Completely free — including the Building-Control-ready PDF. No sign-up, no catch.
Runs entirely in your browser. Watch the wall U-value update as you build.
What a wall U-value is
A wall U-value is the rate of heat loss through the external wall, per square metre per degree — the single biggest fabric element on most dwellings. It depends on the whole build-up: the masonry or framing, the insulation, any cavities, and repeating thermal bridges like timber studs or wall ties.
Build-ups it handles
U-Monkey builds up any wall — full-fill and partial-fill cavity, solid masonry with internal or external insulation, timber frame, steel frame and SIPs — with the bridged-layer fractions (studs, ties) handled to BR 443, and real product conductivities from the 200+ database.
How it's calculated
It calculates the wall U-value by the combined method of BS EN ISO 6946, using BR 443 conventions for bridging, air cavities and the ΔU air-gap correction, and shows the full upper- and lower-limit working.
Every result is traffic-lit live against the Part L 2021 limiting and notional values and the Future Homes Standard targets, so you can see whether the wall passes — and by how much — as you design it.
Free — including the PDF
U-Monkey isn't a paywalled tool. Build up your wall, read the U-value and the full working on screen, and export a branded, Building-Control-ready PDF — all completely free, no sign-up. Prefer the whole SAP done for you? We're happy to help.
Frequently asked questions
What is a wall U-value?
A wall U-value is the rate of heat loss through the external wall, per square metre per degree — the single biggest fabric element on most dwellings. It depends on the whole build-up: the masonry or framing, the insulation, any cavities, and repeating thermal bridges like timber studs or wall ties.
How do I calculate a wall U-value?
It calculates the wall U-value by the combined method of BS EN ISO 6946, using BR 443 conventions for bridging, air cavities and the ΔU air-gap correction, and shows the full upper- and lower-limit working. U-Monkey builds up any wall — full-fill and partial-fill cavity, solid masonry with internal or external insulation, timber frame, steel frame and SIPs — with the bridged-layer fractions (studs, ties) handled to BR 443, and real product conductivities from the 200+ database.
Does it check Part L and the Future Homes Standard?
Every result is traffic-lit live against the Part L 2021 limiting and notional values and the Future Homes Standard targets, so you can see whether the wall passes — and by how much — as you design it.
Is the wall U-value calculator free?
Yes — U-Monkey is completely free, including the Building-Control-ready PDF. No sign-up, no catch.
Other elements
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