How to use U-Monkey — the complete guide

Quick start — a U-value in under a minute

1. Open the calculator. Already have the build-up written down? Type it into Describe it on the first screen — 102.5 brick / 100 DriTherm 32 / 100 block / 12.5 plasterboard on dabs — and press Build it. Otherwise pick your element — wall, roof or floor.
2. Tap a starter template (or “See a worked example”) to load a typical build-up.
3. Swap layers and thicknesses to match your construction. The U-value and the Part L traffic lights update live.

That's genuinely it for a simple element. The rest of this guide covers each feature properly.

1. Choose your element and building-work type

U-Monkey handles external walls, pitched roofs (insulated at ceiling or rafter level), flat roofs (including tapered schemes), exposed floors, solid ground floors and suspended ground floors. Both ground floors follow BS EN ISO 13370, so you'll also enter the exposed perimeter, floor area and wall thickness — a ground-floor U-value depends on plot geometry, not just the layers. A suspended floor adds the ventilated void beneath it. A heated basement has its own two elements: the basement floor (the ground-floor equation with the depth z below ground, which reduces the loss) and the basement wall (the ISO 13370 average over the buried depth, from the wall build-up, the depth and the basement floor's resistance). The part of a basement wall above ground is an ordinary external wall.

For a loft insulated at ceiling level, build the ceiling only: the insulation between and over the joists and the plasterboard. BR 443 (§8) counts everything above the insulation (the ventilated loft, the roof timbers and the covering) as one resistance from BS EN ISO 6946 Table 9, and U-Monkey adds it for you: 0.20 m²K/W for tiles or slates on felt, boards or an underlay, chosen in the roof above the loft box (0.06 with nothing under the tiles, 0.30 with a foil surface under the roof or a boarded and felted roof), with R_se 0.04 beyond it. Joists between the insulation start at BR 443's default of 12.8% timber (47 mm at 400 mm centres plus 1% for cross pieces, §4.5.1); change it if yours are trussed rafters at 600 mm.

A wall or exposed floor next to an unheated space — an integral garage, an unheated stairwell or access corridor in a block of flats, or the loft void behind the wall of a room in roof (an exposed floor takes the garage rows only) — loses less heat than one facing outside air. Pick the space in the On the other side box and U-Monkey adds its resistance the way SAP does: U = 1/(1/U₀ + R_u), where U₀ is the element as if it faced outside air and R_u comes from SAP 10.2 Tables 3.1–3.4 (unchanged in SAP 10.3), or enter your own R_u worked out as BR 443 §5 describes. Only integral garages count; SAP leaves attached garages, and thermally separated conservatories, out of it. For the ceiling of a room below such a loft void, pick the 0.50 row of the roof above the loft box instead.

A party wall is different again: its U-value is not calculated from the layers but assigned by construction, because the loss is air movement inside the cavity rather than conduction to the neighbour (SAP 10.2 Table 3.10, AD L Table 2.1) — solid 0.0, unfilled cavity with no edge sealing 0.5, unfilled but effectively sealed 0.2, fully filled and sealed 0.0. Pick the construction and U-Monkey judges it against the 0.20 limit in England and Wales (Scotland requires a sealed, fully filled cavity, 0.0). Adding the layers is optional and only feeds the cross-section and thermal mass.

