology for Future Homes Standard compliance, and [Approved Document L 2026 takes effect on 24 March 2027](/news/future-homes-standard-adl-2026/). ECaaS is not a route you can use for a submission today. **But start building the junction library now.** If the blanket thermal bridging value is going, then every standard detail in a house type needs a calculated Ψ value with a traceable model behind it — and the time to build that library is while SAP is still the compliance route and a y-value fallback still exists. Doing it under time pressure in 2027, on a plot that has already lost transitional protection, is the expensive version. **Ask your software house where they are on 1.0.0a9.** They are the authorised body integrating with ECaaS. The useful question is not "when will you support HEM" but "which HEM version are you built against, and what happens to my saved models when the API moves". Input schema breaking changes are exactly the kind of thing that invalidates stored data. **Record what the airtightness test engineer did to the underfloor vents.** It sounds trivial. Under 1.0.0a9 it is an input field. If you'd like a set of Ψ values calculated for your standard junctions — to BS EN ISO 10211, reported the way a building control body expects to see them — [get in touch](/contact/) and we'll tell you what we'd need from your details. *Sources: [ECaaS Technical Documentation (MHCLG)](https://docs.building-energy-calculator.communities.gov.uk/) · [Home Energy Model: replacement for the Standard Assessment Procedure (SAP) — consultation (GOV.UK)](https://www.gov.uk/government/consultations/home-energy-model-replacement-for-the-standard-assessment-procedure-sap) · [Energy Calculation as a Service (ECaaS) — Construction Leadership Council](https://www.constructionleadershipcouncil.co.uk/news/energy-calculation-as-a-service-ecaas/) · [The Future Homes and Buildings Standards: Building Circular 01/2026 (GOV.UK)](https://www.gov.uk/government/publications/the-future-homes-and-buildings-standards-building-circular-012026)* --- # Council reorganisation is paused — and building control just got another year of uncertainty URL: https://energycount.co.uk/news/local-government-reorganisation-review-paused-september-2026/ Date: 2026-09-13 > **Quick answer:** On **7 September 2026** the Secretary of State announced in Parliament a **review of Local Government Reorganisation** following "updated and privileged" legal advice. Decisions on **Essex, Hampshire, Norfolk and Suffolk have been withdrawn**. Plans for **14 other areas, plus Cambridgeshire & Peterborough and West Sussex, are paused** while the review runs. **Surrey is unaffected** — East and West Surrey go live in **April 2027**. **May 2027 local elections will be held on existing boundaries.** No end date has been given for the review. ## What was announced The press release, [*Review of local government reorganisation launched*](https://www.gov.uk/government/news/review-of-local-government-reorganisation-launched), was published on GOV.UK on **7 September 2026** following a statement to Parliament. The stated reason is legal. Ministers received "updated and privileged legal advice", and the Secretary of State, Angela Rayner, said she wanted "to satisfy myself firstly that the right process is in place, that it is robust, and of course complies with the law". A second, separate test follows: whether the proposals "meet the priorities of a new administration and the new Prime Minister". So there are two questions in the review, and they are not the same question. One is procedural — was reorganisation done lawfully. The other is political — does the new government still want it in this form. Either could change the answer. The practical effect breaks into three tiers: - **Withdrawn:** Essex, Hampshire, Norfolk and Suffolk — the four Devolution Priority Programme areas. These decisions no longer stand. - **Paused:** 14 other areas, plus Cambridgeshire & Peterborough and West Sussex where decisions had not yet been issued. - **Proceeding:** Surrey. Councillors for the two new unitary authorities were elected in May, and **East and West Surrey go live in April 2027**. The government has also confirmed that **local elections in May 2027 will go ahead on existing council boundaries**, and that a *Rewiring the State* White Paper on devolution will set out the longer-term plan. The decision follows judicial reviews launched by councils in several affected areas in June. ## Why this matters if you build houses It would be easy to file this under local government politics and move on. It sits closer to a scheme than that. **Every reorganising area has a building control service and a planning department inside the reorganisation.** A two-tier area moving to unitary means district building control teams merging, planning services amalgamating, staff deciding whether to stay. Three days after this announcement, MHCLG wrote to local authority chief executives about building control capacity and charging reform — a letter that explicitly welcomed early engagement on new delivery models "particularly in the context of local government reorganisation and wider public service reform". That letter assumed reorganisation was happening. Now, in 16 areas, nobody knows. **Uncertainty is worse for recruitment than either outcome.** A building control manager in a district council that was told it would cease to exist in April 2028, and has now been told the decision is withdrawn or paused with no new date, is not in a good position to hire. Neither are the people they would be hiring. MHCLG's own £55 million programme depends on authorities putting trainee Registered Building Inspectors through an LABC-employed secondment route — which requires an authority confident enough about its own future to say yes at senior level. Against the backdrop of the [planning department staffing gap](/news/planning-department-staffing-gap-2026/) that was already there, another year of structural limbo is not a neutral event. **And the timing collides with the busiest regulatory window in a decade.** The **Building Safety Levy starts on 1 October 2026**, with MHCLG warning of a surge in initial notices and applications through September. **Approved Document L 2026 takes effect on 24 March 2027**, making all of 2027 a dual-standard year — plots with [transitional protection](/news/future-homes-standard-transitional-deadline-commencement/) checked against Part L 2021, plots without it checked against the new Part L and requirement L3. **Approved Document F 2026** lands on the same day. The teams doing that checking are, in 16 areas of England, now working inside an organisation whose future was reopened on 7 September. ## What it does not change Worth being precise, because uncertainty tends to expand to fill available space. **Your building control body has not changed.** Initial notices and applications submitted to a district or borough council go to that council. Nothing about the review alters who has building control jurisdiction today. **Building regulations are unaffected.** Part L, Part F, Part G, Part O and the 2027 commencement dates are set nationally. A paused reorganisation changes nothing about what a dwelling has to achieve. **The Building Safety Levy still starts on 1 October 2026.** Local authorities with building control responsibility remain the collecting authorities for their areas, including where building control is carried out by a registered building control approver or the BSR. **Surrey proceeds.** If you have sites in Surrey, the April 2027 vesting date for East and West Surrey stands. ## What to do about it **If you have schemes in a withdrawn or paused area, don't assume continuity of contact.** Reorganisation programmes have already moved staff, merged teams and changed reporting lines in anticipation of vesting dates that may now not happen. Confirm who your case officer is and who covers them, rather than relying on a contact from six months ago. **Front-load the September and Q1 2027 submissions you were going to make anyway.** This was already the right call because of the levy on 1 October and the Part L change on 24 March 2027. Organisational uncertainty in the receiving authority is one more reason not to leave an application until the deadline. **Make applications easy to check.** It is the same conclusion as that charging reform letter, and it holds under every version of this. An application where the drawings, the specification, the SAP calculation, the U-values, the Ψ values and the ventilation strategy all agree with each other takes less officer time under any structure, in any authority, with any number of vacancies. An application where they don't agree generates a query, and queries are where months go. **Watch for the review's terms and timetable.** No end date has been published. Jim McMahon, the minister for local government, devolution and regional growth, is leading it, and the *Rewiring the State* White Paper is the document to look for. If you'd like the Part L and Part F evidence for a scheme checked for internal consistency before it reaches a building control body — whichever building control body that turns out to be — [get in touch](/contact/). *Sources: [Review of local government reorganisation launched (MHCLG press release, GOV.UK, 7 September 2026)](https://www.gov.uk/government/news/review-of-local-government-reorganisation-launched) · [Local Government Reorganisation (Hansard, House of Commons, 7 September 2026)](https://hansard.parliament.uk/Commons/2026-09-07/debates/BBDE7BC0-92C4-4B04-BFC8-CFB70302CD0E/LocalGovernmentReorganisation) · [Planning news — 10 September 2026 (Planning Portal / The Planner)](https://www.planningportal.co.uk/services/weekly-planning-news/planning-news-10-september-2026) · [Strengthening local building control capacity, capability and financial sustainability (MHCLG divisional letter, 10 September 2026)](https://www.gov.uk/government/publications/strengthening-local-building-control-capacity-capability-and-financial-sustainability)* --- # U-Monkey now works out thermal mass — decrement factor, time lag and κ, on every build-up, free URL: https://energycount.co.uk/news/u-monkey-thermal-mass-decrement-factor-time-lag/ Date: 2026-09-13 > **Quick answer:** every above-ground build-up in [U-Monkey](/tools/u-value-calculator/) now shows three extra numbers under its U-value — **decrement factor**, **time lag** and **areal heat capacity (κ)** — calculated to **BS EN ISO 13786** from the density and specific heat of each layer. κ is the per-element thermal-mass figure a SAP 10 assessment asks for; the other two are what decide whether a room peaks with the afternoon or after midnight. They print on the free PDF. Nothing has been made paid. ## Why we bothered A U-value is a winter number. It says how fast heat leaks out when it is cold outside and warm inside, and Building Control asks for it, so every calculator gives it. It says nothing about summer, and nothing about how the room *feels*: whether the walls soak up the afternoon and hand it back at night, or pass it straight through. Take the pair of walls that turns up on every solid-wall retrofit. Insulate the brick on the outside or insulate it on the inside and you land on the same U-value, around 0.25 W/m²K. On paper they are interchangeable. In the building they are not: the externally insulated one has ten times the usable thermal mass, because the brick is on the warm side of the insulation where the room can use it. U-Monkey used to show them as the same wall. Now it shows the difference. ## What the three numbers mean **Decrement factor** — the share of the outdoor daily temperature swing that reaches the inside face. A 12-degree day outside becomes a 3.6-degree swing inside through a wall with a decrement factor of 0.3, and nearly 10 degrees through one at 0.8. Below about 0.3 is heavyweight behaviour; above about 0.6 is lightweight. **Time lag** — how many hours the outdoor peak takes to work through. Nine or ten hours puts the 3 pm peak after midnight, when the house is cooler and windows are open. One or two hours puts it at teatime, on top of the afternoon. **Areal heat capacity, κ** — how much heat one square metre of the inside face stores over a day, in kJ/m²K. This is the number SAP 10 wants per element to build the dwelling's thermal mass parameter; without it, assessors pick 100, 250 or 450 from Table 1e. U-Monkey now calculates it from the layers you actually specified. ## Seven build-ups, side by side Computed by U-Monkey on the build-ups behind our worked-example pages. Each one opens pre-filled in the calculator from the [thermal mass calculator page](/tools/u-value-calculator/thermal-mass-calculator/). *Update, 12 October 2026: these are the figures as published on 13 September. U-Monkey's conventions have been refined since — the externally insulated wall's brick is now taken as protected (BR 443 §7), which gives κ 103 rather than 120 — and the thermal mass calculator page carries the current table.* | Build-up | U-value | Decrement factor | Time lag | κ | | --- | --- | --- | --- | --- | | Solid brick wall, **external** insulation | 0.26 | 0.17 | 9.0 h | **120** kJ/m²K | | Solid brick wall, **internal** insulation | 0.25 | 0.29 | 8.1 h | **12** kJ/m²K | | Full-fill cavity wall, 150 mm mineral wool | 0.20 | 0.27 | 10.7 h | 33 kJ/m²K | | Timber-frame wall, mineral wool | 0.30 | 0.54 | 5.5 h | 14 kJ/m²K | | Warm flat roof, PIR over timber deck | 0.14 | 0.82 | 3.4 h | 25 kJ/m²K | | Pitched roof, PIR at rafter level | 0.20 | 0.71 | 1.7 h | 12 kJ/m²K | | 270 mm loft quilt at ceiling | 0.16 | 0.86 | 1.9 h | 12 kJ/m²K | The roof rows are why loft conversions overheat: a PIR roof at a perfectly good U-value passes seven-tenths of the day's swing through in under two hours, because nothing in it stores heat. Adding more of the same insulation barely moves that; a heavier internal lining or a ventilated void does. That is a design decision worth making before the plasterboard goes up, and it is not one a U-value can inform. ## How it is done, and what it is not U-Monkey builds the ISO 13786 heat-transfer matrix for your layers at a 24-hour period and evaluates it at the inside face. The hard part was never the maths — it was the data. The method needs the **density and specific heat** of every layer, and U-value libraries do not usually carry them. Ours now does, for all 300+ materials and products, each with a named source and an honest label: the manufacturer's own figure where one is published, a representative value for the material class where not, and — flagged as the weakest — a transfer from an equivalent material where nobody publishes one. The working in the app shows which applies to each layer. A custom material you type in gets no thermal-mass result until you add the two properties; U-Monkey does not guess them, because an invented density gives a confident-looking time lag that is simply wrong. It is not a [Part O overheating assessment](/services/part-o-overheating/). That needs a dynamic model of the whole room — glazing, orientation, ventilation, occupancy — and we do those as a service. What the calculator gives you is the fabric's share of the answer, for free, at the moment you are choosing between two build-ups: which one damps and delays the heat, and by how much. ## Where to find it Under the target pills on every result there is now a **Thermal mass** line; *explain* opens the working with the full Summer performance block and the data sources. The three figures also print under *Result* on the free U-value PDF. The [thermal mass calculator page](/tools/u-value-calculator/thermal-mass-calculator/) has these and three more, each with a link into the pre-filled calculator, and the [how-to guide](/tools/u-value-calculator/how-to-use/#thermal-mass) covers adding density and specific heat to a custom material. --- # MHCLG has revised Approved Documents L and F 2026 — replace your March PDFs URL: https://energycount.co.uk/news/adl-adf-2026-revised-circular-04-2026/ Date: 2026-09-09 > **Quick answer:** On 8 September 2026 MHCLG re-issued the 2026 editions of **Approved Document L Volume 1** (dwellings), **Volume 2** (buildings other than dwellings) and **Approved Document F Volume 1** to correct what [Circular 04/2026](https://www.gov.uk/government/publications/amendments-to-approved-documents-l-energy-and-greenhouse-gas-emissions-and-f-ventilation-circular-042026/amendments-to-approved-documents-l-energy-and-greenhouse-gas-emissions-and-f-ventilation-circular-042026-letter) calls "minor errors". The standard itself does not move and neither do the dates: the revised documents take effect on **24 March 2027**, or **24 September 2027** for higher-risk building work, under the [Circular 01/2026 transitional rules](/news/future-homes-standard-transitional-deadline-commencement/). But if you downloaded the March PDFs, replace them. The corrections touch heat-network sleeving, the Home Energy Model party-wall table, the window U-value convention for existing dwellings, a note to the limiting U-value table, the on-site generation guidance, and flexible ductwork in Part F. ## What was published The circular letter, signed by Keeran Jugdoyal, Deputy Director of MHCLG's New Build Standards and Performance Division, confirms that the revisions are approved under section 6 of the Building Act 1984, that the revised PDFs are on GOV.UK now, and that hard copies from the publisher will follow. The [Approved Document L (2026) page](https://www.gov.uk/government/publications/approved-document-l-2026) and the [Approved Document F (2026) page](https://www.gov.uk/government/publications/approved-document-f-2026) were both updated the same day. The revised PDFs are still labelled "2026 edition", so the quickest way to tell which version you have open is Table 1.2 in Volume 1: if it lists **biogas** and **biomass**, you have the September version. MHCLG also published a separate [FAQ on requirement L3](https://www.gov.uk/guidance/approved-document-l-energy-and-greenhouse-gas-emissions-frequently-asked-questions) on 8 September — the solar PV requirement. That is a bigger story for housebuilders and we cover it in [its own piece](/news/adl-2026-requirement-l3-solar-pv-faq/); the [solar PV calculator](/tools/solar-pv-calculator/) works out the target for a given floor area. ## Volume 1 (dwellings): the six changes | Where | What changed | Why it matters | | --- | --- | --- | | Table 1.2 — low carbon heat sources for a declaration of sleeving capacity | "Biogas" and "biomass" added alongside "biofuels"; "waste heat" becomes "recovered heat" | Widens what a district heat network can count in a sleeving declaration | | Paragraph 2.10b — heat network CO₂ and primary energy factors | New wording: account for the performance of the generation system the sleeving is applied to, and pro-rate distribution and storage losses by the sleeved share of heat | Sleeved heat carries a proportionate share of network losses — it is not the plant's nameplate figure | | Table 2.1 — HEM condition for party walls | The "unfilled cavity with effective sealing… for a concrete frame construction in a building containing flats" row changes from **no heat loss** to **heat loss** | A HEM input for flats; the sealed-but-unfilled concrete-frame party wall no longer gets a free pass | | Paragraphs 3.4b and 3.4c — window U-values in existing dwellings | Standard window is 1.23m (±25%) wide × **1.48m (–25%)** high, not ±25% high | The height tolerance runs downwards only | | Note 4 to Table 3.1 — limiting U-values | "(6mm)" inserted after "window unit with standard thickness" | Defines the equivalent unit when thicker glass is needed for other reasons | | Paragraph 5.68 — on-site electricity generation | A sentence cross-referring other renewable technologies to paragraphs 5.73 and 5.74 is removed | 5.68 applies to all on-site generation; the L3 indicative outputs sit at 5.70 | ### Sleeving: two of the six "Sleeving" is the 2026 edition's route for a dwelling connected to a district heat network to take credit for low-carbon capacity that the network operator has allocated to it. Under paragraphs 1.8 and 1.9, the operator makes a declaration of sleeving capacity to the building control body, validated by a suitably qualified third party, stating among other things that at least 90% of the heat delivered to the dwelling will come from new or unused capacity of the types listed in Table 1.2, and that any new capacity will be connected within five years of the as-built assessment. Only district networks can be sleeved; communal networks cannot. The revised Table 1.2 now lists electrically powered heat pumps, recovered heat (including from power stations), geothermal heat, electric boilers, solar thermal, biogas, biomass and biofuels. The two biomass-family additions are the substantive change; "recovered heat" for "waste heat" is a tidy-up. The new sentences in paragraph 2.10b matter more for the assessor, because they say how the sleeved heat's factors are built up. The calculation should "take account of the performance of the heat generation system that sleeving is being applied to", and the "performance of distribution circuits and storage should be applied on a pro-rated basis based on the proportion of heat being supplied under the sleeving arrangements compared to the heat network as a whole". The existing note is unchanged: a sleeving declaration does not remove the need to calculate the CO₂ emission factor and primary energy factor for the heat. ### The HEM party-wall row Table 2.1 only applies when the approved methodology is the [Home Energy Model](/news/home-energy-model-launch-delayed/). For a SAP 10.3 assessment, paragraph 2.12a still sends you to section 3.7 of the SAP specification for party wall U-values, and nothing there has changed. But HEM will arrive, and flats designed now will be assessed in it later. In the March edition an unfilled cavity party wall with effective edge sealing in a concrete-framed block of flats sat in the "no heat loss" column alongside the fully filled cavity. In the revised table it sits with the other unfilled cavities as a heat-loss condition. If the design intent was to rely on the sealed-but-unfilled wall in a HEM assessment, the choice now is to fully fill the cavity or to take the loss. ### Windows in existing dwellings Paragraph 3.4 lists the ways a window U-value may be determined for an existing dwelling: from the actual size and configuration, from a standard window with the actual configuration, from a standard window with a standard configuration, by hot-box test, or from the SAP 10.3 Table 6e defaults. The March edition gave the standard window as 1.23m (±25%) wide × 1.48m (±25%) high. The revision corrects the height to 1.48m (–25%): a standard-size calculation may be for a window up to a quarter narrower or wider, or up to a quarter shorter, than the reference — but not taller. Because a larger glazed proportion usually pulls the whole-window U-value down, an upward height tolerance would in most cases have flattered the declared figure. For new dwellings nothing changes: paragraph 3.2 already requires the U-value to be measured by hot-box or calculated for the actual size and configuration of the window, as we set out in [our piece on window and doorset U-values under ADL 2026](/news/window-doorset-u-values-adl-2026/). Note 4 to Table 3.1 is the companion correction. Where wind load, safety, security or acoustic performance require thicker glass, the equivalent window unit that has to be shown to meet the limiting U-value is now explicitly one with standard 6mm glazing. The limiting values themselves are unchanged: 0.16 for roofs, 0.26 for walls, 0.18 for floors, 0.20 for party walls, 1.6 for windows and doors, 2.2 for rooflights, and an air permeability no worse than 8.0 m³/(h·m²) at 50Pa (1.57 at 4Pa). ### Paragraph 5.68 The sentence removed from paragraph 5.68 said that where another renewable technology is used to generate electricity, the amount produced by all technologies should be equivalent to the output of the standards in paragraphs 5.73 and 5.74. The circular's explanation is that 5.68 applies to *all* installation of on-site renewable electricity generation — including replacement systems in existing dwellings under requirement L2 — and that guidance on indicative outputs for requirement L3 belongs, and remains, at paragraph 5.70. ## Volume 2 (buildings other than dwellings) Volume 2 receives the same corrections in its own numbering: Table 1.2 (biogas, biomass, recovered heat), paragraph 2.7b (the pro-rated sleeving sentence), paragraphs 3.2b and 3.2c (the 1.48m (–25%) standard window) and Note 8 to Table 3.1 (6mm standard glazing). Appendix G, the list of documents referred to, is also brought up to date: CIBSE **TM39 Building Metering and Monitoring** now cites the 2026 edition rather than 2009, and BSRIA **BG 49/2024 Commissioning Air Systems** replaces BG 49/2015. If your non-domestic commissioning or metering evidence still references the older editions, this is the moment to update the template. ## Approved Document F, Volume 1 One change, in paragraph 1.79. The March text allowed a short length of flexible ductwork for the final connection to fan units only in centralised continuous mechanical extract and MVHR systems. The revision deletes that qualification, and the paragraph now reads: "Flexible ductwork should be limited to 200mm in length and used only for final connections to fan unit spigots." That applies to every system type — intermittent extract fans and decentralised MEV included. The note that follows is untouched: where the fan's automatic control relies on constant pressure in the ductwork, flexible ducting is not permitted anywhere in the system. Our earlier piece on [what Part F 2026 expects by way of ductwork evidence](/news/part-f-2026-ductwork-evidence-building-control/) still stands; this simply removes an inconsistency in it. ## What to do Replace the March PDFs with the September ones and check any internal checklist or template that quotes the affected paragraphs — the window convention for existing dwellings, the party-wall condition for HEM, and the sleeving table and factors are the ones most likely to have been copied out. Nothing here reaches back into a [Part L 2021](/guides/building-regulations-part-l/) job: the 2021 edition continues to apply to work notified before 24 March 2027 and commenced before 24 March 2028. For a scheme heading for the [Future Homes Standard](/guides/future-homes-standard/), the numbers in the SAP 10.3 model do not change, but the evidence you present alongside it should now cite the revised text. One of the corrected paragraphs is the window convention for existing dwellings, which is the sort of detail that quietly changes a U-value rather than announcing itself. If you want to see what the revised convention does to a build-up you have already specified, **U-Monkey** will rebuild it to BS EN ISO 6946 on screen, free, with every layer and correction shown. [Try U-Monkey →](/tools/u-value-calculator/) *Sources: [Amendments to Approved Documents L and F, Circular 04/2026 — letter (MHCLG, 8 September 2026)](https://www.gov.uk/government/publications/amendments-to-approved-documents-l-energy-and-greenhouse-gas-emissions-and-f-ventilation-circular-042026/amendments-to-approved-documents-l-energy-and-greenhouse-gas-emissions-and-f-ventilation-circular-042026-letter) · [Approved Document L (2026), revised 8 September 2026 (MHCLG)](https://www.gov.uk/government/publications/approved-document-l-2026) · [Approved Document F (2026), revised 8 September 2026 (MHCLG)](https://www.gov.uk/government/publications/approved-document-f-2026) · [The Future Homes and Buildings Standards: Building Circular 01/2026 — letter (MHCLG, 24 March 2026)](https://www.gov.uk/government/publications/the-future-homes-and-buildings-standards-building-circular-012026/the-future-homes-and-buildings-standards-building-circular-012026-letter)* --- # North-facing roofs count: MHCLG answers the solar PV questions on requirement L3 URL: https://energycount.co.uk/news/adl-2026-requirement-l3-solar-pv-faq/ Date: 2026-09-09 > **Quick answer:** MHCLG's [FAQ on requirement L3](https://www.gov.uk/guidance/approved-document-l-energy-and-greenhouse-gas-emissions-frequently-asked-questions), published on 8 September 2026, confirms that the L3 target for a house is the **annual output** of a benchmark PV array with a peak power of **0.22 kWp/m² × 40% of the ground floor area** (equation 5.1 of Approved Document L1), converted to kWh through **Appendix M1 of SAP 10.3**. Paragraph 5.73(a) fixes the rest of that benchmark — facing **south-east to south-west**, pitched at **45°**, **not overshaded** — so the target depends only on the ground floor area, and your roof decides how much PV it takes to reach it. The FAQ also confirms that panel efficiency is kWp divided by the whole panel including its frame; that **north-facing and overshaded roof slopes cannot be left out** of the "reasonably practicable roof area" just because they generate less; that a **detached garage** is excluded from both the 40% and the roof area, but may still carry panels; and that a system installed for L3 must also be "appropriately sized for the site and available infrastructure" under **requirement L2** — which is where the DNO's export limit comes in. Our [solar PV calculator](/tools/solar-pv-calculator/) works out both numbers for your own floor area and roof. ## The rule the FAQ is interpreting From 24 March 2027 (24 September 2027 for higher-risk building work, and subject to the [transitional provisions](/news/future-homes-standard-transitional-deadline-commencement/)), requirement L3 of Schedule 1 to the Building Regulations applies whenever a dwelling or a building containing dwellings is erected. A system for on-site renewable electricity generation must be installed on the building or within the boundaries of its curtilage, designed so that the electricity is available to the residents and so that it produces a reasonable output given the building's design and surroundings. The requirement does not apply to a "relevant building" under regulation 7(4) — broadly a building with a storey at least 18m above ground that contains dwellings, an institution or rooms for residential purposes, the same buildings caught by the combustible-materials ban — nor where no reasonable output is possible, nor where an equivalent output is available to the residents from an on-site system that is not on the building or in its curtilage. Approved Document L Volume 1 (2026) says the requirement is met by following paragraphs 5.69 to 5.78. The core of that guidance is paragraph 5.73 for a dwelling-house and 5.74 for a building containing dwellings: an array whose output is at least that of a benchmark array — PV covering the equivalent of 40% of the ground floor area, facing south-east to south-west at 45° and not overshaded — or, where that is not achievable, an array covering the "reasonably practicable roof area". Paragraph 5.77 sets the floor: where the available roof cannot produce 720 kWh a year — "roughly equivalent to less than 2 PV panels", as the government's [consultation response](https://assets.publishing.service.gov.uk/media/69c13592bb0dfe55b83e4b85/Future_Homes_and_Buildings_Standards_Consultation_Response.pdf) of March 2026 put it — it is an exceptional circumstance in which no panels may be required. For flats the floor is 720 kWh per dwelling divided by the number of storeys. That is the framework. The FAQ is MHCLG's first published set of answers on how it works in practice, and it was needed. ## The arithmetic The FAQ walks through equation 5.1. Take the dwelling-house's ground floor area, multiply by 40%, and multiply by 0.22 kWp per square metre — the peak power of PV "with an efficiency of 0.22kWp per m² installed over an area equivalent to 40% of the dwelling-house's ground floor area". That gives the installed peak power of the *benchmark* array, and paragraph 5.73(a) fixes the rest of it: orientated south-east to south-west, a pitch of 45°, not overshaded. Then run that array through Appendix M1 of SAP 10.3 to get its annual output in kWh, and that kWh figure is what the system you actually install must be designed to achieve. For a house with a 45 m² ground floor: 45 × 0.4 × 0.22 = **3.96 kWp**. At the benchmark efficiency that is about 18 m² of panel — around nine 440 W modules of the usual 1.76 × 1.13 m size, which come out at almost exactly 0.22 kWp/m² by the FAQ's own definition of efficiency: the panel's kWp divided by the area of the whole panel including the frame, excluding flashings. SAP's Appendix M formula multiplies the peak power by 0.8, by the annual solar radiation for the array's orientation and pitch, and by an overshading factor. SAP 10.3 runs Part L compliance on UK-average weather (Appendix U), and paragraph 5.72 ties the L3 calculation to the same methodology as the DER and DPER. Because the benchmark's orientation, pitch and shading are fixed, the target does not move with your roof or your region: it works out at about **70.7 kWh a year for every square metre of ground floor** — **3,181 kWh** for the 45 m² house. What your roof changes is how much PV it takes to get there. On a south-facing slope at 35° the array needs 3.68 kWp, slightly less than the benchmark. On an east–west roof at the same pitch it needs 4.46 kWp, about 13% more: ten panels, five a side, produce 3,140 kWh and fall 41 kWh short, where eleven clear it. Because the requirement is expressed as an annual output rather than an area, a developer can meet it with fewer, more efficient panels, or with a better-oriented slope, and the FAQ frames the test for any on-site renewable electricity system — not just PV — as achieving that annual output. Our [solar PV calculator](/tools/solar-pv-calculator/) does the sum for your own floor area, roof slopes and panels. One detail is left open. "South-east to south-west" covers three of SAP's eight orientations. Read as south-east or south-west, the benchmark gives 803 kWh a year per kWp; read as due south, 854 — about 6% more. Which reading the approved software takes will show on the BREL report; until you have seen it, design to clear both. For a building containing one or more dwellings, the FAQ says to use AGRND from equation 5.2 or AGRNDDWELL from equation 5.3 "as appropriate", and paragraph 5.74 sets out the choice. Route (a) sizes the whole building from AGRND, its gross internal area divided by the number of storeys. Route (b) gives each dwelling its own target from AGRNDDWELL, its share of AGRND by floor area — which comes to the flat's floor area divided by the number of storeys. Route (c) is the reasonably practicable roof area. Routes (a) and (b) both use the same fixed south-east-to-south-west, 45°, unshaded benchmark. ## What the "reasonably practicable roof area" has to include This is the part of the FAQ that will change designs. Where the 40% benchmark cannot be achieved, paragraphs 5.73(b) and 5.74(c) require an array covering the reasonably practicable roof area — its target is the output of 0.22 kWp/m² panels over that area, on the slopes you actually have — and the developer is responsible for identifying that area "and justifying that assessment to the relevant building control body". Two questions in the FAQ ask whether north-facing slopes and heavily overshaded slopes can be left out. The answer to both is no. The reasoning is the same in each case: the approved methodology already discounts the output of a panel for its orientation and its overshading, so the lower generation from a north slope or a shaded slope "is unlikely to be sufficient justification to exclude it". A north-facing slope raises the required output by less than a south-facing slope of the same size would — but it raises it, and it has to be covered. If you have been assuming that a north slope simply drops out of the calculation, it does not. Appendix B of Volume 1 gives offset distances from roof edges and features — 600 mm from the ridge; 500 mm from eaves, verges, windows and doors; 750 mm from a party wall's centre line; 300 mm from vents and flues; 1,000 mm from an automatic opening vent. The FAQ says they are "primarily intended for determining the size of the reasonably practicable roof area", are not intended to cover every circumstance, and that different installation offsets may be appropriate provided the panels still cover the reasonably practicable area, comply with other regulatory requirements and follow the manufacturer's instructions. Offsets are measured from the roof feature to the edge of the panel. There is no prescribed panel dimension for laying out the array; housebuilders should use dimensions "consistent with commercially available residential solar PV panels" and appropriate to the building's design. To lay a roof out against the offsets, slope by slope, use the [reasonably practicable roof area calculator](/tools/solar-pv-calculator/reasonably-practicable-roof-area/). An array that misses the benchmark is highlighted on the BREL report (paragraph 5.78), and Appendix B lists the evidence Building Control will expect: roof diagrams with and without the panels, a statement of why more cannot be installed, and the calculations signed by a suitably qualified person — which includes an On Construction Domestic Energy Assessor. What goes in each: [requirement L3 evidence for Building Control](/tools/solar-pv-calculator/building-control-evidence/). ## Garages, curtilage and flats Three of the questions deal with the edges of the building. A **detached garage** does not count towards the 40%: unheated spaces outside the projected main roof area are excluded from the ground floor area, which the FAQ says aligns with the SAP 10 conventions' treatment of attached garages for the notional dwelling. Its roof is not part of the reasonably practicable roof area either, because that phrase refers to the roof of the dwelling. But you *can* put panels on it to meet the required output, provided it is within the curtilage and the other conditions of L3 are met. A detached garage is therefore an option, not an obligation, and not a way to shrink the target. **Curtilage** is given its usual planning meaning: land "so closely and directly associated with a building as to form 'part and parcel' of it" — gardens, garages and driveways for a house; service areas, courtyards and other supporting space for a block of flats — judged case by case on layout, ownership and use. For **buildings containing dwellings**, paragraph 5.75 requires the system to be designed so that generated electricity is available to the residents. The FAQ confirms that supply to communal spaces used for the residents' benefit counts, as does supply to individual dwellings. A landlord's-supply connection serving lifts, corridors and plant is a legitimate design. Equations 5.2 and 5.3 are worked through for three blocks in [solar PV for flats](/tools/solar-pv-calculator/flats/). ## The L2 question: appropriately sized for the site Requirement L2 has been in Part L since 2021 and says that where an on-site electricity generation system is installed, it must be appropriately sized for the site and available infrastructure, have effective controls, and be commissioned so that it produces the maximum reasonable electricity in the circumstances. The FAQ's first two questions deal with how L2 and L3 interact, and the answer is worth reading twice. A system installed to meet L3 must also comply with L2. Having considered the site-specific factors, "it is possible that the output of a system appropriately sized for the site and available infrastructure may differ from the output of a system calculated in accordance with paragraphs 5.73 and 5.74". The factors listed are the on-site energy demand, the capacity and nature of the available infrastructure "including export limits set by the District Network Operator", the relationship between export limits, generation and demand, and other characteristics of the site. Whether reasonable provision has been made "will depend on the facts of the particular case". In plain terms: the DNO's export limit is a relevant factor in sizing, and the FAQ acknowledges that an appropriately sized system may not match the L3 arithmetic. It does not say the export limit overrides L3, and it leaves the judgement with the building control body on the facts. The practical consequence is that the DNO correspondence — the G98 notification for systems up to 16 A per phase, or the G99 application above that — becomes compliance evidence. Get it early, keep it, and put it in the pack. ## What this means for the SAP The L3 output is not a DER or DPER test, but it is not separate from the SAP either: it is worked out in the same approved software, and a shortfall shows on the same BREL report. What has changed is the notional dwelling. Under Part L 2021 it carried a fixed array; the [SAP 10.3 notional dwelling](/news/sap-10-3-what-changed/) carries PV that is the **same as the actual building's, up to** 40% of the ground floor area divided by 4.5 — the same 0.22 kWp/m² (SAP 10.3 Appendix R, Table R1). Below that cap, the target moves with your array. PV no longer buys headroom against the TER and TPER, and a scheme that takes the reasonably practicable route and installs less is not penalised for it in the [DER and DPER](/guides/der-ter/) — the fabric, airtightness, [calculated junction Ψ-values](/tools/psi-calculator/) and the heat pump's seasonal performance carry that comparison either way. Where a scheme installs more than the cap because the roof allows it, the SAP result improves — but the L2 sizing test and the export limit set the ceiling. Requirement L3 is what now makes you fit the array. Design the array and the SAP together, from the first iteration, using the actual roof geometry and the DNO's answer. It is a great deal cheaper than discovering at the as-built stage that a north slope should have been covered, or that the array on the drawings will never be allowed to export. *Sources: [Approved Document L: Energy and greenhouse gas emissions, frequently asked questions (MHCLG, 8 September 2026)](https://www.gov.uk/guidance/approved-document-l-energy-and-greenhouse-gas-emissions-frequently-asked-questions) · [Approved Document L (2026), Volume 1: Dwellings, revised 8 September 2026 (MHCLG)](https://www.gov.uk/government/publications/approved-document-l-2026) — paragraphs 5.69–5.78 and Appendix B · [SAP 10.3 specification, version 12-06-2026 (BRE)](https://bregroup.com/documents/d/bre-group/sap-10-3-full-specification-2026-06-12) — Appendix M1, Appendix U and Table R1 · [The Future Homes and Buildings Standards: consultation response (MHCLG, March 2026), paragraphs 3.10–3.16](https://assets.publishing.service.gov.uk/media/69c13592bb0dfe55b83e4b85/Future_Homes_and_Buildings_Standards_Consultation_Response.pdf) · [Amendments to Approved Documents L and F, Circular 04/2026 — letter (MHCLG, 8 September 2026)](https://www.gov.uk/government/publications/amendments-to-approved-documents-l-energy-and-greenhouse-gas-emissions-and-f-ventilation-circular-042026/amendments-to-approved-documents-l-energy-and-greenhouse-gas-emissions-and-f-ventilation-circular-042026-letter)* *Corrected 29 September 2026: an earlier version of this article said the L3 target moves with the orientation of the actual roof and with the site's region, and that a shortfall on the reasonably practicable route has to be made up in the fabric. Neither is right. Paragraph 5.73(a) fixes the benchmark array's orientation, pitch and shading, SAP 10.3 works on UK-average weather, and the SAP 10.3 notional dwelling's PV follows the actual array up to its cap.* --- # Gateway 2 applications are rising ahead of the levy — and a rejection could cost you the exemption URL: https://energycount.co.uk/news/bsr-gateway-2-applications-rise-ahead-of-levy-september-2026/ Date: 2026-09-08 > **Quick answer:** BSR's latest transparency data, covering **June to August 2026** and published on **4 September**, shows **1,654 Gateway 2 applications in progress** — up by more than 100 in August alone — with BSR attributing the rise to the **Building Safety Levy** coming into force on **1 October 2026**. New-build approvals are running at **92%** with a **22-week median**, but a pre-October application that is **rejected** and resubmitted after 1 October is liable for the levy. Getting the application right first time has just acquired a price tag. ## The surge BSR is expecting The [Building Safety Regulator's building control approval data for June to August 2026](https://www.gov.uk/government/publications/building-safety-regulator-building-control-approval-application-data-june-to-august-2026), with figures as of 31 August, opens with a trend note it hasn't made before: "There has been an increase in gateway 2 applications. This could be because of the building safety levy, which comes into effect in October 2026." The numbers back it up. Across all categories there are now **1,654 Gateway 2 applications in progress**, against 1,547 in the previous dataset — an increase of over 100 in August. For **new higher-risk buildings and conversions** specifically, applications in progress rose from **131 at the end of July to 147**, with **34 new-build applications received in August** alone. BSR says it expects that number "to increase in September, due to the introduction of the building safety levy on 1 October 2026", and that it has a mitigation plan in place. As covered in our [levy explainer](/news/building-safety-levy-1-october-2026/), the levy applies to applications for building control approval submitted **on or after 1 October 2026**. Applications made before that date are not caught, which is why the queue is growing. For higher-risk buildings, the application in question is the Gateway 2 application to BSR. ## The performance numbers: holding steady August 2026 marks twelve months since BSR's Innovation Unit was set up to handle new-build HRB applications exclusively, and the one-year comparison is the one BSR leads with. At that point the median approval time for new builds was **43 weeks** with a **39% approval rate**. Now it is **22 weeks** and **92%**. For the rolling 12 weeks to 31 August, the new-build figures are: **63 applications in**, **50 decisions**, **46 approvals and 4 rejections**, **9,362 residential units approved** and **32,691 units in progress**. Four applications were invalidated and one withdrawn. Complex cases — now tracked separately — number 33 and run to a **33-week median**. Rejections come back faster than approvals, at a median of **8 weeks**. London still dominates: around **65% of in-progress new-build applications** and 61% of the units are there, and 31 of the 50 new-build decisions in the period were London schemes, approved at a **90%** rate with non-complex cases taking 23 weeks. **External remediation** continues to improve: an **89%** approval rate and a **33-week median**, down from 47% and 43 weeks a year ago. Only 12 legacy applications from 2024 remain, down from 42 at the start of 2026, and BSR notes that 2026 approval rates already exceed its 75% target. **HRB internal works** sit at **78%** and 32 weeks, with 457 new applications in — around 43% of all Gateway 2 decisions. **NHS internal works**, reported separately, are at 82% and 22 weeks, with 2026 submissions down to a 13-week median. On resourcing, BSR reports 177 internal staff and 508 people from external organisations involved in building control work, and says it has grown its internal regulatory team by 25 in the past three months, with recruitment continuing for the next six to twelve months. ## Two things that are new **Inspections have found work departing from approved designs.