Completely free — the calculation, the Future Homes Standard checks, the condensation-risk screening and the Building-Control-ready PDF. No sign-up, no catch.
Runs entirely in your browser, online or off. See the U-value update as you build.
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 a neutral database of 200+ real UK products. 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.
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, and we can help with that.
A neutral product database
Search 200+ real UK products — PIR, phenolic, mineral and glass wool, EPS/XPS, wood fibre, blocks, membranes, multifoils and more — each with its declared λ value and source, human-checked. No manufacturer is promoted over another. Prefer a generic material, or your own custom λ? Those are there too.
U-value by element
Jump straight to your element:
- Wall U-value calculator
- Roof U-value calculator
- Flat roof U-value calculator
- Floor U-value calculator
- What U-value do I need?
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.
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.
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 200+ real UK products — PIR, phenolic, mineral and glass wool, EPS/XPS, 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 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.
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-value of a cavity wall with full-fill insulation
- U-value of a cavity wall with PIR partial-fill insulation
- U-value of a timber frame wall
- U-value of a solid brick wall with internal insulation
- U-value of a solid wall with external insulation
- What is the U-value of an uninsulated cavity wall?
- U-value of 270mm loft insulation
- U-value of insulation between rafters (loft conversion)
- U-value of a warm flat roof
- How much insulation does a flat roof need?
- U-value of an insulated concrete ground floor
- U-value of an insulated suspended timber floor
- U-value of a beam and block floor with PIR