Then, under the verdict, say where the building is (England, Wales, Scotland, Northern Ireland or Ireland) and what kind of building it is (a dwelling, or anything else) — or type the site postcode there and U-Monkey sets the nation and the nearest of its 21 climate stations for you (only the first half, e.g. CF10, is looked up). For England and Wales there is also an edition switch: the 2021 / 2022 Approved Documents, or the 2026 ones that take effect on 24 March 2027 (England, the Future Homes Standard) and 4 March 2027 (Wales); work notified earlier can stay on the old edition for a year. The Future Homes Standard keeps the 2021 notional fabric for walls, roofs and floors (0.18 / 0.11 / 0.13), so the result shows one Notional / FHS check. Each combination uses its own tables — Approved Document L Vol 1 for dwellings in England (2021) and Wales (2022: roofs 0.13, walls 0.18, floors 0.15, retained elements Table 11.1); Technical Booklets F1 and F2 in Northern Ireland (2022: dwellings Table 2.2 walls 0.18, floors 0.18, roofs 0.16; extensions Table 3.2 walls 0.28, floors 0.22, roofs 0.16–0.18); Section 6 of the Scottish Technical Handbooks (January 2025: domestic Table 6.2 wall 0.17, floor 0.15, roof 0.12; non-domestic Table 6.3 wall 0.21, floor 0.18, roof 0.16 — one table each for new build, extensions and conversions alike); and Approved Document L Vol 2 for buildings other than dwellings in England (2021: walls 0.26, floors 0.18, pitched roofs 0.16, flat roofs 0.18) and Wales (2022: walls 0.26, floors 0.22, roofs 0.2 for new buildings); and Technical Guidance Document L 2022 for the Republic of Ireland (dwellings Table 1: walls 0.18, floors 0.18, pitched roofs 0.16, flat roofs 0.20, windows 1.4; non-domestic Table 1: walls 0.21, floors 0.21, roofs 0.16 / 0.20, windows 1.6; Tables 5, 10 and 11 for existing buildings). Tick Show the pass / fail verdict on the PDF report and the PDF states which one it used. See the detail pages for Scotland, Wales, Northern Ireland, Ireland and non-domestic buildings.

Then set the building-work type, because it decides the pass mark (table numbers below are the England Vol 1 ones):

• New dwelling — judged against the Part L backstop values.
• New element in an extension — the tighter elemental limits (ADL1 2021 Table 4.2: wall 0.18, roof 0.15, floor 0.18).
• Existing element, as built — appraised against the Table 4.3 threshold values (a survey mode, not a design target).
• Upgrading a retained element — the Table 4.3 “improved” values (e.g. cavity wall 0.55, solid wall 0.30, roof 0.16).

Not sure what you need? See What U-value do I need?

2. Build up the layers, outside → in

Add layers in construction order, outermost first, with ＋ Add layer. The picker is organised in tabs — Insulation, Insulated plasterboard, Boards, Masonry, Concrete & screeds, Timber, Plasters & renders, Membranes, Coverings & cladding, Air cavities, Metals and Special — and the search box looks across everything, including brand names. Building a whole wall? Tick Keep open — add outside → in in the picker: each pick stays in the picker, clears the search and adds the layer inside the last one, with the build-up so far and the live U-value shown in a strip above the list. Type three letters and press Enter for each layer, then Done. The tick is remembered.

The library holds 480+ materials: BR 443 / CIBSE generic values plus real UK products from Kingspan (including the full Kooltherm K-series), SOPRATHERM (formerly Celotex), Recticel, EcoTherm, Unilin, IKO, Rockwool, Knauf, Isover, Superglass, Jablite, Thermalite, Celcon, Durox and many more — each with its declared λ and the source it came from. Entries marked ✓ verified have been human-checked against the manufacturer's datasheet. Every thickness is editable, layers drag to reorder, and each shows its own resistance (R) so you can see what is doing the work.

Can't find a product? ＋ Custom material takes any name, λ and thickness.

Describe it, on the first screen, is the fastest way in when the build-up is already written down. It reads the same text as the paste box below, works out the element from your words (loft, rafters, flat roof, floor; otherwise a wall) — a label such as “Ground floor:” or “Garage roof:” names the kind, and the layers say which one, so joists over a void make it a suspended floor and EPDM a flat roof — and understands framing: “150 mineral wool between joists” becomes a bridged layer at the BR 443 fraction the templates use (studs 15%, rafters 12%, ceiling joists 12.8%, floor joists 10.8%), while “over the joists” is just where the layer sits. Floors are usually written top-down; U-Monkey works outside to inside, and a floor's outside is below, so a floor that starts with its screed or deck is turned over for you — and a ground floor's concrete slab is left out, as BR 443 asks.