** For the first time, the dataset reports that BSR's "growing inspection programme has found a small number of examples where building work has not followed agreed designs and has required some remedial work". BSR says it will monitor this closely and take action "to help make sure that buildings are constructed in line with safe designs". This is the point of the Gateway regime: the design approved at Gateway 2 is the design that has to be built, and the evidence of that is what Gateway 3 tests. **Major change requests are being turned round in seven weeks.** There are **184 live major change requests**, and BSR closed 123 in the period. For new-build projects the median closure time is **7 weeks** — one week over the statutory target. That matters because a major change to an approved HRB design has to be approved before the work continues, so this is the number that governs how quickly a design change on site can be actioned. ## Gateway 3: still nobody home No new-build project that passed through Gateway 2 has yet reached Gateway 3 — the equivalent hard stop before occupation. Of the **346 Gateway 3 applications** received so far, 260 are internal refurbishments, 45 are transitional cases and 23 are NHS internal refurbishments. Over the past 12 weeks 16 Gateway 3 applications were determined at an **88%** approval rate and a **20-week median** (the quickest in 7 weeks). BSR describes this as "an important area", particularly for new build, and expects determination times to reduce as new processes and resource come in. As the first cohort of Innovation Unit approvals completes, this is the stage to watch. ## Why the rejection rate now has a price Put the two halves of this dataset together. A rush of applications is going in to beat the levy. Eight per cent of new-build decisions are rejections, and a further handful of applications are being invalidated for not meeting the submission requirements. Under the levy regulations, an application submitted before 1 October is not liable — but, as our [levy explainer](/news/building-safety-levy-1-october-2026/) set out, if a pre-October application is **rejected and then resubmitted** after the go-live date, the resubmission is liable. An application that is **invalidated** has to be made again, and the date of the resubmission is a question worth putting to the building control authority before you rely on the September date. On a major residential scheme the levy is a per-square-metre charge on the whole chargeable floorspace, so the difference between "approved" and "rejected, resubmit in November" is no longer a programme question alone. There is a second date in play for the same schemes. Under the Future Homes Standard transitional arrangements, a higher-risk building keeps Part L 2021 only where a valid Gateway 2 application is made **before 24 September 2027** and the work starts in time. A rejected or invalidated application does not stop that clock either. ## What to do with this If you have a higher-risk scheme going to BSR in the next three weeks, the priority is validity and completeness, not speed of submission. BSR is explicit that the improvement in approval rates has come from better-quality applications and a shared understanding of what compliance evidence it expects. That includes the Part L, ventilation and fabric evidence — the SAP 10.2 or 10.3 calculation, the U-values and junction Ψ-values behind it, and the Part F strategy — which has to be consistent with the drawings and specification in the same submission. Inspection findings about work departing from approved designs are a reminder that the Gateway 2 package has to be internally consistent — and thermal bridging is where a fabric story most often stops agreeing with itself. **ΨMonkey** gives you calculated Ψ-values to BS EN ISO 10211, free, with a verifiable report behind each one. [Try ΨMonkey →](/tools/psi-calculator/) If you'd like that energy and fabric evidence checked for consistency before a Gateway 2 application goes in, [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [Building Safety Regulator building control approval application data June to August 2026 (GOV.UK, published 4 September 2026)](https://www.gov.uk/government/publications/building-safety-regulator-building-control-approval-application-data-june-to-august-2026) · [Building control approval application data June to August 2026 (PDF)](https://assets.publishing.service.gov.uk/media/6a9a852a5a0c25165ae468ee/Building_Safety_Regulator_building_control_approval_application_data_June_to_August_2026.pdf) · [Building Safety Regulator building control approval application data May to July 2026 (GOV.UK)](https://www.gov.uk/government/publications/building-safety-regulator-building-control-approval-application-data-may-to-july-2026/building-safety-regulator-building-control-approval-application-data-may-to-july-2026)* --- # The wettest week since February. Groundwater still fell at every site. URL: https://energycount.co.uk/news/wettest-week-since-february-groundwater-still-falling/ Date: 2026-09-08 > **Quick answer:** For the week of 28 August to 3 September 2026 England had its **wettest week since early February** — 26mm in the South East to 48mm in central England. Rivers responded at almost every monitored site and abstraction restrictions fell by more than half. **Groundwater fell at every site**, **reservoir storage dropped to 58%** (19.2% below average), **11 reservoirs are under half full** and all **ten areas in drought stayed in drought**. A surface that is finally wet says nothing about where the water table is. ## What the rain did — and didn't do The [Environment Agency's report for 28 August to 3 September 2026](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports/dry-weather-and-drought-in-england-28-august-to-3-september-2026), published on 4 September, is the first this summer to open with good news. Rainfall improved river flows, soil moisture deficits and environmental conditions across much of England. The late-month rain took August to **78% of the long-term average** nationally — although the South East managed only **54%**. **Rivers responded.** Flows increased at almost every site the agency reports on, though most are still normal or lower for the time of year: 25% below normal, 20% notably low and one site exceptionally low. A week earlier, 19% were exceptionally low. **Abstraction restrictions halved.** There are now **608 restrictions on abstraction licences**, down from 1,412 the week before, because flows in some catchments rose back above their triggers. The formal Section 57 restrictions on 302 licences in East Anglia remain until 31 October, and 1,123 voluntary restrictions remain across East Anglia, Thames, Wye and Severn. Then the other half of the report. **Groundwater fell at every site in England.** The seasonal recession continued regardless of the rain. Half of chalk sites are classed as normal for the time of year, including those in the South East and East, but sites in the Wessex Downs, Yorkshire Wolds and the North and South Downs are below normal or lower. Tilshead, in the Upper Hampshire Avon chalk, remains **exceptionally low**, as does Jackaments Bottom in the Cotswold oolite. Skirwith in the Eden Valley sandstone is below normal. **Reservoirs kept draining.** Storage for England was **58%** for the week ending 1 September, down another 1.8 percentage points and now **19.2% below average**. **Eleven reservoirs are less than half full.** All but one are classed below normal or lower, and **nine are classed exceptionally low** — the report's list runs Ardingly, Clatworthy, Clywedog, the Dove Group, Grafham and Rutland, Hanningfield, Haweswater and Thirlmere, and Wimbleball. Haweswater and Thirlmere are new to that list — both United Utilities reservoirs in the North West — while Blithfield has dropped off it. **Nothing changed on the map.** Ten Environment Agency areas remain in drought and four in prolonged dry weather. Temporary use bans remain in force at ten water companies. Wessex Water moved from drought level 1 to **level 1b**. South East Water applied on 28 August for a **drought permit on the River Ouse**; Southern Water's drought order application for the River Test had its public hearing on 25 August. The agency's line is that "further significant rainfall is needed to return to normal water resource conditions for many water companies". ## Why a wet week is the wrong week to test a soakaway We wrote [two weeks ago](/news/bre-365-soakaway-testing-drought-conditions/) about the risk of running BRE Digest 365 infiltration tests in the driest ground in living memory: the test result can flatter the design, and the winter water table — which the method assumes you know — is nowhere to be seen in an August pit. This week's report adds the opposite trap. After 40mm of rain, the top of the profile is wet, the test pit fills with less effort and the temptation is to treat the result as a wet-weather test. It isn't. The rain has wet the soil; it has not touched the aquifer. The Environment Agency's own numbers show groundwater still receding at every monitored site, which means a test carried out this week is still a test at or near the **lowest water table of the year**. Where BRE Digest 365 asks for evidence of the highest likely water table — lowland sites, land near a watercourse, anywhere on the chalk or limestone that the report lists as exceptionally low — that evidence still has to come from somewhere else: borehole records, published aquifer data or a monitoring period through the winter. There is also a smaller, practical point. A single wet week on baked ground produces uneven wetting — the agency has noted all summer that rain on hard ground runs off rather than soaking in. A pit dug into partially re-wetted soil can give a less repeatable result than either a dry-season or a genuinely wet-season test. BRE Digest 365's rule of three fills, taking the lowest, exists for exactly this reason. Record the antecedent rainfall with the test date, and if the three results diverge, treat that as information rather than noise. ## And on the water supply side For water efficiency and water neutrality evidence, the report changes nothing about the argument set out in our [last drought update](/news/drought-spreads-north-west-reservoirs-below-60-percent/): reservoirs at 58% and falling, two North West reservoirs now exceptionally low, and a growing list of suppliers in formal drought response. Demand has eased with the cooler weather but the agency notes it remains high for some companies, particularly in the South East. If you are preparing a Part G calculation or a water neutrality statement for a site in the Wessex, Solent, Kent, Thames, Anglian or — increasingly — United Utilities areas, the supplier's drought status this autumn is worth a line in the report. It is context a planning officer will recognise. ## Three things to do this month **Don't rebrand a September test as a wet-season test.** If the site needs a high-water-table check, it needs winter monitoring or third-party groundwater data, whatever the weather did last week. **Log antecedent conditions with every test.** Rainfall in the preceding fortnight and the Environment Agency area status on the day. It costs nothing and it answers the question a reviewer will ask in March. **Size with the lowest of three.** It is the method, and in a year with this much variability it is also the only defensible instinct. [SoakMonkey](/tools/soakaway-calculator/) runs BRE Digest 365 from your test results and gives you a printable design for the drainage strategy — it just needs the right test going in. *Sources: [Dry weather and drought in England: 28 August to 3 September 2026 (Environment Agency, GOV.UK, 4 September 2026)](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports/dry-weather-and-drought-in-england-28-august-to-3-september-2026) · [Dry weather and drought in England: 21 to 27 August 2026 (Environment Agency, GOV.UK)](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports/dry-weather-and-drought-in-england-21-to-27-august-2026) · [BRE Digest 365 — Soakaway design (2016)](https://bregroup.com/store/bookshop/soakaway-design-dg-365-2016-download)* --- # Three weeks left: BRE's Section 6 energy certification schemes close on 29 September URL: https://energycount.co.uk/news/bre-section-6-certification-schemes-close-29-september-2026/ Date: 2026-09-07 If you certify energy designs in Scotland under a BRE Global scheme — or you rely on someone who does — there is a hard date in the diary. **On 29 September 2026 BRE Global's Approved Certifier of Design (Section 6 – Energy) schemes close**, both the domestic and the non-domestic one. The announcement went to members on 2 April, so it is not news in itself, but the deadline is now just over three weeks away and the closure notice contains several traps for projects that straddle it. ## What BRE has said BRE's letter to members is short on ceremony: the decision is "driven by commercial considerations relating to scheme operations", the schemes "will remain in operation until 29 September 2026, after which they will close", no new applications are being accepted, and existing members "will not be able to certify projects after this time". Membership is not being transferred to another provider — BRE is "not entering into any agreements with another Scheme Provider" — so a renewal that falls before the date only buys membership up to 29 September, and one that falls after it simply lapses. BRE says it has been working with the Scottish Government's Building Standards Division, the Scottish Building Standards Hub and LABSS on processes to limit the disruption. ([BRE Global closure notice and FAQs, 2 April 2026](https://www.bre.co.uk/filelibrary/Scotland/acod/FAQ-BRE-ACoD-scheme-close-down.pdf)) Approved Bodies can keep producing Certificates of Design in the usual way right up to the closing date. The complications are for anything that isn't finished by then. ## Why it matters beyond the certifiers themselves Under the Building (Scotland) Act 2003 a Certificate of Design from an appointed scheme is confirmation that the design meets Section 6 (Energy) of the Building Regulations. LABSS puts it plainly: verifiers "must accept" a BRE or RIAS energy certificate, with its accompanying drawings and details, as complying with Section 6 ([LABSS, Certifiers of Design](https://www.labss.org/certification/certifiers-design)). That is why the route exists — a certified Section 6 design earns a discount on the building warrant fee and avoids the verifier re-checking the SAP or SBEM work. Take the certificate away and the full Section 6 design information goes to the local authority verifier for checking instead, with everything that does to programme. Only two schemes are appointed for Section 6 energy: BRE's and the RIAS scheme, which covers both domestic and non-domestic buildings. From 30 September, RIAS is the only one left. BRE's FAQs point departing members to the [Scottish Building Standards Certification portal](https://www.sbsc.uk.net/) for the alternative, and warn that membership criteria and application processes "may be different" — and that the other scheme may ask BRE whether an applicant left with any outstanding audit issues. ## Staged certificates: the trap The sharpest warning in the notice concerns **staged certification**. Where a parent certificate and early stage certificates have been produced under the BRE scheme, no further stage certificates can be generated after 29 September. A project certified at foundations and superstructure this summer cannot have its final design certified by BRE this autumn. BRE's advice is to think about this from the outset for anything that will span the date, and it lists three ways out: - take the project through a **non-certified route** — full Section 6 information to the verifier; - switch to a **single, traditional certificate** if you are confident the whole design can be certified before 29 September; or - certify the project, including the remaining stages, through **another scheme provider**. The consequences of leaving a staged project stranded are spelt out. Any discounted warrant fee received for the Section 6 element has to be repaid to the local authority, and the authority will not issue the warrant or stage approval until it has been. The complete final Section 6 design then goes to the verifier for checking, and the designer, applicant or agent has to make sure it complies before submitting it. Moving to the other scheme is possible but not a shortcut: the project and all its earlier stages would typically have to be registered afresh, with fees, and the new Approved Body has to satisfy itself that the earlier stages comply — it cannot certify only the final stage. ## Schedule 1 items, records and the members' area Three smaller points from the FAQs that are easy to miss: - **Schedule 1 items.** Where a certificate depends on items listed on a Schedule 1 form, a certificate produced before 29 September remains valid provided those items meet the specified performance. The certifier should complete a **Form Q** for the verifier, keep the evidence of compliance and give the verifier a copy. - **Records.** Certificates of Design and the evidence behind them must be kept for **at least seven years**. That obligation does not close with the scheme. - **The members' area shuts on 29 September.** BRE will not be able to provide project information after that date, so anything you need — certificates, registrations, audit correspondence — has to be extracted and saved before it. BRE closes with a reminder that compliance is the building owner's or developer's responsibility, and that a building warrant application must "clearly and fully" demonstrate compliance with the mandatory standards whether or not it is certified. For a Section 6 submission that means the SAP or SBEM calculation and its inputs — U-values, junction Ψ-values, air permeability, services — will be read by a verifier rather than signed off by a certifier. ## What to do in the next three weeks If you are a BRE Approved Body or certifier: list every live project, flag the staged ones, and decide for each whether it can be fully certified by the 29th, goes non-certified, or moves to RIAS. Download everything from the members' area now rather than on the last day. If you are a designer or developer using a BRE certifier: ask which of those three routes applies to your job, and budget time for a verifier check if it is the first. If you are a verifier: expect a run of Form Qs and a rise in non-certified Section 6 submissions from October. Whichever route a project takes, the calculation underneath it has to stand up on its own. ΨMonkey gives you calculated junction Ψ-values for the common details in your browser, so the fabric inputs in a Section 6 SAP submission are ones a verifier can trace rather than defaults. Want the whole Section 6 calculation done? [Send us the drawings](/contact/). *Sources: [Important Notice regarding the BRE Global Approved Certifier of Design Schemes — letter and FAQs, 2 April 2026 (BRE Global)](https://www.bre.co.uk/filelibrary/Scotland/acod/FAQ-BRE-ACoD-scheme-close-down.pdf) · [Latest ACD News and Updates (BRE Group)](https://tools.bregroup.com/acd/page.jsp?id=1165) · [Certifiers of Design (LABSS)](https://www.labss.org/certification/certifiers-design) · [Scottish Building Standards Certification portal](https://www.sbsc.uk.net/)* --- # The nature restoration levy now has a rulebook — and Parliament is waving it through URL: https://energycount.co.uk/news/nature-restoration-levy-guidance-parliament-approval/ Date: 2026-09-07 Three things happened in the first week of September that, between them, turn the Nature Restoration Fund from a policy into a procedure. On **1 September 2026** Defra, MHCLG and Natural England published the first user guidance on gov.uk — how a developer gets a levy quote, commits to pay, assumes liability and what happens if they don't. On **2 September** a Commons Delegated Legislation Committee agreed the draft **Nature Restoration Levy Regulations 2026**. On **3 September** the House of Lords approved them too, though not without attaching a regret motion about viability. [Back in June](/news/nature-restoration-fund-nutrient-regulations-laid/) we said laying the regulations was a step towards the fund rather than the switch being flicked. The switch is now wired up. It still hasn't been flicked — but we can now see exactly how it will work. ## What Parliament did The levy regulations are subject to the draft affirmative procedure, so both Houses have to approve them before they can be made. The **Fifth Delegated Legislation Committee** considered them on 2 September and agreed the motion without a division. In the Lords on 3 September, the government's approval motion passed — but only after peers voted **41 to 29** for Lord Roborough's amendment regretting that the regulations "will not require Natural England to take into consideration the financial viability of development", and in particular the affordable housing, education, health and infrastructure contributions already loaded onto the same schemes. A regret amendment doesn't stop an instrument; the motion as amended was agreed. As of 7 September the Commons had still to take the short formal approval motion that follows a committee's decision — the last step before the regulations can be signed and given a commencement date. ([Lords business, 3 September 2026](https://lordsbusiness.parliament.uk/ItemOfBusiness?itemOfBusinessId=175110§ionId=38&businessPaperDate=2026-09-03)) The regulations sit alongside the [Environmental Delivery Plans (Appropriate Prioritisation) Regulations 2026](https://www.legislation.gov.uk/uksi/2026/655/contents/made), which have been in force since 9 July. ## The process, step by step The useful document is [Using the nature restoration levy](https://www.gov.uk/guidance/using-the-nature-restoration-levy). Read together with the [overview guidance](https://www.gov.uk/guidance/understanding-the-nature-restoration-fund) and the [enforcement and appeals page](https://www.gov.uk/guidance/nature-restoration-levy-enforcement-actions-and-appeals-process), it gives a seven-stage process. None of it operates until an Environmental Delivery Plan (EDP) covering your site has been made and come into force. 1. **Get a quote.** Through an online "nature restoration levy service" — which the guidance says will be piloted on a test-and-learn basis — you enter the development's location and the number of units (dwellings), and the service prices it against the EDP's charging schedule. A quote commits you to nothing, reserves nothing, and you can run as many as you like. You won't get one at all if the development would exceed the EDP's remaining capacity; capacity figures are to be published on gov.uk "at regular intervals". 2. **Request to use the levy.** Before you submit the planning application, you ask Natural England to accept the development into the EDP. Natural England checks eligibility, capacity and completeness. If it accepts, it issues a **commitment certificate** and books the capacity against your scheme. 3. **Submit the certificate with the planning application.** The certificate tells the local planning authority that the impact covered by the EDP — for the first EDPs, nutrient pollution — can be disregarded for the relevant Habitats Regulations obligations. The LPA still has to deal with everything the EDP doesn't cover. **The commitment expires after six months** if no application has been made, and the capacity goes back into the pool; you can immediately re-quote and re-request if capacity is still there. 4. **A statutory pre-commencement condition.** Where permission is granted to a development using the levy, the regulations require a condition that the levy — in full, or the first instalment — is paid before work starts. EDPs may also carry their own conditions that the decision-maker is expected to apply. 5. **Assume liability.** After permission and before commencement, somebody has to file an assumption of liability notice: usually the committed developer, but it can be anyone, and more than one person, jointly and severally. Natural England then issues a **liability notice** setting the amount, adjusted for inflation since the charging schedule came into force using the RICS CIL index, and the payment dates. Liability can be transferred with the new party's consent; overdue sums stay with whoever was liable when they fell due. If nobody assumes liability before work starts, Natural England **imposes** it on the owners of material interests in the land — the freehold, or a lease with more than seven years to run — apportioned by value, plus a surcharge. 6. **Pay.** An invoice follows the liability notice. Payment in full, or the first instalment where Natural England has published an instalment plan for that EDP, is what discharges the pre-commencement condition; Natural England tells the planning authority once it has the money. Later instalments are inflation-adjusted again at the point of payment. The liability is registered as a **local land charge** until the last instalment clears. 7. **Cancel, if it doesn't happen.** A commitment can be cancelled — and the capacity released — where no application is made within six months, permission is refused and not appealed, the permission lapses or is quashed, the EDP is revoked, or you simply decide not to proceed. Cancel it and you can no longer rely on the levy; the Habitats Regulations obligations come straight back. ## The small print that bites The enforcement page is where the regulations show their teeth, and it is worth knowing the numbers before anyone on site assumes the levy is a soft obligation. | Failure | Consequence | | --- | --- | | Nobody assumes liability before commencement | Liability imposed on landowners, plus a surcharge of **2% of the levy or £300**, whichever is greater | | Payment 30 days overdue | **5%** surcharge (minimum £300; none if the levy is under £1,000) | | Still unpaid at 6 months / 12 months | Further **5%** at each point, on the principal | | Material change to the development not notified | Surcharge of £300 or 30% of the levy, whichever is lower (only where the recalculated levy is at least £300 higher) | | Any overdue amount | Interest at **2.5 percentage points above Bank of England base rate**, simple not compound | | Development under way with an overdue amount | Warning notice (3–28 days), then a **stop notice** requiring work to cease until paid | | Persistent non-payment | Injunctions, enforcement of the land charge, debt recovery | There is a review route — 28 days from the liability notice to challenge the calculation or the apportionment, with Natural England answering within 21 days — and an appeal to the Environment Secretary on calculations, apportionment, surcharges, interest and warning or stop notices. Nothing falls due while a review or appeal is open. ## What ministers said, and why it matters Matthew Pennycook's answers to the Commons committee fill in gaps the guidance leaves ([Hansard, 2 September 2026](https://hansard.parliament.uk/Commons/2026-09-02/debates/6b6c12d8-d1a6-4db5-b382-8b6e1f34c7dd/DraftNatureRestorationLevyRegulations2026)): - **Your rate is locked when Natural England accepts your commitment.** If the charging schedule later goes up, "the new rate will take effect only for future requests made to the EDP." That makes the timing of the request, and the six-month expiry that follows it, a commercial decision rather than an administrative one. - **Full cost recovery, ringfenced.** Charging schedules will be "based primarily on the cost of delivering the conservation measures", consulted on as part of each EDP, and levy receipts can only be spent on that EDP's measures and its administration. Natural England will decide whether to charge per dwelling or per hectare. - **No payments in kind.** Offering a wetland site instead of cash is not something the regulations provide for. - **Voluntary, in almost all cases.** Developers keep the option of the existing routes if the levy doesn't "represent good value for money"; mandatory EDPs are reserved for exceptional, bespoke cases. - **The first EDP consultation is close.** "In the very near future we will launch the consultation on the first EDP, which is on nutrient neutrality." The Defra Secretary of State will decide whether to make it, and the government has committed to report back to Parliament on the first nutrient EDPs before any other kind is made — great crested newts are next in the queue. In the Lords, Baroness Taylor confirmed that instalments will exist only where Natural England publishes an instalment plan for an EDP, and that the first instalment must be paid before commencement. Baroness Coffey told the House the first EDP would be for nutrient neutrality in **Norfolk** — the minister did not confirm the catchment, though the Broads and River Wensum are on Natural England's published list of sixteen candidates. Two other contributions are worth logging: Baroness Parminter said a Wildlife Trust and local authority scheme in the Solent that had enabled around 2,500 homes had "effectively stopped" since EDPs were announced, and Lord Fuller put the going rate for phosphate credits at around £5,000 per dwelling. That second number is the one every levy charging schedule will be compared against. ## What hasn't changed Nothing is live. Until an EDP for your catchment has been consulted on, approved and brought into force, nutrient neutrality applies exactly as it does today: a nutrient budget, mitigation secured, and a Habitats Regulations assessment the planning authority can rely on. The implementation plan is clear that third-party credit schemes can carry on selling credits and bespoke mitigation remains available; Natural England's own Nutrient Mitigation Scheme will fold into the relevant EDP when it arrives. The local routes we have written about — [Stour Environmental Credits in Kent](/news/stour-nutrient-credits-stodmarsh/), for instance — remain the way consents are actually moving this autumn. ## What to do now Three practical things. First, run the [nutrient budget](/services/nutrient-neutrality-assessment/) regardless: the levy quote will be priced on dwellings and location, but whether it is good value only becomes clear when you know what your scheme's nitrogen or phosphorus load would cost to mitigate the current way. Second, if you are in a catchment where a draft EDP appears, read the charging schedule and the capacity figure together — a low rate with thin capacity is a race, and the six-month commitment window means you don't want to request before the application is genuinely ready. Third, brief whoever runs the site: the levy is a pre-commencement condition backed by stop notices, surcharges and a land charge, not a contribution to be tidied up later. NutrientMonkey gives you the budget for any of the affected catchments in your browser, so you have your own number before the first charging schedule lands. If you'd rather we ran it and produced the planning report, [send us the site details](/contact/). *Sources: [Understanding the Nature Restoration Fund (gov.uk, 1 September 2026)](https://www.gov.uk/guidance/understanding-the-nature-restoration-fund) · [Using the nature restoration levy (gov.uk, 1 September 2026)](https://www.gov.uk/guidance/using-the-nature-restoration-levy) · [Nature restoration levy enforcement actions and appeals process (gov.uk, 1 September 2026)](https://www.gov.uk/guidance/nature-restoration-levy-enforcement-actions-and-appeals-process) · [Environmental Delivery Plans collection (gov.uk)](https://www.gov.uk/government/collections/environmental-delivery-plans) · [Draft Nature Restoration Levy Regulations 2026 — Fifth Delegated Legislation Committee, Hansard, 2 September 2026](https://hansard.parliament.uk/Commons/2026-09-02/debates/6b6c12d8-d1a6-4db5-b382-8b6e1f34c7dd/DraftNatureRestorationLevyRegulations2026) · [Nature Restoration Levy Regulations 2026 — House of Lords business, 3 September 2026](https://lordsbusiness.parliament.uk/ItemOfBusiness?itemOfBusinessId=175110§ionId=38&businessPaperDate=2026-09-03) · [Nature Restoration Levy Regulations 2026 — UK Parliament SI tracker](https://statutoryinstruments.parliament.uk/instrument/puKlIVNF) · [Implementing the Nature Restoration Fund (Defra/MHCLG/Natural England, December 2025)](https://www.gov.uk/government/publications/nature-restoration-fund-implementation-plan/implementing-the-nature-restoration-fund)* --- # SAP 10.3: what actually changed, and which jobs it applies to URL: https://energycount.co.uk/news/sap-10-3-what-changed/ Date: 2026-09-04 > **Quick answer:** SAP 10.3 is the approved methodology for the **Future Homes Standard** — schemes notified from **24 March 2027**. Anything notified before then, and started before 24 March 2028, stays under Part L 2021 and **may continue in SAP 10.2**. The change that matters is the grid electricity factor pair: carbon **0.136 → 0.086 kgCO₂e/kWh** and primary energy **1.501 → 1.969**. Gas barely moves (0.210 → 0.214; 1.130 → 1.12). Electric heating gets a stronger carbon case and a weaker primary-energy one; a heat pump still wins both by a distance, a panel heater now loses the primary-energy test badly. ## Which version applies to which job The government's [Notice of Approval of 24 March 2026](https://www.gov.uk/government/publications/methodologies-for-calculating-the-energy-performance-of-new-buildings-in-england-notice-of-approval-24-march-2026) approves "the Government's Standard Assessment Procedure (SAP) for the Energy Rating of Dwellings, 10.3 edition" for new dwellings in England, superseding the June 2025 notice that approved 10.2. It takes effect on 24 March 2026 — but it carries the same transitional clause as the Future Homes Standard itself: > This approval does not apply in relation to building work on a particular building, where a building notice or an initial notice has been given to, or full plans have been deposited with, a local authority, in respect of that building, before 24 March 2027, provided that the building work on that building is started before 24 March 2028. In such cases, the earlier versions of the methodologies may continue to be used. So the practical rule is the one you already use for the edition of Part L: a building notified before 24 March 2027 and commenced before 24 March 2028 is a [Part L 2021](/guides/building-regulations-part-l/) building and is assessed in SAP 10.2; a building under [Approved Document L 2026](/guides/future-homes-standard/) is assessed in SAP 10.3 — or, once it is approved, the Home Energy Model. For higher-risk buildings the cut-off is a valid Gateway 2 application before 24 September 2027. Protection is per building, not per site: the plot that starts late on a phased scheme moves to the new edition and the new SAP on its own. The [edition checker](/guides/building-regulations-part-l/#part-l-checker) works this out from three dates. ## The fuel factors, from Table 12 These are read from the SAP 10.3 specification (version dated 12 June 2026), Table 12, set against the SAP 10.2 values from BRE's technical paper on the factors. Both editions quote carbon as CO₂ **equivalent** — methane and nitrous oxide included. | Fuel factor | SAP 10.2 — Part L 2021 work | SAP 10.3 — Future Homes Standard | | --- | --- | --- | | Grid electricity, CO₂e | 0.136 kgCO₂e/kWh (annual average of monthly factors) | **0.086 kgCO₂e/kWh** | | Grid electricity, primary energy | 1.501 | **1.969** | | Mains gas, CO₂e | 0.210 kgCO₂e/kWh | 0.214 kgCO₂e/kWh | | Mains gas, primary energy | 1.130 | 1.12 | Two details from the small print. The 10.3 electricity figure is a **forward-looking** average — the grid the dwelling will run on, not last year's — and it is the same for every tariff. And the monthly variation SAP 10.2 introduced has gone: the footnotes to Table 12 say that in SAP 10.3 "the average figure given in this table is used directly in the SAP worksheet", with the monthly tables kept "for comparison purposes only". One number, applied flat. ## What the arithmetic does Divide the factors by the efficiency of the heating and you get the cost of a kilowatt-hour of **heat** under each version — which is what the carbon and primary energy targets are built from. | Per kWh of heat delivered | SAP 10.2 carbon | SAP 10.3 carbon | SAP 10.2 primary energy | SAP 10.3 primary energy | | --- | --- | --- | --- | --- | | Gas boiler at 89.5% (the 2021 notional) | 0.235 | 0.239 | 1.26 | 1.25 | | Heat pump at SCOP 2.5 (the FHS notional) | 0.054 | 0.034 | 0.60 | 0.79 | | Heat pump at SCOP 3.0 | 0.045 | 0.029 | 0.50 | 0.66 | | Direct electric (panel heaters, electric boiler) | 0.136 | 0.086 | 1.50 | 1.97 | kgCO₂e per kWh of heat, and kWh of primary energy per kWh of heat. On **carbon**, everything electric gets better: a heat pump at SCOP 3 delivers heat at an eighth of a gas boiler's emissions, and even a panel heater comes in at a third. That is the [DER against TER](/guides/der-ter/) test, and it moves in electricity's favour. On **primary energy** — the [DPER against TPER](/guides/der-ter/) test, the headline test since 2021 — it goes the other way. Every kilowatt-hour of electricity now costs 1.969 kWh of primary energy instead of 1.501, a rise of 31%. A heat pump still beats the boiler comfortably (0.66 against 1.25 at SCOP 3), but the margin is narrower than it was, and **direct electric heating now costs more primary energy than gas** (1.97 against 1.25) where under 10.2 it was already the wrong side of the line. A design that leaned on resistance heating and a lot of PV to scrape a 10.2 pass should not assume the same trick works in 10.3. The same 31% applies to every other kilowatt-hour of electricity in the calculation — lighting, fans, pumps, MVHR — which is why the notional dwelling's fixed lighting jumps from 80 to 120 lumens per circuit-watt. PV pulls less weight on carbon, because each kilowatt-hour it generates displaces 0.086 kg rather than 0.136 kg. That is not an accident: under the Future Homes Standard PV is a legal requirement in its own right ([requirement L3](/tools/solar-pv-calculator/)), and the notional dwelling no longer carries a fixed array of 40% of the ground-floor area ÷ 6.5. It has the **same PV as the actual dwelling, up to 40% of the ground-floor area ÷ 4.5** (Table R1) — so below that cap your array moves the target with it, and PV no longer buys headroom against the TER and TPER. ## The notional dwelling in SAP 10.3 Appendix R of the specification defines the notional dwelling the Future Homes Standard targets come from — a single specification, with the Home Energy Model version differing only in how it sizes the notional heat pump. The fabric is the Part L 2021 notional's: walls 0.18, floors 0.13, roofs 0.11, windows 1.2, doors 1.0, rooflights 1.7 W/m²K. What changes is everything around it: air permeability **4** m³/(h·m²) with decentralised mechanical extract, a **heat pump at a SCOP of 2.5** in place of the gas boiler, waste water heat recovery on every shower except in single-storey dwellings, lighting at 120 lm/W, and the larger PV array above. The full set, dated and sourced, is in the [building regulations reference](/reference/#sap). Table R2 of the same appendix lists the junction Ψ-values the notional dwelling assumes — 0.05 for lintels, sills and jambs, 0.16 for a ground floor, 0.06 for eaves — and Appendix K's Table K1 lists the defaults a junction gets when nothing is calculated: 1.00 for a lintel, 0.32 for a ground floor. With a tighter air permeability and a heat pump in the notional, there is less slack elsewhere in the calculation, and the gap between the default and a calculated value is often the pass. The [reference page's Ψ table](/reference/#thermal-bridging) now shows default, notional and calculated side by side for the common junctions. ## What to do with this If a scheme is already notified, check the commencement date against 24 March 2028 and carry on in SAP 10.2; nothing about SAP 10.3 reaches back into it. If a scheme will be notified from 24 March 2027, run the design-stage SAP in 10.3 from the start rather than converting a 10.2 model later — the notional, the factors and the PV rule all change together, and a design that passed in 10.2 is not evidence that it passes in 10.3. Either way, the levers are the ones they have always been: fabric, airtightness, [calculated junction Ψ-values](/tools/psi-calculator/) instead of defaults, and a heat pump that is sized and controlled to reach its SCOP. *Sources: [SAP 10.3 specification, version 12-06-2026 (BRE)](https://bregroup.com/documents/d/bre-group/sap-10-3-full-specification-2026-06-12) — Table 12, Appendix K Table K1, Appendix R Tables R1 and R2 · [Notice of Approval of the methodologies of calculation of the energy performance of new buildings in England, 24 March 2026 (MHCLG)](https://www.gov.uk/government/publications/methodologies-for-calculating-the-energy-performance-of-new-buildings-in-england-notice-of-approval-24-march-2026) · [SAP 10 Technical Paper S10TP-15: CO₂ and primary energy factors (BRE)](https://bregroup.com/documents/d/bre-group/s10tp-15_-_co2_and_primary_energy_factors_for_sap_v1_1_10_1-pdf) · [Future Homes and Buildings Standards: Building Circular 01/2026 (MHCLG)](https://www.gov.uk/government/publications/the-future-homes-and-buildings-standards-building-circular-012026)* --- # Four weeks from the levy, nine in ten SME builders say sites will stop stacking up URL: https://energycount.co.uk/news/hbf-sme-building-safety-levy-viability-research/ Date: 2026-09-03 The [Building Safety Levy starts on 1 October](/news/building-safety-levy-1-october-2026/). Four weeks out, the industry has put a number on how it expects that to land. Research published on **18 August 2026** by the **Home Builders Federation** and **Quantum Development Finance**, based on a survey of SME house builders across England, found that **91% believe the levy will make developments financially unviable**, and that **36% have already delayed, redesigned or cancelled schemes** in anticipation of it. HBF is asking the government to pause the levy's introduction pending a full assessment. Survey findings are advocacy as well as evidence, and it's worth reading them as such. But the underlying arithmetic is checkable, and for anyone pricing a scheme this autumn it's the arithmetic that matters. ## What the survey found Beyond the headline figure: - **69%** of SME builders said the levy would make them **less likely to invest** in new development opportunities; a further 9% said it was too early to say. - The effect is most pronounced in **London**, where **86.7%** said it would make them less likely to invest — notable given the capital delivered 32,680 homes against a standard-method need of around 85,000 a year. - Elsewhere: **West Midlands 85.7%**, **South West 82%**, **East Midlands 78.9%**, **Yorkshire 76.9%**. Two structural complaints sit underneath the numbers. First, **cash flow**: SME builders pay the levy when the **first home on a site completes**, rather than as sales come through — which puts the charge at one of the tightest points in a project's lifecycle. Second, **scope**: no exemption was made for medium-sized developments, so builders who have never built a high-rise are contributing to remediating high-rise defects. HBF's argument for a pause rests on the money already in the system: the industry has committed around **£6 billion** towards historic building safety costs, and more than **£2.5 billion of the existing £5.1 billion Building Safety Fund remains unallocated**. The government's position, unchanged, is that the levy is needed to fund remediation at the scale required and that it has been through consultation and impact assessment. ## Where the levy actually sits in the cost stack This is the part worth knowing regardless of what you make of the campaign. HBF's **Viability Crunch** report (published May 2026, updated July) breaks down what has been added to the cost of building a typical home over five years. Its total is **£76,000** — around **20% of an average new home value of £365,000**. The components, as HBF estimates them, per unit: | Cost line | HBF estimate per home | | --- | --- | | Material and labour inflation | £37,000 | | **Future Homes Standard** (incl. 2021 Part L uplift) | **£10,200** | | Other Building Regulations changes | £7,770 | | Nutrient neutrality mitigation | £7,000 | | Biodiversity Net Gain | £5,700 | | **Building Safety Levy** | **£2,320** | | Landfill Tax | £2,000 | | Taxes (NI, Corporation Tax, RPDT) | £2,055 | | Section 106 inflation | £985 | For high-rise, HBF adds a further **£22,272** per unit — of which **up to £22,000** is attributed to the second staircase requirement taking effect on 30 September 2026 and **£272** to Building Safety Regulator gateway application costs. So the levy is one of the **smaller** lines. On HBF's own figures it is roughly a fifth the size of the Future Homes Standard line and under a third of the "other Building Regulations" line. It's also worth noting the levy is charged per square metre of **gross** residential floorspace, so on apartment schemes it captures circulation and other unsaleable space — which is why HBF puts the London apartment figure nearer **£3,000** per unit. ## The line that should interest assessors The single largest **regulatory** item in that table is the one we work on every day. HBF attributes **£10,200 per home** to the 2021 Part L uplift and the Future Homes Standard together, driven principally by the solar PV requirement, which HBF describes as panels covering an area equivalent to **40% of ground floor area**. HBF's own analysis suggests around **60% of homes may not practically achieve that coverage**, which is a claim worth treating as a starting point for debate rather than a settled fact, but it points at something real: where the roof cannot carry the assumed array, the deficit has to be made up elsewhere in the calculation. The rest of that table breaks down as follows, on HBF's numbers: **Part G** water efficiency changes at £3,100, **Part M(1)** accessibility at £1,800, **Part S** EV charge points at £870, and **Part R** gigabit infrastructure within a £2,000 cap. None of these are arguments against the standards. But they do change where the effort should go. If FHS compliance is the largest single regulatory cost on a plot, then the accuracy of the inputs feeding that calculation stops being a technical nicety and starts being a commercial question. Default thermal bridging values, unmodelled junctions and conservative U-value assumptions all push the required specification upwards — more insulation, more glazing spend, more PV — to close a gap that may not exist in the built form. ## What to do in the next four weeks **Confirm which side of 1 October each live scheme sits on.