Free U-value Calculator with Condensation Check

Completely free — the calculation, the Future Homes Standard checks, the condensation-risk screening and the Building-Control-ready PDF. No sign-up, no catch.

Launch U-Monkey — free, no sign-up

Issuing reports to clients? U-Monkey Pro puts your practice on them →

Runs entirely in your browser, online or off. See the U-value update as you build.

U-Monkey U-value calculator: a full-fill cavity wall built up with 250mm mineral wool, showing a live cross-section and a 0.13 W/m²K result that meets the Future Homes Standard — U-Monkey in action — stack up the layers, watch the U-value and the Part L / FHS checks update live. Watch the 28-second demo →

How do you calculate a U-value? Add up the thermal resistance of every layer in the build-up — thickness in metres ÷ the material's λ (W/m·K) — plus the fixed internal and external surface resistances and any air cavity, then U = 1 ÷ total R (W/m²K). Layers bridged by timber studs, joists or wall ties are combined by the upper/lower resistance-limit method in BS EN ISO 6946, with BR 443 conventions for cavities, fixings and insulated plasterboard; ground floors instead follow BS EN ISO 13370, where the answer depends on the floor's perimeter-to-area ratio, not just the layers. U-Monkey does all of that live as you stack the layers and traffic-lights the result against the Part L 2021 and Future Homes Standard targets. Free, including the Building Control-ready PDF — no sign-up.

A neutral product database — 300+ materials, no favourites

300+materials & products

70+UK brands

195+λ values human-verified

0manufacturers promoted

Build from the products you actually specify: Kingspan (the full Kooltherm K-series and Therma range), Celotex, Recticel, EcoTherm, Unilin, IKO, Rockwool (including NyRock), Knauf, Isover, Superglass and Jablite; insulated plasterboards from Kooltherm K118 to Gyproc ThermaLine — added as core + facing so the physics stays honest; aircrete blocks from Thermalite, Celcon, Durox and Mannok; plus EPS, XPS, wood fibre, hemp, cork, multifoils, aerogel and vacuum panels. Every entry carries its declared λ and the source it came from, and entries marked ✓ have been human-checked against the manufacturer's datasheet.

The neutrality is the point. Manufacturer calculators only ever recommend their own boards; U-Monkey lists everyone side by side, no one pays to appear, and no one is promoted over another — so the cheapest build-up that passes is allowed to win. Organised in clear tabs (insulation, insulated plasterboard, boards, masonry…) with search across every brand and product name. Prefer generic BR 443 / CIBSE values, or your own custom λ? Those are there too.

Why this one, not just any U-value calculator?

It checks condensation, not just heat. Every build-up gets a free interstitial condensation-risk screening to BS EN ISO 13788 with BS 5250 vapour data and UK climate — so you find out the wall that passes Part L won't quietly rot before anyone builds it. We haven't found another free UK calculator that does this.
It already speaks Future Homes Standard. Targets verified against the published March 2026 standard, for both the SAP 10.3 and HEM routes — plus a solver that works backwards from the U-value you need to the thickness that gets you there.
The evidence is free too. A Building-Control-ready PDF — full layer table, method statement, report ID — at no charge, drawn from the neutral 300+ material database above. No favourites, no upsell, no sign-up.

Need a U-value, fast?

U-values sit behind half of a SAP submission and every Building Regs fabric check. U-Monkey lets you work one out yourself in seconds: pick an element, stack up the layers, and read the U-value straight off — with the full working shown, so it stands up to scrutiny. No dated software, no spreadsheet. Just need to turn a U-value into an R-value, or a layer's thickness and conductivity into a resistance? The U-value to R-value converter does that on its own.

The method Building Control expects

U-Monkey uses the combined method of BS EN ISO 6946:2017 with BR 443 conventions — bridged layers (timber and steel fractions), air cavities, the ΔU air-gap correction and ground floors to BS EN ISO 13370. Every result shows its upper- and lower-limit resistances and its working, and it reproduces the official gov.scot worked example exactly.

Current with the Future Homes Standard

The Future Homes Standard was published in March 2026 — so U-Monkey traffic-lights your result against the Part L 2021 limiting and notional values and the Future Homes Standard targets, for both the HEM and SAP 10.3 routes. You see whether a build-up passes, and by how much, as you design it.

The condensation check no one else gives away

A build-up can pass Part L on paper and still fail in the wall: add insulation in the wrong place and vapour condenses inside the construction — wet timber, saturated insulation, mould. U-Monkey runs a free interstitial condensation screening to BS EN ISO 13788 on every build-up, using BS 5250 vapour resistivities (cross-checked against ISO 10456) and monthly UK climate data. Where published values genuinely vary — membranes, foils, OSB — it says so, and lets you enter the manufacturer's declared value instead. It's a screening check, honestly labelled as one: for unusual or historic build-ups you still want a full hygrothermal assessment.

The on-screen screening is free. When Building Control wants it in writing, the full condensation-risk report to BS EN ISO 13788 is £25 +VAT (£30 inc VAT) — the layer-by-layer working, monthly interface analysis and clear PASS/FAIL, ready to submit alongside the free U-value PDF. See an example condensation report before you pay a penny. Full detail: the condensation risk calculator, explained.