Two shortcuts worth knowing. The picker opens on Recently used (your last picks, newest first) and Common in … — a short list of what usually goes into the element you are building — so the fifteen things your practice actually specifies are one tap away, with the whole library below, A–Z within each type. Search ignores case, hyphens and word order (“celcon hi-strength” finds H+H’s High Strength Grade), and Enter adds the row marked Add ↵. And 📋 Paste a build-up as text takes a whole construction the way you'd write it in an email — 102.5 brick / 50 cavity / 100 PIR / 100 block / 12.5 plasterboard, outside first — and turns it into layers. It shows you what it recognised before adding anything, prefers generic materials so a bare word never quietly picks someone else's branded product, and lists anything it couldn't place rather than guessing at it. If there is already a build-up on screen it replaces it by default (with an Undo on the confirmation), or tick Add after them to append. 🗑 Clear build-up next to it empties every layer to start again — also undoable for a few seconds. Write on dabs after a lining ("12.5 plasterboard on dabs") and the dot-and-dab air gap is added as its own layer, the BR 443 way, with the dabs bridging it; on battens does the same for a battened lining, while cladding, tiles or slates on battens get the ventilated batten space behind them, and "battened out with 50 PIR" puts the PIR between the battens (BR 443's 11.8% timber in a wall). "2 x 50 PIR" is two layers; "47 x 150 rafters" on its own is the frame, not a layer, so it is listed back asking what fills it. A steel frame is added unbridged for you to set, since the timber fractions don't apply to it. Anything Describe can't place stays listed above the layers until you add it or dismiss the note. Name an insulated plasterboard ("37.5 Kingspan K118") and it arrives as core plus facing with the overall board size. Product codes with a slash work as typed ("75 SR/PR", "XT/TL 52.5", and "K 108" or "TW 50" with a space), "ventilated", "vented", "partially ventilated" or "not ventilated" set the cavity, and a brand on its own or with only generic words ("100 Kingspan", "Kingspan insulation board") is listed back to you rather than guessed.

Branded products only offer real thicknesses

Pick a branded product and the − / + buttons step through the thicknesses that manufacturer actually makes, stopping at each end of the range. SOPRATHERM GA4000 (formerly Celotex GA4000) goes 50, 60, 70, 75, 80, 90, 100 — so those are your options. Type something else and it snaps to the nearest real size and tells you the range; the layer says what it's made in, and the picker tells you before you even add it.

The reason is simple: a U-value quoted for 137mm of a board nobody sells isn't a specification anyone can build to. Generic materials are unaffected and still step freely — and so do products that are laid or sprayed to depth rather than sold in boards. Spray foam, blown bead and fibre, loose fill, poured foam-glass gravel and wet lime or clay renders all stay adjustable, because for those the listed figures are typical depths, not a product range.

Insulated plasterboard

Thermal laminates (Kingspan K118, SOPRATHERM PL4000 (ex-Celotex), EcoTherm Eco-Liner, Unilin XT/TL, Gyproc ThermaLine, Knauf laminates) live in their own tab, and picking one adds two layers at once — the insulation core and the plasterboard facing. That keeps both the U-value and the ISO 13788 vapour profile physically correct, which a single "effective λ" layer cannot do.

You don't do any arithmetic. The core is set for you — a 52.5mm K118 is a 40mm core plus a 12.5mm facing — and the − / + buttons on the core step straight through the real board sizes, so core plus facing is always a board you can order. The facing itself is locked to its own sheet thickness.

3. Bridging, cavities and foil facings

Bridged layers

Insulation between studs, rafters or joists is bridged: the timber conducts more than the insulation. Open the layer's ⋯ menu, choose bridging, and either type the fraction or enter the stud width and centres — U-Monkey computes the percentage and combines the two paths per ISO 6946. (Steel studs are different — see Light steel frames below.) Blockwork is bridged too: BR 443 counts the mortar joints in aircrete and lightweight blocks, so U-Monkey adds them for you as a 6.7% mortar bridge (standard blocks with 10mm joints). For thin-joint blockwork, edit that bridge to about 2%.

Leave the timber out and U-Monkey asks. A rafter, stud, joist or cold flat-roof build-up with insulation and no bridging drawn gets a suggestion under the result that adds softwood at BR 443's default fraction (studs 15%, rafters 12%, ceiling joists 12.8%, floor joists 10.8%) and shows the U-value it gives, with Undo; a single loft quilt between 150 and 250 mm is split at a 100 mm joist depth with only the lower part bridged. Tap Not between if the insulation is over the members instead.

