A ground-bearing concrete floor with 100mm PIR typically achieves a U-value of about 0.13–0.15 W/m²K — but the honest answer depends on the floor’s perimeter-to-area ratio, because ground floors are calculated to BS EN ISO 13370, where the ground itself provides much of the resistance. U-Monkey does the real 13370 calculation; most free calculators don’t.
Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report
The build-up (outside → inside)
- 75mm sand/cement screed
- 100mm PIR
- Concrete slab
- (Ground resistance via ISO 13370 — P/A dependent)
The pre-filled link opens with U-Monkey’s default 30m perimeter / 50m² area (P/A 0.6) — edit both to your floor.
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| 100mm PIR, typical semi (P/A ≈ 0.6) | ≈ 0.13–0.15 | Current new-build spec |
| 75mm PIR, same floor | ≈ 0.16–0.18 | Older spec / extensions |
Figures computed with the BS EN ISO 6946 combined method (bridging included) using BR 443 conventional lambda values — indicative for the generic build-up shown. Your products, thicknesses and (for floors) footprint will move the number: that's what the calculator is for.
What changes the number
Perimeter/area ratio is the hidden variable: a big square floor loses proportionally less than a narrow extension. That is why two "identical" floor build-ups get different U-values — and why a tool that ignores ISO 13370 gives the wrong answer. Enter your real perimeter and area in U-Monkey.
Next: check the junctions
A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £15 +VAT):
Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.
Frequently asked questions
What is the U-value of an insulated concrete ground floor?
A ground-bearing concrete floor with 100mm PIR typically achieves a U-value of about 0.13–0.15 W/m²K — but the honest answer depends on the floor’s perimeter-to-area ratio, because ground floors are calculated to BS EN ISO 13370, where the ground itself provides much of the resistance. U-Monkey does the real 13370 calculation; most free calculators don’t.
Why does my floor U-value change with the room size?
Because ground-floor heat loss is dominated by the edges. BS EN ISO 13370 captures this through the perimeter-to-area ratio — the same build-up in a narrow extension performs worse than in a wide house. U-Monkey asks for P and A and calculates it properly.
Is this calculator free?
Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard) and the condensation-risk screening check are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.
Does it follow the official calculation method?
Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging and air-gap corrections, ISO 13370 for ground and suspended floors, and ISO 13788 for the condensation screening check.
Other common build-ups
- 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 suspended timber floor
- U-value of a beam and block floor with PIR
Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are.