How to use U-Monkey — the complete guide

Quick start — a U-value in under a minute

1. Open the calculator and 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.

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 or Ireland) and what kind of building it is (a dwelling, or anything else). The eight combinations each use their 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); 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). The PDF states which one the verdict used. See the detail pages for Scotland, Wales, 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 300+ materials: BR 443 / CIBSE generic values plus real UK products from Kingspan (including the full Kooltherm K-series), 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.

Two shortcuts worth knowing. The materials you use most sit as Recently used chips at the top of the picker, so the fifteen things your practice actually specifies are one tap away. 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; name an insulated plasterboard ("37.5 Kingspan K118") and it arrives as core plus facing with the overall board size.

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. Celotex GA4000 goes 25, 40, 50, 75, 100, 120, 150 — 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, Celotex PL4000, 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 framing needs the BRE Digest 465 method; U-Monkey warns rather than pretending.)

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.

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 are the level 0 / 1 / 2 choice above the result. 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, K5 or K103) are correctly excluded. 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 example exactly. 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).

Missing the target? The target solver tells you what thickness of your chosen insulation would get you there — try it before redesigning the whole element.

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). 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, to see the working. 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) and the humidity class (class 3 suits most dwellings), and optionally the BS 5250 1-in-10 severe year. You get a month-by-month verdict — no condensation, forms but evaporates, or accumulates — plus the vapour-pressure chart showing exactly where the build-up runs closest to its dew point, the fRsi surface-mould check, and a robustness scenario for cold-roof conversions. Vapour data comes from BS 5250/ISO 10456; where a value is genuinely product-specific you can enter the declared μ or sd on the layer. If condensation would land on structural timber, U-Monkey also interprets what the moisture uptake means 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. 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.

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.

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).
• Multifoils are modelled by certified system R — U-Monkey deliberately won't pretend a first-principles number is compliant.
• 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.