U-value of an insulated concrete ground floor

PIR under screed on a slab — calculated properly to ISO 13370.

The current new-build standard — 150mm PIR under a 75mm screed — gives a U-value of about 0.12 W/m²K on a typical semi footprint, while the older 100mm build-up shown below comes out nearer 0.17 on the same floor. The honest answer always 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.

Open this build-up in U-Monkey — pre-filled, free

Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report

The build-up (outside → inside)

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

SpecificationU-value (W/m²K)Notes
150mm PIR, typical semi (P/A ≈ 0.6)≈ 0.12Current new-build standard
100mm PIR, same floor≈ 0.17Previous standard spec
75mm PIR, same floor≈ 0.21Older 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 £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 an insulated concrete ground floor?

The current new-build standard — 150mm PIR under a 75mm screed — gives a U-value of about 0.12 W/m²K on a typical semi footprint, while the older 100mm build-up shown below comes out nearer 0.17 on the same floor. The honest answer always 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

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.

Your build-up won't be exactly this one.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.