Light steel frames — BRE Digest 465

Steel studs are entered the BR 443 way (§4.10): a bridge through the insulation at the web fraction — web thickness ÷ centres, plus noggings, λ 50 (for example 1.2 mm at 600 mm centres is 0.2%). As soon as steel bridges a layer, a Light steel frame panel appears: a warm frame (all the insulation outside the steel) is the normal ISO 6946 calculation, while a cold or hybrid frame weights the upper and lower limits by p, which depends on the flange width, stud centres and stud depth you enter there. The working shows p and the plain ISO 6946 figure it replaces, and the PDF names the method.

Metal cladding — SCI P312 and composite panels

A wall or roof with insulation between two thin metal sheets shows a Metal cladding panel, because a layer-by-layer sum misses what makes metal cladding lose heat (BR 443 §4.9). For built-up (twin-skin) cladding, enter the outer sheet, the quilt at its full depth (as one layer or as the rolls that make it up) and the liner, choose Built-up and type in your system’s rail spacing, brackets per m² and bracket size: U-Monkey works it out by SCI P312, which BR 443 §4.9.1 gives as an alternative for rail-and-bracket systems, and the working shows p, R_T and the bracket correction. Z-spacers and standing-seam clips are outside the method: use the system maker’s figures. A factory-made composite panel takes the maker’s declared U-value instead: add your maker’s panel from the materials — Kingspan’s QuadCore KS1000RW and AWP, Tata Steel’s Trisomet and Joris Ide’s PIR and stone wool panels carry their makers’ tables — and pick its core thickness in its row, and U-Monkey fills in the declared U for that core. For any other panel, add “Insulated composite panel (the maker’s declared U-value)” and type the U for your panel and core thickness into its row. See the built-up metal roof and composite panel wall worked examples.

Edge insulation on a ground-bearing slab

A solid ground floor has an Edge insulation option in its geometry panel: a horizontal strip under or over the slab edge, or vertical insulation down the foundation, with its width or depth, thickness and λ. It is worked to BS EN ISO 13370 Annex D — U = U₀ + 2Ψ/B′ — and the working shows U₀, Ψ and the reduction.

Air cavities

Cavities come from the Air cavities tab — masonry cavity, dot-and-dab gap, service void, the ventilated loft of a cold roof. Ventilation is set per cavity: unventilated, slightly ventilated (with the opening area in mm²/m, interpolated per ISO 6946 §6.9.3) or well-ventilated — a well-ventilated cavity and everything outside it is disregarded, exactly as the standard requires. The ventilated loft of a cold roof is the exception: it and the roof above it become the roof-space resistance described above, rather than nothing.

The search box finds them by the names people use — air gap, airgap, void, batten gap, ventilated gap — and lists them first, the likeliest at the top. An air gap is never a custom material: it has no λ of its own (still air conducts about 0.025 W/mK, but radiation and convection carry most of the heat across a gap), so BS EN ISO 6946 gives it a resistance from its thickness, the direction of heat flow and its surfaces. Name one “air gap” in the custom-material box and U-Monkey offers the sealed or ventilated air layer instead; one already entered as a material is flagged, with a one-tap fix.

Wall ties, EWI fixings and inverted roofs — the Annex F corrections

BS EN ISO 6946 Annex F (applied through BR 443) adds three corrections on top of the layer arithmetic, and U-Monkey now carries all three. Air gaps follow BR 443 by default: level 0 where a second layer of insulation covers the joists and any gaps in the first, as in a loft with quilt over the joists or PIR under the rafters, and level 1 otherwise. The Fitting quality setting above the result overrides it with level 0, 1 or 2. Mechanical fixings — stainless wall ties through a full-fill or partial-fill cavity, steel screw fixings through external wall insulation — are set from the insulation layer's ⋯ menu (“Fixings through this layer…”): pick a preset or enter the fixing's conductivity, number per square metre and cross-section, and the correction ΔU_f = α·λ_f·n_f·A_f/d₀·(R₁/R_T,h)² is applied and itemised on the report. Ties across an unfilled cavity and basalt, GRP or plastic fixings (λ below 1 W/mK) are exempt and U-Monkey says so. An insulated masonry cavity wall with no fixings set gets a one-tap “Wall ties” prompt under the result — typical stainless wire ties add about 0.003 W/m²K, under the 3 % rule, but the report then shows they were considered. For a flat roof with the insulation above the waterproofing, tick “Inverted roof” in the roof options and point at the waterproofing layer; the heating-season rainfall p is filled in from the climate station chosen for the condensation check (October–May mean daily rainfall from the Met Office 1961–1990 normals — from about 1.5 mm/day in Kent to 4.4 in the Southern Uplands) and can be switched to any of the 21 stations or typed from your own data. Choose the drainage factor for the board joints (0.04 butt-jointed, 0.03 rebated, 0.02 under a green roof or paving, or the product's certified figure); the correction ΔU_r = p·f·x·(R₁/R_T)² is often the largest term on such a roof. Where the total of all corrections is under 3 % of the U-value the standard lets you omit them; U-Monkey applies them anyway and tells you.