Every report — free U-value PDFs included — carries a printed report ID, so Building Control can verify it's genuine in seconds.

U-value by element

Jump straight to your element:

How it works

Pick the element — wall, roof (rafter or ceiling), flat roof, exposed floor or ground floor.
Build the layers — start from a template or from scratch; add materials and thicknesses; the cross-section and U-value update as you go.
Check the targets — the result is traffic-lit against the Part L limit, notional and Future Homes Standard values.
Export the PDF — a free, branded, Building-Control-ready report with the layer table, method statement and a report ID.

Want the whole thing explained properly? How to use U-Monkey — the complete guide walks through every feature in order: choosing the element and building-work type, bridged layers, cavities and insulated plasterboard, the Part L traffic lights, the condensation-risk check and exporting the free PDF.

Who it's for

Energy assessors who need a defensible U-value and evidence for a SAP submission; architects and technologists specifying a build-up to hit a target; and builders and self-builders checking "will 100mm of PIR do it?" on site. Want the whole SAP done for you instead? We're happy to help.

Reports you can issue as your own — U-Monkey Pro

The calculator is free and always will be. Pro is for people who issue these reports to clients rather than just checking a number: every export carries your project name, number, client and revision, your practice details lead the header, and our marketing comes off. Set it up once and every report after that comes out ready to send.

Project references — project, number, client and revision on every report, so Building Control resubmissions track properly
Set up once — your details live on your account, not re-typed on every export
No Energycount marketing — it's your document, not an advert for ours
Your practice on the header — logo, practice name, address and contact details
Still verifiable — every report keeps its reference and verification QR
Condensation reports at £10 +VAT — the member rate, instead of £25 +VAT

What Pro adds in full, and when it pays for itself →

See what Pro adds to a report — £19/month +VAT Or £190/year — or £49/month with condensation and thermal-bridge reports included. Cancel any time, online. The free calculator stays free.

Frequently asked questions

What does the U-value calculator work out?

It calculates the thermal transmittance (U-value) of a wall, roof or floor — how much heat passes through it — from the layers you build up. You add each material and thickness, and U-Monkey returns the U-value live, with a full breakdown of the working, ready for a SAP submission or Building Control.

Which method and standards does it use?

The combined method of BS EN ISO 6946:2017 using BR 443 (2019) conventions — the method UK Building Control and SAP assessors expect. It handles bridged layers (timber and steel fractions), air cavities, ΔU air-gap corrections, and ground floors to BS EN ISO 13370. It reproduces the official gov.scot worked example exactly.

Does it check Part L and the Future Homes Standard?

Yes. Every result is traffic-lit against the Part L 2021 limiting and notional values and the Future Homes Standard targets (published March 2026), for both the HEM and SAP 10.3 routes — so you can see at a glance whether a build-up passes, and by how much.

Can I use real insulation products, or just generic materials?

Both. There's a neutral database of 300+ materials — 250+ real UK products from 70+ brands (PIR, phenolic, mineral and glass wool, EPS/XPS, insulated plasterboard, wood fibre, blocks, membranes and more) — each with its declared λ value and source, human-checked. No manufacturer is promoted over another. You can also use generic values or add your own custom materials.

Does it check condensation risk?

Yes — every build-up gets a free interstitial condensation-risk screening to BS EN ISO 13788, using BS 5250 vapour resistivities cross-checked against ISO 10456 and monthly UK climate data. Where published vapour values genuinely vary (membranes, foils, OSB), U-Monkey flags the assumption and accepts the manufacturer's declared value instead. It's a screening check — for unusual or historic constructions, commission a full hygrothermal assessment.

Is U-Monkey free? What does the PDF cost?

It's completely free — including the Building-Control-ready PDF. Build up any element, check it against the targets, and download the report with the full layer table, method statement and a unique report ID. No sign-up, no catch.

Which elements can it do?

External walls; pitched roofs insulated at rafter or ceiling level; flat roofs; exposed floors; and ground floors to ISO 13370 (which also asks for the exposed perimeter and floor area, since a ground-floor U-value depends on the plot geometry).

Is the method reliable?

It implements the ISO 6946 combined method in full and has been checked against the official gov.scot Part 6.B worked example and an independent re-implementation. The product λ values have been human-signed-off against the manufacturers' datasheets.

Can I get a refund if something goes wrong?

Yes — our refund practice is deliberately generous. An undelivered report refunds automatically, normally within the hour. If a report is faulty — it doesn't match your inputs or misapplies the stated methodology — we'll repair, replace or refund it under the Consumer Rights Act 2015, and minor corrections can be re-exported free for 24 hours after purchase.

Will Building Control accept the report?

It calculates to BS EN ISO 6946 / BR 443 (and BS EN ISO 13788 for condensation) with the full upper/lower-limit working shown, so a building control body can check every line. Every report — the free U-value PDF included — carries a printed report ID that can be verified in seconds at energycount.co.uk/verify, proving the document is genuine and unaltered — and if anything raises a question, we're a real consultancy on 01202 623236, not just a website.

Common build-ups — start pre-filled

Jump straight in with a typical UK construction already loaded — every layer editable, with the typical U-value and what changes it explained on each page.

U-Monkey — the free U-value calculator