** The levy applies to applications for building control approval submitted on or after that date. Applications made before it are outside the charge, even if varied afterwards — but a pre-October application that is rejected and resubmitted becomes liable. **Pull your local authority's rate.** Rates are set per authority and weighted by local house prices, ranging from around £13/m² to over £100/m². The brownfield discount is 50%. On a scheme of any size, the difference between authorities is material. **Check the reliefs before assuming liability.** Social and supported housing, and all works by non-profit registered providers, are outside the charge entirely. **Don't rush an incomplete submission to catch a date.** A thin application invites questions, and questions cost more time than the levy costs money. If a scheme is going in before 1 October, the compliance evidence needs to be ready to go with it. ## The bigger point The cost stack is real, and the industry is right that it has been assembled department by department without anyone adding it up. The response available to a developer, though, isn't to argue individual lines down — it's to make sure the ones that are calculated rather than fixed are calculated properly. **ΨMonkey** models thermal bridging PSI values to BR 497 and BRE IP 1/06, so the junctions in your SAP assessment carry their real heat loss rather than the pessimistic defaults. On an FHS plot, the difference between calculated and default y-values routinely moves the specification — and therefore the build cost — by more than the Building Safety Levy will. [Try ΨMonkey →](/tools/psi-calculator/) And if you'd like the Part L, Part G and thermal bridging package for a scheme sorted before it goes to Building Control this month, [get in touch](/contact/). *Sources: [Nine in ten SME house builders warn Building Safety Levy will make even more sites unviable — Home Builders Federation, 18 August 2026](https://www.hbf.co.uk/news/nine-in-ten-sme-house-builders-warn-building-safety-levy-will-make-even-more-sites-unviable/) · [The Viability Crunch — Home Builders Federation, May 2026](https://www.hbf.co.uk/research-insight/viability-crunch/) · [Building Safety Levy: Guidance, GOV.UK](https://www.gov.uk/guidance/building-safety-levy-guidance) · [The Building Safety Levy (England) Regulations 2025, legislation.gov.uk](https://www.legislation.gov.uk/uksi/2025/1236/contents/made)* --- # Nine in ten planning departments are short-staffed URL: https://energycount.co.uk/news/planning-department-staffing-gap-2026/ Date: 2026-09-02 The gap between what the planning system is being asked to do and what it is staffed to do got measurably wider this year. The Home Builders Federation's [**Planning on Empty**](https://www.hbf.co.uk/research-insight/planning-on-empty-2026) report, published in August 2026 with Paragon Bank, is based on a Freedom of Information survey of councils across England and Wales covering the 2025/26 financial year. It asked a narrow, checkable question — how many planning staff do you have, and how many do you say you need — and the answers are worse than last year's. ## The numbers **90% of local planning authorities are operating below full staffing capacity.** A year ago that figure was 80%. **The average department runs at 80% strength.** The typical planning department in England and Wales has just under 36 staff. On its own assessment it needs 44. **Nearly half are worse than that.** Of the councils below capacity, 44% are at 80% or less. In 2025 it was under a quarter. **The national shortfall is 2,660 planners** — up 20% on last year's 2,200. That works out at an average of 8.4 additional planners per authority. The government's commitment is 1,400 additional planning officers across this Parliament, or roughly four per authority. **Recruitment isn't closing it.** A third of authorities have no vacancies advertised at all, and 45% have fewer active vacancies than they would need to reach full capacity — which the report reads, reasonably, as budget constraint rather than contentment. Planning officers account for 28% of vacancies and team leads or principal planners another 21%. Average turnover is 11%, and 30% of councils are at 15% or higher. **Agency staff are filling the gap expensively.** 55% of responding councils use agency workers for planning services, at an average spend of nearly £200,000 per council across all respondents. Charnwood spent over £1.7 million; North Hertfordshire £1.2 million. ## What it does to a programme Two figures in the report do more work than the staffing numbers themselves. **Only around 20% of major applications — 10 or more units — are decided within the statutory 13 weeks.** Not 20% late. 20% on time. **Section 106 agreements take an average of 515 days.** That is HBF's own earlier research, and it sits alongside £9 billion in Section 106 contributions currently held unspent by local authorities, partly because councils lack the staff to monitor and allocate them. We covered the government's proposed [standard s106 templates for 10–49 unit sites](/news/standard-s106-templates-medium-sites-consultation/) last week; this is the resourcing problem those templates are trying to route around. The report is blunt about where the delay accumulates: "Delays to planning decisions, Section 106 negotiations and discharge of conditions are becoming entrenched across the development process." ## Why this lands on technical evidence in particular The report identifies the thing that has changed underneath the staffing numbers: the job got bigger. It lists Biodiversity Net Gain, nutrient neutrality, flood risk and building safety as duties added at the planning stage in recent years, on top of the [August 2026 NPPF](/news/nppf-2026-clean-energy-and-water-w1-w4/), the Planning and Infrastructure Act, local government reorganisation and a new 30-month timetable for local plans. Each of those arrives as a document someone in a planning department has to read and form a view on. When the department is at 80% strength and a fifth of its vacancies are for the senior officers who would normally handle the technical ones, the queue for that reading gets longer. There is a specific consequence for anyone producing drainage, water efficiency, nutrient or energy evidence. The August 2026 Framework already moved a lot of that material forward — [drainage evidence into the validation list under policy DM2](/news/nppf-2026-dm2-validation-lists-suds-statement/), rather than into a condition to be discharged later. That change was designed to front-load evidence. In a fully staffed authority it front-loads scrutiny too. In an understaffed one, it front-loads the queue. The result is that a weak or incomplete technical submission now costs more than it used to. A request for further information does not go to the back of a short queue. It goes to the back of a long one, staffed in part by agency officers who may not know the catchment, the local plan or the site. ## Four things worth doing differently **Submit complete, not nearly complete.** The single cheapest thing you can do in an understaffed system is not to generate a second round of correspondence. Check your submission against the authority's own validation list, item by item, before it goes in — including the items you think are disproportionate. Arguing about a validation requirement costs weeks; supplying it costs an afternoon. **Make the technical documents readable by a generalist.** The officer reading your infiltration testing or your water efficiency calculation may not be a drainage engineer or an energy assessor. State the method, state the standard you are working to, state the result, and put the workings in an appendix. A drainage strategy that answers the question in its first page gets determined faster than one that requires the reader to reconstruct it. **Assume conditions take longer to discharge than they used to.** If your programme has a condition being discharged in six weeks because that is what the condition says, build in the real number. Discharge of conditions is named in the report as one of the places delay has become entrenched, and it is the stage with the least political visibility, so it is the stage that slips. **Do the evidence that can be done early, early.** Infiltration testing depends on ground conditions and season. Nutrient budgets depend on credit availability. Water efficiency depends on which optional requirement applies. None of these get faster because you left them late, and all of them can stall a determination that is otherwise ready. ## The wider point The report's recommendations are aimed at government: long-term funding for planning teams, a resourcing assessment attached to every new policy requirement, and annual public reporting of LPA capacity. Those are sensible and slow. What is not slow is the adjustment applicants can make themselves. Planning reform has moved a great deal of evidence from the condition stage to the application stage. The authorities receiving that evidence are, on their own account, a fifth short of the staff they need to assess it. The practical response is to send them something that is easy to say yes to. ## Get the drainage evidence right before it goes in **SoakMonkey** runs a BRE Digest 365 soakaway calculation in your browser — the standard method, your own infiltration test data, and a clear result you can put in front of a planning officer or Building Control without further explanation. [Try SoakMonkey →](/tools/soakaway-calculator/) If you have a scheme where the drainage, water efficiency or nutrient evidence needs to stand up at validation rather than at condition stage, [get in touch](/contact/) and we'll tell you what we'd want to see before it's submitted. *Sources: [Home Builders Federation — Planning on Empty, August 2026](https://www.hbf.co.uk/research-insight/planning-on-empty-2026) · [HBF — Planning staff shortages worsen despite Government reforms, further delaying housing delivery, 28 August 2026](https://www.hbf.co.uk/news/planning-staff-shortages-worsen-despite-government-reforms-further-delaying-housing-delivery/)* --- # The second staircase deadline is four weeks away URL: https://energycount.co.uk/news/second-staircase-deadline-30-september-2026/ Date: 2026-09-02 There is a Building Control deadline on 30 September 2026 and another on 1 October. They are unrelated, they were set by different parts of government, and they land within twenty-four hours of each other. If you have a residential scheme over 18m in the pipeline, both of them apply to you. This piece is about the first one. ## What changes on 30 September The **2026 Approved Document B amendment booklet**, issued with [MHCLG Circular 04/2024](https://www.gov.uk/government/publications/amendments-to-approved-document-b-fire-safety-circular-and-letter/approved-document-b-fire-safety-new-updates-to-support-enhanced-fire-safety), comes into force on 30 September 2026. Its content is short: second staircase provisions in residential buildings over 18m in height. From that date, the guidance in Approved Document B expects new residential buildings at or above 18m to be served by two staircases. The policy has been settled since 2024 — the height threshold was lowered from the originally proposed 30m following consultation — and the long lead-in was deliberate. That lead-in is now nearly over. Note what the amendment booklet is amending: Approved Document B 2019, incorporating the 2020, 2022 and 2025 amendments. If you are working from a version of ADB and are not sure which one, that is the question to resolve this month. ## The transitional route, precisely The old guidance continues to apply — but only on two conditions, and both have to be met. **Condition one: the submission.** A building notice or an initial notice must have been given to, or a building control approval application with full plans made to, the relevant authority **before 30 September 2026**. Those three terms have the meanings given in the Building Regulations 2010. This is a date on a submission, not a date on a design freeze or a planning permission. **Condition two: sufficiently progressed.** The work must have started and be sufficiently progressed either before 30 September 2026, or within the period of 18 months beginning on that day — so by the end of March 2028. The circular defines "sufficiently progressed" tightly, and it is worth quoting the substance rather than paraphrasing it: - **New building:** when the pouring of concrete for the permanent placement of the trench, pad or raft foundations has started, or the permanent placement of piling has started. - **Work to an existing building:** when that work has started. - **Material change of use:** when work to effect that change of use has started. That is the same test used elsewhere in the building safety transitional arrangements, so it should be familiar. What it means in practice is that a submission alone does not preserve your position indefinitely. You get eighteen months to get concrete or piling in the ground. An 18-month window sounds generous. Against a scheme that still needs its Gateway 2 approval, its discharge of conditions and its enabling works, it is not necessarily generous at all. ## The other deadline, one day later The [Building Safety Levy starts on 1 October 2026](/news/building-safety-levy-1-october-2026/). It applies to building control applications and initial notices submitted **on or after** that date, for major residential development — broadly 10 or more dwellings, or 30 or more purpose-built student bedspaces — and is charged per square metre of residential floorspace created. So for a residential scheme over 18m at 10 units or more, the same submission date does two things at once. Made before 30 September, it opens the second staircase transitional route and falls outside the levy. Made on or after 1 October, it does neither. That is not a reason to rush an incomplete application. A thin submission made to catch a date is a submission that comes back with questions, and the questions do not stop the clock on the 18-month progression test. But it is a reason to know, this week, which side of the line each of your live schemes sits on. ## What to check before the end of September **Confirm the submission date on record, not the one in your programme.** For each live 18m+ residential scheme, establish whether a building notice, initial notice or full plans application has actually been made — and get the date confirmed by the building control body, not from an internal tracker. **Check the height measurement.** 18m is a threshold, and schemes sit close to it. If your scheme is near the line, resolve how the height is being measured under Approved Document B before you rely on being under it. **Work backwards from March 2028.** If you are relying on the transitional route, put the foundation or piling milestone on the programme as a hard date and identify what could move it. Gateway 2 timescales, discharge of conditions and ground remediation are the usual candidates. **Don't assume a redesign is a non-material amendment.** Adding a second core generally changes unit numbers and mix. That is normally a Section 73 variation rather than a non-material amendment, which means a fresh round of consultation and, in London, potential referral to the Mayor — and it reopens the scheme to whatever the current policy position is, not the one you were consented under. **Sort the compliance evidence that will be needed anyway.** Whichever side of the date a scheme falls, a complete Building Control submission is the thing that makes the rest of the programme predictable. The fabric, Part L and ventilation evidence is part of that package, and it is the part that can be prepared in advance rather than discovered late. ## The point The second staircase requirement is not new information. What is new is that the exit route from it closes this month, and the exit route is defined by an administrative act — a submission made to a building control body before a specific day — rather than by anything about the design. Four weeks is enough time to establish where every live scheme stands. It is not enough time to fix a scheme you discover is on the wrong side of the line on 1 October. If you have a higher-risk residential scheme where the Part L, SAP and fabric evidence needs to be complete and defensible before it goes to Building Control, [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [GOV.UK — Approved Document B (Fire Safety): new updates to support enhanced fire safety, MHCLG Circular 04/2024, 2 September 2024](https://www.gov.uk/government/publications/amendments-to-approved-document-b-fire-safety-circular-and-letter/approved-document-b-fire-safety-new-updates-to-support-enhanced-fire-safety) · [GOV.UK — Fire safety: Approved Document B](https://www.gov.uk/government/publications/fire-safety-approved-document-b) · [GOV.UK — Consultation response: Amendments to Approved Document B, second staircases](https://www.gov.uk/government/consultations/sprinklers-in-care-homes-removal-of-national-classes-and-staircases-in-residential-buildings/outcome/consultation-response-amendments-to-approved-document-b-second-staircases)* --- # Rain fell. Flood alerts went out. The drought didn't lift. URL: https://energycount.co.uk/news/drought-spreads-north-west-reservoirs-below-60-percent/ Date: 2026-09-01 If you watched the last week of August from a window, you would have concluded the drought was over. Low pressure moved in, rain fell across most of England, and on 27 August the Environment Agency issued **more than 40 flood alerts**. The [Environment Agency's report for 21 to 27 August 2026](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports/dry-weather-and-drought-in-england-21-to-27-august-2026), published on 28 August, says otherwise. Nothing recovered. Several things got worse. ## What the numbers actually did **Reservoir storage fell below 60%.** England's storage was **59.8% on 25 August**, down 2.7 percentage points in a single week and **18.2% below average** for the time of year. **Nine reservoirs** are now classed as having exceptionally low storage — two more than the week before. Clatworthy, Clywedog, the Dove Group, Blithfield, Hanningfield, Ardingly, Grafham, Rutland and Wimbleball are all on that list. **Groundwater kept falling.** The seasonal recession continued **at every site in England**. Tilshead, in the Upper Hampshire Avon chalk, is now classed as exceptionally low; Jackaments Bottom in the Cotswolds remains so. **Rivers didn't recover.** Flows fell at 58% of monitored sites. Only 7% are normal for the time of year. Nineteen per cent are exceptionally low and a further 39% notably low. **The rain was regionally lopsided.** For England as a whole, August rainfall to the 25th was **34% of the long-term average**. In south-east England it was **15%**. Weekly totals ranged from 6mm in the South East and South West to 23mm in the North East. The Environment Agency also makes a point worth holding onto: after months of dry weather, **rainfall on hard ground is less likely to soak in**, which raises run-off. That is why flood alerts and a drought can share a day. ## Two suppliers moved — and one of them is northern **Bristol Water** announced a temporary use ban on 26 August, in force from 27 August, following an Environment Agency drought order application to protect flows on the River Severn. Around **30 million people** in England now have restrictions on their water use. More significant for anyone working outside the usual water-stressed belt: **United Utilities' Strategic Resource Zone entered drought level 1** on 26 August, after Haweswater reservoir storage fell below the trigger. Its Carlisle zone moved from drought level 1 to enhanced monitoring. Formal Environment Agency drought status still covers 71% of England by land and did not change last week. But four areas — the North East, Yorkshire, Cumbria and Lancashire, and Greater Manchester, Merseyside and Cheshire — remain in **prolonged dry weather**, the stage before drought. Abstraction is being squeezed accordingly. There are **1,412 restrictions on abstraction licences** (a small improvement on the previous week), plus mandatory Section 57 spray irrigation reductions in East Anglia to 31 October cutting 288 licences by 50%, and 14 abstractors in the Wensum catchment required to stop entirely. Confirmed drought-related environmental incidents reached **266 for the year**, up 40 in a week. ## Why this matters for assessors and housebuilders Water efficiency has, for years, been treated as a South East issue — the Part G optional requirement of 110 l/p/d showing up in Cambridgeshire, Sussex and the Thames Valley, and largely nowhere else. That framing is getting harder to defend. Three things line up here. **The pending Part G reform.** Defra has proposed cutting the baseline from 125 to 105 l/p/d, with the optional standard falling from 110 to 100. Water UK has backed it. The government response is still awaited, with implementation intended during 2026 and a six-month transition. **The August 2026 NPPF.** Local plans may apply the tighter Part G optional requirement where justified, and exceptionally a more stringent local standard in areas of serious water stress. That is a test of local evidence — and the evidence base in the North West and Yorkshire looks different this year than it did two years ago. **Water neutrality and capacity evidence.** NPPF policies W1 and W4 push development plans towards early engagement with water companies on supply, wastewater and drainage capacity. A supplier in drought level 1 is a supplier with a harder conversation to have about new connections. None of this means northern authorities will adopt tighter standards this autumn. It does mean that if you are preparing a water efficiency calculation, a water neutrality statement or a capacity position for a site outside the traditional water-stressed areas, the assumption that "water isn't an issue here" is worth checking rather than inheriting. ## Three practical points **Check the optional requirement before you calculate, not after.** Whether a site is 125 or 110 l/p/d is a planning-condition question, and the answer is increasingly local. Getting it wrong means recalculating a whole fittings schedule. **Keep the calculation auditable.** Building Control wants the fittings schedule, not the headline number. Record the flow rates and capacities you actually specified, and note where they came from. **Watch for the Part G response.** If 105 l/p/d lands with a six-month transition, schemes already through design will need a route to comply or to demonstrate transitional protection. Knowing where your current designs sit against 105 costs very little now and a great deal later. The National Drought Group is now meeting weekly. Its next meeting was scheduled for 3 September. ## Run the Part G calculation in minutes **WaterMonkey** does the Part G water efficiency calculation in your browser — the official methodology, a full fittings schedule, and live PASS/FAIL against 125 or 110 l/p/d. Free to use, with a Building Control-ready PDF when you need one. [Try the free Part G water calculator →](/tools/part-g-water-calculator/) *Sources: [GOV.UK — Dry weather and drought in England: 21 to 27 August 2026](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports/dry-weather-and-drought-in-england-21-to-27-august-2026) · [GOV.UK — Dry weather and drought in England 2026: summary reports](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports) · [Water Magazine — Water UK backs tougher water efficiency standards for new homes](https://www.watermagazine.co.uk/2026/08/03/water-uk-backs-tougher-water-efficiency-standards-for-new-homes/)* --- # Plug-in solar goes on sale — what assessors need to know URL: https://energycount.co.uk/news/plug-in-solar-legal-27-august-2026/ Date: 2026-08-31 A fortnight ago we covered [the rewrite of the domestic solar permitted development rules](/news/gpdo-solar-permitted-development-27-august-2026/), which wrote plug-in solar into English planning law for the first time. The other half of that story landed on **26 August 2026**, when DESNZ announced that plug-in solar panels are **legal to buy and use across Great Britain from 27 August** — the same day the planning changes came into force. This is the sort of change that reaches assessors sideways. You won't see it in an Approved Document, but you will get asked about it: by homeowners on an RdSAP visit, by developers wondering whether it helps a Part L result, and by landlords eyeing an EPC band. ## What the kits actually are A plug-in solar kit is exactly what it sounds like: one or two panels that stand on a balcony, terrace, wall or patch of garden and connect to the home through an ordinary 13-amp plug and socket. No installer, no consumer-unit work, no scaffolding. The government's numbers: - Each kit produces up to **800W** of power. - That is enough for **up to 20%** of an average home's electricity use — DESNZ's illustration is a fridge, TV, router, laptop, phone charger and games console running together while the sun is out. - Estimated saving: **up to £110 a year**. The legal change follows independent safety testing across the key electrical elements, and DESNZ says the resulting UK safety specification is more stringent than Germany's — the market where the technology is already mainstream, with around half a million devices plugged in last year. Major retailers have worked with government on the rollout. **Currys, B&Q, Screwfix and Amazon** are named as delivery partners, with **Amazon, Argos, Currys and Wickes** committing to stock the panels in stores and online. ## The permission checks that still apply "Legal to buy" is not "install it anywhere". Before buying or fitting a kit, the government asks people to check whether their property needs: - permission from a **landlord**, building owner or **freeholder**; - **planning permission**; or - **listed building consent**. Whether those permissions are granted is a matter for the landlord, freeholder or council — government has simply encouraged them to consider each case on its merits. Layered on top of that, in England, is the permitted development framework in **SI 2026/896**, which amended Part 14 of Schedule 2 to the GPDO with effect from 27 August 2026. Two limits catch plug-in solar in particular: - Plug-in solar is **not** permitted development where any part of it would be installed on a **wooden wall, balcony, fence, gate or other enclosure**, or on any **timber-clad** part of a house or block of flats. - The usual heritage carve-outs bite: nothing on a listed dwelling or block of flats, nothing on a scheduled monument, and in a conservation area or World Heritage Site nothing on a wall, balcony or roof enclosure fronting a highway. The combustible-substrate exclusion is the one worth remembering, because timber fences and balcony balustrades are precisely where a lot of people will instinctively want to hang a panel. One jurisdictional wrinkle: the legality of the products applies across **Great Britain**, but the GPDO amendment is **England only**. Scotland and Wales have their own permitted development regimes. ## Does it count in SAP, RdSAP or an EPC? Short answer: nothing in this announcement changes the assessment methodology. There is no new convention, no new SAP input, and no change to Approved Document L. That matters, because the obvious client question — "will this improve my EPC?" — doesn't have a comfortable answer yet. Plug-in solar is user-installed, moveable, and typically has no commissioning paperwork, no MCS certificate and no fixed orientation or pitch. Those are exactly the things an assessment relies on to credit a PV array: declared capacity, orientation, pitch and overshading, evidenced at the time of the visit. A kit that can be unplugged and taken to a new flat on moving day is a poor fit for a certificate that lasts ten years. For **new build**, the point is sharper still. The Future Homes Standard notional dwelling assumes a meaningful area of roof-mounted PV, and Part L compliance is demonstrated against a designed, fixed, documented array. An 800W plug-in kit is not a substitute for specifying the real thing — and it isn't something Building Control will accept as part of a compliance package. Our reading: treat plug-in solar as a bill-saving consumer product, not an energy-assessment input, unless and until a convention says otherwise. If you're asked, that's a defensible line to take, and it's worth putting in writing to the client. ## The market context The announcement lands in an already busy year for solar. MCS figures cited by DESNZ show almost **150,000 certified solar installations in the first six months of 2026** — up 17% on the previous record start to a year in 2025, or roughly one installation every 74 seconds. Certified battery installations reached **36,000**, close to double the same period last year. The Energy Networks Association's response also flagged something to watch: network operators expect the **registration process for microgeneration** to evolve as these devices spread. That's the kind of quiet administrative change that eventually shows up in what assessors and installers are asked to evidence. ## What to do with this If you advise homeowners, the useful checklist is short: check the tenure and heritage permissions first, keep the panels off timber, and don't promise an EPC improvement. If you're working on new build, the change doesn't move your compliance model at all — which is rather the point. Solar only ever earns its keep in a Part L calculation when the fabric underneath it is right; PV bolted on to compensate for a leaky envelope and untested junctions is an expensive way to fail a design-stage assessment. If you'd like the **PSI values** and fabric numbers modelled properly so your renewables aren't papering over losses you could have designed out, [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [Households can save as plug-in solar panels come to market, DESNZ, 26 August 2026, GOV.UK](https://www.gov.uk/government/news/households-can-save-as-plug-in-solar-panels-come-to-market) · [The Town and Country Planning (General Permitted Development) (England) (Amendment) Order 2026 (SI 2026/896), legislation.gov.uk](https://www.legislation.gov.uk/uksi/2026/896/made) · [Plug-in solar panel checklist, Energy Saving Trust](https://energysavingtrust.org.uk/plug-in-solar-panel-checklist) · [Record number of UK households combining renewables, MCS](https://mcscertified.com/record-number-of-uk-households-combining-renewables-to-improve-energy-resilience/)* --- # Scotland has moved EPC reform to April 2028 — and it says something about the UK timetable URL: https://energycount.co.uk/news/scottish-epc-reform-delayed-april-2028/ Date: 2026-08-30 If you only work in England, a Scottish commencement date is easy to scroll past. This one is worth two minutes, because the Scottish Government has published its reasoning — and that reasoning is about the **UK-wide** Home Energy Model programme that everyone's compliance work eventually depends on. On **27 August 2026** the Scottish Government published an update confirming that the **Energy Performance of Buildings (Scotland) Regulations 2025** will now come fully into force on **30 April 2028**, rather than the 31 October 2026 date set when the Regulations were laid. ## What's actually changed The Energy Performance of Buildings (Scotland) Amendment Regulations 2026 do two things: - Move the full commencement date of the 2025 Regulations from **31 October 2026 to 30 April 2028**, and shift the saving and transitional provisions to match. - Allow the **lodgement fee** charged by the Keeper of the Register to rise **as already planned on 31 October 2026** — from **£2.60 to £6.00** for domestic properties, and from **£12.10 to £15.50** for other properties. Nothing else about the substance of the reforms changes. The new rating systems, the shorter five-year validity period, the redesigned certificate and Property Report and the strengthened quality-assurance regime all remain as approved by the Scottish Parliament in December 2025. The majority of the 2008 Regulations, and the EPC system assessors use today, stay in place until spring 2028. One part of the 2025 Regulations is already live: **Regulations 11 and 12**, covering the new Scottish EPC Accreditation Scheme and the appointment of Approved Organisations, came into force on **1 January 2026** and remain in force. The scheme is open to applicants now, ahead of the rest of the Regulations landing in 2028. ## Why the date moved — and why it matters in England The Scottish Government is explicit that this follows "extensive engagement with the UK Government and with the EPC assessor industry" on the delayed launch of new EPCs in England and Wales. Three points stand out: 1. **The UK Government's public commitment is still "the second half of 2027."** Scotland notes the UK Government had not reached final decisions on detailed implementation at the point Scotland had to lay its amending regulations to stop the 2025 Regulations biting on 31 October 2026. 2. **Scotland deliberately built in contingency.** Rather than match the second half of 2027, it planned for April 2028 to reduce the risk of having to amend the regulations again — and the market uncertainty that would cause. 3. **The underlying programme is shared.** The plan to finalise **HEM**, build and test the new **EPC Register** and the **Energy Calculation as a Service (ECaaS)** infrastructure, and train the assessor industry, "will remain the same across the UK." That third point is the useful one. The assessor market operates UK-wide and is partly covered by the UK Internal Market Act 2020, so the two governments are working to a common technical timetable even where commencement dates diverge. Scotland's stated expectation is that the infrastructure will be **tested and available for assessors to train on during 2027**. That's a reasonable read on when English assessors should expect HEM training to become real work rather than a diary note. ## The Scottish transitional arrangements, in brief If you do work north of the border, the transitions are worth knowing: - **Buildings for sale or let (other than short-term lets):** a one-year grace period from **30 April 2028 to 30 April 2029**. Old-style EPCs issued under the 2008 Regulations can still be *used* during that window, but from 30 April 2028 assessors can only *issue* new-style certificates. If a property is sold or let again within the transition, a new-style EPC is then required. After 30 April 2029, only new-style EPCs are allowed. - **Short-term lets:** the same full one-year grace period, recognising that around 30,000 Scottish short-term lets are continuously advertised and couldn't all be re-certified on day one. - **Large non-domestic buildings frequently visited by the public** (the display duty under Regulation 13): can keep displaying their existing old-style EPC until it expires or until **30 April 2033**, whichever comes first. - **New construction:** for buildings completed on or after **30 April 2028**, the developer must give the owner a valid new-style EPC and Property Report within **seven days** of submitting the completion certificate to the local authority verifier. This moves the requirement out of standard 6.9 of Schedule 5 to the Building (Scotland) Regulations 2004 and into the EPC regulations. Elmhurst also reports that the Scottish Government is aiming to launch the new **onsite audit regime in the second half of 2027** — ahead of the main reform date — and that the next changes to Scottish Building Standards, expected to use HEM, are currently planned for autumn 2029. That would leave an interim period where SAP is used for Building Standards compliance while HEM produces EPCs, which will need working through for the new-build sector. ## What to do about it For most English assessors and housebuilders, the practical answer is: nothing changes today. **SAP 10.3 remains the compliance methodology** for Part L and the Future Homes Standard, and the fabric work underneath it — U-values, air permeability, and thermal bridging — is exactly the same set of numbers whether the calculation engine is SAP or HEM later. That's the quiet advantage of a slipping methodology timetable. The inputs are stable even when the tool isn't. Junction detail and **PSI values** in particular carry more weight under the Part L 2026 targets than most schemes budget for, and modelled junctions beat default values by a margin that usually pays for itself. If you'd like your junctions modelled properly now — so the fabric is right whichever engine ends up running the numbers — [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [Energy Performance Certificate Reform: updates — 24 August 2026 update, gov.scot (published 27 August 2026)](https://www.gov.scot/publications/energy-performance-certificate-reform-updates/) · [Energy Performance of Buildings (Scotland) Regulations 2025 — EPC reform consultations: SG response, gov.scot](https://www.gov.scot/publications/energy-performance-buildings-scotland-regulations-2025-update-government-response-epc-reform-consultations/) · [The Energy Performance of Buildings (Scotland) Regulations 2025 (SSI 2025/417), legislation.gov.uk](https://www.legislation.gov.uk/ssi/2025/417/contents/made) · [Scottish Government confirms EPC reform delayed until April 2028, Elmhurst Energy](https://www.elmhurstenergy.co.uk/blog/2026/08/28/scottish-government-confirms-epc-reform-delayed-until-april-2028/)* --- # Strategic sites get their own NPPF policy — and phasing is the whole point URL: https://energycount.co.uk/news/nppf-2026-strategic-sites-ho4-ho13/ Date: 2026-08-29 Last week we wrote about the [national section 106 templates aimed at 10–49 unit sites](/news/standard-s106-templates-medium-sites-consultation/). This is the other end of the same telescope. The **National Planning Policy Framework published on 17 August 2026** creates, for the first time, a distinct planning category for the very largest housing schemes — and the policies that govern it are almost entirely about phasing. For anyone doing SAP work on a strategic site, that is not a planning-nerd detail. It is the thing that decides which edition of Part L each phase is built to. ## What counts as a strategic site The Framework's plan-making policy, **HO4: Land for strategic site development**, describes them by type rather than by a bare number: > "The development plan should, where appropriate, identify suitable locations for strategic sites for housing-led development (such as new settlements, new urban quarters or significant extensions to existing settlements)." The Planning Portal, summarising the government's position on publication, reports that strategic sites will typically be those with capacity for **1,500 homes or more**, though the figure can vary with the mix of uses on the site. Treat that as the government's working scale rather than a hard threshold in the policy text. HO4 then sets three tests for the locations chosen. They should be capable of being supported by **the necessary infrastructure and facilities at appropriate points in the development's delivery**; capable of supporting a sustainable community with access to services and employment "without expecting an unrealistic level of self-containment"; and they should > "Address strategic environmental opportunities and safeguards, including those set out in Local Nature Recovery Strategies." Local plans are then expected to identify specific sites with their infrastructure and site-specific requirements, set design expectations through **masterplans and design codes**, make "a realistic assessment of the likely rates of delivery", and identify routes to rapid implementation such as joint ventures, land assembly or locally led development corporations. ## HO13 is where it gets practical The decision-making counterpart is **HO13: Build out of residential and strategic sites**. Paragraph 1 sets the general expectation that land with permission is developed without unnecessary delay, and paragraph 2 invites councils to consider conditions requiring development to begin sooner than the statutory default. Paragraphs 3 and 4 apply specifically to strategic sites. Paragraph 3 says the consenting framework should: > "a. Set out the parameters and requirements for the design, infrastructure and other features of the development (including the quantity and type of affordable housing to be provided), with the level of initial and subsequent detail tailored to the proposed scale and phasing of the development; and > > b. Be flexible enough to respond positively to changing circumstances as phases are brought forward, including changes to housing need, infrastructure requirements, viability and design." Paragraph 4 adds that where an emerging plan — "however advanced" — proposes to allocate strategic sites, development proposals should not be inconsistent with the proposed scale, location and phasing of those sites. So: parameters set early, detail scaled to phasing, and an explicit instruction that the consent should flex as phases come forward. ## Why this matters for Part L Here is the bit that lands on assessors' desks. A strategic site is, by definition, a scheme that will still be building out in ten years. The Future Homes Standard transitional arrangements are not that generous. As we've [written before](/news/future-homes-standard-transitional-deadline-commencement/), protection under **The Building Regulations etc. (Amendment) (England) Regulations 2026** (SI 2026/335) requires two things: a building notice, initial notice or full-plans application before **24 March 2027**, *and* the work on that individual building commenced before **24 March 2028** — with "commenced" meaning a completed sub-structure including ground floor structure, plot by plot. On a 1,500-home site, that is a small fraction of the programme. Phases one and possibly two might be protected. Everything after that is being built to ADL 2026 — heat pumps, on-site generation under requirement L3, and the tighter fabric that goes with them. HO13(3)(b) is helpful here, in a backhanded way. A consenting framework that is required to "respond positively to changing circumstances as phases are brought forward, including changes to... infrastructure requirements, viability and design" is one that ought to be able to absorb a change of Part L edition mid-build-out without a full section 73 fight. But that only works if the parameters set at outline are drafted to allow it. **The trap is a parameter plan or design code written to the fabric and services of the current regulations.** Fixed roof pitches and orientations that don't work for the PV array L3 now expects. Plot-wide glazing ratios set against a 2021 overheating assessment. External wall build-ups locked into a design code at a thickness that won't take the insulation ADL 2026 needs. Heat network capacity sized on the assumption every plot has a gas boiler. None of those are hard to fix at masterplan stage. All of them are expensive to fix at phase four. ## The other long-programme problems Two more that phasing makes worse rather than better. **Nutrient budgets.