Low-emissivity (foil-faced) cavities — automatic

A cavity bounded by a shiny foil surface loses far less heat by radiation: its effective surface emissivity drops from about 0.9 to 0.2, and BR 443 §4.7.2 raises its resistance accordingly (0.44 instead of 0.18 for a 50mm wall cavity). U-Monkey detects this automatically — put an unventilated cavity next to a foil-faced PIR, phenolic or multifoil board and the cavity is flagged “low-e (auto — foil-faced neighbour)”, with a toast explaining what happened. Tissue-faced boards (like Kingspan TR27, TR24, K5 or K103) are correctly excluded. Where the product next to the cavity has a certificate declaring a lower emissivity, the cavity is calculated from that instead, by BS EN ISO 6946 Annex D: 0.665 rather than 0.44 for a 50mm wall cavity at 0.05, the same figure Kingspan's TW50 certificate uses. That covers most foil-faced boards — Kingspan Therma and Kooltherm, SOPRATHERM (Celotex), Unilin Thin-R, EcoTherm, Recticel and IKO all certify 0.05 to 0.07, and K118's rear face 0.13 — and multifoils such as SuperFOIL or TLX Silver. The row names the product and its certificate. Boards without a certified figure keep BR 443's 0.2, and a few plasterboards whose certificates take the facing at 0.9 (PL4000, XT/TL, ThermaLine PIR) and K106, whose cavity face is fleece, are not low-e at all. Behind dot-and-dab only a certified emissivity counts (K118's 0.13); an uncertified foil gets no credit there, as in the certificates and the makers' own figures. It stays editable: open the cavity's ventilation sheet and untick it, and your choice is remembered.

4. Read the result

The headline U-value is computed by the full combined (upper/lower limit) method of BS EN ISO 6946:2017 with BR 443 conventions — not a simple resistance sum — including ΔU air-gap corrections and the standard's own reliability check. It reproduces the official gov.scot worked examples 6.B.2 and 6.B.3, every cell of the guidance's ground-floor Tables 1 and 5, and 157 cells of the thickness tables in BBA certificates for six insulation systems; how U-Monkey is tested lists every case, and the few cells left out and why. Under the number you get a plain-English verdict, traffic-light pills for each applicable target, the cross-section drawing, and any warnings (oversized layers, multifoil caveats, steel bridging and so on).

You don't have to scroll to see it while you edit: on a desktop a compact copy of the result sits above the cross-section, and on a phone a bar along the bottom carries the U-value and the verdict — tap either to jump to the full result.

Missing the target? The one-tap fix under the verdict, and the target solver, tell you what thickness of your chosen insulation would get you there — in the sizes a branded product is actually made in, or, for a generic material, rounded up to a thickness at least two manufacturers make. Want to see what else would do the job? 🔎 Compare insulation options lists every product in the library made for that position — full-fill cavity, between rafters, over a slab and so on — each solved to the target in its real sizes and ranked by the thickness it needs. Nobody pays to be in it, and Use swaps it in, keeping your bridging and fixings.

Check the maths yourself

Under the result, Show the working gives the layer table and the ISO 6946 upper/lower limits. Inside it, Show the arithmetic — every number substituted writes the calculation out in full: the internal surface resistance, then R = d / λ for each layer with the real figures in place, then the external surface resistance and how they assemble into the U-value. A ground or suspended floor shows its ISO 13370 terms instead — R_f, the characteristic dimension B′ = A/(½P), the equivalent thickness d_t, and which form of the equation applied.

