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What a floor U-value is
A floor U-value is the heat loss through the ground or exposed floor. Ground floors are a special case: the U-value depends not only on the build-up but on the floor's geometry — its exposed perimeter and area — because heat is lost to the ground around the edges.
Build-ups it handles
U-Monkey handles solid and suspended ground floors — insulated slabs, beam-and-block and insulated timber floors — and exposed floors over an open space. For a ground floor it asks for the exposed perimeter and area, because the method needs them.
How it's calculated
Ground floors are calculated to BS EN ISO 13370, which sits alongside ISO 6946 for the build-up itself; exposed floors use the ISO 6946 combined method. Both show the full working.
Results are traffic-lit against the Part L 2021 and Future Homes Standard floor targets live, so the floor's pass margin is clear as you design it.
Free — including the PDF
U-Monkey isn't a paywalled tool. Build up your floor, 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 floor U-value?
A floor U-value is the heat loss through the ground or exposed floor. Ground floors are a special case: the U-value depends not only on the build-up but on the floor's geometry — its exposed perimeter and area — because heat is lost to the ground around the edges.
How do I calculate a floor U-value?
Ground floors are calculated to BS EN ISO 13370, which sits alongside ISO 6946 for the build-up itself; exposed floors use the ISO 6946 combined method. Both show the full working. U-Monkey handles solid and suspended ground floors — insulated slabs, beam-and-block and insulated timber floors — and exposed floors over an open space. For a ground floor it asks for the exposed perimeter and area, because the method needs them.
Does it check Part L and the Future Homes Standard?
Results are traffic-lit against the Part L 2021 and Future Homes Standard floor targets live, so the floor's pass margin is clear as you design it.
Is the floor U-value calculator free?
Yes — U-Monkey is completely free, including the Building-Control-ready PDF. No sign-up, no catch.
Common floor build-ups
Worked examples with typical U-values — each one opens in U-Monkey pre-filled, ready to edit:
- U-value of an insulated concrete ground floor — typically 0.17 W/m²K
- U-value of an insulated suspended timber floor — typically 0.30 W/m²K
- U-value of a beam and block floor with PIR — typically 0.17 W/m²K
- U-value of a ground floor with insulation below the slab — typically 0.13 W/m²K
- U-value of a floor over a garage — typically 0.14 W/m²K
- U-value of an uninsulated concrete ground floor — typically 0.98 W/m²K
- U-value of an uninsulated suspended timber floor — typically 0.81 W/m²K
- Underfloor heating on a ground floor: insulation and U-value — typically 0.17 W/m²K
- U-value of insulation laid over an existing concrete floor — typically 0.28 W/m²K
- U-value of a garage conversion floor — typically 0.17 W/m²K
- U-value of a basement floor (heated basement) — typically 0.16 W/m²K
- U-value of an extension floor (and the whole-house perimeter rule) — typically 0.18 W/m²K
- U-value of an insulated raft foundation (EPS raft) — typically 0.11 W/m²K
- U-value of a limecrete floor (insulated with foamed glass) — typically 0.33 W/m²K
- U-value of a garden room (garden office) floor — typically 0.25 W/m²K
- U-value of a solid floor with edge insulation — typically 0.68 W/m²K
- U-value of a floor over an unheated cellar — typically 0.21 W/m²K
- U-value of an exposed floor over a passage, undercroft or overhang — typically 0.15 W/m²K
- U-value of a battened timber floor over a concrete slab — typically 0.34 W/m²K
- U-value of a suspended timber floor insulated with wood fibre or sheep’s wool — typically 0.29 W/m²K
- U-value of a concrete floor over a car park — typically 0.17 W/m²K
- U-value of a warehouse floor slab — typically 0.16 W/m²K
Other elements
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