** In a protected catchment, the nutrient budget for a strategic site is calculated on the whole scheme, but the mitigation is usually delivered and paid for in tranches. If the budget was struck on an occupancy assumption or a wastewater treatment consent that changes between phase one and phase five, the arithmetic moves. The [new NPPF chapter on clean energy and water](/news/nppf-2026-clean-energy-and-water-w1-w4/) pushes catchment capacity into plan-making, which should mean fewer surprises — but only on plans prepared after August 2026. **Drainage.** HO4(1)(a) asks that infrastructure be capable of support "at appropriate points in the development's delivery". Surface water is the classic case where phase-by-phase attenuation designed in isolation adds up to a strategy that doesn't work at full build-out, and where the [national SuDS standards](/news/suds-strategy-templates-august-2026/) now expect a scheme-wide answer. ## What we'd suggest doing **Map your transitional cliff-edge against your phasing plan, on paper, now.** Which plots realistically get a full-plans application in before 24 March 2027 and a completed sub-structure before 24 March 2028? Everything to the right of that line is an ADL 2026 scheme. If nobody has drawn that line, the design code is being written blind. **Write the design code so it survives a regulation change.** Ask for parameter ranges rather than fixed values on the things Part L touches — wall thickness, roof form and orientation, plant space, glazing ratio. HO13(3)(b) gives you the policy hook to argue for it. **Get the fabric strategy tested against ADL 2026 before the code is fixed, not after.** A notional-building run on a representative house type from each phase is a day's work and will tell you whether the code you're about to adopt is buildable under the standard that most of the site will actually be built to. **Thermal bridging is the cheapest margin you have.** On a scheme with repeated house types across ten phases, bespoke PSI values are calculated once and used thousands of times. Default y-values on a 1,500-home site are, in aggregate, a very expensive convenience. **ΨMonkey** calculates bespoke PSI values for your junctions to BS EN ISO 10211, following BR 497 and BRE IP 1/06 conventions, in your browser — so your SAP model reflects the junctions in your design code rather than the worst case the software assumes. [Try ΨMonkey →](/tools/psi-calculator/) If you're masterplanning a site that will still be building out well past the Future Homes Standard transitional dates and you'd like the Part L strategy stress-tested before the design code is signed off, [get in touch](/contact/) — that conversation is much cheaper at outline than at phase four. *Sources: [National Planning Policy Framework, August 2026 (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/6a8334c03bd75b81e2329ac4/National_Planning_Policy_Framework.pdf) — chapter 6, policies HO4 and HO13 · [National Planning Policy Framework guidance page, MHCLG, 17 August 2026](https://www.gov.uk/guidance/national-planning-policy-framework) · [Government publishes latest NPPF with new strategic sites policy, Planning Portal Blog, 19 August 2026](https://blog.planningportal.co.uk/2026/08/19/government-publishes-latest-nppf-with-new-strategic-sites-policy/) · [The Building Regulations etc. (Amendment) (England) Regulations 2026 (SI 2026/335), legislation.gov.uk](https://www.legislation.gov.uk/uksi/2026/335/made)* --- # The NPPF now asks you to design against wildfire URL: https://energycount.co.uk/news/nppf-2026-wildfire-mitigation-cc3/ Date: 2026-08-29 England has had a summer marked by heatwaves, drought and wildfire damage to homes and habitats. The **National Planning Policy Framework published on 17 August 2026** responds with something national planning policy has never had before: a policy telling applicants to design against fire spreading in from the landscape. It is a short paragraph. It is also, for a certain kind of site, the thing that changes your landscape strategy and your boundary treatment specification. ## What CC3(1)(e) actually says Policy **CC3, "Adaptation to climate change"**, is a national decision-making policy. It opens by requiring development proposals to take into account the current and potential impacts of climate change *over the lifetime of the scheme*, then lists what that means in practice — flood risk, coastal change, SuDS, overheating, and now this: > "Incorporate proportionate measures to mitigate against wildfires, where opportunities to do so exist and wildfires pose a particular risk (for example as a result of the combined effect of topography, prevailing wind direction and being located within or adjacent to heavily vegetated areas). Such measures should be designed to limit fuel loads for fires, and to create defendable spaces, such as by avoiding timber panel fences and incorporating firebreaks into development layouts and planting schemes." Three things are worth pulling out of that. **It is conditional, not universal.** The duty bites "where opportunities to do so exist *and* wildfires pose a particular risk". Most urban infill will not engage it. The trigger the policy gives is a combination — topography, prevailing wind direction, and being within or adjacent to heavily vegetated areas. Heathland, forestry, moorland edge, gorse-covered slopes, plantation boundaries. **It names a specific product.** "Avoiding timber panel fences" is unusually prescriptive for the NPPF. Close-boarded timber fencing running from a vegetated boundary into a housing layout is a continuous fuel path, and the Framework says so in as many words. If your standard boundary detail on a rural-edge site is a 1.8m closeboard fence, that is now a policy conversation. **"Defendable space" is a design concept, not a buffer strip.** The wording asks for measures "designed to limit fuel loads" and to create defendable spaces, including **firebreaks in development layouts and planting schemes**. That is a masterplanning and landscape input, not something you bolt on at reserved matters — which means it needs to be in the layout at outline stage. ## And CC1 puts it into plan-making The plan-making counterpart, **CC1: Planning for climate change**, requires development plans to take a proactive approach to adapting to climate change > "taking into account the implications of extreme weather and long-term climate trends including overheating, wildfires, drought, flood risk, coastal change, water supply, biodiversity and landscapes" — and then to address "any specific risks from climate change in their proposed allocations for development, and necessary adaptations, both of which should be considered for the anticipated lifetime of the development". So wildfire is now something an emerging local plan is expected to have thought about when it allocates land. Expect it to start appearing in site-specific allocation policies and in the evidence base for rural and urban-fringe allocations over the next plan-making round. ## Where this sits next to the rest of the climate chapter Chapter 5 is compact — CC1 for plan-making, CC2 for mitigation, CC3 for adaptation — and CC3 now bundles four adaptation duties into one list: flooding, coastal change, [sustainable drainage under policy F8](/news/nppf-2026-suds-national-standards-policy-f8/), [overheating under DP3(2)(b)](/news/nppf-2026-overheating-policy-cc3/), and wildfire. That grouping matters. It means a single planning officer, reading one policy, is now looking for evidence on all four. On a rural-edge site in a hot, dry catchment you could plausibly be asked at validation for a drainage strategy, an overheating assessment and a note on wildfire mitigation — three things that until this month sat in three different places, two of them in Building Regulations rather than planning. There is also a tension worth naming. **CC3(1)(d)** asks for green infrastructure and tree planting to minimise overheating risk. **CC3(1)(e)** asks you to limit fuel loads. On most sites those pull in the same direction — shade trees near dwellings, managed grassland, irrigated planting. On a dry heathland edge they do not, and the policy gives no hierarchy. Someone will have to reconcile that in a design and access statement, and the sooner your landscape architect is in the room the cheaper that is. ## What we'd suggest doing **Work out whether you're actually in scope.** The test is topography plus prevailing wind plus adjacency to heavy vegetation. If your red line abuts forestry, heath, moorland or unmanaged scrub on the upwind side of a slope, assume the question will be asked and get ahead of it. If you're in a town centre, it won't be. **Look at your boundary treatment standard detail now.** Timber closeboard is named in national policy as the thing to avoid. Substituting masonry, metal railings or hedge-on-post on the vegetated boundary is cheap at design stage and expensive at condition-discharge stage. **Put firebreaks in the layout, not the landscape schedule.** The policy talks about "development layouts and planting schemes" together. Layout is fixed at outline; planting is not. If the mitigation lives entirely in the soft landscaping, you have less room to move later. **Don't let it undercut your overheating design.** Part O has not changed — the [2021 edition is still the one in force](/news/record-summer-2026-part-o-overheating-gap/) while the promised full review runs its course — and CC3(1)(d) still expects overheating to be designed out. Wildfire mitigation should reshape *where* the planting goes and *what species*, not delete it. ## If you're already doing the overheating work For most of our clients, the practical effect of chapter 5 is that climate adaptation evidence is now something planning asks for, months before Building Control does. Overheating is the one that costs money if you get it wrong late — a TM59 failure at Part O stage on a scheme already consented with a fixed glazing ratio is an expensive conversation. If you have a site coming forward on a rural or urban-fringe edge and you'd like the overheating, drainage and adaptation evidence lined up before validation rather than after, [get in touch](/contact/) and we'll tell you what we'd want to see modelled first. *Sources: [National Planning Policy Framework, August 2026 (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/6a8334c03bd75b81e2329ac4/National_Planning_Policy_Framework.pdf) — chapter 5, policies CC1 and CC3 · [National Planning Policy Framework guidance page, MHCLG, 17 August 2026](https://www.gov.uk/guidance/national-planning-policy-framework) · [Revised National Planning Policy Framework published: what it means for ecology and environmental management, CIEEM, 20 August 2026](https://cieem.net/revised-national-planning-policy-framework-published-what-it-means-for-ecology-and-environmental-management/)* --- # National s106 templates are coming for 10–49 unit sites URL: https://energycount.co.uk/news/standard-s106-templates-medium-sites-consultation/ Date: 2026-08-28 If you work on schemes of 10 to 49 homes, the section 106 agreement is often the thing that decides whether permission arrives in three months or nine. On **25 August 2026** MHCLG published a consultation on **four standard s106 templates** aimed squarely at that size of site. It runs for eight weeks, closing at **11:59 on 20 October 2026**, and applies to **England only**. ## What's being consulted on The consultation covers four draft documents, drafted for MHCLG by **Town Legal LLP** working with the Planning Advisory Service: 1. **Bilateral agreement** — standard clauses, plus schedules for Financial Contributions and the Council's Covenants. 2. **Unilateral Undertaking** — standard clauses, plus a schedule for Financial Contributions. 3. **Affordable Housing Schedule for Full Planning Permissions**, including a discretionary cascade mechanism. 4. **Affordable Housing Schedule for Outline Planning Permissions**. The draft templates are published alongside the consultation document, and there are 80 questions to respond to. ## "Medium-sized" now has a formal definition The threshold comes from the **new NPPF published on 17 August 2026**, which, as the consultation puts it, "formally introduces the category of 'medium' development (schemes of up to 2.5 hectares in area and between 10 and 49 units)." That's a meaningful change from the earlier proposal, which had floated 10–49 units on sites of up to **1.0 hectare**. The area limit has more than doubled, so the templates will catch a lot more of the typical SME site than first proposed. ## These aren't optional in practice The templates are not legally mandatory, but the new NPPF gives them real weight. The consultation points to **policy DM6(4)**, which > "sets out … that where national model planning obligations are relevant to the development, they should be used unless there are strong reasons for using a different obligation." MHCLG is explicit about the intent: the templates are "intended to become the default for applications in the future, reducing time spent agreeing standard provisions and freeing up local capacity to focus negotiations on more complex obligations." Government also says it "intends to publish further model obligations and planning conditions to support consistency in decision making in due course." The one carve-out is the affordable housing **cascade mechanism**, which is proposed as discretionary for councils to use where they think it appropriate. ## The bit that matters for environmental obligations Here's the part worth understanding properly. **The templates are content-neutral shells.** Affordable housing is the only substantive obligation actually drafted. Everything else — nutrient mitigation payments, SuDS and drainage maintenance sums, biodiversity net gain contributions, open space, education, highways — is left to be inserted into **Schedule 2** as a named "Contribution". We checked the consultation document carefully, and it does not mention energy efficiency, carbon, Part L, the Future Homes Standard, water efficiency, nutrient neutrality, phosphates or nitrates, biodiversity net gain, SuDS, sustainable drainage, flood risk, overheating or EV charging anywhere. Not once. None of the 80 questions asks about them either. That isn't necessarily a criticism — a template that tried to draft every environmental obligation for every catchment in England would be unusable. But it does mean the standardisation stops exactly where the technical work starts. Schedule 2 works as a repeatable pro-forma: as the consultation explains, "for each contribution, or instalment of a contribution, sub clauses (a) and (b) will need to be copied and completed with the name of the contribution and the relevant date." For outline permissions, where the sum depends on the quantum approved at reserved matters, "the formula for calculating the contribution should be included." So on a nutrient-constrained site, your phosphate or nitrogen mitigation payment is still a bespoke figure that someone has to calculate, justify against the **Regulation 122 CIL tests** (necessary, directly related, and fairly and reasonably related in scale and kind), and drop into a blank line in an otherwise standard document. ## Other practical points - **Payment triggers.** Schedule 2 provides for payment before specified points such as commencement or first occupation, and MHCLG acknowledges instalments linked to build-out, occupation of a set number of dwellings, or phases. - **Indexation.** There's a dedicated indexation clause, and question 30 asks which indices are appropriate for particular types of contribution. If you've ever watched a nutrient credit price move between agreement and payment, that question is worth answering. - **Unspent money.** The bilateral agreement's Schedule 3 requires contributions to be used solely for the purpose secured and held in an interest-bearing account, with a default five-year repayment window for unspent sums. Question 56 asks whether councils should instead be able to spend unclaimed sums on other mitigation in the vicinity. - **Still site-specific.** MHCLG is clear that "agreements must therefore be considered on a site-by-site basis to ensure that they are appropriate for a particular development." ## Why this is worth ten minutes of your time The stated problem is a familiar one. In MHCLG's words, s106 negotiation "has become synonymous with inefficiency and delay, which demands significant local authority resources and can have a disproportionately negative impact on Small and Medium Enterprise (SME) builders." If that lands with you, the consultation is a genuine chance to shape the drafting — particularly on indexation, payment triggers and how environmental contributions get expressed. Responses go through Citizen Space, with enquiries to standards106consultation2026@communities.gov.uk. For most schemes at this size, though, the s106 was never the hard part. The hard part is arriving at a defensible mitigation figure in the first place — and on a nutrient-constrained catchment that means a nutrient budget that stands up to Natural England scrutiny, for both nitrogen and phosphorus where the catchment demands it. If you'd rather have that number settled before the s106 conversation starts, [get in touch](/contact/) and we'll tell you how we'd approach your site. *Sources: [Standard planning agreements for medium-sized sites, GOV.UK (published 25 August 2026)](https://www.gov.uk/government/consultations/standard-planning-agreements-for-medium-sized-sites) · [Consultation document (full text), GOV.UK](https://www.gov.uk/government/consultations/standard-planning-agreements-for-medium-sized-sites/standard-planning-agreements-for-medium-sized-sites) · [Letter from MHCLG to Chief Planning Officers, 25 August 2026, GOV.UK](https://www.gov.uk/guidance/planning-guidance-letters-to-chief-planning-officers) · [The Planning Jungle summary, 27 August 2026](https://planningjungle.com/2026/08/27/the-government-publishes-a-consultation-until-20-10-2026-relating-to-standard-planning-agreements-for-medium-sized-sites/)* --- # Your soakaway test was done in the driest ground for years URL: https://energycount.co.uk/news/bre-365-soakaway-testing-drought-conditions/ Date: 2026-08-27 Here's a scenario worth thinking about before autumn submissions go in. A soakage test carried out in August 2026 comes back with a healthy infiltration rate. The soakaway is sized accordingly, the drainage strategy is approved, and the thing is built the following spring — into ground that behaves nothing like it did in the test pit. ## Why this year is different The Environment Agency's report for **14 to 20 August 2026** records that the **seasonal recession of groundwater levels is continuing**, with levels in the Oolite Limestone aquifers **exceptionally low in places** and some chalk sites in the Wessex Downs, Yorkshire Wolds and the North and South Downs classed as below normal or notably low. A quarter of monitored river sites are exceptionally low, and **71% of England by land area is in drought** ([GOV.UK](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports/dry-weather-and-drought-in-england-14-to-20-august-2026)). That follows what the Environment Agency has called the **driest July in 190 years**. The agency also notes a related effect worth holding onto: with ground conditions this dry, **water does not easily soak into baked and compacted soils**, increasing run-off risk. Very dry ground can behave in ways that don't map neatly onto either summer or winter design assumptions. ## What BRE Digest 365 actually asks of you [BRE Digest 365, *Soakaway design* (2016 edition)](https://bregroup.com/store/bookshop/soakaway-design-dg-365-2016-download) is still the standard reference for infiltration testing — our [BRE Digest 365 soakaway calculator](/tools/soakaway-calculator/bre-digest-365/) runs the full method, and the [percolation test calculator](/tools/soakaway-calculator/percolation-test/) turns the pit timings into the infiltration rate. Two features of the method matter here. First, it is deliberately conservative about the test itself: the design rate is taken from **the lowest of up to three tests** in a pit, not an average of them. That guards against one flattering fill-and-drain cycle carrying the design. Second — and this is the one that gets skipped — the method assumes you know where the water table is going to be. **Groundwater levels move seasonally**, so evidence of the **highest likely water table** is normally needed alongside the test result. Standard practitioner guidance for sites with a seasonally high water table, such as lowland areas or land near a watercourse, is to test in **winter or spring, when the table is highest** — precisely the opposite of the conditions available across much of England right now. There is a wider professional argument running about how reliable BRE 365 results are in general — susdrain hosts an [Environmental Protection Group briefing](https://www.susdrain.org/files/resources/briefings/EPG_Briefing_SoakwayTesting_V2.pdf) asking whether the test has had its day. You don't need to take a side in that debate to accept the narrower point: a test done in the driest ground in living memory carries more uncertainty than usual. ## Why the stakes just went up Drainage evidence used to be something you could largely settle after permission. That's changed. Under the August 2026 NPPF, **policy F8** requires SuDS on all development with drainage implications to be designed in accordance with Defra's National Standards, and **policy DM2** puts a site-specific flood risk assessment and a SuDS statement on a national validation list. The Environment Agency and CIRIA have published **national SuDS Strategy Templates** for major and non-major development to standardise what gets submitted. So the infiltration rate is now load-bearing at validation, not just at discharge of conditions. If it turns out to be optimistic, you're not correcting a detail — you're revisiting an approved strategy, potentially with a different discharge route and a different land take. ## Five things worth doing **Record the test date and antecedent conditions.** Note rainfall in the preceding weeks and the groundwater context. A reviewer six months from now needs to know the test was run in a drought, and so do you. **Get separate groundwater evidence.** Don't infer the winter water table from an August pit. Borehole records, published aquifer data or a monitoring period give you something defensible. **Consider retesting in the wet season** on marginal sites, or designing with an explicit margin and stating why. **Know your fallback.** If the rate is genuinely marginal — broadly, once you're heading below about 10⁻⁶ m/s, infiltration stops being practical — establish the next step in the discharge hierarchy early rather than discovering it at construction. **Keep three tests, and use the lowest.** It's the method, and in a year like this one it's also the only sensible instinct. ## Size it properly, quickly **SoakMonkey** does BRE Digest 365 soakaway sizing in your browser — enter your test results, get a defensible design and a printable output for the drainage strategy. It won't tell you whether your test was done in the right season, but it will make the rest of the job take minutes. [Try SoakMonkey →](/tools/soakaway-calculator/) *Sources: [GOV.UK — Dry weather and drought in England: 14 to 20 August 2026](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports/dry-weather-and-drought-in-england-14-to-20-august-2026) · [BRE Digest 365 — Soakaway design (2016)](https://bregroup.com/store/bookshop/soakaway-design-dg-365-2016-download) · [susdrain — EPG briefing on soakaway testing](https://www.susdrain.org/files/resources/briefings/EPG_Briefing_SoakwayTesting_V2.pdf) · [Environment Agency blog, 12 August 2026](https://environmentagency.blog.gov.uk/2026/08/12/how-the-environment-agency-manages-abstraction-to-protect-the-environment/)* --- # 71% of England is in drought — and it's now a planning problem URL: https://energycount.co.uk/news/drought-71-percent-england-water-efficiency-august-2026/ Date: 2026-08-27 Water efficiency has spent years as the quiet part of a Building Regulations submission — a fittings schedule, a calculation, a number under 125. That framing is getting harder to sustain. ## What the Environment Agency published On **21 August 2026** the Environment Agency published its weekly summary covering **14 to 20 August** ([GOV.UK](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports/dry-weather-and-drought-in-england-14-to-20-august-2026)). The headline numbers are worth reading properly: - **71% of England by land area is in drought**, with no change in area status that week. - **Reservoir storage for England was 62.6% on 18 August** — down 3.4% in a single week, and **16.3% below average** for the time of year. Seven major reservoirs are classed as exceptionally low. - **August rainfall to date was 17% of the long-term average** for England, ranging from 5% in the south east to 33% in the north west. - **A quarter of monitored river sites are classed as exceptionally low**, predominantly across southern and eastern England. - **Around 29 million people** are under temporary use bans, after Affinity Water extended its ban to its eastern region on 17 August and Wessex Water went company-wide on 18 August. - **1,562 restrictions on abstraction licences** are in place, including mandatory Section 57 spray irrigation reductions in parts of East Anglia running to **31 October**, cutting abstraction by 50% across 288 licences. - **226 drought and dry-weather incidents** have been confirmed since the start of 2026. The **National Drought Group was reconvened on 20 August** and is now meeting weekly. This follows a July the Environment Agency has described as the **driest in 190 years** ([EA blog, 12 August 2026](https://environmentagency.blog.gov.uk/2026/08/12/how-the-environment-agency-manages-abstraction-to-protect-the-environment/)). Areas in drought now include East Anglia, Thames, Kent and South London, Hertfordshire and North London, Solent and South Downs, Wessex, the West Midlands, the East Midlands, Lincolnshire and Northamptonshire, and Devon, Cornwall and the Isles of Scilly. Four further areas are in prolonged dry weather. ## Why this lands on your desk Most of those catchments are exactly where housing is being delivered — and where water is already a live constraint on consent. Three connections matter. **Part G is moving.** The current national standard is 125 litres per person per day, with a 110 l/p/d optional requirement available through planning conditions. Defra has consulted on tightening these to **105 and 100 l/p/d**. Nothing in the drought changes the legal position today, but it does change the political weather around the response. **Local standards survived the NPPF rewrite.** The August 2026 Framework confirms that local plans may apply the tighter Part G optional requirement where justified, and exceptionally a more stringent local standard in areas of serious water stress. That patchwork isn't going away this autumn — if anything, expect more authorities to reach for it. **Water availability is being tested at application stage.** New NPPF policies require plan-making to be informed by early engagement with water and wastewater providers on capacity, and several catchments already operate water neutrality or offsetting regimes. A scheme that clears Part G on paper can still stall on supply. ## The practical version If you're producing water efficiency evidence this autumn, a few things are worth doing now rather than at discharge-of-conditions: Check whether the authority has adopted the 110 l/p/d optional requirement, and whether a local plan policy goes further — don't assume 125 is the target. Design to the standard you're likely to be held to, not the one that was in force when the site was first appraised. Keep the fittings schedule and the calculation together in one auditable pack, because Building Control and the LPA are increasingly asking to see the same evidence. And if the site sits in a water neutrality or offsetting area, treat that as a separate workstream with its own lead time — it is not a Part G calculation with extra steps. None of this is new regulation. It's the same rules being applied in a much less forgiving environment. ## Get the water calculation right first time **WaterMonkey** runs the Part G water efficiency calculation in your browser — 125, 110 or a local standard — and gives you a fittings schedule you can hand straight to Building Control. One less thing to redo when the target moves. [Try the free Part G water calculator →](/tools/part-g-water-calculator/) *Sources: [GOV.UK — Dry weather and drought in England: 14 to 20 August 2026](https://www.gov.uk/government/publications/dry-weather-and-drought-in-england-2026-summary-reports/dry-weather-and-drought-in-england-14-to-20-august-2026) · [Environment Agency blog — managing abstraction, 12 August 2026](https://environmentagency.blog.gov.uk/2026/08/12/how-the-environment-agency-manages-abstraction-to-protect-the-environment/) · [Defra — Review of water efficiency standards in the Building Regulations](https://consult.defra.gov.uk/water-efficiency-demand/review-of-water-efficiency-standards)* --- # Mayors are getting call-in powers — and 150 homes is the threshold URL: https://energycount.co.uk/news/mayoral-planning-powers-consultation-october-2026/ Date: 2026-08-27 If you work on schemes of any size, the question of *who decides* is about to change in a lot of England. On **23 August 2026** the Prime Minister's Office announced that mayors across England will be given "call in" powers over the most important planning decisions in their areas ([GOV.UK press release](https://www.gov.uk/government/news/mayors-given-powers-to-green-light-developments)). The detail landed the following day: MHCLG's consultation, **Planning powers for mayors in England**, opened on **24 August 2026** and runs for six weeks, closing at **23:59 on 5 October 2026**. ## The thresholds This is the first time national thresholds for strategic call-in have been set outside London. Under the announcement, applications in scope are: - **more than 150 homes** - **more than 15,000 m² of commercial floorspace** - **any building over 30 metres tall** — roughly ten storeys A mayor will be able to take over a qualifying application and direct the council to approve or refuse it. Councils will continue to determine the large majority of applications, every decision must still follow planning law, the development plan and national policy, applicants keep their right of appeal, and ministers retain a backstop power to intervene. Worth noting for anyone modelling risk on a marginal site: the consultation observes that the 150-dwelling and 15,000 m² categories would **align with the thresholds for the Secretary of State's new and proposed consultation directions** where an authority proposes to refuse. In other words, the same scale of scheme is picking up attention from two directions at once. ## The part that changes when your evidence is due The consultation has four parts — call-in of applications of potential strategic importance, **Mayoral Development Orders (MDOs)**, **Mayoral Community Infrastructure Levy (MCIL)**, and some further changes to Local Development Orders. The MDO section is the one worth reading if you produce technical evidence. An MDO is an **upfront grant of planning permission by a mayor for development on a particular site** — no planning application needed. The powers sit at [sections 61DA to 61DE of the Town and Country Planning Act 1990](https://www.legislation.gov.uk/ukpga/1990/8/part/III/crossheading/mayoral-development-orders), originally introduced for the Mayor of London by the Infrastructure Act 2015 and updated by the English Devolution Act. The proposed process is modelled on Local Development Orders: a draft order plus a **statement of reasons**, a minimum **30-day** publicity and consultation period, notification of owners, occupiers and the local planning authority, and a duty on the mayor to consider representations before adopting the order. The consultation is explicit that key planning matters have to be settled early. It says it will be important to ensure that **environmental mitigations and the likely infrastructure requirements to be secured through planning obligations** are considered early in the process, along with the mechanism for discharging any subsequent conditions and who the approving authority for those will be. Translated into working terms: on an MDO site, the drainage strategy, the overheating position, the nutrient or water position and the energy standard don't sit behind a pre-commencement condition to be argued about later. They inform whether the order gets made at all, and the consenting framework it sets. That's the front-loading everyone has been talking about since the NPPF restructure, made concrete. ## Timing The government says it wants the call-in and MDO powers **in place by early next year**, and — subject to consultation and parliamentary scheduling — intends to **lay the MDO secondary legislation by the end of 2026**. MCIL outside London depends on a Spatial Development Strategy being in place in the relevant area, and would fund a broader range of strategic infrastructure than the London model, which is limited to strategic transport. Two limitations are proposed on MDO scope, including an exclusion for **minerals development**. ## What to do about it If you regularly work on schemes at or above 150 homes, this is worth twenty minutes. The thresholds, the MDO procedure and the question of who discharges conditions under an order are all live questions in the consultation, and the answers will shape how early your technical evidence has to be finished. Responses close at **23:59 on 5 October 2026** via the [consultation page on GOV.UK](https://www.gov.uk/government/consultations/planning-powers-for-mayors-in-england/planning-powers-for-mayors-in-england). If your view is that upfront consenting only works when the drainage, overheating and water evidence is genuinely resolved at order stage, that's exactly the kind of practitioner feedback Part 2 is asking for — so say it. *Sources: [GOV.UK — Planning powers for mayors in England (consultation)](https://www.gov.uk/government/consultations/planning-powers-for-mayors-in-england/planning-powers-for-mayors-in-england) · [GOV.UK — Mayors given powers to green light developments, 23 August 2026](https://www.gov.uk/government/news/mayors-given-powers-to-green-light-developments) · [MHCLG in the Media, 26 August 2026](https://mhclgmedia.blog.gov.uk/2026/08/26/coverage-of-plans-to-strengthen-mayoral-powers-over-planning-decisions/)* --- # A small planning change aimed at grid delays — and it closes on 4 September URL: https://energycount.co.uk/news/grid-ground-investigations-permitted-development-consultation/ Date: 2026-08-26 Ask a housebuilder what's actually stopping a site right now and grid connection comes up as often as anything in the planning system. It's a strange constraint to explain to a client: the homes are consented, the design is fixed, and the thing holding it up is a substation upgrade with a date three years out. There's a small consultation open at the moment that's aimed squarely at one corner of that problem, and it closes soon. ## What's proposed On **6 August 2026**, the **Department for Energy Security and Net Zero** published a consultation on **new permitted development rights for ground investigations and surveys** carried out to support electricity network infrastructure projects in England. It was flagged to every local planning authority in the **Chief Planner's Planning Update Newsletter of 18 August 2026**, which asked councils to engage with it. The proposal is narrow. It would introduce a permitted development right for **certain temporary, low-impact ground investigations and surveys** undertaken by **electricity network developers** — specifically the distribution and transmission network operators licensed to carry out works relating to electricity networks. It does not grant permission for the infrastructure itself. The consultation is **open until 4 September 2026**. ## Why a survey PD right matters to a housing programme This looks like a technical change for utilities, and in the strict sense it is. But it sits on a chain of events that developers wait on. Before a network operator can design and cost a reinforcement — a new substation, a cable route, an upgraded feeder — it needs ground information: trial pits, boreholes, geophysical and environmental surveys along the route. If each of those needs its own planning application, the front end of the process gains weeks or months before any design work starts, and every one of those weeks lands somewhere in a connection date that a housebuilder is planning around. Bringing that survey work inside permitted development is an attempt to take that step off the critical path. It's a modest change. But delays of this kind compound, and this is the sort of unglamorous fix that shortens programmes without changing the substantive planning tests on anything. It also fits a pattern. In April 2026 the government confirmed it will raise the permitted development threshold for **electricity substations in England from 29 to 45 cubic metres**, bringing England into line with Scotland so network operators can deliver capacity upgrades with less administrative friction. This consultation extends the same logic upstream, to the investigation work that precedes the design. ## The Future Homes Standard connection This matters more from **24 March 2027** than it did before. Under [Approved Document L 2026](/news/future-homes-standard-adl-2026/), new homes will no longer use fossil fuel heating. Heating and hot water move to electricity — heat pumps or heat networks — with on-site renewable generation under the new PV requirement. Which means a new development's electrical load profile is fundamentally different from a gas-heated equivalent: higher peak demand, heat pumps and EV charging, and PV export back onto the network. In other words, the Future Homes Standard makes every new site a bigger ask of the local network at exactly the point when connection queues are already the binding constraint. Anything that shortens network reinforcement lead times is, indirectly, a Part L delivery measure. The new [NPPF chapter on securing clean energy and water](/news/nppf-2026-clean-energy-and-water-w1-w4/), published 17 August 2026, made the same point from the planning side: policies W1 to W4 put network capacity into plan-making and give substantial weight to the infrastructure that fixes it. This consultation is the deregulatory half of the same argument. ## Should you respond? If you're a housebuilder or developer with sites where a connection date is driving the programme, this is a short consultation with a specific question, and the case for it is one you can evidence better than most people — you know what a slipped connection date costs. Worth saying plainly: this is a proposal about surveys, not about capacity. It will not conjure headroom on a constrained network, and nobody should tell a client it will. But the front end of a reinforcement is one of the few parts of the chain where planning reform can actually move the date, and the consultation is open for another week and a bit. Local planning authorities have been asked to engage with it directly. If you deal with an LPA that has been vocal about grid constraints in its local plan evidence base, it's worth checking they've seen it. **The consultation closes on 4 September 2026.** --- If you're planning around a Future Homes Standard changeover and want to know where your plots sit on the [24 March 2027 and 24 March 2028 transitional dates](/news/future-homes-standard-transitional-deadline-commencement/), [get in touch](/contact/) — that's the deadline we get asked about most, and the definition of "commenced" catches people out. *Sources: [Letter from MHCLG to Chief Planning Officers — Planning Update Newsletter, 18 August 2026 (PDF)](https://planningjungle.com/wp-content/uploads/Letter-from-MHCLG-to-Chief-Planning-Officers-Planning-Update-Newsletter-18-August-2026.pdf) · [Planning guidance letters to chief planning officers, GOV.UK](https://www.gov.uk/guidance/planning-guidance-letters-to-chief-planning-officers) · [Electricity network infrastructure: consents, land access and rights — government response, GOV.UK](https://www.gov.uk/government/consultations/electricity-network-infrastructure-consents-land-access-and-rights/outcome/electricity-network-infrastructure-consents-land-access-and-rights-government-response) · [DESNZ publishes a consultation (until 04/09/2026) relating to permitted development rights for ground investigations and surveys for electricity network infrastructure, The Planning Jungle, 12 August 2026](https://planningjungle.com/2026/08/12/desnz-publishes-a-consultation-until-04-09-2026-relating-to-permitted-development-rights-for-ground-investigations-and-surveys-for-electricity-network-infrastructure/)* --- # The hottest summer on record — and a Part O that hasn't moved since 2021 URL: https://energycount.co.uk/news/record-summer-2026-part-o-overheating-gap/ Date: 2026-08-26 Overheating assessments get done against a fixed rulebook. The weather does not read it. On **11 August 2026** the Met Office published provisional statistics showing the UK on course for its **warmest summer on record for mean temperature**. It's worth reading the numbers properly, because they're the backdrop to every Part O conversation you'll have this autumn. ## What the Met Office actually published From **1 June to 10 August 2026**, the UK mean temperature stood at **16.48°C — 1.88°C above the 1991–2020 average**. The existing UK record is **16.12°C, set in 2025**. To merely *equal* that record, the rest of August would have had to average 0.21°C below the long-term summer average. Anything warmer sets a new record, in a series that starts in **1884**. England and Wales are further ahead again: | Area | Mean temp (1 Jun – 10 Aug 2026) | Difference from average | Current record | | --- | --- | --- | --- | | UK | 16.48°C | +1.88°C | 16.12°C (2025) | | England | 18.19°C | +2.46°C | 17.44°C (2025) | | Wales | 16.80°C | +2.17°C | 16.09°C (2025) | Met Office senior scientist Mike Kendon noted this followed the **second-warmest June and second-warmest July on record**, with 38°C reached in June and 35°C in both late May and July. The 1976 record for average *maximum* temperature was also described as under threat. Two things are worth separating here. A single hot summer is weather, not a methodology change. But the direction is consistent — and 2025 held the previous record, which tells you something about how quickly the reference points are moving. ## Meanwhile, the compliance position hasn't moved Here's the awkward part for anyone designing homes right now. **Approved Document O is still the 2021 edition.** When MHCLG published the Future Homes Standard package on 24 March 2026, Part O was pulled out of it. Instead of an updated AD O alongside the new Parts L and F, the government announced a **standalone full review of Part O**, reflecting the volume of issues raised at consultation. That review has **no published date**, and because Part O sits outside the Future Homes Standard transitional window, it does not automatically share the 24 March 2027 in-force date that applies to Parts L and F. **The dynamic modelling methodology has moved on without it.** CIBSE published a substantially revised [**TM59:2026**](/news/cibse-tm59-2026-overheating-update/) in July — a passive-first three-stage method with four assessment criteria and a rewritten bedroom night-time test. Approved Document O still points at the 2017 edition. If you run a dynamic assessment, you now have to be explicit about which edition you've used and why, because your Building Control body may not assume the same one you did. **Planning has moved too.** Since **17 August 2026**, [NPPF policy **CC3**](/news/nppf-2026-overheating-policy-cc3/) requires development proposals to use design approaches which minimise risks from overheating. That's a decision-making policy applying at planning — years ahead of any revised AD O. So the practical position is a widening gap: the evidence base is getting hotter, planning has an overheating test, the modelling methodology has been rewritten, and the statutory guidance is five years old. ## What this means on a live job A few things worth doing now rather than in a year's time: - **Don't treat the simplified method as free.** The Part O simplified method was written against a different climate baseline. Where a plot is borderline — south or west glazing, single aspect, urban site with restricted openable area — a dynamic assessment gives you evidence you can defend at both planning and Building Control. - **State your TM59 edition.** Put it in the report front sheet. If you use TM59:2026, say so; if you use the 2017 edition because AD O references it, say that too. Ambiguity is what gets queried. - **Deal with overheating at planning, not after.** Under CC3 a planning officer can now ask the question, and retrofitting a fix after permission — deeper reveals, external shading, changed window schedules — is expensive and often unpopular with the elevations. - **Watch the interaction with Part L.** The FHS package already recognises this: where air conditioning or reduced g-value glazing is installed to comply with Part O, the Part L 2026 standards account for the resulting energy use, so you aren't penalised twice for the same design decision. That's a change in your favour — use it. - **Remember airtightness cuts both ways.