It's there so you never have to take the number on trust. If a figure looks wrong, you can see exactly which line produced it.

Thermal mass, decrement factor and time lag

Under the target pills, a one-line Thermal mass row gives three numbers for the build-up, calculated to BS EN ISO 13786 (the 24-hour periodic heat-transfer-matrix method):

• κ, areal heat capacity (kJ/m²K) — how much heat the inside face of the element stores over a day. This is the thermal mass parameter input for SAP 10; give it to your assessor instead of letting them pick 100, 250 or 450 from Table 1e.
• Decrement factor — the fraction of the outdoor daily temperature swing that reaches the inside. Below about 0.3 is heavyweight; above 0.6 is lightweight.
• Time lag (hours) — how long the outdoor peak takes to work through.

Click explain and the working opens at the Summer performance block, which also lists the mass per m² and where each density and specific heat came from — the product datasheet, a material-class value, or a transfer from an equivalent material. Every library material carries these two properties with a source, so the row appears without you doing anything. For a custom material it says which layer is missing them: open that layer's ⋯ menu → Thermal mass and enter density (kg/m³) and specific heat (J/kgK) from the datasheet. U-Monkey will not guess them — an invented density gives a confident-looking time lag that is simply wrong.

The three numbers print on the free PDF under the result. They compare build-ups and feed SAP; they are not a Part O overheating assessment, which models the whole room dynamically. Ten build-ups compared, with what the numbers mean for design: the thermal mass calculator page.

5. Check condensation risk

Every build-up gets a free interstitial condensation screening to BS EN ISO 13788 (monthly Glaser method), plus the f_Rsi surface-mould check. The card sits folded under the result with its verdict in the header — a fail opens it automatically; otherwise click the header, or the verdict chip in the result. Pick the nearest of the 21 climate stations (BS 5250 Table D.1 for Heathrow, Ringway and Turnhouse; Met Office 1961–1990 normals for the rest, from Lerwick to Plymouth), the humidity class (class 3, the default, covers dwellings with high or unknown occupancy) and the design year (1-in-10 suits most buildings; 1-in-20 or 1-in-50 for sensitive ones). Roofs are checked with the surface 2 K below the air, for night-sky cooling, as the standard asks. Vapour data comes from BS 5250/ISO 10456; where a value is product-specific, enter the declared μ or sd on the layer.

The verdict is one of seven, and each tells you what to do next:

• Pass — no condensation in any month, and it holds with each assumed vapour value tested across its plausible range and each vapour control layer at a third of its rating, one at a time and then together.
• Pass (seasonal) — condensation forms in winter and dries out fully in summer, which BS EN ISO 13788 does not count as accumulation; check the amount against the material it forms on.
• Conditional pass — it passes only while a named value holds, usually a membrane or board entered at a typical figure. Enter the manufacturer’s declared value and it firms up either way.
• Not a clear pass — the condensation dries out, but lands on timber in a quantity that eats into its decay margin, or on a material that can’t absorb it in run-off quantities.
• Not a pass — the calculation isn’t the right test. BS 5250:2021 settles cold pitched and cold flat roofs by prescriptive rules, and expects an air and vapour control layer on the warm side of a warm pitched roof (and a ventilated void under a high-resistance underlay).
• Screening only — internal insulation on a solid wall, where a BS EN 15026 hygrothermal assessment by a specialist is the right next step, or a humidity class 5 building such as a pool, which needs its own design conditions.
• Fail — condensate builds up year on year, or the inside surface is cold enough for mould.

