The current new-build standard — 150mm PIR under a 75mm screed — gives a U-value of about 0.12 W/m²K calculated to BS EN ISO 13370 §9.2 on a typical dwelling footprint, which clears the 0.13 notional. The older 100mm build-up shown below comes out nearer 0.17 on the same floor: still inside the 0.18 limiting value, but no longer notional-standard. It is the default suspended floor of volume housebuilding.
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
- Beam & block deck
- (Ventilated void + ground via ISO 13370)
The pre-filled link opens with U-Monkey’s default footprint (P/A 0.6) — edit to your floor.
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| 150mm PIR + 75 screed, typical semi (P/A ≈ 0.6) | ≈ 0.12 | Current new-build standard |
| 100mm PIR, same floor | ≈ 0.17 | Previous standard spec |
| 75mm PIR, same floor | ≈ 0.21 | 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
As with all ground floors, perimeter-to-area does the quiet work — check the same spec on your actual footprint. Edge insulation and the wall-floor junction detail (a ΨMonkey job) decide how much of the paper performance you keep.
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 £25 +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 a beam and block floor with PIR?
The current new-build standard — 150mm PIR under a 75mm screed — gives a U-value of about 0.12 W/m²K calculated to BS EN ISO 13370 §9.2 on a typical dwelling footprint, which clears the 0.13 notional. The older 100mm build-up shown below comes out nearer 0.17 on the same floor: still inside the 0.18 limiting value, but no longer notional-standard. It is the default suspended floor of volume housebuilding.
Does the insulation go above or below a beam and block deck?
Above, almost always — rigid boards over the deck, screed or chipboard over the boards. It keeps the insulation continuous and the calculation clean. Model it in U-Monkey with your real perimeter and area for the ISO 13370 figure.
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 concrete ground floor
- U-value of an insulated suspended timber floor
Different build-up? See the full floor U-value calculator, or start from scratch in the main U-value calculator.
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.