** Tighter fabric under FHS makes purge and night ventilation strategies more important, not less. A home that can't dump heat overnight is a home that fails a night-time criterion. ## The honest summary Nothing in the Met Office release changes a single input in your model this week. What it changes is the risk profile of the assumption that a marginal pass today will still look marginal in ten years — and the likelihood that when the Part O review does land, it lands harder rather than softer. If you've got a scheme where overheating is close to the line, that's the one to model properly now. Summers like this one make the fabric's summer behaviour worth a look alongside the glazing and shading. The [thermal mass calculator](/tools/u-value-calculator/thermal-mass-calculator/) gives the decrement factor and time lag for any build-up to BS EN ISO 13786, free — the two numbers that decide how much of the afternoon reaches a bedroom at night. If you've got a plot where overheating is close to the line — flats, single aspect, constrained urban sites, or anywhere a planning officer has already asked the question under CC3 — [get in touch](/contact/) and we'll tell you how we'd approach the [Part O assessment](/services/part-o-overheating/) and what evidence is worth putting in front of Building Control. *Sources: [Warmest UK summer on record "increasingly likely" as temperatures stay well above average, Met Office, 11 August 2026](https://www.metoffice.gov.uk/blog/2026/warmest-uk-summer-on-record-increasingly-likely-as-temperatures-stay-well-above-average) · [The Future Homes and Buildings Standards — written statement, UK Parliament, 24 March 2026](https://questions-statements.parliament.uk/written-statements/detail/2026-03-24/hcws1445) · [Government announces full review of Overheating Regulations, Elmhurst Energy, 9 April 2026](https://www.elmhurstenergy.co.uk/blog/2026/04/09/government-announces-full-review-of-overheating-regulations-following-industry-feedback/) · [National Planning Policy Framework, August 2026 (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/6a8334c03bd75b81e2329ac4/National_Planning_Policy_Framework.pdf) — policy CC3* --- # Your drainage evidence just became a validation document URL: https://energycount.co.uk/news/nppf-2026-dm2-validation-lists-suds-statement/ Date: 2026-08-25 Every developer has a story about validation. One council wants a drainage strategy up front; the next takes the application and conditions it; a third asks for something nobody has heard of because a policy officer added it to a list in 2019. The **National Planning Policy Framework published on 17 August 2026** takes a run at that problem — and in doing so quietly moves drainage evidence forward in the programme. ## What policy DM2 says DM2 sits in the new decision-making chapter, alongside DM1 (preparing proposals) and DM3 (determining them). It has two limbs, and both matter: > "1. To ensure a clear and consistent approach to the information required to determine development proposals, local validation lists setting out the information required in support of an application for development should include the information specified in the relevant national decision-making policies (summarised in Annex C). > > 2. Local validation lists should only include additional information requirements if there is a policy in the development plan requiring a specific further assessment. Any such additional information requirements should not be applied equally to all applications but should be proportionate to the scale of development and its potential impact. Where appropriate, the requirements should clearly distinguish between what is required for major, medium and other types of development proposal." Limb 1 sets a floor. Limb 2 sets a ceiling — and it is a genuine constraint on local practice. A validation requirement that isn't in Annex C and isn't traceable to a development plan policy is now, on the face of national policy, one the council should not be applying. Blanket requirements applied equally to a 200-home scheme and a pair of semis are specifically called out. ## Annex C: what's on the national list Annex C summarises the information requirements that flow from the Framework's own decision-making policies. For anyone working on drainage, three entries matter: - a **site-specific flood risk assessment**; - a **SuDS statement** — a statement showing how the national standards have been met; - a **coastal change vulnerability assessment**, where relevant. That is the shift. A SuDS statement is not a document you produce when a condition is discharged. It is a document the application needs in order to be valid. It joins up with **policy F8**, which requires sustainable drainage on development with drainage implications to be designed in accordance with **Defra's 2025 National Standards for SuDS**. F8 tells you what the design has to comply with; DM2 and Annex C tell you that you have to show it at the front door. ## Why this changes the programme, not just the paperwork Under the old pattern, a lot of drainage work happened after permission. Outline consent came with a condition requiring a detailed drainage strategy; infiltration testing got done when the site was accessible; if the ground turned out to be worse than assumed, the layout absorbed it. Front-loading the SuDS statement breaks that sequence in a useful and an uncomfortable way at the same time. **The useful part.** Drainage constraints surface before the layout is fixed by a planning permission. If a soakaway won't work on this site, finding out at application stage costs a drawing revision. Finding out at discharge-of-conditions stage costs a redesign, a Section 73 and a delay. **The uncomfortable part.** You need real infiltration data earlier. A SuDS statement that leans on a desk-study assumption about permeability is a statement waiting to be unpicked. **BRE Digest 365** soakaway testing is not something you can do retrospectively once the layout is committed. ## What a defensible SuDS statement needs to show Nothing here is new engineering — what has changed is when it has to exist. Expect to need: - **The drainage hierarchy applied and evidenced.** Infiltration first, then discharge to a watercourse, then to a sewer — with a reason for each step you rule out. "Not feasible" without test data is not a reason. - **Infiltration testing to BRE Digest 365**, in the right locations, at the right depth, with the results and the design infiltration rate stated. If testing is genuinely not possible yet, say so explicitly and say what happens if the assumption fails. - **Compliance with the 2025 National Standards** stated against the standards themselves, not against a generic paragraph about sustainable drainage. - **Storage volumes, discharge rates and climate change allowances** shown, with the design event and the exceedance route. - **Maintenance and adoption** — who maintains what, for how long, funded how. - **Proportionality.** DM2 explicitly distinguishes major, medium and other development. A small scheme should not be asked for a major scheme's evidence pack, and you are entitled to say so. ## The other half of the policy: pushing back DM2(2) is the clause worth keeping in your back pocket. If a council's validation list asks for a document that is neither in Annex C nor required by a specific development plan policy, national policy says it should not be there. Two practical caveats before you rely on that: 1. **Local validation lists take time to catch up.** Councils will be revising them over the coming months, and the Planning Practice Guidance published alongside the Framework had topics still referring to the previous NPPF immediately after publication. Check what version you are arguing against. 2. **"Proportionate" is a judgement, not a threshold.** DM2 gives you a principle, not a number. It is a good argument at pre-application; it is a weaker argument once validation has been refused. ## What to do about it 1. **Move the soakaway testing earlier in your programme.** If the drainage strategy is now a validation document, the test results that underpin it have to exist before submission. 2. **Write the SuDS statement against the National Standards explicitly**, clause by clause. A statement that references them generally invites a request for more information — which is exactly the trigger DM3 gives an authority for pausing a decision. 3. **Be honest about assumptions.** If you are designing on an assumed infiltration rate, state it, state the sensitivity, and state the fallback. An acknowledged assumption is a far better position than one discovered later. 4. **Review the local validation list against DM2 before you accept it.** Particularly on smaller schemes, where blanket requirements are most likely to be disproportionate. 5. **Don't assume a condition will save you.** The whole direction of the new Framework is that the evidence arrives with the application. ## The bigger picture Sustainable drainage has spent years in an odd position in England: mandatory in Wales through Schedule 3, non-statutory here, and delivered through whatever the planning system could be persuaded to require. The August 2026 Framework has not commenced Schedule 3 — there is still no SuDS Approving Body in England — but it has done something with a similar practical effect. It has named the standards, required compliance with them, and made the evidence a condition of your application being accepted at all. For most developers that means one thing: the drainage engineer needs to be in the room earlier. If you have a site where infiltration is the open question — or a SuDS statement that needs to stand up at validation rather than at condition stage — [get in touch](/contact/) and we'll tell you what testing and evidence we'd want to see before it goes in. *Sources: [National Planning Policy Framework, August 2026 — policies DM2, DM3 and F8, and Annex C, GOV.UK](https://www.gov.uk/guidance/national-planning-policy-framework) · [National standards for sustainable drainage systems (SuDS), GOV.UK](https://www.gov.uk/government/publications/national-standards-for-sustainable-drainage-systems/national-standards-for-sustainable-drainage-systems-suds) · [NPPF August 2026 changes: flood risk and drainage implications for planning, Unda](https://www.unda.co.uk/news/nppf-august-2026-flood-risk-drainage-changes/)* --- # Natural England's duty to advise on planning applications has gone URL: https://energycount.co.uk/news/statutory-consultee-reform-natural-england-environment-agency/ Date: 2026-08-25 If you have ever waited eleven weeks for Natural England to comment on a nutrient budget, or watched an application stall while the Environment Agency worked through a queue, the government has just changed the rules of that game — though not in the direction most people expected. **MHCLG published its response to the "Reforms to the statutory consultee system" consultation on 17 August 2026**, the same day the new National Planning Policy Framework landed. It attracted far less attention than the NPPF, but for anyone whose schemes depend on environmental sign-off, it matters just as much. ## The headline change: Natural England no longer has to answer The most significant line in the document is easy to miss, because it is a change made by primary legislation rather than by this response: > "The Planning and Infrastructure Act 2025 amends Section 4(1) of the Natural Environment and Rural Communities (NERC) Act 2006 to remove the requirement for Natural England to provide advice on any request from LPAs relating to development under the TCPA. This will allow Natural England to prioritise its advice and focus their input on high risk and high opportunity casework." That is a real shift. Natural England will still respond where the law separately requires it — the Town and Country Planning (Development Management Procedure) Order still prescribes it as a consultee on certain applications — but the open-ended duty to advise whenever an authority asks is gone. What replaces it is a **prioritisation framework**, to be set out in an **Operational Statement** produced with Defra. That document is not published yet. The response says only that it is "proposed to be in place shortly", and adds: > "Until the Operational Statement is published, Natural England's existing approach to dealing with planning casework continues to apply." So there is a live gap: the duty has been removed, but the replacement rules for what Natural England will and won't look at have not been written down. If you are relying on Natural England advice for a nutrient neutrality case, that Operational Statement is the document to watch. ## The Environment Agency: no change to criteria, but explicit triage The response makes no change to when the Environment Agency must be consulted. What it does confirm is that the Agency intends to ration its own effort: > "The Environment Agency is committed to effective triaging to ensure efficiency of responses whilst ensuring environmental protection and sustainable development by focussing resource on more complex casework. This includes recruiting and training staff, improving guidance, developing new case management systems, data standards, alongside utilising Artificial Intelligence (AI) to triage and process casework, and enhanced geospatial tools to support users." Respondents were blunt about the risk. The response records "strong cross-sector consensus that complex environmental risks, including flooding, water quality, hydrology, contaminated land and cumulative ecological impacts, require specialist scrutiny", and warns that "reframing objections as 'advice' or relying too heavily on standing advice could weaken safeguards". Money is going in alongside the triage: **£100 million** to modernise the environmental planning services of Natural England, the Environment Agency and the Forestry Commission, plus a Defra **Lead Environmental Regulator** model being piloted on eight major projects to stop developers receiving conflicting advice from different Defra bodies. ## The bit that bites: NPPF policy DM3 None of the above would matter much if authorities simply waited for the advice anyway. The new NPPF closes that door. Policy **DM3(1)(d)** tells local planning authorities to: > "Consult statutory or internal consultees only where it is necessary to do so. Decisions on development proposals should not be delayed in order to secure advice from a statutory or internal consultee beyond their statutory deadlines unless there is insufficient information to make the decision, there are public safety risks from proceeding without advice, or more detailed advice may enable an approval rather than a refusal" Read that alongside the consultee reforms and the direction is clear. Consultees will comment on less; authorities are told not to wait; and the exception that keeps the door open is **"insufficient information to make the decision"**. That exception is the whole ballgame. If your submission is thin, the authority now has a named national policy reason to pause. If your submission is complete, the same policy pushes them to decide. ## What else the response confirms - **No further removals.** "At this stage, we are not proposing to remove any additional statutory consultees", and the moratorium on creating new ones stays. - **The Gardens Trust and Theatres Trust lose statutory consultee status**, replaced by notification duties. **Sport England is retained** but with a narrowed list of referral triggers — the government expects its referrals to "fall by half or more" from around 1,100 applications a year. - **The Mining Remediation Authority will stop routinely commenting on discharge of conditions** — worth knowing if you work on coalfield sites, where MRA comments at condition stage have been a familiar delay. - **Water companies will not become statutory consultees on planning applications.** The government's position is that "strategic issues, such as water capacity, drainage, and wastewater management are best dealt with at a strategic level" — through plan-making, "Requirement to Assist" regulations and the forthcoming Clean Water Bill. If wastewater headroom is your constraint, the fight has been moved to the local plan. - **A wider aim to trim advice.** The ministerial foreword states the government "will also take forward changes to reduce the scope of advice provided by some of the largest statutory consultees." ## Timing There is no commencement date. The foreword says only that "regulations will follow in due course to bring these reforms into force", with Planning Advisory Service support for authorities. A further consultation is promised on a **planning fee surcharge** to fund key statutory consultees. The NPPF policies, by contrast, apply to decisions **now**. ## What to do about it 1. **Stop treating consultee advice as a design service.** The clear policy direction is that consultees respond to complete, well-evidenced submissions rather than helping you work out what the answer should be. Bring the assessment to them, not the question. 2. **Front-load the technical evidence.** Nutrient budgets, drainage strategies, infiltration testing, contamination assessments — anything that would previously have been left to a condition is now the thing that decides whether "insufficient information" applies to your application. 3. **Watch for Natural England's Operational Statement.** It will tell you which categories of case still get individual advice and which fall to standing advice. Until it appears, existing practice continues — so cases submitted in this window sit in an unusually uncertain space. 4. **Check whether your nutrient mitigation route depends on Natural England capacity.** Credit-based schemes and bespoke on-site mitigation carry different exposure to a consultee that is being told to prioritise. 5. **If wastewater capacity is the constraint, engage at plan-making stage.** The response is explicit that this is where the government wants the issue resolved. ## The bigger picture This is deregulation of process rather than of standards. The Habitats Regulations still apply. Nutrient neutrality obligations still apply. Flood risk policy has just been strengthened, not weakened, in the new NPPF. What has changed is who does the thinking — and the answer is increasingly the applicant, not the regulator. For schemes in nutrient-affected catchments that is a meaningful shift. A nutrient budget that would once have been iterated with Natural England now needs to be right when it is submitted. If you have a scheme in a nutrient catchment and want the budget checked before it goes in — or you would like a second opinion on mitigation that was calculated some time ago — [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [Reforms to the statutory consultee system: government response, GOV.UK, 17 August 2026](https://www.gov.uk/government/consultations/reforms-to-the-statutory-consultee-system/outcome/reforms-to-the-statutory-consultee-system-government-response) · [National Planning Policy Framework, August 2026 — policy DM3, GOV.UK](https://www.gov.uk/guidance/national-planning-policy-framework) · [The government publishes its response to the consultation relating to statutory consultees, The Planning Jungle, 18 August 2026](https://planningjungle.com/2026/08/18/the-government-publishes-its-response-to-the-consultation-until-13-01-2025-relating-to-statutory-consultees/)* --- # Half of England's councils failed the Housing Delivery Test — what the 2025 results mean URL: https://energycount.co.uk/news/housing-delivery-test-2025-results/ Date: 2026-08-24 Buried in the noise of NPPF week was a data release with real consequences for where the next few years of housing schemes will come forward. On **17 August 2026**, MHCLG published the **2024 and 2025 Housing Delivery Test (HDT) measurements** — two years at once, after a delay, to bring the test back up to date and re-establish the annual cycle. The **2025 measurement is the current one** for decision-making, and the Chief Planner's newsletter directs authorities to use it. ## What the test measures The HDT is the government's backward-looking check on delivery: net homes actually completed in an authority's area over a rolling three-year period — for the 2025 measurement, **2022/23 to 2024/25** — divided by the homes required, published as a percentage. It measures completions, not permissions. An authority can be granting consents freely and still fail if the homes aren't getting built. It's the counterpart to the forward-looking **five-year housing land supply**: one asks whether homes were delivered, the other whether enough deliverable sites exist for the next five years. Either can now expose an authority to national policy consequences. ## The 2025 numbers Of **301 authorities with a numerical result** (the Isles of Scilly has none): - **147 — nearly half — scored below 95%**, triggering at least the requirement to publish an action plan; - **35** sat between 85% and 94%; - **19** sat between 75% and 84%, where a **20% buffer** is added to the authority's housing land supply requirement; and - **93 — almost a third of England's planning authorities — scored below 75%**, the most serious threshold. The remaining 154 authorities (just over half) reached 95% or above. ## The 75% line now works differently Under the pre-2026 framework, a sub-75% HDT result switched on the old paragraph 11 "tilted balance". The **August 2026 NPPF rewires this**: a score below 75% now counts as **evidenced unmet housing need** for the purposes of policy **S5(1)(j)**, feeding the restructured **S3–S6 presumption in favour of sustainable development**. For a suitable housing scheme outside a settlement in a sub-75% district, that evidence can bring the proposal within the favourable S5 policy category — and the same unmet-need evidence is relevant to the grey-belt route in Green Belt authorities. In plain terms: in the 93 districts below the line, the policy case for housing on suitable unallocated sites just got materially stronger, and refusals in those places will be harder to defend at appeal. A low score doesn't override design, highways, heritage, flood risk or Green Belt policy — but it changes the starting point. The **HDT Rule Book was also updated on 17 August** to reflect unmet-need adjustments to local housing need and transitional arrangements for the revised standard method, and those changes were applied to both new measurements. ## What this means if you build — or assess — new homes Expect activity to cluster where the test was failed. Promoters will move on sites in sub-75% districts while the evidence is fresh; authorities under the 20% buffer will be looking to grant permissions that shore up their supply; and appeal decisions over the next year will show how inspectors handle the new S5 mechanics. The 2025 results are the current measurement until the next annual release. For assessors and housebuilders, the timing matters. Schemes unlocked by these results will be designed and submitted straight into the [Future Homes Standard transition](/news/future-homes-standard-transitional-deadline-commencement/) — building regulations applications from 24 March 2027 will be under the FHS, with SAP 10.3 as the compliance methodology at launch. A site that gets its planning permission because of a weak HDT score still has to get through Part L, and under the FHS notional dwelling the fabric and bridging detail will be doing more of the work than ever. If you've got schemes coming forward — especially in a sub-75% district where the planning window has just opened — [get in touch](/contact/) about your Part L and SAP 10.3 strategy early. Bespoke thermal-bridging PSI values instead of punitive defaults remain one of the cheapest compliance margins available, and that's exactly what **ΨMonkey** and our assessor team are for. *Sources: [Housing Delivery Test: 2025 measurement, MHCLG, GOV.UK](https://www.gov.uk/government/publications/housing-delivery-test-2025-measurement) · [Housing Delivery Test: 2024 measurement, MHCLG, GOV.UK](https://www.gov.uk/government/publications/housing-delivery-test-2024-measurement) · [Housing Delivery Test measurement rule book, GOV.UK](https://www.gov.uk/government/publications/housing-delivery-test-measurement-rule-book) · [Housing Delivery Test results 2024 and 2025 published, Planning Geek, 17 August 2026](https://www.planninggeek.co.uk/2026/housing-delivery-test-results-2025/) · [Planning Update Newsletter, 18 August 2026 — Letter from MHCLG to Chief Planning Officers, GOV.UK](https://www.gov.uk/guidance/planning-guidance-letters-to-chief-planning-officers)* --- # Your SuDS strategy now has a standard form — national templates published URL: https://energycount.co.uk/news/suds-strategy-templates-august-2026/ Date: 2026-08-24 A week after the new NPPF made Defra's National Standards the named rulebook for sustainable drainage, the paperwork has caught up. Tucked into the **Chief Planner's Planning Update Newsletter of 18 August 2026** is a practical announcement that will touch almost every planning application with drainage implications: the **Environment Agency has published national SuDS Strategy Templates**, produced with **CIRIA, the Association of SuDS Authorities (ASA), Arup** and a working group of SuDS practitioners. The stated aim is disarmingly simple: help applicants "submit the right information on SuDS first time", and make it easier for local planning authorities and Lead Local Flood Authorities to check whether a proposed drainage system has been designed in accordance with **Defra's National Standards for sustainable drainage systems**. ## Two templates, picked by scale To reflect the proportionate approach in the new National Planning Policy Framework, there are two versions: - a **SuDS Strategy Template for Major Development** — broadly, schemes of 10 or more homes or the non-residential equivalent; and - a **SuDS Strategy Template for Non-major Development** — everything smaller, where a full major-scheme strategy would be overkill. Applicants are told to pick the template that best fits the scale of the proposal, and to download it fresh each time from the **Planning Portal or the CIRIA website** so they're always working from the latest iteration rather than a saved copy. ## Why this matters more than it sounds Templates are rarely news. This one is, for two reasons. First, the **Environment Agency is explicitly encouraging LPAs and LLFAs to adopt the national templates** and drop locally produced versions. Anyone who submits drainage strategies across several authorities knows the current reality: each LLFA has its own pro-forma, its own appetite for detail and its own pet requirements. A single national format — like the standardised decision-making policies in the new NPPF — is aimed at squeezing out exactly that local variation. Second, a standard form cuts both ways. It tells you precisely what to include, and it makes what's missing impossible to hide. Expect the template to walk through the things the National Standards already ask for: where your site sits on the **discharge hierarchy** (reuse, then infiltration, then a watercourse, then a surface water sewer, with a combined sewer strictly last), the evidence behind that choice, runoff rates, storage volumes, the treatment train and the maintenance arrangements. A drainage strategy that asserts "infiltration is not viable" without percolation test results to show for it is going to look thin in a box that asks for the data. For anyone sizing soakaways, that means the familiar routine — trial pits and infiltration testing to **BRE Digest 365**, storage checked across a range of storm durations at the design return period with a climate change uplift — is no longer just good practice. It's the evidence the checking officer's form now expects to see, in the place they expect to see it. ## Free SuDS training for planners Alongside the templates, the Environment Agency and the **Town and Country Planning Association** have launched a free, interactive e-learning module on securing good SuDS through planning, developed with experts from ASA and with input from CIRIA and CIWEM. It's aimed at planners and flood-risk practitioners, self-paced, and [open to anyone to register](https://learning.tcpa.org.uk/courses/intro-suds-for-planners). If the person determining your application has just done a course on what a compliant SuDS strategy looks like, it's worth knowing what they were taught. ## The bigger picture This lands as part of a deliberate sequence. The [August 2026 NPPF's policy F8](/news/nppf-2026-suds-national-standards-policy-f8/) requires SuDS on all development with drainage implications to be designed in accordance with the National Standards; Schedule 3 of the Flood and Water Management Act stays uncommenced, with the government betting instead on this strengthened planning-led approach. Named standards, a national submission format, trained checking officers — the direction is consistent, and it all points to drainage strategies being marked against a rubric rather than negotiated case by case. The practical advice is unchanged but sharper: **test early, and test properly**. An infiltration rate assumed at layout stage and disproved at discharge-of-conditions stage still means a redesign — only now the gap will be visible on a standard form at validation rather than surfacing quietly later. **SoakMonkey** runs the BRE Digest 365 soakaway method in your browser — infiltration rate from your percolation results, storage sizing tested across storm durations, and a Building Control-ready report to drop straight into your SuDS strategy. Sizing on screen is free; the PDF export is £25 +VAT (£30 inc VAT). [Try SoakMonkey →](/tools/soakaway-calculator/) And if a planning condition is asking for a SuDS strategy to the National Standards and you're not sure how your site will behave, [get in touch](/contact/) — a trial pit and an hour of sizing usually settles it. *Sources: [Planning Update Newsletter, 18 August 2026 — Letter from MHCLG to Chief Planning Officers, GOV.UK](https://www.gov.uk/guidance/planning-guidance-letters-to-chief-planning-officers) · [National standards for sustainable drainage systems (SuDS), Defra, GOV.UK](https://www.gov.uk/government/publications/national-standards-for-sustainable-drainage-systems/national-standards-for-sustainable-drainage-systems-suds) · [Introduction to SuDS for Planners (free e-learning), TCPA](https://learning.tcpa.org.uk/courses/intro-suds-for-planners)* --- # The EPC accuracy audit: homes use less energy than the model says URL: https://energycount.co.uk/news/epc-accuracy-research-performance-gap/ Date: 2026-08-23 If you lodge EPCs for a living, there's a government research report you should know about — because it quantifies, home by home and month by month, how far the modelled numbers drift from what the meter actually records. The **EPC accuracy research**, published by DESNZ on **26 May 2026** and produced by the **UCL Energy Institute with Alan Pither Limited**, compared EPC-modelled energy use against smart-meter and temperature data from **over 1,100 homes**. It was commissioned to feed the EPC Action Plan and the development of the **Home Energy Model** — which makes it, in effect, the evidence base the next methodology will be judged against. It didn't make much noise when it landed, but with HEM held back for final assurance, it's exactly the kind of evidence being pored over right now. ## The headline numbers On average, **gas-heated homes used 16.0% less energy than their EPC models predicted**. Electrically heated homes showed a much bigger gap: **31.4% less** on average, peaking at **47% in December** — midwinter homes using barely half the energy the model says they should. The 16% figure isn't fixed, and the way it shrinks is telling. When the researchers rebuilt each model with the current RdSAP version, the actual weather during monitoring, the occupants' real heating patterns and any improvements installed since the certificate was lodged, the gas gap narrowed to **10.9%**. More than half of that correction came from a single source: work done to the home *after* the EPC was issued. The biggest "inaccuracy" in the EPC stock isn't the calculation — it's certificates that no longer describe the building. For homes whose boiler had been replaced since lodgement, updating the model shrank the gap from 22.1% to 9.5%. ## Where the model drifts Because the study broke energy use down by fuel and season rather than comparing annual totals, it could spot offsetting errors that headline figures hide: - **Homes are warmer than SAP assumes.** Measured internal temperatures ran above modelled ones, even in band F and G homes — so underheating can't explain the gap. The data also gave little support to SAP's two-zone assumption that the non-living area sits up to 3°C cooler; in practice the zones track each other closely. - **Ventilation defaults inflate heating demand.** RdSAP 2012 assumes an average heating-season air change rate of 1.02 ach for F and G rated homes against 0.69 for A and B. Field measurements suggest real rates are often lower — sometimes below SAP's 0.5 ach minimum, which the report recommends reviewing for both SAP and HEM. - **Solar gains are underpredicted — by up to 54%.** April 2021, the sunniest April on record, acted as a natural experiment, and RdSAP underpredicted the effect of passive solar heating by 54% against monitored data. Age-band glazing assumptions are a suspected culprit; RdSAP 10's measured glazing should close part of the gap. - **The summer fuel split is wrong.** Total summer delivered energy comes out almost exactly right, but metered summer gas use is 34% higher than modelled while electricity use in gas-heated homes runs 18.2% lower year-round. The errors cancel in a delivered-energy metric — but not in a cost or primary-energy metric. ## The new-build sting One finding runs against the grain of everything above, and it's the one that matters most for our corner of the industry: **older homes outperform their models, but new homes underperform theirs**. In the existing stock, age-based fabric defaults are pessimistic — real walls, roofs and floors lose less heat than assumed. In new builds the gap reverses: **measured heat loss is higher than modelled**, consistent with every as-built performance study of the last decade. Design-stage assumptions about fabric, junctions and airtightness are flattering what's actually delivered on site — which is precisely the gap the FHS's tighter fabric standards, mandatory testing culture and (eventually) HEM's half-hourly engine are meant to squeeze. There's a second new-build wrinkle: homes assessed as built with full SAP get a *worse* EPC when they're later reassessed for sale under RdSAP, because the reduced-data procedure falls back on defaults in place of construction data that existed all along. The report's fix is sensible — give RdSAP assessors access to the original as-built SAP calculation for any home built since 2008. ## What it means for HEM and reformed EPCs Four modelling assumptions are flagged for review in HEM as well as SAP: the two-zone heating assumption, seasonal electricity demand, the heat transfer coefficient assumptions, and the 0.5 ach ventilation minimum. The study also demonstrates that smart-meter energy signatures can validate a model version against measured reality — including half-hourly models, which is exactly what HEM produces. SAP ran for three decades without a standing accuracy audit; HEM could launch with one built in. Two recommendations read as direct endorsements of the reform direction: drop primary energy as a headline certificate metric, and make EPCs dynamic — updated automatically when regulated work like a boiler swap is recorded, rather than frozen for ten years. That lines up with the [four-metric reformed EPC](/news/reformed-epcs-four-metrics-home-energy-model/) due in the second half of 2027, and with the measured-performance direction set out in the [SMETER Strategic Guide](/news/smeter-strategic-guide-measured-performance/). ## What to do with this For assessors, the direction of travel is unmistakable: modelled assessments are heading into an era where they'll be routinely checked against metered reality. The practical response is the same one we'd give for any compliance model — get the inputs right. Assessor error alone shifted predicted space and water heating by 6% on average in the resurveyed subsample, mostly through misrecorded dimensions, heating systems and wall insulation. And on the new-build side, the finding that as-built heat loss exceeds the model is a reminder that default junction values and optimistic PSI assumptions are exactly where paper performance leaks away. If you'd rather your junctions were calculated than assumed, that's our patch — modelled PSI values that stand up when someone eventually puts a meter on the result. [Get in touch](/contact/) and we'll tell you how we'd approach your scheme. *Sources: [Energy Performance Certificate (EPC) accuracy research, DESNZ, 26 May 2026](https://www.gov.uk/government/publications/energy-performance-certificate-epc-accuracy-research) · [EPCs Overestimate Energy Use by 16%: What the Government's Accuracy Research Found, HEM Guide](https://home-energy-model.co.uk/news/2026-07-15-epc-accuracy-research/)* --- # Stodmarsh movement: Stour nutrient credits are now on sale URL: https://energycount.co.uk/news/stour-nutrient-credits-stodmarsh/ Date: 2026-08-23 If you've got a scheme stuck in east Kent, this is the one to read. **Stour Environmental Credits (SEC)** — the not-for-profit company owned and overseen by **Ashford Borough Council and Canterbury City Council** — announced on **30 July 2026** that it has begun issuing **nutrient credits** for the Stour catchment. That gives developers in one of the country's most prominent stalled catchments a locally run, council-backed route to demonstrate nutrient neutrality and get planning permissions over the line. ## What's actually on offer SEC's initial mitigation programme is expected to support the delivery of **more than 1,500 new homes** across the catchment. Credits are being released in tranches rather than all at once: the **first release enables up to 30 dwellings** to progress, with further releases to follow "on a regular basis" as existing mitigation schemes progress and new projects come forward. Credits are generated from verified local mitigation projects that reduce nutrient pollution in line with Natural England guidance, and the scheme sits alongside Kent County Council's Stodmarsh Catchment Nutrient Mitigation Strategy. Because SEC is a not-for-profit, the proceeds are recycled — into buying more mitigation, developing new schemes, and monitoring the existing mitigation in perpetuity. Pricing and a register-your-interest form are on the [SEC website](https://www.stourenvironmentalcredits.co.uk/). ## Why Stodmarsh matters The **Stodmarsh SAC/SPA/Ramsar** near Canterbury is the protected site behind one of the largest nutrient neutrality blockages in England — advice that has held up thousands of homes across the Stour valley, taking in Canterbury, Ashford and neighbouring districts. And it's a **dual-nutrient catchment**: schemes have to be neutral for **both nitrogen and phosphorus**, which means a compliant scheme needs a credit position for each, not just one headline number. That's worth dwelling on before you pick up the phone. A tranche of credits is only useful once you know what your scheme actually needs — and that starts with the **nutrient budget**: occupancy, water use, wastewater treatment works, land-use change, the lot. With credits being drip-fed in releases, schemes that arrive with a calculated budget and a clear credit requirement will be the ones best placed to secure an allocation early. ## The bigger picture: local schemes still carry the load The strategic fix for nutrient neutrality — the **Nature Restoration Fund**, with Natural England preparing **Environmental Delivery Plans** so developers can discharge nutrient obligations through a single levy payment — [took its first legal steps in June](/news/nature-restoration-fund-nutrient-regulations-laid/). But the first EDPs are still being prepared, with each draft facing a 28-day public consultation before it can be approved; officials have indicated the first will launch by the end of 2026. Until an approved EDP covers your catchment, site-by-site neutrality remains the law, and locally led credit schemes like SEC are what's actually keeping consents moving. For assessors and consultants, that makes the current period a hybrid one: keep calculating budgets and securing mitigation under the existing rules, while watching where the first EDPs land — Natural England notified 23 planned EDPs back in December 2025, 16 of them for nutrients. ## What to do about it If you're promoting a scheme in the Stour catchment: run the nutrient budget now, work out your nitrogen and phosphorus credit requirement, and register your interest with SEC early — the first release is small, and allocation will favour schemes that know their numbers. NutrientMonkey covers the Stodmarsh catchment, so you can get an instant first-pass budget for your site before committing to anything. Want the full calculation done properly, ready for validation? [Send us the details](/contact/) and we'll take it from there. *Sources: [Stour Environmental Credits begin selling nutrient credits, Ashford Borough Council, 30 July 2026](https://www.ashford.gov.uk/news/latest-news/stour-environmental-credits-begin-selling-nutrient-credits-and-opening-building-opportunities-in-the-stour-catchment/) · [Stour Environmental Credits](https://www.stourenvironmentalcredits.co.uk/) · [Natural England will 'launch' first EDP by the end of year, Planning Resource](https://www.planningresource.co.uk/article/1960547/natural-england-will-launch-first-edp-end-year-official-reveals)* --- # Overheating is now a named planning policy — not just a Part O problem URL: https://energycount.co.uk/news/nppf-2026-overheating-policy-cc3/ Date: 2026-08-22 Overheating has been a Building Regulations problem since Approved Document O landed in 2021. You design the home, you run the simplified method or a TM59 dynamic model, you hand the results to Building Control, and planning barely touches it. The **National Planning Policy Framework published on 17 August 2026** changes where that conversation starts. Overheating was not absent from the previous Framework — the December 2024 version mentioned it in its general climate framing (paragraph 161) and in its plan-making paragraph on long-term risks (162). What it did not have was a decision-making policy telling applicants to design for it. The 2026 rewrite does. ## What the new policies actually say The Framework's climate chapter contains three coded policies, **CC1 to CC3**. Two of them mention overheating directly. **CC1** is the plan-making policy. It requires development plans to take a proactive approach to adapting to climate change, "taking into account the implications of extreme weather and long-term climate trends including **overheating**, wildfires, drought, flood risk, coastal change, water supply, biodiversity and landscapes." **CC3** is the national decision-making policy on adaptation, and it is the one that will show up on your desk. It requires development proposals to take account of the current and potential impacts of climate change over the lifetime of the scheme and, where relevant, to: > "Use design approaches which minimise risks from overheating in accordance with policy DP3(2)(b), and include green infrastructure and suitable tree planting in accordance with policies DP3(2)(c) and N3" Two things follow from that wording. First, overheating sits alongside flood risk, coastal change and SuDS in the same list — the company it keeps tells you how seriously it is meant to be taken. Second, it routes through **DP3**, the design policy in the well-designed places chapter, which means overheating is being framed as a design-quality issue rather than a bolt-on calculation. ## Why this matters now Because the Building Regulations side has not caught up. **Approved Document O (2021)** remains in force and still references the **2017** edition of CIBSE TM59. The government confirmed in its Future Homes and Buildings Standards consultation response on **24 March 2026** that Part O would not be amended alongside Parts L and F, and would instead get its own standalone technical review — covering, among other things, adoption of the updated TM59 methodology, fixes to the simplified method, noise and security guidance, extension to material change of use, and updated weather files. Meanwhile **CIBSE published TM59:2026** in July 2026, a substantial rewrite of the methodology that Part O does not yet point to. So the sequence is now: planning policy names overheating in August 2026; the industry methodology was rewritten in July 2026; and the Approved Document that governs compliance still references a 2017 document. That gap is where arguments happen. ## What it means in practice Nothing here creates a new numerical standard. CC3 does not set a temperature threshold, and it does not require a TM59 model. What it does is give a local planning authority a clear, named national policy hook to ask how a scheme deals with overheating — at application stage, before Building Control ever sees it. Expect that to surface as: - **Validation and consultee questions** on larger or higher-risk schemes — single-aspect flats, high glazing ratios, urban sites where opening windows is constrained by noise or air quality. - **Design-and-access or sustainability statements** being expected to say something specific about overheating strategy, rather than deferring it to "Part O at technical design". - **Planning conditions** requiring an overheating assessment or a mitigation strategy to be submitted and approved. Once that condition exists, the assessment method and the acceptance criteria become negotiable in a way they are not under Part O. - **Tension between policies.