When the result isn’t a pass, a How to fix it panel appears. U-Monkey tries the usual remedies on a copy of your build-up — a vapour control layer on the warm side (added, upgraded or moved), a more vapour-open outer layer, a ventilated void under a roof underlay, turning a cold flat roof into a warm deck, extra insulation for a mould fail — and lists only the ones that pass the whole check, with the value needed: a VCL rated at least so many metres sd, say. Where the remedy is a change of layers, tap Apply and U-Monkey makes it for you: the new layer goes in as the performance to ask for — “Vapour control layer (sd ≥ 120 m)” — and Undo takes it out again. Where a result hangs on an assumed vapour value, it opens that layer’s vapour resistance for the datasheet figure instead. They are calculated options, not a design, so check them against the manufacturers’ instructions and the rest of the design, and swap in the actual products before you specify them. The vapour-pressure chart shows exactly where the build-up runs closest to its dew point, and if condensation would land on structural timber, U-Monkey interprets the moisture uptake against the BS 5250 decay threshold. For the full method, see the condensation risk calculator page.

6. Do the whole job, not one element

A building isn't one element. Tap ＋ Add another element and U-Monkey keeps the wall, the floor and the roof open together — a row of chips at the top, each with its own U-value and pass/fail dot. Click any one to go back to it; nothing is lost when you switch. (Build one element and you'll never see the row — it only appears when there's a second thing to hold.)

＋ Window / door adds the openings. A window's U-value is certified, not calculated — it comes off the BFRC label or the manufacturer's data — so you type the figure and U-Monkey checks it against the right Approved Document L limiting value: 1.6 W/m²K for windows and doors in a new dwelling, 1.4 for a new window in an existing one, and 2.2 for rooflights (which are judged more leniently than windows — an easy one to get wrong).

Element schedule then lists the whole job — every element, its U-value, its own target and whether it meets it, with a count of how many pass. It's the summary a spreadsheet was doing for you. Your project is kept in your browser and offered back the next time you open the tool.

Each element still exports its own free PDF. A single combined project report is a Monkey Pro feature.

7. Save, share, resume and export

• Everything lives under 📁 My build-ups in the header: 💾 Save to a file (Ctrl+S) downloads a project file, 📂 Open a file reopens it, 🕐 Recent build-ups lists your latest builds and ⭐ Saved build-ups is your named library. ⭐ Keep in My build-ups at the foot of the result adds the current one.
• The app autosaves as you work and offers to restore when you return.
• Share this result → Copy permanent link makes a link with the whole build-up encoded in the URL, so it never expires — send it to a colleague or your assessor. The 🔗 Get Link button makes a shorter 30-day resume link for picking up on another device.
• My build-ups keeps a named library of favourites in your browser — and ⬇ Export all downloads that library as a file you own. Worth doing before you clear site data or move to a new machine; ⬆ Import merges it back without overwriting anything.
• Get U-value PDF exports the free Building-Control-ready report: layer table, full ISO 6946 working, corrections, method statement. It states the U-value with no pass/fail verdict unless you tick Show the pass / fail verdict on the PDF report just above the button. The submittable condensation report is a paid extra.

If another calculator gives you a different answer for the same build-up, the arithmetic view is where you find out why — and why U-value calculators disagree walks through the five things that actually cause it.

Retrofit: before and after

Set the work type to Upgrading existing (or Existing element) and a Before and after panel appears under the result. Give it what was there before in one of three ways: an RdSAP 10 default by age band and construction (with the measured wall thickness where RdSAP calculates from it, and the loft insulation depth for a roof) — clearly marked as an assumption; the build-up as it was built, modelled first and then frozen with one button; or a U-value you already have. It shows the U-value before and after, the heat through each square metre, and the inside-surface factor f_Rsi for plane areas against 0.75. Internal insulation on a solid masonry wall is flagged: PAS 2035:2023 notes that a BS EN ISO 13788 check might not be sufficient there, and a BS EN 15026 hygrothermal assessment is the next step. Choosing a solid wall type also switches Part L's “improved” target to the solid-wall figure. Monkey Pro members can download the pair as a before-and-after PDF.

Home Energy Model inputs

Under the thermal-mass line, Home Energy Model inputs lists what HEM asks for this element that U-Monkey already knows: the U-value, the areal heat capacity of the whole element (Σρcd — HEM counts the full thickness, unlike the inside-face κ SAP uses), a suggested mass distribution class (ISO 52016-1: I, E, IE, D or M, which you can change) and, for a ground floor, its type, perimeter, wall thickness, floor-construction R and any edge insulation. Monkey Pro members can copy or download it as JSON in HEM's field names.

Linking straight into a build-up

You can write a U-Monkey link by hand, which is useful in an email, a specification note or a teaching example:

/tools/u-value-calculator/app/?el=wall&layers=brick:102.5,cavity:50,pir:100,block:100,plasterboard:12.5

el is the element (wall, pitched_roof_rafter, pitched_roof_ceiling, flat_roof, floor, ground_floor, suspended_floor) and layers is a comma-separated list, outside first, each written name:thickness_mm. Names are matched the same way pasted text is, so anything unrecognised is reported rather than guessed. ?el= on its own is a handy bookmark if you always calculate the same kind of element.

Every report can be verified — free ones included

Every report U-Monkey issues — the free U-value PDF as well as the paid condensation report — carries a unique report ID printed on the document. Anyone holding the report can enter that ID at energycount.co.uk/verify and confirm in seconds that it's genuine and matches what was originally issued.

Why that matters: a plain PDF is just a file. It can be edited after the event, a thickness can be "adjusted" somewhere along an email chain, and the person receiving it — a building control officer, a SAP assessor, a main contractor — has no way to tell. The report ID closes that gap. It gives your calculation provenance: Building Control can check it without taking anyone's word, your client can rely on it in a handover pack, and it still verifies years later — during a sale, a retrofit or a warranty query — when the original email is long gone. It's the difference between "here's a PDF someone made" and a document that stands on its own, which is exactly what you want attached to a Building Regulations submission.

Put a U-value box on your own website

One snippet adds a small U-value checker to any page — the same engine with generic materials, the Part L checks for England, Wales, Scotland or Northern Ireland, and a button that opens the build-up in the full U-Monkey.

Power tips

• Keyboard: / opens the material picker with the search ready, Enter adds the first result, ↑ / ↓ change the thickness of the layer you're on (stepping through real product sizes where that applies), Ctrl+Z undoes the last change to the build-up (any change — a swapped material, a thickness, a deleted layer; Ctrl+Shift+Z redoes), and Esc closes anything open. Building a dozen elements in a sitting is much faster this way.
• Search the picker by brand ("celotex", "nyrock", "thermalite", "webertherm", "jetfloor") — it looks across every tab.
• Boards whose declared λ changes with thickness (Thermaroof TR24, Safe-R, Isolair, Bauder XPS 500 and others) appear once in the picker; the layer switches band by itself when the thickness crosses the manufacturer's boundary, and the row says which band it is on — so you can't be on 0.027 at 120 mm by mistake.
• Layers reorder by dragging the ⋮⋮ grip — or focus it and use the arrow keys.
• The dot-and-dab entry is the air gap only — add the plasterboard as its own layer (the picker note reminds you). U-Monkey bridges the gap with the plaster dabs, 20% at λ 0.43 as BR 443 §4.6.1 sets out, and a batten space behind a wall lining with the battens (11.8%, §4.6.2); both show on the row and can be edited or removed in ⋯ → Bridge. In Describe, write on dabs or on battens after a lining and the gap is added for you.
• Multifoils follow BR 443 §3.10: a certified product is held by its core R, so draw the air spaces either side as their own layers and U-Monkey calculates them from the product's own declared emissivity (BS EN ISO 6946 Annex D; BR 443's ε 0.2 where a product declares none). A multifoil without accredited test data, such as EcoQuilt65 or Triso-Super, takes BR 443's R 1.7 for the foil and both air spaces together, so leave those air spaces out; U-Monkey flags them if you draw them. A thin foil laid straight on a slab or under a screed with no test behind it, such as SuperFOIL SFUF, has no air spaces for that default to stand on: it counts at R 0, and its row and the note under the result give the figure its maker quotes and why it is not used. Type a certified core R into its R box if you have one. In a warm pitched roof, a multifoil that does most of the insulating (the build-up's thermal midpoint falls within it), laid unbroken across the rafters with a declared sd of 10 m or more, counts as the air and vapour control layer the condensation check looks for, and the result reminds you to tape every lap and penetration; cut to fit between the rafters, it does not count.
• Every material shows its source; ⚠ flags mean "check the datasheet before you rely on it".

Frequently asked questions

Ready? Open U-Monkey and build your element — or start from the U-value calculator overview.