** CC3's answer to overheating is passive design, shading and green infrastructure — which can pull against glazing, daylight, density and elevational treatment expectations elsewhere in the Framework. Conflicts between Part O and other parts of the Building Regulations are one of the workstreams in the government's own Part O review, so this is a known problem rather than a theoretical one. The NPPF does not resolve it for you. ## What to do about it If you are working on schemes going into planning now: 1. **Do the overheating thinking at concept stage, not at Building Regs stage.** Orientation, glazing ratio, aspect and shading are cheap to change on a layout drawing and expensive to change once the planning permission fixes the elevations. 2. **Be explicit in the planning submission.** A short, honest paragraph on the overheating strategy — passive measures first, what you have assumed about openable windows, what you would do if the dynamic model came back tight — is much easier than answering it cold after a consultee objection. 3. **Watch which TM59 you are being asked for.** Approved Document O still means the 2017 edition. A planning condition drafted loosely as "in accordance with CIBSE TM59" now arguably means the 2026 edition, which is a different test with different criteria. Get that pinned down in the wording rather than discovering it at discharge. 4. **Do not assume Part O compliance settles the planning question.** It usually will. But CC3 is drafted around risk and design approach, not around a compliance certificate, so a scheme that scrapes through the simplified method may still attract questions. ## The bigger picture This is a familiar pattern. Water efficiency and sustainable drainage both moved from technical guidance into named planning policy before the Building Regulations changed, and both ended up being fought over at application stage. Overheating now looks set to follow the same route — with the added complication that the review of Part O is still open, so the technical standard the planning system is implicitly reaching for has not yet been written. CC3 arrives at planning stage, when the cheapest fixes are still orientation, glazing area and construction. The last of those is easy to overlook: a heavyweight build-up delays and flattens the afternoon peak in a way a lightweight one does not. The [thermal mass calculator](/tools/u-value-calculator/thermal-mass-calculator/) gives the decrement factor and time lag for any construction to BS EN ISO 13786, free, if you want that comparison before the drawings are fixed. If you have a scheme where overheating is likely to be the awkward bit — flats, constrained urban sites, high glazing, or anywhere a planning officer has already asked the question — [get in touch](/contact/) and we'll tell you how we'd approach the assessment and what evidence is worth putting in front of the planners. *Sources: [National Planning Policy Framework, August 2026 (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/6a8334c03bd75b81e2329ac4/National_Planning_Policy_Framework.pdf) — chapter 5, policies CC1–CC3 · [The Future Homes and Buildings Standards — written statement, UK Parliament, 24 March 2026](https://questions-statements.parliament.uk/written-statements/detail/2026-03-24/hcws1445) · [Future Homes and Buildings Standards consultation response (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/69c13592bb0dfe55b83e4b85/Future_Homes_and_Buildings_Standards_Consultation_Response.pdf) · [Government announces full review of Overheating Regulations, Elmhurst Energy, 9 April 2026](https://www.elmhurstenergy.co.uk/blog/2026/04/09/government-announces-full-review-of-overheating-regulations-following-industry-feedback/)* --- # Retrofit just got a stronger planning policy — 'substantial weight' under CC2 and CC3 URL: https://energycount.co.uk/news/nppf-2026-substantial-weight-existing-buildings/ Date: 2026-08-22 Most of the attention on the **17 August 2026 National Planning Policy Framework** has gone to housing numbers, stations and the rewritten flood risk chapter. There is a smaller change in the climate chapter that matters if you ever put a heat pump, solar array or fabric upgrade in front of a planning officer. ## The change The December 2024 Framework said this, at paragraph 167: > "Local planning authorities should also give **significant weight** to the need to support energy efficiency and low carbon heating improvements to existing buildings, both domestic and non-domestic (including through installation of heat pumps and solar panels where these do not already benefit from permitted development rights)." The August 2026 Framework says this, at policy **CC2(2)**: > "**Substantial weight** should be given to the benefits of improving the energy efficiency of existing buildings and/or drawing energy from district heat networks, renewable and low carbon sources (including through the installation of heat pumps and solar panels where these do not already benefit from permitted development rights) **where this would be achieved through proposals for development**." Three differences are worth pulling out. **Significant became substantial.** Planning weight is not a formally defined scale, and lawyers will argue about whether "substantial" genuinely sits above "significant". But it is not accidental drafting — the Framework uses "substantial weight" deliberately and repeatedly across its new decision-making policies, and a decision-maker now has to explain why a benefit given substantial weight by national policy was outweighed. **The benefit is the thing weighed, not the need.** The old wording weighed "the need to support" improvements. The new wording weighs "the benefits of improving". That is a subtle shift from a policy aspiration to a material planning benefit sitting on the balance. **There is a new limiting clause.** "Where this would be achieved through proposals for development" scopes the policy to what the application actually delivers. It is not a free-floating credit for being an energy-conscious applicant; the efficiency or low-carbon gain has to be part of the scheme in front of the authority. ## And a new one for resilience Policy **CC3(2)** adds a parallel provision that has no real predecessor: > "Substantial weight should be given to the benefits of improving the resilience of existing buildings and public spaces to anticipated climate change impacts where this would be achieved through proposals for development." So adaptation work — shading, ventilation strategy, surface water management, cooling resilience — now attracts the same policy weight as carbon reduction when it is delivered on existing stock. That is new, and it is likely to be useful in exactly the cases where retrofit and heritage collide. ## Where this actually bites Permitted development rights already cover a large share of domestic heat pumps and solar, and the **GPDO amendment coming into force on 27 August 2026** rewrites and broadens the rights for domestic solar specifically. This policy is aimed at everything that falls outside those rights: - **Conservation areas and listed buildings**, where external wall insulation, glazing replacement, solar panels and air source heat pumps routinely need consent. CC2(2) does not override the heritage policies — the Framework's heritage chapter still applies — but it strengthens the public-benefit side of the balancing exercise. - **Flats and mixed-use buildings**, where permitted development rights are narrower or absent. - **Non-domestic retrofit**, which is explicitly in scope and is going to matter more as MEES tightens. - **Larger schemes with a refurbishment component**, where fabric upgrades to retained buildings can now be presented as a weighted planning benefit rather than a neutral fact. ## What to do about it The practical move is to stop treating the energy performance improvement as background detail in a planning submission and start presenting it as a benefit, with numbers. - **Quantify it.** "Improves the fabric" is weak. A modelled reduction in heat demand, a before-and-after U-value schedule, an estimated bill saving or an EPC uplift is something a case officer can put on the balance and defend in a report. - **Cite the policy by name.** CC2(2) for efficiency and low-carbon heat; CC3(2) for resilience. Coded policies are easier for officers to reference than paragraph numbers, which is rather the point of the rewrite. - **Show it is delivered by the development.** Given the new limiting clause, make the link explicit between the works applied for and the performance gain claimed. - **In heritage cases, do the balance for them.** Set out the harm, set out the public benefit, and note that national policy requires substantial weight on the benefit side. That is a considerably better position than it was under paragraph 167. None of this makes a bad application good. But if you have been losing marginal retrofit and low-carbon-heat applications on the balance, the balance has just moved. Where that improvement is a renovation rather than an extension, Part L has its own view of it: Table 4.3 sets the threshold at which a retained wall, roof or floor has to be upgraded, and the upgrade only stands if it pays back inside 15 years. **RenoMonkey** works both tests for any element, free on screen, so you can see which parts of a refurbishment the regulations will actually require before the planning statement claims them as benefits. [Try RenoMonkey →](/tools/thermal-element-calculator/) If you need the performance side of a retrofit or refurbishment quantified properly — fabric modelling, U-values, thermal bridging and the evidence to back a planning statement — [get in touch](/contact/) and we'll tell you what we'd put together. *Sources: [National Planning Policy Framework, August 2026 (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/6a8334c03bd75b81e2329ac4/National_Planning_Policy_Framework.pdf) — chapter 5, policies CC2 and CC3 · [National Planning Policy Framework (December 2024), chapter 14, paragraph 167, GOV.UK](https://www.gov.uk/guidance/national-planning-policy-framework/14-meeting-the-challenge-of-climate-change-flooding-and-coastal-change) · [National Planning Policy Framework guidance page, MHCLG, 17 August 2026](https://www.gov.uk/guidance/national-planning-policy-framework)* --- # The NPPF now has a chapter about the pipes and the grid URL: https://energycount.co.uk/news/nppf-2026-clean-energy-and-water-w1-w4/ Date: 2026-08-21 Ask most developers what actually stops a site, and the answer is rarely design. It's a grid connection date, or a treatment works with no headroom, or a catchment where the nutrient position hasn't moved in three years. The **National Planning Policy Framework published on 17 August 2026** gives those problems their own chapter for the first time. **Chapter 10, "Securing clean energy and water"**, contains four policies — two for plan-making, two for decisions — and its stated objective is to support energy and water infrastructure "in ways which align with wider development, clean power and net zero objectives". ## W1: capacity has to be understood before growth is located The plan-making policy is the substantive one. **W1** requires the development plan to be informed by **early engagement between the plan-making authority, utility providers, regulators and network operators**, so there is a clear understanding of: - energy supply and network capacity - water supply, drainage and **wastewater capacity** - the additional infrastructure provision those imply That engagement has to consider planned growth, changing consumption patterns and climate change, and must draw on the relevant strategic infrastructure plans — Water Resource Management Plans, Drainage and Wastewater Management Plans, and, once published, the Strategic Spatial Energy Plan, Centralised Strategic Network Plan and Regional Energy Strategic Plans. The plan then has to reflect it by taking those factors into account **in the location and phasing of planned growth**, making provision for the new or enhanced infrastructure needed, and setting out measures to stop neighbouring development constraining the operation or future expansion of existing networks. That last limb is quietly useful. Sites near a treatment works or a substation have historically been allocated without much thought about whether the allocation forecloses the operator's expansion land. W1 asks plans to think about it. ## W2 and W3: the energy side **W2** asks plans to identify areas suitable for renewable and low carbon energy development and electricity network infrastructure — including for **re-powering and life extension** of existing sites — and to identify opportunities for development to draw heat or energy from decentralised networks such as district heat networks. **W3** is the decision-making counterpart. Substantial weight goes to the benefits of energy security, economic development and the net zero transition; to the additional benefit of re-using an established site for re-powering; and to the contribution of small-scale and community-led projects. Crucially: > "Applicants should not be required to demonstrate the need for renewable or low carbon energy development and electricity network infrastructure." Where time-limited development is proposed, applications should come with decommissioning and site restoration proposals. ## W4: water infrastructure gets the same treatment **W4** mirrors W3 for water. In considering proposals for water supply, drainage and wastewater development, substantial weight should be given to the benefits of: - providing the capacity needed to serve proposed development, and/or improving security of supply and capacity for existing users, and - **improving water quality and reducing water-borne pollution** And again: applicants should not be required to demonstrate the need for water infrastructure developments. ## Why this matters if you work in a nutrient catchment The wording of W4(1)(b) is the bit to notice. Improving water quality and reducing water-borne pollution now attracts substantial weight in its own right — which is the same problem nutrient neutrality has been solving one housing site at a time since 2018. None of this changes the Habitats Regulations. A scheme in a protected catchment still needs its nutrient budget and its mitigation, and the Nature Restoration Fund route is still being built out through Environmental Delivery Plans. But the planning framework is now more clearly on the side of the treatment works upgrade or the wetland scheme that fixes the catchment rather than mitigating around it — and W1 pushes catchment constraints into plan-making, where they belong, rather than leaving every applicant to discover them individually. For solar, W2 and W3 sit alongside the **Future Homes Standard's requirement L3** for on-site renewable electricity generation and the [new permitted development rights for domestic solar](/news/gpdo-solar-permitted-development-27-august-2026/) that take effect on 27 August. The direction is consistent: generation and network capacity are being treated as things the planning system should be actively enabling, not merely tolerating. ## What we'd suggest doing Two practical points. **Check the catchment before the offer, not after.** A nutrient budget is a short piece of work relative to what it tells you, and the answer determines whether a site is a twelve-month project or a four-year one. If you're bidding on land in the Solent, Poole Harbour, the Wensum, the Tees or any of the other affected catchments, run the numbers before you commit. **Read the local plan evidence base, not just the policy map.** W1 means the infrastructure engagement behind an emerging plan is now formally part of how growth is located and phased. If your site is in a plan and the utility engagement says the network can't serve it until 2032, that will show up in the phasing — and it is much better to know that at option stage. **NutrientMonkey** implements Natural England's published nutrient-budget methodology across all 27 NE catchments in your browser — nitrogen, phosphorus or both, with the 20% buffer included. Building your assessment on screen is free; the planning-ready PDF is £79 +VAT (£94.80 inc VAT). It's a planning-support tool, not the statutory NE calculator, and the report says so. [Try NutrientMonkey →](/tools/nutrient-neutrality-calculator/) If you've got a site in a protected catchment and you're trying to work out what the mitigation actually costs, [get in touch](/contact/) and we'll run the budget with you. *Sources: [National Planning Policy Framework, August 2026 (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/6a8334c03bd75b81e2329ac4/National_Planning_Policy_Framework.pdf) — chapter 10, policies W1–W4 and footnote 33 · [National Planning Policy Framework guidance page, MHCLG, 17 August 2026](https://www.gov.uk/guidance/national-planning-policy-framework) · [New planning framework puts water infrastructure at heart of future development, Water Magazine, 18 August 2026](https://www.watermagazine.co.uk/2026/08/18/new-planning-framework-puts-water-infrastructure-at-heart-of-future-development/)* --- # Your soakaway design now has a named rulebook — NPPF policy F8 URL: https://energycount.co.uk/news/nppf-2026-suds-national-standards-policy-f8/ Date: 2026-08-21 Drainage has always been the quiet part of a planning submission. You do the infiltration tests, size the soakaway, write the strategy, and hope the Lead Local Flood Authority agrees with your assumptions. What has been missing is a single document everyone is measuring against. The **National Planning Policy Framework published on 17 August 2026** goes some way to fixing that. Flood risk and coastal change now has its own chapter — **nine coded policies, F1 to F9** — plus a dedicated **Annex F**, and the Framework is explicit that "the annexes included with this Framework are also national planning policy". The policy that changes day-to-day work is **F8**, on sustainable drainage. ## What F8 does Two things, both practical. **It names the standard.** Development proposals with drainage implications should incorporate sustainable drainage systems, and those systems should be **designed in accordance with the National Standards for Sustainable Drainage Systems**. That's Defra's guidance, published 19 June 2025 and updated 30 July 2025. Until now it sat in an awkward position — a serious technical document that nobody was formally required to follow. National planning policy now points at it by name. **It brings the Lead Local Flood Authority into the frame for major development.** Major schemes are expected to take account of LLFA advice, which formalises a conversation most competent applicants were having anyway — but gives the LLFA's position more weight when it isn't. There's also a watercourse limb: development should not enclose existing watercourses without compelling reasons, and de-culverting and re-naturalising river channels is encouraged where possible. The Framework's climate policy reinforces all of this. Policy **CC3** on adaptation asks development to: > "Incorporate sustainable drainage systems to manage surface water flow rates and reduce volumes of runoff in accordance with policy F8" So SuDS is now cross-referenced from the climate adaptation policy as well as sitting in the flood chapter. It is harder than it used to be to treat drainage as a detail to be resolved after layout. ## What the National Standards actually ask for If you haven't read them since they landed, this is the part worth knowing. The 2025 standards replaced the thin 2015 non-statutory technical standards with **seven standards** that go well beyond peak flow control: 1. **Runoff destinations** — a drainage hierarchy. Collect for non-potable use first, then **infiltrate to ground**, then discharge to a surface water body, then a surface water sewer, and only last a combined sewer. Evidence is expected to justify anything lower down the list. 2. **Management of everyday rainfall** — intercept the first **5 mm of rainfall** so it doesn't leave the site as runoff. 3. **Management of extreme rainfall and flooding** — design requirements for infiltration features and allowable discharge rates. 4. **Water quality** — a risk assessment tailored to the land use and pollution risk. 5. **Amenity** — multi-functional places and landscapes. 6. **Biodiversity** — including links to Biodiversity Net Gain and Local Nature Recovery Strategies. 7. **Design for construction, operation, maintenance, decommissioning and structural integrity** — long-term maintenance planned in, not assumed. Look at the first standard again. **Infiltration sits second in the hierarchy**, behind rainwater reuse and ahead of every piped option. On most sites that means the honest answer to "can we soak away?" needs to be established with real infiltration testing, and it needs to be established early enough to influence the layout — not after the plots are drawn and there is nowhere left to put a soakaway at the required distance from foundations. ## What hasn't changed Worth being clear about the limits. **Schedule 3 of the Flood and Water Management Act 2010 is still not commenced in England.** There is no SuDS Approving Body, no separate statutory approval, and no legal duty in the way there is in Wales. Ministers have said repeatedly that better delivery may come through planning policy, adoption and maintenance rather than commencement, and that a final decision will be made "in due course". **The National Standards themselves remain non-statutory guidance.** What F8 does is give them a route into every planning decision, because the NPPF is a material consideration of critical importance. That's a real change in weight, not a change in legal status. The practical effect is the same either way: if your drainage strategy departs from the standards, you now need a reason you're willing to write down. ## What we'd suggest doing Test early, and test properly. The single most common cause of a drainage strategy unravelling is an infiltration rate assumed at layout stage and disproved at technical approval. **BRE Digest 365** sets out the trial pit method — three fills, the rate taken between 75% and 25% of the effective depth — and it takes a day on site plus a return visit. Doing it before the layout is fixed is cheap. Doing it after is a redesign. The second is soakaway sizing checked at a single storm duration. The worst case isn't always the shortest, heaviest storm — a long, moderate storm can fill a slow-draining soakaway that a cloudburst wouldn't. BRE 365 asks you to check a range of durations, at the design return period with a climate change uplift, and take whichever needs the most storage. **SoakMonkey** runs the BRE Digest 365 soakaway method in your browser — infiltration rate from your percolation results, storage sizing tested across storm durations, and a Building Control-ready report. Sizing on screen is free; the PDF export is £25 +VAT (£30 inc VAT). [Try SoakMonkey →](/tools/soakaway-calculator/) If you've got a planning condition asking for a SuDS strategy to the National Standards and you're not sure your site will infiltrate, [get in touch](/contact/) — a trial pit and an hour of sizing usually settles it. *Sources: [National Planning Policy Framework, August 2026 (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/6a8334c03bd75b81e2329ac4/National_Planning_Policy_Framework.pdf) — chapter 18 (policies F1–F9), Annex F, policy CC3 · [National standards for sustainable drainage systems (SuDS), Defra, GOV.UK](https://www.gov.uk/government/publications/national-standards-for-sustainable-drainage-systems/national-standards-for-sustainable-drainage-systems-suds) · [Sustainable Drainage Systems (SuDS), House of Commons Library briefing CBP 10483, 4 February 2026](https://commonslibrary.parliament.uk/research-briefings/cbp-10483/) · [New planning framework puts water infrastructure at heart of future development, Water Magazine, 18 August 2026](https://www.watermagazine.co.uk/2026/08/18/new-planning-framework-puts-water-infrastructure-at-heart-of-future-development/)* --- # Part F 2026 puts ventilation ductwork in front of Building Control URL: https://energycount.co.uk/news/part-f-2026-ductwork-evidence-building-control/ Date: 2026-08-20 Ventilation ductwork has never been the glamorous end of Building Regulations. It gets designed late, squeezed into whatever space is left, and finished with a metre of concertina'd flexible duct behind a hatch. **Approved Document F Volume 1, 2026 edition** takes a much firmer line — and unusually, it asks for paperwork before the ceiling goes on. It's a short change, but it alters how jobs get run. ## Rigid is now the default The 2026 guidance is specific about what ducts should be made of: - **Centralised continuous MEV and MVHR** systems should use **rigid or smooth semi-rigid ducts**. - **Intermittent extract fans and decentralised (d-MEV) fans** should use **rigid ducting**, with a maximum duct length of **2.0 m where practical**. - **Flexible ductwork is limited to 200 mm in length**, and only for final connections to fan unit spigots on centralised MEV or MVHR systems. - Where a system's fan control relies on constant pressure within the ductwork, **flexible ducting is not permitted anywhere in the system**. Any flexible duct that is used has to be installed so the full internal diameter is maintained and flow resistance is minimised — no sagging, no crushing, no lazy bends. ## A hard ceiling on static pressure AD F 2026 introduces **Table 1.8**, which caps the *designed* system static pressure for centralised continuous MEV and MVHR by total flow rate: | Total air flow rate | c-MEV (multiple extract spigots) | MVHR / MEV (single extract spigot) | | --- | --- | --- | | Up to and including 37 l/s | 30 Pa | 60 Pa | | 38 to 53 l/s | 50 Pa | 110 Pa | | 54 l/s and above | 80 Pa | 160 Pa | Paragraph 1.81 sets out how you're expected to hit those numbers: minimise overall duct length, minimise bends and offsets, and size the ducts appropriately for the flow rate. None of that is new engineering advice — it's just now written down as guidance you can be held to. ## The evidence requirement This is the one to flag with site teams. **Paragraph 1.78**: for all systems with a duct length greater than **2.0 m**, design calculations and drawings should be provided **to the building control body** to demonstrate that the fans, the connected ducting and the external air terminal can achieve the required flow rates, and that the static pressure principles have been met. That evidence should come from **a suitably competent person**. Two metres is not a lot of duct. In practice, most centralised MEV and MVHR installations — and plenty of d-MEV runs — will clear that threshold, which means a ventilation design package becomes a routine Building Control submission rather than something worked out on site. ## The insulation rule that reads backwards Paragraph 1.83 asks for **more** duct insulation in heated space than in cold. Intake and exhaust ductwork through conditioned areas — the heated part of the dwelling — needs the equivalent of at least **50 mm** of material at ≤0.04 W/(m·K), fully vapour sealed. Through unheated areas such as lofts and sub-floor voids, it drops to **25 mm**. Read as a Part L heat-loss rule that looks like a typo. It isn't one: it's condensation control. Intake and exhaust ducts are the two runs between the unit and the outside world, so they carry air at roughly outdoor temperature. Push a duct full of 2°C air through a warm, humid hallway and its surface sits below the room's dew point — you get condensation on the *outside* of the duct, and it finds its way through the ceiling. In a cold loft, duct and surroundings are at much the same temperature, so there's little driving force. The exhaust run is arguably the greater risk of the two: after heat recovery that air has been cooled and is close to saturation. **Mind the terminology.** In Part F, "intake" means the outdoor air intake — Section 2 is all about siting intakes away from traffic and downwind of exhaust outlets. It does *not* mean the supply ducts delivering tempered air to habitable rooms. An MVHR unit has four runs: intake and exhaust (unit to outside, covered by 1.83), and supply and extract (unit to rooms, not covered). Supply air leaves the heat exchanger close to room temperature, so it isn't a condensation risk indoors. That leaves a gap worth closing yourself. An MVHR supply or extract duct crossing a cold loft carries near-room-temperature air through an unheated space — a genuine heat-loss and condensation risk that paragraph 1.83, read strictly, doesn't cover. The installation checklist at paragraph 4.19(g) is looser, asking that "all ductwork is insulated following the guidance in paragraph 1.83", so expect some Building Control bodies to read it as covering everything. Insulating supply and extract runs in unheated spaces is sensible practice regardless of which reading you take. ## The rest of the detail A few more things worth having on the checklist: - **External air terminals** should have a free area of at least **90%** of the free area of the duct they serve. A restrictive grille can undo an otherwise good design. - **Connections** should be both mechanically secured and adequately sealed; flexible ducts need mechanical fastenings such as rigid connectors or jubilee clips. - **Access for maintenance** must be reasonable, and on MVHR units filter access should not require tools or removal of the main front cover. ## Why it matters Industry evidence has said the same thing for years: the gap between designed and delivered ventilation performance is usually an *installation* gap, not a design one. A well-specified MVHR system with 3 m of crushed flexible duct on the kitchen extract will underperform quietly, and nobody finds out unless it's measured. AD F 2026 attacks that from both ends — tighter rules on what gets installed, and a requirement to show your working to Building Control before it's buried. The amended Approved Documents L and F come into force on **24 March 2027** for most new building work (**24 September 2027** for higher-risk building work), with transitional protection to 24 March 2028 for qualifying plots. Systems being designed now for 2027 starts should already be working to these rules. ## Worth doing now Pull the ventilation design forward in the programme. Decide unit locations and duct routes early enough that they can be short and straight, allow space for insulation, and make sure someone owns the design calculations that Building Control will ask for. It's a lot cheaper than re-routing ductwork through a plasterboarded ceiling. If you're working through an FHS-compliant plot and want a second pair of eyes on how the Part L and Part F requirements stack up together, [get in touch](/contact/). *Sources: [Approved Document F Volume 1, 2026 edition (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/69c12224d588c92c483e4b6a/ADF1_2026.pdf) — paragraphs 1.75 to 1.86 and Table 1.8 · [CIBSE Journal CPD Module 266: Ventilation design under the Future Homes Standard, July 2026](https://www.cibsejournal.com/cpd/modules/2026-07-env/) · [The Building Regulations etc. (Amendment) (England) Regulations 2026 (SI 2026/335), legislation.gov.uk](https://www.legislation.gov.uk/uksi/2026/335/made)* --- # Part F 2026 has a definition that could catch you out — 'highly airtight' URL: https://energycount.co.uk/news/part-f-2026-highly-airtight-dwellings/ Date: 2026-08-20 Most of the Future Homes Standard conversation has been about Part L — the notional dwelling, heat pumps, PV, U-values. But the change that's most likely to bite on site sits in the other document. **Approved Document F Volume 1, 2026 edition** introduces a formal split between "less airtight" and "highly airtight" dwellings, and that single definition decides what ventilation system you're allowed to use. ## The definition AD F 2026 defines it in the glossary. A **highly airtight dwelling** is one that achieves one or both of the following: - a **design air permeability lower than 5 m³/(h·m²) at 50 Pa**, or - an **as-built air permeability lower than 3 m³/(h·m²) at 50 Pa**. Everything else is a "less airtight dwelling". That's the whole test — two numbers. Now put it next to Part L. The FHS notional dwelling under the SAP 10.3 route assumes an air permeability of **4 m³/(h·m²)**, against a legal backstop that stays at 8. So a dwelling designed to match the notional specification is, by definition, a highly airtight dwelling under Part F. The two documents are deliberately joined at the hip. ## Why the label matters Table 1.6 of AD F 2026 sets out which ventilation systems the guidance covers for which dwellings: - **Natural ventilation** — background ventilators plus intermittent extract fans — is listed as suitable for **less airtight dwellings only**. - **Continuous mechanical extract ventilation** (centralised c-MEV or decentralised d-MEV) covers **all dwellings**. - **Mechanical ventilation with heat recovery** covers **all dwellings**. In other words, once a home is highly airtight, trickle vents and a humidistat fan in the bathroom stop being a route to compliance. You need continuous mechanical ventilation, or a specialist design. That's not surprising in itself — squeeze out uncontrolled infiltration and something has to replace it. What's new is that the guidance now says so explicitly, with a number attached. ## The bit that catches people out Here's the paragraph worth reading twice. **AD F 2026, paragraph 1.46**: where a dwelling has natural ventilation and a *measured* air permeability that differs from the *design* air permeability, so that it becomes defined as a highly airtight dwelling, one of the following applies: - expert advice should be sought, or - a continuous mechanical extract ventilation system should be installed. Read that as a site scenario. A plot is designed at, say, 5.5 m³/(h·m²) with a natural ventilation strategy — perfectly reasonable, less airtight dwelling, trickle vents and intermittent extract. The build team does a genuinely good job on the tapes and seals. The as-built test comes back at 2.8. That plot is now a highly airtight dwelling, and the ventilation system that's already installed is no longer the one the guidance supports. Building tighter than you designed used to be a small bonus on the SAP result. Under AD F 2026 it can also be a compliance problem — and it lands right at the end of the programme, after first fix. The practical answer is to decide early. If there's any prospect of the as-built result coming in under 3, design for continuous mechanical ventilation from the start rather than hoping the test lands in the right band. ## What continuous MEV actually asks for If you do land in continuous extract territory, a few of the numbers are worth having to hand: - **Whole-dwelling ventilation rate** — the higher of 0.3 l/s per m² of internal floor area (all floors) and the bedroom-based figure in Table 1.3: 19 l/s for one bedroom, 25 for two, 31 for three, 37 for four, 43 for five, adding 6 l/s per bedroom beyond that. If the dwelling has only one habitable room, 13 l/s. - **Total continuous extract** should be at least the whole-dwelling ventilation rate. - **Background ventilators are still required** with c-MEV and d-MEV. They should not be in wet rooms, should provide a minimum equivalent area of 4,000 mm² per habitable room, and the total number should equal **the number of bedrooms plus two** — so a one-bed needs three, a two-bed needs four. They're meant to be left open. - **Internal doors** need a free area equivalent to a 10 mm undercut in a 760 mm door, measured above the finished floor. MVHR is the exception on background ventilators: because the unit supplies filtered air to habitable rooms directly, they aren't required — but you then need room for two sets of ductwork, and AD L 2026 wants at least 73% heat recovery efficiency plus a summer bypass. ## When this applies The amended Approved Documents L and F come into force on **24 March 2027** for most new building work, and **24 September 2027** for higher-risk building work, with transitional protection running to 24 March 2028 for plots that qualify. If you're designing now for a 2027 start, you're designing to these rules. ## What to do about it Three things worth doing before the deadline gets close: 1. **Pick the ventilation strategy from the target air permeability, not the other way round.** If the design figure is under 5, natural ventilation is off the table from day one. 2. **Stress-test the "we built it too well" scenario.** Ask what happens if the as-built test comes back under 3, and make sure the answer isn't "retrofit a ducted system into a finished house". 3. **Get the junctions right anyway.** Airtightness and thermal bridging live in the same places — junctions, penetrations, service voids. Tighter fabric makes both your psi-values and your air test result more important, and neither is something you want to discover late. **ΨMonkey** calculates bespoke PSI values for your junctions to BR 497 and BRE IP 1/06 conventions, so the details you're actually building show up in your SAP model instead of the worst case the software assumes. [Try ΨMonkey →](/tools/psi-calculator/) And if you've got a plot where the air test has come in tighter than the design figure and you're not sure where that leaves the ventilation strategy, [get in touch](/contact/) — it's a conversation worth having before first fix, not after. *Sources: [Approved Document F Volume 1, 2026 edition (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/69c12224d588c92c483e4b6a/ADF1_2026.pdf) — glossary definition of "highly airtight dwellings", Tables 1.3 and 1.6, and paragraphs 1.20, 1.21, 1.46 and 1.64 · [CIBSE Journal CPD Module 266: Ventilation design under the Future Homes Standard, July 2026](https://www.cibsejournal.com/cpd/modules/2026-07-env/) · [The Building Regulations etc. (Amendment) (England) Regulations 2026 (SI 2026/335), legislation.gov.uk](https://www.legislation.gov.uk/uksi/2026/335/made)* --- # Councils can still beat Part L — but only as a percentage uplift on TER URL: https://energycount.co.uk/news/nppf-2026-local-energy-standards-ter-uplift/ Date: 2026-08-19 If you've ever run a SAP calculation twice — once for Building Control and once for a local plan policy demanding "a 35% improvement on Part L" — you'll have followed the NPPF consultation with some interest. The **December 2025 draft NPPF** proposed, at policy **PM13**, that local plans should not set quantitative standards covering matters already addressed by Building Regulations. Read literally, that would have ended local energy, carbon and overheating targets: Building Regulations would have become both the floor and the ceiling. The consultation closed on 10 March 2026 and drew a lot of noise from councils and the low-energy sector. The **final Framework was published on 17 August 2026**, replacing the December 2024 version. On this point, the government changed its mind. ## What PM13 actually says now Policy PM13 still tells plan-makers that quantitative standards should "not cover matters that are already addressed by Building Regulations" — but it now carves out three exceptions. The energy one is worth reading properly: > "Energy efficiency, for which any standards that go beyond the current or proposed Building Regulations should have a clear and robustly costed rationale which shows that there will not be an adverse impact on the viability and deliverability of development. Any such standards should be expressed as a percentage uplift of a dwelling's Target Emissions Rate (TER) calculated using a specified version of the Standard Assessment Procedure (SAP) or other approved calculation methodology." The other two exceptions are **accessibility** (M4(2)/M4(3), set in accordance with policy HO5) and **water efficiency** (the tighter optional requirement where justified, or exceptionally something more stringent in areas of serious water stress). So local ambition survives. But it has to arrive in a specific currency. ## Why the "percentage uplift on TER" wording matters This is the part that lands on assessors' desks rather than planners'. At the moment, local energy policies are written in whatever unit the authority fancied at the time: kgCO₂/m²/yr targets, space-heating demand limits in kWh/m²/yr, "net zero carbon" definitions, energy use intensity caps, Passivhaus-equivalent wording. Some of those can't be evidenced from a SAP output at all, which is how you end up with a planning condition that no compliance calculation can cleanly discharge. PM13 pushes all of that back into a single, familiar metric — **DER against TER, as a percentage** — and requires the plan to name **which version of SAP** the percentage is measured against. That last clause is not a footnote. A "31% uplift" means something quite different under SAP 10.2 than it does against the **SAP 10.3** notional building used for the Future Homes Standard, where the target has already moved a long way. A policy that doesn't pin its version down is a policy that will be argued about at appeal. There's also a viability test attached: the rationale must be "clear and robustly costed" and show no adverse impact on the viability and deliverability of development. Expect that to be where most of the examination argument happens. ## The Future Homes Standard collision Worth holding both dates in your head. The **Future Homes and Buildings Standards** come into force on **24 March 2027** for non-higher-risk work, with transitional protection for plots where a valid application was made before that date and work is commenced before **24 March 2028**. From then, the notional dwelling gets very demanding indeed — and requirement L3 mandates on-site renewable electricity generation. A local policy demanding a percentage uplift on a *post-FHS* TER is a materially different ask from the same percentage against the 2021 baseline. Many existing local plan policies were written against Part L 2013 or 2021 targets and will need revisiting; PM13 gives examiners a clear basis for asking authorities to show their working. ## What this means in practice **If you're a housebuilder or developer:** check the energy policies in the local plans covering your live sites, and specifically whether they name a SAP version. Where they don't, the new PM13 wording is a reasonable basis for asking the authority what baseline it thinks it is measuring against — before you price the specification. **If you're an assessor:** expect more requests for a *dual* output — the Part L compliance case, plus a stated percentage improvement of DER over TER for the planning condition. Getting both from a single, consistent model saves an argument later. **If you're in Building Control:** nothing changes for you directly. PM13 is a plan-making policy. But it should, over time, reduce the number of planning conditions that ask for something Building Control has no way of verifying. ## What we'd suggest doing now Don't rewrite specifications on the strength of a planning policy that may itself be revised. Do find out where you actually sit. For a percentage-uplift policy, the number that decides everything is the **DER/TER margin** — and on most schemes the cheapest place to find margin is the thermal bridging. Default PSI values are deliberately pessimistic; calculated junction values routinely recover several percent of the emissions rate without changing a single product on the specification. **ΨMonkey** calculates bespoke PSI values for your junctions to BR 497 and BRE IP 1/06 conventions, so your SAP model reflects the details you're actually building rather than the worst case the software assumes. [Try ΨMonkey →](/tools/psi-calculator/) If you've got a site where a local plan energy policy and the Part L case are pulling in different directions — or a condition that names no SAP version at all — [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [National Planning Policy Framework, August 2026 (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/6a8334c03bd75b81e2329ac4/National_Planning_Policy_Framework.pdf) · [Full steam ahead to fast-track more homes near stations — MHCLG press release, 17 August 2026](https://www.gov.uk/government/news/full-steam-ahead-to-fast-track-more-homes-near-stations) · [The Building Regulations etc. (Amendment) (England) Regulations 2026 (SI 2026/335), legislation.gov.uk](https://www.legislation.gov.uk/uksi/2026/335/made)* --- # The new NPPF keeps local water efficiency standards on the table URL: https://energycount.co.uk/news/nppf-2026-local-water-efficiency-standards/ Date: 2026-08-19 There's a running theme in water efficiency policy: everyone agrees the standard should be tighter, and nobody agrees on who should set it. Earlier this month we covered **Water UK backing Defra's proposed cut from 125 to 105 l/p/d**, and its argument for a **single national standard** rather than a patchwork of locally set numbers. The **National Planning Policy Framework published on 17 August 2026** has now answered the second half of that question, and the answer is: not yet. ## What the Framework says Policy **PM13** tells plan-makers that quantitative standards in local plans should not cover matters already addressed by Building Regulations. Water efficiency is one of only three carve-outs: > "Water efficiency, for which it may be appropriate to apply the tighter Building Regulations optional requirement where justified, or exceptionally a more stringent local standard in areas of serious water stress" The plan-making climate policy reinforces it. Policy **CC1** requires development plans to take a proactive approach to climate mitigation and adaptation, including by "setting local water efficiency standards for new development where these are justified in accordance with policy PM13". Two things follow from that wording. **First, the 110 route is explicitly endorsed.** The Part G optional requirement of **110 l/p/d** remains available to authorities that can justify it — which, in practice, is most of the south and east of England. **Second, and more interesting: "exceptionally, a more stringent local standard".** That is national policy sanctioning numbers *below* 110 in areas of serious water stress. It doesn't say what those numbers should be, or how they'd be calculated, which leaves the door open for the 100 and 95 l/p/d policies that a handful of authorities have already pushed for. The check on all of this is the same one applied to energy standards: the rationale must be justified and proportionate, and PM13 as a whole requires evidence that standards won't undermine viability and deliverability. ## Why this matters now The timing is awkward in a useful way. Defra consulted on cutting the mandatory Part G standard from **125 to 105 l/p/d**, with a fittings-based compliance route, and the government response is still pending. If that lands, the national baseline moves below today's optional requirement — and the whole optional-requirement architecture will need rethinking, because a 110 l/p/d "tighter" standard makes no sense once the mandatory floor is 105. The NPPF doesn't pre-empt that. It just keeps the local mechanism in place in the meantime. So for the next stretch, you're likely to be working with: - A **mandatory** standard of 125 l/p/d, heading for 105 - An **optional** standard of 110 l/p/d applied by many local plans - A small number of authorities pressing for **below 110** on serious water stress grounds - No national consolidation of any of it If you build across more than one authority, that's three different numbers to track per site, plus the possibility of a fourth. ## What it means for your calculations The practical consequences are unchanged in kind, but sharper in degree. The levers are the same as ever — **shower flow rate, bath capacity, WC flush volumes, basin and kitchen tap flow rates**, with rainwater and greywater reuse as the heavier tools. What changes is headroom. A specification that clears 110 comfortably may have very little left if a local policy lands at 100, and a bath plus a generous shower head will use most of that margin between them. There's also an evidence point worth noting for anyone responding to a local plan consultation. PM13 requires standards to be justified with proportionate evidence, and the "exceptionally" in the water clause is doing real work — it signals that sub-110 standards are meant to be rare and well-evidenced, not a default ambition. If a draft policy in your area proposes one without that evidence, the Framework now gives you clear language to point at. ## What we'd suggest doing Know your number per authority, not per company. Run each standard house type through the official methodology at the standards that actually apply to your sites — 125, 110, and whatever the local plan is proposing — and find out which types have margin and which are living on the edge. One dwelling type covers every identical plot, so this is a small piece of work that maps your whole exposure. **WaterMonkey** runs the full official Part G water efficiency methodology with live PASS/FAIL as you type at 125 or 110 l/p/d, rainwater and greywater included, free in your browser. Building Control-ready PDF reports are £25 +VAT, and one dwelling type covers every identical plot on the site. We'll add the new standard as soon as Defra confirms it. [Try the free Part G water calculator →](/tools/part-g-water-calculator/) If you've got a planning condition asking for something tighter than 110 l/p/d and you're not sure it's achievable with the specification you've priced, [get in touch](/contact/) and we'll tell you where the margin is. *Sources: [National Planning Policy Framework, August 2026 (PDF), GOV.UK](https://assets.publishing.service.gov.uk/media/6a8334c03bd75b81e2329ac4/National_Planning_Policy_Framework.pdf) · [Full steam ahead to fast-track more homes near stations — MHCLG press release, 17 August 2026](https://www.gov.uk/government/news/full-steam-ahead-to-fast-track-more-homes-near-stations) · [Review of Water Efficiency Standards in the Building Regulations 2010 — Defra consultation](https://consult.defra.gov.uk/water-efficiency-demand/review-of-water-efficiency-standards)* --- # The Future Homes Standard transitional trap — 'commenced' doesn't mean what you think URL: https://energycount.co.uk/news/future-homes-standard-transitional-deadline-commencement/ Date: 2026-08-18 Most housebuilders we speak to have the first date in their heads: **24 March 2027**. That's when the Future Homes and Buildings Standards bite, and the received wisdom is "get the application in before then and you're fine". It's the second date that catches people out — and the definition sitting behind it. Getting that right now beats a nasty surprise in 2028. ## The two-part test The Future Homes Standard is delivered by **The Building Regulations etc. (Amendment) (England) Regulations 2026** (SI 2026/335), made on 23 March 2026 and laid before Parliament the following day. The Regulations come into force on **24 March 2027**, except that regulations 3, 4, 6, 7, 8 and 9 come into force for **higher-risk building (HRB) work and work to an existing HRB on 24 September 2027**. For ordinary, non-HRB work, **regulation 5** sets out the transitional protection. It applies to building work on a particular building where: 1. a **building notice, an initial notice, or an application for building control approval with full plans** has been given to the relevant authority in respect of that building **before 24 March 2027**; **and** 2. the building work to which that notice or application relates is **commenced in relation to that building before 24 March 2028**. Both limbs have to be satisfied. Miss either one and the amendments made by regulations 3 and 4 — the new Part L, requirement L3 and the rest — apply to the work. Two things are easy to miss here. The first is that this is expressed **per building**, not per site. An early full-plans application does not throw a blanket over a whole scheme; each individual building has to have commenced in time. The second is that "commenced" has a legal meaning, and it isn't a generous one. ## What "commenced" actually means Regulation 5(5) is explicit: paragraphs (2) to (5) of **regulation 46A** of the Building Regulations 2010 apply to determine whether work is regarded as commenced. Regulation 46A was written for a different purpose — the lapse of building control approval — but it's now doing double duty as the FHS cut-off test. For **the construction of a building** (and for a horizontal extension), regulation 46A(3) says work is regarded as commenced when: > the sub-surface structure of the building or the extension including all foundations, any basement level (if any) and the structure of ground floor level is completed. Read that again. Not "started on site". Not "foundations poured". The sub-structure has to be **complete, including the ground floor structure**, on that specific building, before 24 March 2028. There are two variants: - **Complex buildings** (regulation 46A(2)) — commencement is when the foundations supporting that building and the structure of its lowest floor level are completed. A "complex building" means one sharing a foundation plinth or podium with another building or structure, one with more than one storey below ground, or one intended primarily as a public building with capacity for 100 or more visitors. - **Any other building work** (regulation 46A(4)) — commencement is when the work identified in the client's statement as amounting to **15% of the proposed work** is completed. Site clearance, service diversions, roads and sewers, hoarding — none of that gets a plot over the line. ## Higher-risk buildings work differently For HRB work, work to an existing HRB, and stages of HRB work, regulations 6 to 8 use a different mechanism: protection depends on a **valid building control approval application having been submitted to the regulator before 24 September 2027** and not being rejected before or after that date. There's no separate commencement deadline — but a rejection kills the protection, which given Gateway 2 rejection rates is not a theoretical risk. Helpfully, where protection is triggered by an application for a *stage* of HRB work, it carries through to subsequent stages provided the original application hasn't been rejected or lapsed. ## And the older transitional arrangements are going Regulation 10 revokes the transitional arrangements in the 2013 Regulations and removes part of the 2021 equivalent, subject to savings. If you have plots that have been sitting under a 2013 saving, they need to be started before 24 March 2028 too, or they fall into the new regime. ## What you're falling into if you miss it It's worth being clear about the size of the gap. From 24 March 2027 the amended Part L is retitled **"Energy and greenhouse gas emissions"**, requirement L1 is rewritten around minimising greenhouse gas emissions as well as conserving fuel and power, and a brand-new functional requirement **L3** applies: when a building is erected which is or contains one or more dwellings, a **system for on-site renewable electricity generation** must be installed on the building or within its curtilage, designed so residents can use the electricity and capable of a reasonable output given the building's design and surroundings. There are limited exemptions — buildings exempt under regulation 7(4), buildings where a reasonable output genuinely isn't possible, and cases where an equivalent off-building on-site system already serves the residents. The definition of "fixed building services" also widens to include lifts, escalators and moving footways in new buildings (excluding those within individual dwellings), and a new regulation 40C requires that information owed to new-dwelling owners under regulations 39 to 40B is actually provided **in an appropriate format**. Alongside all this sit the new **Approved Document L Volumes 1 and 2 (2026)** and **Approved Document F Volume 1 (2026)**. That's not a tweak. A plot that slips from the old regime to the new one is a redesign, not a re-run of the calculation. ## The practical move Two things are worth doing now: 1. **Map your plots against both dates**, building by building — application in before 24 March 2027, sub-structure complete before 24 March 2028. Any plot where the programme has the ground floor slab landing in early 2028 is a plot at risk. 2. **Model the FHS case for the plots you know won't make it**, rather than discovering the shortfall late. Fabric, thermal bridging and the [L3 renewables requirement](/tools/solar-pv-calculator/) all move together, and the cheapest time to find the answer is before the house types are frozen. If you'd like the Part L and SAP 10.3 modelling done properly — including bespoke PSI values for your junctions rather than defaults that quietly wreck the DER — [get in touch](/contact/) and we'll tell you how we'd approach your scheme. *Sources: [The Building Regulations etc. (Amendment) (England) Regulations 2026 (SI 2026/335), legislation.gov.uk](https://www.legislation.gov.uk/uksi/2026/335/made) · [Regulation 46A, The Building Regulations 2010, legislation.gov.uk](https://www.legislation.gov.uk/uksi/2010/2214/regulation/46A) · [The Future Homes and Buildings Standards: Building Circular 01/2026, GOV.UK](https://www.gov.uk/government/publications/the-future-homes-and-buildings-standards-building-circular-012026/the-future-homes-and-buildings-standards-building-circular-012026-letter)* --- # New-build EPCs held up while the rest of the market fell — the Q2 2026 numbers URL: https://energycount.co.uk/news/new-dwelling-epc-statistics-q2-2026/ Date: 2026-08-18 Quarterly statistics rarely make anyone's day, but this set is worth a look — partly for what it says about new-build volumes heading into the Future Homes Standard, and partly because MHCLG has changed how it counts. The **Energy Performance of Buildings Certificates statistical release for April to June 2026** was published on 30 July 2026, followed on **12 August** by the companion release, *Energy efficiency characteristics of new dwellings*, which is MHCLG's Part L monitoring series. Here's the plain-English version of the headline data. ## New build held up; the rest of the market didn't In England in Q2 2026, **440,000 domestic EPCs** were lodged on the Register — an 11% fall on the same quarter in 2025. But that fall is almost entirely on the existing-stock side: - **New dwellings: 51,000 EPCs, up 2%** on Q2 2025 - **Existing dwellings: 389,000 EPCs, down 12%** Over the **12 months to June 2026**, England recorded **203,000 new-dwelling EPCs, a 3% increase** on the previous year. Across England and Wales combined, new-dwelling lodgements rose 2% to 53,000 in the quarter while existing-dwelling lodgements fell 12% to 411,000. Because EPCs for new dwellings are lodged as a matter of course on completion, they're a reasonably good early read on housing supply — MHCLG uses them, adjusted for demolitions, as an indicator of progress towards the 1.5 million homes target. The divergence between the two lines is essentially the sales-and-lettings market cooling while completions hold steady. Regionally, the biggest year-on-year rise in new-dwelling lodgements was in the **West Midlands (up 24%)**; the biggest fall was in the **South East (down 11%)**. In Wales, 2,000 new-dwelling EPCs were lodged, up 5%, against 22,000 for existing dwellings, down 6%. ## Ratings: 89% A or B, and stuck there **89% of new dwellings in England were rated A or B** — unchanged from the same quarter in 2025. Wales came in at the same 89%, with 10% at C or D. That flatness is the interesting part. Under the 2021 Part L uplift, band B has become the default outcome rather than a stretch, and the ratings distribution has essentially stopped moving. The 2026 Approved Document L, with the new requirement L3 for on-site renewable electricity generation, is what shifts it again — and those homes won't start appearing in these statistics in volume until 2027–28. ## The methodology change worth noting Buried in the "changes to this release" section: MHCLG has carried out **additional de-duplication of new-dwelling EPCs that lack a UPRN**. New dwellings often have more than one EPC on the Register — one lodged against the design and another after a change during construction — and the extra cleaning strips more of those duplicates out. The practical effects: - New-dwelling figures are **slightly lower (less than 1%) across all quarters**, historically as well as currently. - The **live tables have been revised** back through the series; **previously published statistical releases have not** been changed. - **Existing dwellings, non-domestic EPCs and DECs are unaffected.** If you quote new-dwelling EPC counts in bids, land appraisals or board packs, make sure you're pulling from the revised live tables rather than an older release, or your year-on-year comparison will be built on two different methodologies. ## What to take from it For assessors, the message is that new-build SAP work is holding up while RdSAP volumes on the existing stock are visibly softening — a useful thing to know if you're deciding where to put your training and accreditation budget over the next year. For housebuilders, a flat 89% A-or-B tells you the current regime has been fully absorbed and the next step change is a regulatory one, not a market one. The next quarterly release, covering July to September 2026, is expected on **29 October 2026**. If you'd like your new-build Part L and SAP 10.3 work handled properly — including bespoke thermal-bridging PSI values instead of punitive defaults — [get in touch](/contact/) and we'll tell you how we'd approach your scheme. *Sources: [Energy Performance of Buildings Certificates Statistical Release: April to June 2026, England and Wales, GOV.UK](https://www.gov.uk/government/statistics/energy-performance-of-building-certificates-in-england-and-wales-april-to-june-2026/energy-performance-of-buildings-certificates-statistical-release-april-to-june-2026-england-and-wales) · [Energy efficiency characteristics of new dwellings: April to June 2026 (release announcement), GOV.UK](https://www.gov.uk/government/statistics/announcements/energy-efficiency-characteristics-of-new-dwellings-april-to-june-2026)* --- # RdSAP Conventions v12.2 are live — what changed on 6 August URL: https://energycount.co.uk/news/rdsap-conventions-v12-2-6-august-2026/ Date: 2026-08-16 If you produce EPCs on existing homes, the ground shifted slightly under you this month. The **RdSAP 10 Conventions v12.2** — the MHCLG-approved rulebook that tells domestic energy assessors how to interpret what they find on site — came into effect on **6 August 2026**, superseding v12.1 (which had run since August 2025). It's not a new methodology, but the detail matters: the revision history lists **16 amended conventions, four brand-new ones, and the deletion of Appendix 4**. Most of the changes are about squeezing out inconsistency between assessors, replacing "use your judgement" with a rule. Here are the ones worth reading twice. ## Bungalows and self-contained annexes The definition of a **bungalow** has been tightened: a property should only be recorded as a bungalow where **all habitable accommodation is at ground-floor level**. Chalet bungalows and anything with a room in the roof are now treated as houses. There's also clearer guidance on when an annexe counts as a **self-contained dwelling** needing its own EPC. It must have fixed cooking and bathing facilities — the cooking provision expected to include a fixed oven, a sink and either two fixed hot rings (for a one-bedroom dwelling) or four (for anything larger) — plus its own access, either from outside or via a communal corridor. An internal connecting door to the main house does not, on its own, stop the unit being self-contained. ## Rooms in roof: Convention 2.06 rewritten This is one of the biggest single changes, aimed at loft conversions and rooms in the roof: - **A new 25% rule for connected gable walls.** Where only part of a gable wall is connected to another part of the dwelling, it's treated as *exposed* if less than 25% is connected, and *connected* if more than 25% is. - **A new 20% dormer rule.** Small dormers taking up less than 20% of the room-in-roof floor area can be ignored when measuring the roof. Larger dormers must be measured separately — vertical faces as stud walls, roofs as flat ceilings. - **Hidden insulation.** Where there's clear evidence that insulation runs from the rafters through to the eaves, an unobservable stud wall can now be assumed to share the same insulation level, rather than defaulting to uninsulated. - **Complex geometry can be merged**, and in certain constructions you can now take U-values straight from **Table 18 of the RdSAP 10 Specification** rather than calculating each element. That last point is tied to the removal of **Appendix 4**, which previously covered room-in-roof U-values — those values are now inferred from Table 18 where the stated conditions are met. ## Conservatories with solid roofs Convention 2.28 now spells out the definition: for RdSAP purposes a conservatory is a building part where **50% or more of the external heat-loss walls are glazed and 75% or more of the roof is glazed**. Crucially, where a conservatory's roof has been replaced with a solid roof (or a false ceiling added) so it no longer meets that test, it must be **recorded as an extension**, with clear rules for how to enter the roof depending on pitch and the evidence available. This finally standardises one of the most argued-over judgement calls in the job. ## Heating, hot water and ventilation Several amendments pin down how systems are recorded: - **Heat-as-a-Service** gets its own new convention: where a provider owns or operates the heat generator, it's treated as **community heating** regardless of where the generator sits — entered as a community heat pump (electric) or community boiler (otherwise), with documentary evidence required. - **Electric combi boilers** (Convention 4.22) are now entered manually as a standard boiler, direct acting, fuel type electricity. - **Incomplete or condemned systems** (Convention 4.03) now explicitly covers **hot water** as well as heating — a fixed but non-working hot water source still counts when deciding the primary heating and hot water type. - **Two mechanical ventilation systems** get a new Convention 7.03e setting a clear order of precedence (MVHR first, then MV, MEV, PIV from outside, PIV from loft) where the software will only take one entry. ## Chimneys, evidence and PV batteries A few smaller but practical clarifications round it out. **Decommissioned chimneys with air bricks** (Convention 9.01) must now be considered: a ventilation opening under **30,000 mm²** is treated as a blocked chimney (20 m³/hr), larger openings as open (80 m³/hr). On **documentary evidence** (9.02), where you can see insulation and have evidence of intent but can't measure the thickness, you now record "insulated — insulation thickness unknown" rather than overstating or ignoring it. **PV batteries** over 5 kW (9.05) are split into a 5 kW entry plus a second battery for the excess. And an **air-tightness certificate** becomes invalid for RdSAP entry if material changes have been made to the property since the test (10.01). ## What to do about it None of this rewrites the RdSAP engine, but plenty of it changes the inputs — and inconsistent inputs are exactly what drives variation between assessors on the same house. The practical move is to read the full v12.2 document rather than rely on a summary, pay particular attention to the room-in-roof and conservatory rules if you survey a lot of older stock, and check that your software and accreditation scheme guidance reflect the new edition before you lodge. RdSAP's conventions exist because a DEA usually cannot see inside a wall. Where you can — an opened-up survey, a known build-up, a retrofit specification — a calculated U-value beats an assumed one, and **U-Monkey** will do it to BS EN ISO 6946 free, showing every layer, correction and source so the figure can be defended later. [Try U-Monkey →](/tools/u-value-calculator/) If you'd like the modelled, new-build side of the picture handled properly — SAP 10.3, Part L compliance and PSI-value thermal-bridging work — that's our patch. [Get in touch](/contact/) and we'll tell you how we'd approach your scheme. *Sources: [RdSAP Conventions v12.2 (MHCLG-approved), BRE Group](https://bregroup.com/documents/d/bre-group/rdsap-conventions-v12-2-mhclg-approved) · [Conventions (v12.1) for RdSAP 10, BRE Group](https://bregroup.com/documents/d/bre-group/rdsap-conventions-v12-1-21-august-2025-final) · [RdSAP Conventions v12.2: Key Changes for Energy Assessors, Energy Trust](https://energy-trust.co.uk/rdsap-conventions-v12-2-key-changes/)* --- # Domestic solar permitted development is being rewritten — what changes on 27 August 2026 URL: https://energycount.co.uk/news/gpdo-solar-permitted-development-27-august-2026/ Date: 2026-08-13 Solar rarely makes the news in our world, but a change landing this month is worth a look — especially if you specify PV on new homes or advise clients on what they can install without a planning application. On **27 August 2026**, a new order rewrites the permitted development (PD) rights for domestic solar in England, and for the first time writes **plug-in "balcony" solar** into the rules. ## What's actually changed The change comes via **The Town and Country Planning (General Permitted Development) (England) (Amendment) Order 2026** (SI 2026/896), which was made on 29 July 2026, laid before Parliament on 30 July, and comes into force on **27 August 2026**. It amends **Part 14 (renewable energy) of Schedule 2** to the 2015 GPDO — the part that grants planning permission for certain solar installations without the need for a planning application. The headline structural change is that the rules for **dwellinghouses** and for **blocks of flats** have been separated out and set down explicitly, where the old text bundled them together. On top of that, the order adds a set of clearer dimensional limits and — the genuinely new bit — a definition and rules for plug-in solar. ## Solar on houses: the new limits For solar PV or solar thermal on a **dwellinghouse** (Class A), the order spells out when an installation falls *outside* permitted development and therefore needs a planning application. The limits to have in mind: - **On a wall:** not permitted if the equipment protrudes more than **0.2 metres** beyond the wall where that wall abuts a highway, or more than **0.4 metres** in any other case. - **On a balcony or roof enclosure:** the same 0.2 m (fronting a highway) / 0.4 m (otherwise) protrusion limits. - **On a pitched roof:** not permitted if it protrudes more than **0.2 metres** beyond the roof slope, or if the highest part sits above the ridge (excluding the chimney). - **On a flat roof:** not permitted if the highest part is more than **0.6 metres** above the highest part of the roof (excluding the chimney). - **Heritage constraints:** in a conservation area or World Heritage Site, not permitted on a wall, balcony or roof enclosure that fronts a highway; and not permitted at all on a scheduled monument or on a **listed** dwellinghouse. **Stand-alone** (ground-mounted) solar in the curtilage (Class B) gets its own height table — ranging from **1 metre** where it sits forward of the principal elevation and within 5 m of the boundary, up to **4 metres** in the most unconstrained case — plus a cap keeping the panel area to no more than **9 square metres**, and a new **prior-approval** step for stand-alone solar placed on article 2(3) land within 5 m of the boundary. ## Plug-in "balcony" solar is now in the rules The most talked-about change is the arrival of **plug-in solar** — defined in the order as solar PV "intended for connection to the relevant domestic premises' electricity installation by means of a standard plug and socket". It's now folded into the definition of solar PV, which brings the plug-in, stand-it-on-your-balcony kits that are common across Europe into the English PD framework as they become legal to self-install here. There's an important safety-driven limitation, though: plug-in solar is **not** permitted development where any part of it would be installed on a **wooden wall, balcony, fence, gate or enclosure**, or on a **timber-clad** part of a house or block of flats. So the combustible-substrate carve-out that assessors already recognise from other parts of the regs shows up here too. ## The one-year grace period If you have a scheme or product that was permitted under the old wording but no longer qualifies under the new rules, the order includes a **transitional provision**: previously permitted development can still be carried out until the **end of 26 August 2027**. That gives roughly a year of overlap rather than a hard cliff-edge — useful if something is already designed and in the pipeline. ## Why this matters for Part L and the Future Homes Standard This is a planning change, not a Building Regulation — but it lands right next to the energy work we do. Under the **Future Homes Standard**, on-site solar PV is a central part of how new homes hit their Part L targets; the FHS notional dwelling assumes a meaningful area of roof-mounted PV. So the PD rules that govern where and how PV can go on a roof, a wall or the ground directly shape what's deliverable on a plot without a separate application — and, by extension, how easily a design reaches compliance. The practical takeaways are straightforward: if you're specifying roof or wall-mounted PV, sense-check the new protrusion and ridge-height limits early so the array you're relying on for the Part L result is actually permitted development; watch the heritage and listed-building carve-outs on constrained sites; and if plug-in kit is in the mix, keep it off timber substrates. Solar is only ever one lever in the Part L calculation — the fabric, glazing and thermal-bridging numbers have to be right underneath it, or the PV ends up compensating for losses you could have designed out. If you'd like the fabric and **PSI-value** side modelled properly so your renewables aren't papering over a weak envelope, [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [The Town and Country Planning (General Permitted Development) (England) (Amendment) Order 2026 (SI 2026/896), legislation.gov.uk](https://www.legislation.gov.uk/uksi/2026/896/made) · [Explanatory Memorandum to SI 2026/896, legislation.gov.uk](https://www.legislation.gov.uk/uksi/2026/896/memorandum/contents) · [Town and Country Planning (General Permitted Development) (England) Order 2015, Part 14 of Schedule 2, legislation.gov.uk](https://www.legislation.gov.uk/uksi/2015/596/schedule/2/part/14)* --- # Scotland's 'Passivhaus equivalent' standard slips to 2029 — what the Stage 2 consultation proposes URL: https://energycount.co.uk/news/scotland-passivhaus-equivalent-stage-2-consultation/ Date: 2026-08-13 This one is for our Scottish readers first — but anyone tracking where UK new-build standards and the Home Energy Model are heading should take a look. On **24 July 2026** the Scottish Government published its **Stage 2 consultation** on the next revision to Scotland's energy standards, part of its work to introduce a **'Passivhaus equivalent'** standard for new homes through the Building Standards. The most immediate headline is a slipped timetable; the more interesting detail is what's proposed underneath. ## The date has moved: autumn 2029, not 2028 The earlier Stage 1 partial response pointed to the new standards coming into force on **31 March 2028**. That's now moved. After reviewing the programme and the time needed to develop the supporting guidance, assessment methodologies and software, the Scottish Government intends to publish guidance and approved methodologies for **Standard 6.1 in autumn 2027**, with the new requirements now expected to take effect in **autumn 2029**. It's also keeping its options open on *how* the standard lands — whether in full, in phases, or as a voluntary route before it becomes mandatory. ## Three options for the energy uplift The consultation sets out three routes for future energy performance: - **Good Practice** — roughly a **6% cut** in carbon emissions against the current 2023 standards. Improved fabric, typically with an air source heat pump; PV required where an alternative heating system (such as direct electric or a heat network) is used. - **Best Practice** — roughly a **37% cut** in carbon emissions. A bigger step up in fabric performance plus **mechanical ventilation with heat recovery (MVHR)**, and PV alongside a heat pump. - **Current Standard** — retain the existing 2023 requirements, with no uplift. Both of the uplift options strengthen the fabric; the Best Practice route is the more demanding, bringing in MVHR and, with a heat pump, PV. ## New metrics and a HEM-based method Compliance would be judged on two metrics: a **Delivered Energy Rate** (energy delivered to the home, excluding unregulated use and grid exports) and a **Space Heat Demand Rate** (driven by fabric performance — insulation, **thermal bridging** and airtightness). If that second metric sounds familiar, it should: it puts fabric and junction detailing right at the heart of compliance. On calculation, Standard 6.1 will be based on the **Home Energy Model with a Scottish wrapper** — the same underlying engine England is moving towards, tailored for Scotland. A modified version of the **Passivhaus Planning Package (PHPP)** is also proposed as an approved compliance methodology. The current SAP 10.2 notional-dwelling approach to setting targets would be replaced with a series of **target performance tables**, selected by the home's location, dwelling type, orientation and heating system. ## Fabric and overheating The maximum backstop U-values in Standard 6.2 would stay unchanged, but a new maximum **air permeability of 5 m³/(h·m²) at 50 Pa** would be introduced — so every new home would need an airtightness test at or below that level. On overheating, the Government is reviewing Standard 3.28 and considering whether PHPP could be used to assess overheating risk, with updated guidance to follow. ## Why it's worth reading in England too Scotland and England run separate Building Standards, so none of this applies to an English Part L job. But the direction of travel is shared — HEM as the calculation engine, fabric-led metrics, tighter airtightness and a hard look at thermal bridging — and Scotland is arguably setting out its stall in more detail. If you work across the border, or you want a preview of the arguments likely to shape future English uplifts, it's a useful read. The consultation **closes on 16 October 2026**. Whichever standard you're working to, the common thread is fabric: tighter airtightness targets and a fabric-driven space-heat metric both reward getting the junctions and PSI-values right at design stage. If you'd like your thermal bridging and fabric performance modelled properly — in Scotland or England — [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [Scottish building regulations: proposed changes to energy and environmental standards — Stage 2 consultation, gov.scot](https://www.gov.scot/publications/scottish-building-regulations-proposed-changes-energy-environmental-standards-stage-2-consultation-bria/) · [Stage 2 consultation (consult.gov.scot)](https://consult.gov.scot/local-government-and-housing/building-regulations-passivhaus-equivalent-stage-2/) · [Elmhurst Energy: Scottish Government outlines revised timeline in Stage 2 building regulations consultation](https://www.elmhurstenergy.co.uk/blog/2026/08/07/scottish-government-outlines-revised-timeline-in-stage-2-building-regulations-consultation/)* --- # The government just backed measuring how homes really lose heat URL: https://energycount.co.uk/news/smeter-strategic-guide-measured-performance/ Date: 2026-08-13 Most of what we do rests on *modelled* performance — SAP, HEM, U-values and PSI-values that predict how a home should behave. This month the government put its weight behind measuring how a home *actually* behaves. The Department for Energy Security and Net Zero (DESNZ) has published a **Smart Meter Enabled Thermal Efficiency Ratings (SMETER) Strategic Guide**, along with a supporting evidence base and case studies. It won't change a compliance calculation tomorrow, but it's a clear signal of where things are heading. ## What SMETER actually measures A SMETER assessment looks at how well an *occupied* home holds onto heat. Most methods combine actual energy-use data, indoor and outdoor temperatures and local weather to work out how quickly heat escapes the building. The result is expressed as a **Heat Transfer Coefficient (HTC)** — in watts per kelvin — for the whole dwelling. The rule of thumb is simple: a **high HTC** means the home is leaking heat quickly, while a **low HTC** means it's retaining heat well. Crucially, this is a measured, in-use figure rather than a value derived from construction assumptions. To be clear about what it is *not*: a SMETER assessment does not replace an EPC or a detailed survey. It sits alongside modelled assessments, giving real-world evidence of how a building performs against how it was predicted to perform. ## Why this matters: the performance gap For years, decisions about energy efficiency have leaned on modelled assessments and assumptions about how a building was built. Those models remain essential — but hidden defects, unknown or missing insulation and air leakage all mean the real building can behave differently from the one on paper. The difference between predicted and measured performance is the **performance gap**, and it's exactly the thing measured HTC can expose. The guide's evidence base is the interesting part. It highlights the **MEASURED project** — a field trial run by Build Test Solutions, Veritherm UK and Elmhurst Energy across 56 homes — which tested whether measured heat loss could improve heat-pump surveys and design. The headline finding is striking: the traditional heat-loss calculation matched the measured result in only **30%** of homes. It **over**estimated heat loss in 59% of cases and **under**estimated it in 11%. In other words, for roughly **70%** of the homes, relying on the standard BS EN 12831 calculation alone could have led to a heat pump being sized incorrectly. That's a useful reminder for anyone specifying low-carbon heating under the Future Homes Standard: oversized kit costs more and can run inefficiently, while undersized kit leaves homes cold. Measured data offers a way to sense-check the assumptions. ## An important caveat Measured HTC tells you *how much* heat a building is losing overall — not *where* it's being lost. So it's best used to calibrate and strengthen detailed, room-by-room calculations, not to replace the wider heating-design process. If the number looks off, the follow-up is the familiar toolkit: airtightness testing, thermal imaging and in-situ U-value measurement to pin down where the losses actually are. ## Where it's heading — EPCs and policy This is where assessors should pay attention. The government has consulted on a **voluntary option to record validated, quality-assured SMETER HTC values on future EPCs**, alongside an HTC produced by the Home Energy Model, and is analysing the responses. DESNZ is also building the validation and quality-assurance systems that measured performance would need, and has signalled interest in using SMETER within future policy, including the Warm Homes Plan. None of that makes measured HTC a general requirement today. But taken together with **CIBSE TM71** — the new framework for measuring and reporting HTC values across different methods — it points to a future where calculated and measured figures are used side by side: the model for a consistent assessment framework, the measurement for evidence of how a specific home really performs. With smart meters now in most homes, the raw data is already there. ## What to do with this now There's nothing to comply with yet, so treat this as horizon-scanning rather than a deadline. The practical takeaways: expect measured performance to feature more heavily in retrofit funding, quality assurance and eventually EPCs; be ready for clients to ask how a modelled result compares with real-world data; and remember that a strong measured HTC still comes down to the fundamentals — fabric, airtightness and well-detailed junctions. That last point is the one we'd flag hardest. Measured performance will increasingly hold designs to account, and the homes that measure well are the ones where the thermal bridging and fabric detailing were right on paper first. If you'd like your **PSI-values and fabric performance** modelled properly so the as-built numbers stand up to real-world measurement, [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [Smart-meter-enabled thermal efficiency ratings (SMETER): Strategic Guide, GOV.UK](https://www.gov.uk/government/publications/smart-meter-enabled-thermal-efficiency-ratings-smeter-strategic-guide) · [SMETER case studies and evidence base annex, GOV.UK](https://assets.publishing.service.gov.uk/media/6a4790e28effd97622f53c04/smeter-case-studies-evidence-base-annex.pdf) · [CIBSE TM71: Measuring Heat Transfer Coefficients in Buildings (2026)](https://www.cibse.org/knowledge-research/knowledge-portal/tm71-measuring-heat-transfer-coefficients-in-buildings-2026/) · [Elmhurst Energy: New SMETER guidance — from measurement to action](https://www.elmhurstenergy.co.uk/blog/2026/08/11/new-smeter-guidance-from-measurement-to-action-the-future-of-measured-building-performance/)* --- # Biodiversity Net Gain just got lighter for small sites — what changed on 6 August URL: https://energycount.co.uk/news/biodiversity-net-gain-exemptions-6-august-2026/ Date: 2026-08-10 This one isn't a SAP input, but it lands on the same desks we work with every day — housebuilders, small developers and the planning teams around them. On **6 August 2026** a first batch of changes to **Biodiversity Net Gain (BNG)** came into force in England, and if you bring forward smaller sites, the paperwork just got lighter. ## The quick recap on BNG Since February 2024, most developments needing planning permission under the Town and Country Planning Act have had to deliver a **10% biodiversity net gain** — leaving habitat measurably better than before, on-site where possible, or off-site or via statutory credits where not. It's been a real workload for small schemes in particular, where the assessment cost can be out of proportion to the site. The government consulted on easing that burden last year, responded in **April 2026**, and has now made the first legislative changes through **The Biodiversity Gain (Town and Country Planning) (Amendment) Regulations 2026 ([SI 2026/790](https://www.legislation.gov.uk/uksi/2026/790/contents/made))**, laid before Parliament on 13 July and in force from 6 August ([Defra Environment blog, 14 July 2026](https://defraenvironment.blog.gov.uk/2026/07/14/biodiversity-net-gain-amendments-and-transitional-arrangements-published/)). ## What actually changed on 6 August Four things, per Defra's confirmation: - **A new 0.2-hectare exemption.** The smallest developments — sites of 0.2 ha or less — no longer have to do mandatory BNG, *unless* on-site priority habitat would be negatively impacted. This is the headline: it takes a large slice of minor applications out of the regime entirely. - **A new temporary-development exemption.** Development on land that will be reinstated within five years or less is exempt — again, unless on-site priority habitat is negatively impacted. - **A relaxed hierarchy for minor development.** Minor sites that *aren't* exempt (for example, those over 0.2 ha) can now go straight to off-site provision in the first instance, rather than having to work through the on-site-first hierarchy. - **The self- and custom-build exemption is removed.** The old blanket carve-out for self- and custom-build is gone. Those projects now sit under the ordinary rules — so a small self-build may instead pick up the new 0.2-hectare exemption, but it isn't exempt simply for being self-build. Defra is clear that exempt developments are still expected to incorporate nature-friendly features, and that existing environmental protections and the planning mitigation hierarchy continue to apply — permission should still be refused where significant harm to biodiversity can't be avoided, mitigated or, as a last resort, compensated ([updated Planning Practice Guidance](https://www.gov.uk/guidance/biodiversity-net-gain-exempt-developments)). ## The date that matters: applications from 6 August This is the bit to get right. The changes apply **only to new applications for planning permission made from 6 August 2026**. Anything already submitted before that date continues under the existing BNG rules — the exemptions don't retro-apply. So the question for any given scheme is simply: when was (or will) the application be made? ## What's still coming This is a first batch, not the whole reform. Defra has signalled further changes to follow from the April response, including exemptions for development whose primary purpose is to conserve or enhance biodiversity, a targeted exemption for works to parks, playing fields and public gardens, and updates to the **statutory biodiversity metric**. Separately, the consultation on a possible exemption for certain residential **brownfield** development closed on 10 June 2026, with a government response still to come. In short: more movement to expect, but nothing else in force yet. ## What this means for you If you advise on or appraise smaller residential sites, it's worth re-checking your live pipeline against the 0.2-hectare threshold and the application date — some schemes that were budgeting for BNG assessment and units may no longer need them, while anything already in the system stays as it was. And keep priority habitat front of mind: it's the condition that switches every one of these exemptions off. Working on the environmental side of a stalled or marginal site? Our nature and nutrient tools can help you scope obligations before you commit spend — [take a look here](/tools/). --- # The EPC is being rebuilt — four metrics, and the Home Energy Model behind it URL: https://energycount.co.uk/news/reformed-epcs-four-metrics-home-energy-model/ Date: 2026-08-09 The Energy Performance Certificate has looked more or less the same for years: one coloured dial, one A–G band, one number that's meant to sum up a whole building. That's about to change. The government has confirmed a wholesale reform of the domestic EPC — a move from a single headline rating to **four separate metrics**, produced by the **Home Energy Model (HEM)** rather than RdSAP. ## What's actually changing In its partial response to the Energy Performance of Buildings consultation (updated **9 March 2026**), the Ministry of Housing, Communities and Local Government confirmed that domestic EPCs will replace the current single cost-based headline with **four headline metrics** ([GOV.UK, partial government response](https://www.gov.uk/government/consultations/reforms-to-the-energy-performance-of-buildings-regime/outcome/reforms-to-the-energy-performance-of-buildings-regime-partial-government-response)): - **Energy cost** — an estimate of running costs, the nearest thing to today's headline. - **Fabric performance** — a measure of the building envelope that doesn't move with energy prices; effectively a proxy for how well a home holds its heat. - **Heating system** — a rating reflecting the efficiency and carbon intensity of the heat source, rewarding a move to low-carbon heating. - **Smart readiness** — how well the home can work with smart meters and flexible tariffs. Sitting behind those, the certificate will also carry a secondary **energy demand** figure (based on delivered energy) and a secondary **carbon** metric. To keep comparisons possible during the switchover, the legacy Energy Efficiency Rating stays on the certificate for now. Non-domestic EPCs are treated differently: they keep the single carbon-based **Environmental Impact Rating** as their headline. The thinking is straightforward. One number was always doing too much work — a cost metric penalises electric heating on price, blurs fabric quality, and shifts with the energy market. Splitting it out gives owners a clearer, fabric-first picture: insulate well, then electrify. ## The bit that matters to assessors: HEM produces it This isn't just a redesign of the certificate. Reformed domestic EPCs are intended to be **generated using the Home Energy Model**, through a separate DESNZ consultation on how new-style EPCs will be produced and how the four metrics will be banded. For existing homes, that points to a more **modular** approach to assessment than the RdSAP data set you use today. If you've been watching HEM purely as the SAP replacement for new-build compliance, this is the other half of the story — HEM is also the engine intended to sit under the domestic EPC. The current RdSAP-based system remains the only route for existing-dwelling EPCs until the reformed methodology goes live. ## When Originally the government wanted new-style EPCs from **October 2026**. On 9 March 2026 that was pushed back: the launch now moves to the **second half of 2027**, with government committing to agree a specific date and a **shared implementation plan** with industry and the devolved administrations **by summer 2026**. So the plan itself is due about now — worth keeping an eye on. A final consultation response, covering the remaining questions (EPC data, EPC quality, DEC validity periods and more), is still expected during 2026. ## A few other changes worth banking Alongside the metrics, the response confirmed some practical points that will land on assessors and agents: - The **10-year validity period** is retained for both existing and reformed EPCs. - An EPC will be required at the **point of marketing** a property, not up to 28 days after — so the certificate needs to be in hand before a home goes on the market. - Scope widens: a valid EPC for the **whole HMO** when a single room is let, EPCs for **short-term lets** regardless of who pays the bills, and the **heritage exemption** removed. And the deadline that gives all this its edge hasn't moved: the private-rented MEES **EPC C by 2030** target still stands, which compresses the window between the new methodology arriving and landlords having to meet it. ## What to do now Nothing changes on your current EPCs today — anything issued now stays valid for ten years, well past 2030. But the direction is set: a four-metric certificate, produced by HEM, with fabric performance pulled out as a headline in its own right. If you assess existing homes, this summer's implementation plan is the one to watch, and it's a good moment to get comfortable with HEM's approach before the retraining rush. Fabric performance becoming a headline metric in its own right is the change worth preparing for: it stops being something averaged away inside a rating and starts being a number on the certificate. **U-Monkey** calculates it element by element to BS EN ISO 6946, free and in the browser, and shows the working rather than just the answer. [Try U-Monkey →](/tools/u-value-calculator/) We'll keep tracking the reform as the implementation plan and final response land. If you'd like the plain-English version of each step as it happens — and a heads-up on what it means for your assessments — keep an eye on the Energycount news feed. --- # The Building Safety Levy goes live on 1 October — what developers and Building Control need to know URL: https://energycount.co.uk/news/building-safety-levy-1-october-2026/ Date: 2026-08-08 This one isn't a SAP input, but it lands squarely on the desks of the people we work with every day — housebuilders, developers and Building Control. On **1 October 2026** the **Building Safety Levy** comes into operation in England, and if you're bringing forward anything of ten homes or more, it's a number you'll want in your appraisal now rather than as a surprise near completion. ## What the levy is The Building Safety Levy is a tax on new residential buildings in England, brought in under the **Building Safety Act 2022**. The money raised — expected to run to billions of pounds over the next decade — goes towards fixing building safety defects such as unsafe cladding across the country. Crucially, it's collected by **local authorities**, in their role as the local guardians of the building control process, rather than by a new national body. The rules sit in **The Building Safety Levy (England) Regulations 2025** (made on 19 November 2025), with a set of minor amendment regulations laid before Parliament on 2 July 2026 and the official guidance last updated in July 2026. ## Who pays, and who doesn't The levy bites where three conditions are all met: the works form part of a **major residential development**, they create new residential floorspace, and the client isn't an exempt person. "Major residential development" means **10 or more new dwellings, or 30 or more bedspaces** in purpose-built student accommodation. Schemes below that threshold are outside the levy altogether — and you can't dodge it by slicing a large permission into sub-ten-dwelling building control applications, because liability follows the planning permission for the wider site. A number of developments are excluded. **Social housing** (including affordable, social and intermediate rent, shared ownership, and First Homes sold at no more than 70% of market value) and **supported housing** are exempt, as is anything built by a **non-profit registered provider of social housing** — for those providers, all their works are exempt even where the homes would otherwise be chargeable. Also outside the charge: care homes and hospices, hospitals, hotels and hostels, children's homes, refuges for domestic abuse victims, armed forces accommodation and similar. Straightforward extensions and improvements that don't create a new dwelling aren't caught either. ## How the charge is worked out The levy is a **rate per square metre** applied to the chargeable floorspace, measured as Gross Internal Area under the RICS Code of Measuring Practice. Each local authority has its own rate, weighted by average house prices in the area — so the figures vary enormously, from around £13 per square metre in the lowest-value areas to over £100 per square metre in the priciest. There's a **50% discount for previously developed ("brownfield") land**, to reflect the higher cost of building there. It's worth actually pulling your council's rate from the published schedule before you price a scheme, because on a site of any size the difference between a low-value and a high-value authority is real money. ## Where the teeth are Here's the bit to flag to your commercial team: payment is due **before the earlier of completion or occupation**, and it's enforced through the certificate you can't do without. If the levy isn't paid, the building control authority will **withhold the completion certificate** (or reject the final certificate). No certificate, no sign-off — so this isn't a bill you can leave to argue about later. One point of timing relief: the levy only applies to applications for building control approval submitted **on or after 1 October 2026**. Applications submitted before that date aren't caught, even if they're varied afterwards — but if a pre-October application is rejected and then resubmitted after the go-live, it will be liable. ## What we'd suggest doing If you've got schemes crossing the 1 October line, three quick jobs are worth doing now: check your local authority's rate against the published schedule, confirm whether any part of the scheme qualifies for the social-housing or brownfield reliefs, and make sure whoever manages your building control applications knows the levy has to be cleared before the completion certificate is issued. Of the costs landing this autumn, the levy is the one you cannot calculate your way out of. Thermal bridging is not: SAP's default y-value is deliberately pessimistic, and **ΨMonkey** models junctions to BS EN ISO 10211 free on screen, which on a marginal plot often buys back more specification than the levy takes. [Try ΨMonkey →](/tools/psi-calculator/) The levy sits alongside the compliance work we do help with — the **Part L**, **Part G** and thermal-bridging numbers that also have to be right before Building Control signs a scheme off. If you'd like a hand getting the energy and water side buttoned down while the commercial team handles the levy, [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [Building Safety Levy: Guidance — Section 1: Introduction (gov.uk, updated July 2026)](https://www.gov.uk/guidance/building-safety-levy-guidance/section-1-introduction) · [Building Safety Levy: Guidance — contents (gov.uk)](https://www.gov.uk/guidance/building-safety-levy-guidance) · [The Building Safety Levy (England) Regulations 2025 (legislation.gov.uk)](https://www.legislation.gov.uk/uksi/2025/1236/contents/made)* --- # TM59 has been rewritten — what the 2026 overheating methodology changes URL: https://energycount.co.uk/news/cibse-tm59-2026-overheating-update/ Date: 2026-08-08 Overheating has been quietly climbing up the design agenda for years, and this month it took a big step. In **July 2026**, CIBSE — working with Arup and Loughborough University — published a substantially revised second edition of **TM59**, the standard methodology for assessing overheating risk in homes. It's been retitled *Overheating Risk in Dwellings: A Design Stage Methodology*, and it's less a refresh than a rewrite. ## Why TM59 matters to assessors If you touch Part O, you already know TM59. **Approved Document O** offers two compliance routes — a simplified prescriptive check, and the dynamic thermal modelling route built on TM59. Where a design can't pass the simplified method (and plenty can't), the TM59 model is what decides whether a home overheats on paper. So a new edition of the methodology changes the reference point for a large slice of overheating work. ## What's actually changed The headline is a shift to a **passive-first, three-stage assessment**. The 2017 edition was essentially a pass/fail check against the criteria. The 2026 edition makes you demonstrate the design in a strict hierarchy: **Stage 1** assesses the dwelling under an unconstrained passive scenario; **Stage 2** only allows enhanced mechanical ventilation where real site issues — external noise, poor air quality, security — genuinely restrict opening windows; and **Stage 3** treats mechanical cooling as a last resort. It's the same cooling hierarchy London assessors will recognise from the GLA's energy guidance. The number of assessment criteria has grown from two to **four**. Criterion A covers daytime comfort in naturally ventilated living rooms, kitchens and bedrooms — and now home offices. Criterion C formalises the rules for mechanically ventilated or cooled spaces. And a new **Criterion D makes communal circulation areas a mandatory part of compliance**, requiring them to stay below 28°C for all but 3% of occupied hours — previously a flagged risk, now a potential fail. The biggest technical change is the **bedroom night-time test (Criterion B)**. The old rule failed a bedroom if it exceeded 26°C for more than 1% of annual sleeping hours. The new method instead counts **overheated nights** — a maximum of four between May and September — using mean operative temperature, with sleeping hours shifted to 11pm–8am and thresholds that vary by occupant category. It's backed by Loughborough University sleep research, and it's designed to reflect real disruption rather than a blunt hourly count. A few other changes worth noting: **ceiling fans** are now formally recognised as a mitigation measure, with specified air speeds and temperature uplifts (though not for sleeping bedrooms or communal circulation, and with rules to stop double-counting in the model); a dedicated **home office** occupancy profile has been added; the scope now explicitly extends to sheltered housing, care homes, student accommodation, hotels, hospital bedrooms and even retrofit projects; and the reporting requirements are considerably more detailed, expecting you to document and justify every stage, occupant category and ventilation or shading assumption. ## The bit that trips people up: the regs haven't caught up Here's the nuance that matters for compliance. **TM59:2026 is a CIBSE methodology, not a Building Regulation.** Approved Document O 2021 — in force since 15 June 2022 — still references the **2017** edition of TM59. The government confirmed in its March 2026 Future Homes Standard consultation response that Part O will get a **full technical review**, and adopting the updated TM59 is explicitly on that review's list — but no revised Approved Document O and no date have been published yet. So there's a genuine gap to manage. For strict Building Regulations sign-off, the 2017 methodology remains the referenced document until Part O is revised. But the 2026 edition is now the current industry best practice, and London schemes, planning conditions and many clients' own standards will increasingly expect it. On a lot of projects you'll effectively be working to both — which makes it worth reading the new edition now rather than when the regs finally point at it. ## What this means for design The direction of travel is clear and it rewards the same things good assessors already push for: real passive design, effective external shading, sensible glazing ratios and genuine thermal mass, rather than a marginal pass that leans on opening windows at night on a noisy or insecure elevation. Designs built on those assumptions will weather the transition — and any future Part O revision — a lot better than those that don't. There's a knock-on into the energy calculation, too. Where a scheme leans on reduced g-value glazing or mechanical cooling to pass overheating, that choice shows up in the **Part L** numbers — one more reason to get the fabric and glazing strategy right early, before it's fighting your SAP result. TM59 is a dynamic model and there is no shortcut to it, but two fabric numbers feed the behaviour it simulates: how much of the day's heat a build-up lets through, and how long it takes. Our [thermal mass calculator](/tools/u-value-calculator/thermal-mass-calculator/) gives the decrement factor and time lag for any construction to BS EN ISO 13786, free — useful for sanity-checking a specification early, though not a substitute for the model. If overheating is the sticking point on one of your sites, or you want the fabric, glazing and thermal-bridging side modelled properly so it doesn't undermine your Part L result, [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [CIBSE TM59 — Overheating risk in dwellings: a design stage methodology (2026), CIBSE Knowledge Portal](https://www.cibse.org/knowledge-research/knowledge-portal/tm59-overheating-risk-in-dwellings-a-design-stage-methodology-2026/) · [TM59:2026 — Overheating in Residential Buildings; what's changed, Housebuilder & Developer, 17 July 2026](https://www.hbdonline.co.uk/news/tm59-2026-overheating-in-residential-buildings-whats-changed-from-the-2017-edition/) · [Guidance to help tackle overheating risk in homes, Loughborough University, July 2026](https://www.lboro.ac.uk/news-events/news/2026/july/overheating-homes/) · [Government Confirms Full Review of Approved Document O (HEM Guide, 9 April 2026, verified against the FHS consultation response)](https://home-energy-model.co.uk/news/2026-04-09-part-o-overheating-review/) · [Approved Document O, Overheating (gov.uk)](https://www.gov.uk/government/publications/overheating-approved-document-o)* --- # A water label for taps and showers is coming — and it lands next to Part G URL: https://energycount.co.uk/news/mandatory-water-efficiency-labelling-2026/ Date: 2026-08-07 You already read a lot of labels: SAP bands, energy ratings, the U-value on a window schedule. There's a new one on the way, and this time it's on the fittings themselves — the taps, showers and WCs that decide whether your Part G calculation passes. The government is introducing a **mandatory water efficiency label**: an A–F, colour-coded rating — much like the energy label on a fridge — that has to be shown on water-using products at the point of sale. ## What's actually being introduced The scheme is the **Mandatory Water Efficiency Labelling Scheme (MWELS)**, brought in under powers in the Environment Act 2021. In short, it puts a standard, comparable water-efficiency rating on the products people buy. The headline points: - A single **A–F colour-coded label** showing how much water a product uses, so two showers or two WCs can be compared like for like. - It covers the usual suspects — **taps, showers, toilets and urinals, dishwashers and washing machines**. - The obligation sits with the trade: **suppliers** test their products, work out the banding and produce the label; **dealers** (retailers and merchants) have to display it clearly at the point of sale. - It's built on the international standard **ISO 31600:2022**, and it applies **UK-wide** — England, Scotland, Wales and Northern Ireland. The reasoning is the same one driving the Part G reform: England is short of water, and demand from new homes has become a genuine constraint on where and how much you can build. A label won't save a drop on its own, but it makes the efficient choice the obvious one on the shelf. ## Where it stands right now This is coming, not here. The regulations were **laid before Parliament on 6 May 2026** as a draft affirmative instrument — meaning both Houses have to approve them before they take effect — with the intention of the scheme **coming into force by the end of 2026**. So nothing changes on your current jobs today. But the direction is set, and the timing is worth noting: the label is arriving in the same window as the proposed cut to the Part G standard from **125 to 105 litres per person per day**. Two separate pieces of policy, pointing the same way, landing at roughly the same time. ## It's not landing without argument Worth being straight about: the industry isn't uniformly behind the detail. The **Builders Merchants Federation** has flagged the risk of consumer confusion, given there's already an established voluntary label (the Unified Water Label) in the market — and a second, mandatory scheme sitting alongside it could muddy rather than clarify. Appliance manufacturers have raised concerns about cost and about whether a single metric fairly represents how products like dishwashers actually perform. There's a deeper point too, and it's one assessors will recognise. A recent University of Surrey study (with input from Mira Showers) made the case that **flow rate on a label doesn't equal real-world consumption**: push flow rates too low and people take longer showers, or tamper with the fitting, and the saving the label promised quietly disappears. The label tells you what a product can do, not what a household will actually do with it — which is exactly the gap the Part G calculator's fixed-use assumptions try, imperfectly, to bridge. ## What it means for you For assessors and specifiers, the useful bit is the join between this and Part G. The water calculator has always run off the specified fittings — shower flow rate, bath capacity, WC flush volume, tap flow rates. A consistent, mandatory label makes those inputs **easier to source and easier to evidence**: instead of digging through manufacturer datasheets for a flow rate, you'll increasingly have a standard rating to point to. It also sharpens the compliance question that Defra has already raised — whether the fittings on the drawings are the fittings that actually get installed. When every product on site carries a visible rating, the gap between the specified spec and the fitted spec gets harder to hide. None of this changes your method today. **125 l/p/d remains the standard you calculate to**, with 110 l/p/d where a Local Plan requires it, until the Part G reform is confirmed. But if you're specifying fittings for schemes that will complete in 2027 and beyond, it's worth getting into the habit now of choosing products on their water rating — because the margin between a comfortable pass and a fail lives entirely in those numbers. ## What we'd suggest doing Know your number before the standards tighten. Run your standard house types through a proper Part G calculation and see which ones still pass with headroom — a fittings spec is far cheaper to adjust on paper than after a Building Control inspection. **WaterMonkey** does exactly that: the full official water efficiency methodology, live PASS/FAIL as you type at 125 or 110 l/p/d, rainwater and greywater included, free in your browser. Building Control-ready PDF reports are £25 +VAT, and one dwelling type covers every identical plot on a site — so a whole estate needn't cost the earth. We'll factor the new labelling in as the scheme and the revised Part G methodology are confirmed. [Try the free Part G water calculator →](/tools/part-g-water-calculator/) If water is already the sticking point on one of your sites, [get in touch](/contact/) and we'll tell you how we'd approach it. *Sources: [UK mandatory water efficiency labelling — Defra consultation and outcome (gov.uk)](https://www.gov.uk/government/consultations/uk-mandatory-water-efficiency-labelling) · [United Kingdom: New Mandatory Water Efficiency Labelling (MWEL) Regulations — Product Compliance Institute, 6 February 2026](https://www.productcomplianceinstitute.com/2026/02/06/united-kingdom-new-mandatory-water-efficiency-labelling-mwel-regulations/) · [The Mandatory Water Efficiency Labelling (MWEL) Regulations 2026 — TÜV Rheinland](https://www.tuv.com/regulations-and-standards/en/united-kingdom-the-mandatory-water-efficiency-labeling-mwel-regulations-2026.html) · [Water efficiency labelling alone won't deliver Britain's water-saving ambitions — Builders Merchants Journal, 27 July 2026](https://buildersmerchantsjournal.net/water-efficiency-labelling-alone-wont-deliver-britains-water-saving-ambitions/)* --- # Part G is heading for 105 litres — and the water industry has backed it URL: https://energycount.co.uk/news/part-g-105-litres-water-efficiency/ Date: 2026-08-06 If you've done a Part G water calculation in the last decade, you know the number: **125 litres per person per day**. It's been the baseline since the standard came in, with an optional tighter standard of 110 l/p/d that councils can require through their Local Plans in water-stressed areas. Both figures are on the table — and this month the water industry came out publicly in favour of changing them. ## What's actually proposed Defra's [review of water efficiency standards in the Building Regulations 2010](https://consult.defra.gov.uk/water-efficiency-demand/review-of-water-efficiency-standards) put forward three changes: - Cutting the minimum Water Efficiency Standard from **125 l/p/d to 105 l/p/d** - Cutting the optional technical standard from **110 l/p/d to 100 l/p/d**, for areas of water stress or other local need - **Revising the water calculator and the fittings-based approach**, to reflect how water use and fitting technology have moved on since the standard was written There's also a call for evidence attached, looking at whether rainwater harvesting and grey/black water reuse should play a bigger role in future. The driver is blunt, and it isn't really about the environment: water scarcity has become a housing supply problem. Defra's own consultation puts it in cash terms — failing to manage water demand could mean **over 61,000 homes not being built** this parliamentary term, at a cost of some £25 billion. Cambridge and parts of Sussex have already seen schemes held up because demand outstripped supply. Twenty litres per person per day, multiplied across a site, is the difference between a scheme that fits inside the water resource envelope and one that stalls. ## Where it stands right now The consultation opened on **23 September 2025** and closed on **16 December 2025**. Defra has not yet published its response, so nothing in Approved Document G has changed: **125 l/p/d remains the standard you design and calculate to today**, with 110 l/p/d where a Local Plan policy or planning condition requires it. The stated intention is to implement the Part G changes **during 2026**, subject to the consultation outcome and parliamentary approval, followed by a **six-month transitional period** ([CIH briefing, September 2025](https://www.cih.org/news/cih-briefing-on-defra-s-consultation-on-updating-part-g-of-the-building-regulations-on-water-efficiency-in-new-homes/)). The news this month is **Water UK** — the industry's trade association — publishing its response backing the 105 l/p/d figure. It also supports the fittings-based route to demonstrating compliance as simpler and more consistent, and argues for a **single national standard** rather than a patchwork of locally set numbers ([Water Magazine, 3 August 2026](https://www.watermagazine.co.uk/2026/08/03/water-uk-backs-tougher-water-efficiency-standards-for-new-homes/)). If ministers agree, that last point would be a quiet relief to anyone who's had to work out which of three water policies a particular district council is currently applying. Water UK's caveat is worth noting too: tighter new-build standards alone won't deliver the demand reductions England needs — retrofit, smart metering and more efficient appliances have to do the heavy lifting on the existing stock. Useful context for how this is likely to be framed when the response lands. ## What it means in practice The jump from 125 to 105 isn't enormous, but it's enough to bite on schemes that currently scrape through. If you've been passing at 118 or 120 with a fairly standard fittings spec, that margin disappears. The pressure lands where it always does: **shower flow rate, bath capacity, WC flush volumes, and basin and kitchen tap flow rates**. A generous bath and a 9 l/min shower head will eat the new headroom between them without any help. The other lever is rainwater or greywater reuse, which the calculator already recognises — and which the call for evidence suggests the government is interested in leaning on harder in future. There's a compliance angle too. The consultation notes that actual consumption in some new homes runs as high as **195 l/p/d** — either because people use water in unexpected ways, or because dwellings aren't being built to the standard on the drawings. Expect more attention on whether the fittings that were specified are the fittings that got installed. ## What we'd suggest doing Don't redesign anything yet. Do know your number. Run your standard house types through the calculation and see which ones would still pass at 105. One dwelling type covers every identical plot on a site, so this is a small piece of work that tells you a lot about your exposure — and it's far cheaper to fix a specification on a spec sheet than after a Building Control inspection. **WaterMonkey** does exactly that: the full official water efficiency methodology, with live PASS/FAIL as you type at 125 or 110 l/p/d, rainwater and greywater included, free in your browser. Building Control-ready PDF reports are £25 +VAT, and one dwelling type covers every identical plot on the site — so a whole estate needn't cost the earth. We'll update the calculator as soon as the revised standard and methodology are confirmed. [Try the free Part G water calculator →](/tools/part-g-water-calculator/) If you've got a site where water is already the sticking point — or a planning condition asking for something tighter than 110 — [get in touch](/contact/) and we'll tell you how we'd sort it. *Sources: [Review of Water Efficiency Standards in the Building Regulations 2010 — Defra consultation](https://consult.defra.gov.uk/water-efficiency-demand/review-of-water-efficiency-standards) · [Water UK backs tougher water efficiency standards for new homes — Water Magazine, 3 August 2026](https://www.watermagazine.co.uk/2026/08/03/water-uk-backs-tougher-water-efficiency-standards-for-new-homes/) · [CIH briefing on Defra's Part G consultation](https://www.cih.org/news/cih-briefing-on-defra-s-consultation-on-updating-part-g-of-the-building-regulations-on-water-efficiency-in-new-homes/)* --- # Your window U-values are about to get harder (and more honest) URL: https://energycount.co.uk/news/window-doorset-u-values-adl-2026/ Date: 2026-07-06 Here's a change hiding in plain sight inside the Future Homes Standard: the way window and door **U-values** are calculated and declared is being overhauled. It's not the headline the trade press ran with, but it lands squarely on assessors' desks — because these are numbers you type straight into your SAP model. ## What's actually changing Today, a window U-value is typically calculated on a **standard reference size** — a fixed benchmark pane that rarely matches what's actually fitted. It's a tidy simplification, but it flatters the paperwork: a real window is seldom that exact size, so the declared figure and the installed reality drift apart. From **24 March 2027** in England (4 March 2027 in Wales), Approved Document L 2026 requires the declared U-value to reflect the **actual size and configuration** of every window and doorset installed — a far more rigorous, product-by-product approach calculated in line with **BR 443** and **EN ISO 10077-1/-2** ([Specification Online, 10 June 2026](https://specificationonline.co.uk/articles/2026-06-10/bsi/new-certification-launched-to-help-industry-comply-with-the-future-homes-standard)). ## Why the numbers go up This is the bit worth internalising: smaller windows generally have a **worse** U-value, not a better one. Frames conduct heat more readily than the glazed unit, so the more frame you have relative to glass — as in a small window — the higher the whole-window U-value climbs. In practice, a product currently declared at around **1.30 W/m²K** on a standard reference size can recalculate closer to **1.40–1.50 W/m²K** once the real dimensions and all the correct components are accounted for ([British Woodworking Federation](https://www.bwf.org.uk/latest-news/changes-to-u-values-explained/)). Nobody's product got worse overnight — the methodology just stopped rounding in the specifier's favour. ## What it means for assessors A useful line to hold: **you set the target, the supplier certifies the value.** As the assessor you can specify the U-value a dwelling needs — say, windows at 1.30 W/m²K to make the fabric energy balance work — but you can't calculate or certify the window's U-value on the manufacturer's behalf. That declared figure is the supplier's responsibility, produced with the correct Future Homes Standard methodology. For **SAP 10.3** — still the sole compliance route at Future Homes Standard launch — the practical upshot is simple: the U-value inputs you use must reflect each window's **as-designed** configuration, not a generic default or an optimistic reference-size figure. Ask suppliers for U-values calculated to the new method, and treat any old standard-size declaration with caution. To help the market get this right, BSI has just launched two new **Certificates of Conformity** for the underlying calculation data and software, and ran an introductory webinar on 1 July 2026 ([Specification Online](https://specificationonline.co.uk/articles/2026-06-10/bsi/new-certification-launched-to-help-industry-comply-with-the-future-homes-standard)). ## The bigger fabric picture Tighter fabric standards mean every element of the heat-loss calculation gets more scrutiny — glazing today, and the **junctions** around it just as much. As walls and openings improve, the heat escaping through poorly-detailed thermal bridges becomes a bigger share of the total, and leaning on the default junction Psi-value of 0.15 W/m·K carries a real penalty in your assessment. That's where **ΨMonkey** earns its keep — a browser-based Psi-value calculator that does proper **BS EN ISO 10211** thermal modelling to **BR 497** conventions, without the desktop-software faff or the consultancy invoice. Get your window U-values honest and your junction Psi-values calculated, and your SAP figures have a lot less to prove. [Try ΨMonkey →](/tools/psi-calculator/) *Sources: [New certification launched to help industry comply with the Future Homes Standard — Specification Online, 10 June 2026](https://specificationonline.co.uk/articles/2026-06-10/bsi/new-certification-launched-to-help-industry-comply-with-the-future-homes-standard) · [Changes to U-Values Explained — British Woodworking Federation](https://www.bwf.org.uk/latest-news/changes-to-u-values-explained/) · [Approved Document L Volume 1 2026 (gov.uk)](https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf)* --- # The Nature Restoration Fund just took its first legal step URL: https://energycount.co.uk/news/nature-restoration-fund-nutrient-regulations-laid/ Date: 2026-06-24 If you've had a housing scheme stuck in a protected catchment, this one matters. On **18 June 2026** the government laid the first two pieces of secondary legislation for the **Nature Restoration Fund (NRF)** before Parliament — the machinery that's meant to give developers a simpler way to deal with nutrient pollution than the site-by-site mitigation we've all been wrestling with ([Defra Environment blog, 19 June 2026](https://defraenvironment.blog.gov.uk/2026/06/19/laying-the-groundwork-for-the-nature-restoration-fund/)). ## What actually got laid Two sets of regulations, both now on the statute books in draft: - **The Nature Restoration Levy Regulations 2026** — the framework for how the nature restoration levy will work, i.e. the route by which developers will eventually be able to discharge certain environmental obligations with a payment. ([legislation.gov.uk](https://www.legislation.gov.uk/ukdsi/2026/9780348284379/contents)) - **The Environmental Delivery Plans (Appropriate Prioritisation) Regulations 2026** — how Natural England must prioritise different "conservation measures" when it writes an Environmental Delivery Plan, honouring the mitigation hierarchy commitment made during the Bill's passage. These regulations **came into force on 9 July 2026**. ([legislation.gov.uk](https://www.legislation.gov.uk/uksi/2026/655/contents/made)) Both flow from the **Planning and Infrastructure Act 2025**, which got Royal Assent in December 2025 and created the NRF in the first place. ## How the NRF is meant to work The idea is straightforward, even if the delivery is anything but. Rather than commissioning your own mitigation for every site, you'll be able to pay a **nature restoration levy** into a central fund. Natural England pools those payments and delivers nutrient mitigation at a landscape scale — wetlands, river restoration, that sort of thing — through area-specific **Environmental Delivery Plans (EDPs)**. Where an EDP is in place and covers your impact, the levy payment replaces the bespoke nutrient budgeting and credit-hunting that has held up tens of thousands of homes across affected catchments. ## The important caveat: nothing is live yet Here's the bit to be clear-eyed about. Laying regulations is a step *towards* the fund, not the switch being flicked. Natural England has said it will **shortly launch the formal consultation on the first draft EDP — which will deal with nutrient pollution** — and every draft EDP must go through a public consultation of at least 28 days. Only once the legislation is in place and consultations are complete will finalised EDPs go to the Secretary of State for approval before they "go live" for developers. So for now, **nutrient neutrality still applies exactly as it does today.** If your site sits in one of the affected catchments, you still need a nutrient budget and mitigation to get a planning consent — the NRF is the future route, not yet the present one. ## What this means for you Two practical takeaways. First, keep doing your nutrient calculations properly — the current regime is unchanged until an approved EDP actually covers your area, and a sound nutrient budget remains the foundation of any consent or mitigation deal. Second, watch for that first nutrient EDP consultation; when it lands it'll set out the proposed conservation measures *and* the levy charging schedule, which is the number every developer will want to compare against the cost of buying credits today. Either way, it starts with knowing your numbers. **NutrientMonkey** works out the nitrogen and phosphorus load for your scheme across England's affected catchments — in your browser, no sign-up — so whether you're mitigating the old way or sizing up the levy when it arrives, you're working from a defensible budget rather than a guess. [Try NutrientMonkey →](/tools/nutrient-neutrality-calculator/) *Sources: [Laying the groundwork for the Nature Restoration Fund — Defra Environment blog, 19 June 2026](https://defraenvironment.blog.gov.uk/2026/06/19/laying-the-groundwork-for-the-nature-restoration-fund/) · [The Nature Restoration Levy Regulations 2026 (legislation.gov.uk)](https://www.legislation.gov.uk/ukdsi/2026/9780348284379/contents) · [The Environmental Delivery Plans (Appropriate Prioritisation) Regulations 2026 (legislation.gov.uk)](https://www.legislation.gov.uk/uksi/2026/655/contents/made) · [Nature Restoration Fund Implementation Plan (gov.uk)](https://www.gov.uk/government/publications/nature-restoration-fund-implementation-plan)* --- # The Future Homes Standard is real — what ADL 2026 changes URL: https://energycount.co.uk/news/future-homes-standard-adl-2026/ Date: 2026-06-19 After years of "it's coming, honest," the Future Homes and Buildings Standards are no longer a rumour. On **24 March 2026**, MHCLG published the new statutory guidance — **Approved Document L 2026** (Volumes 1 and 2) plus an updated **Approved Document F** — alongside The Building Regulations etc. (Amendment) (England) Regulations 2026. ## The dates that matter The headline you need to diarise: the changes **come into force on 24 March 2027**. There's a one-year buffer too — for non-HRB work, projects with a building notice, initial notice or full-plans application submitted **before 24 March 2027** can still build to current standards, provided work starts **before 24 March 2028**. Higher-risk buildings get a slightly later switch-on of **24 September 2027** with their own transitional rules. In short: you've got runway, but the clock is now genuinely ticking. ([Building Circular 01/2026, gov.uk](https://www.gov.uk/government/publications/the-future-homes-and-buildings-standards-building-circular-012026/the-future-homes-and-buildings-standards-building-circular-012026-letter)) ## What's new in the rules The big-ticket changes amend the Building Regulations 2010 to bake in low-carbon heating and high-efficiency fabric, so new homes won't need retrofitting to hit zero-carbon-in-use as the grid decarbonises. Notable bits: - A **new functional requirement (L3)** mandating on-site renewable electricity generation (think solar PV) for new dwellings and buildings containing dwellings. Our [solar PV calculator](/tools/solar-pv-calculator/) works out how much a home needs. - Part L of Schedule 1 reworded to cover **both** cutting greenhouse gas emissions **and** conserving energy. - "Fixed building services" widened to include lifts, escalators and moving footways in non-dwelling buildings. - A new **regulation 40C** requiring homeowner information (the Home User Guide) in a usable format. And — importantly for the carbon maths — the old 2013 and 2021 transitional arrangements that let people keep building to 2010 energy standards are being revoked. ## What about SAP and the Home Energy Model? One thing that trips people up: **at launch, SAP 10.3 is the only methodology** you can use to demonstrate compliance. The much-discussed Home Energy Model (HEM) isn't the approved tool on day one — it follows a minimum of three months after the standard goes live, with a confirmed dual-running period before it eventually replaces SAP. So for now, plan around SAP 10.3. ## What this means for you (and your junctions) Here's the bit competitors gloss over. Tighter fabric standards make **thermal bridging** matter *more*, not less. As walls, roofs and floors get better insulated, the heat that sneaks out through poorly-detailed junctions becomes a bigger slice of the total loss. Lean on the default junction Psi-value of 0.15 W/m·K and you'll be carrying a hefty, avoidable penalty in your calculation. The fix is the same as it's always been, just higher-stakes: calculate your junction Psi-values properly to **BS EN ISO 10211**, following **BR 497** conventions, rather than accepting worst-case defaults. Get the details right on paper and your SAP figures breathe a lot easier. That's exactly what **ΨMonkey** is for — a browser-based Psi-value calculator that does ISO 10211 thermal modelling without the stuffy desktop software or the eye-watering consultancy invoice. Model your junctions, drop the real numbers into your assessment, and give the Future Homes Standard the respect it deserves (while not taking the paperwork too seriously). [Try ΨMonkey →](/tools/psi-calculator/) *Sources: [Building Circular 01/2026 (gov.uk)](https://www.gov.uk/government/publications/the-future-homes-and-buildings-standards-building-circular-012026/the-future-homes-and-buildings-standards-building-circular-012026-letter) · [Approved Document L Volume 1 2026 (gov.uk)](https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf) · [The Building Regulations etc. (Amendment) (England) Regulations 2026 (legislation.gov.uk)](https://www.legislation.gov.uk/uksi/2026/335/made)* --- # The Home Energy Model just hit snooze (and that's fine) URL: https://energycount.co.uk/news/home-energy-model-launch-delayed/ Date: 2026-06-19 Quick one for anyone watching the Future Homes Standard like a hawk: on **8 June 2026** the government quietly pressed pause on the **Home Energy Model (HEM)** launch. HEM was widely expected to arrive around now — the Future Homes Standard documents had flagged it would be approved "no earlier than three months after publication," which pointed to a June 2026 window. Instead, it's being held back for final internal assurance, and is now expected "in the coming months" rather than on any fixed date ([HEM Guide timeline, verified against GOV.UK](https://home-energy-model.co.uk/timeline/)). So, no panic. Nothing has gone backwards. ## What this actually changes Practically? Very little changes *today* — and that's the point. **SAP 10.3 remains the sole approved methodology for Future Homes Standard compliance.** It was always going to be the only route at the FHS launch on **24 March 2027**, with HEM following at least three months later and then running alongside SAP 10.3 for a minimum of 24 months. This delay just nudges the start of that dual-running period a little further out. In other words: the methodology you'll be using for FHS submissions on day one hasn't changed. SAP 10.3 — updated with the new FHS notional dwelling specs and forward-looking carbon factors — is still the tool of the moment, and now for a touch longer than some had pencilled in. ## Why a delay is genuinely good news We'd rather have a robust calculation engine than a rushed one. HEM is a big leap — half-hourly modelling (17,520 timesteps versus SAP's monthly 12), centralised calculation through the **ECaaS** cloud platform, and a serious step up in the volume of input data it expects. Getting that right before it becomes a compliance route matters far more than hitting an arbitrary launch month. A delay for "final assurance" is the regulator doing the sensible thing. ## What this means for you If you're an assessor or developer, here's the chilled-out version: Keep building your SAP 10.3 confidence — it's the route that counts at launch and for a good while after. Don't down tools on HEM prep, though: when it arrives, HEM will *punish* missing data with deliberately punitive defaults, so the teams who start collecting detailed product, geometry and junction data now will be the ones gliding through later. And one detail that's easy to miss in all the heat-pump-and-solar headlines: **thermal bridging still matters under both routes.** Whether you're in SAP 10.3 today or HEM tomorrow, junction psi-values feed straight into your fabric performance — and HEM expects a psi-value for *each* junction or an accredited construction detail reference. Sloppy thermal bridging is one of the quietest ways to fail your fabric energy efficiency target. ## Sort your psi-values the easy way That's exactly what **ΨMonkey** is for. It calculates thermal bridging psi-values to BS EN ISO 10211 in your browser — no clunky desktop software, no PhD required — so your junctions are nailed down whichever compliance route you end up taking. Give it a spin and take one variable off your plate while everyone else waits for HEM to wake up. [Try ΨMonkey →](/tools/psi-calculator/) *Sources: [Home Energy Model timeline & status (HEM Guide)](https://home-energy-model.co.uk/timeline/) · [FHS compliance pathways](https://home-energy-model.co.uk/future-homes-standard/compliance/) · [GOV.UK — Home Energy Model consultation](https://www.gov.uk/government/consultations/home-energy-model-replacement-for-the-standard-assessment-procedure-sap)*