U-value of a ground floor with insulation below the slab

PIR under a ground-bearing concrete slab, calculated to BS EN ISO 13370.

A ground-bearing concrete slab cast on 150mm of PIR achieves a U-value of about 0.13 W/m²K on a typical semi footprint (perimeter 30m, area 50m²), meeting the 0.13 notional value; 100mm of PIR gives about 0.18, inside the limiting value. With EPS, often used below slabs, the same 0.13 takes about 250mm. Above or below the slab makes almost no difference to the U-value, because BR 443 and BS EN ISO 13370 leave the dense slab’s own resistance out — its conductivity is close to the ground’s. U-Monkey does the full 13370 calculation from your perimeter and area.

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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
Uninsulated slab, same floor≈ 0.98The starting point
100mm PIR below the slab≈ 0.18Inside the 0.18 limiting value (0.176)
150mm PIR below the slab, typical semi (P/A ≈ 0.6)≈ 0.13Meets the 0.13 notional value (0.126)
200mm EPS below the slab≈ 0.16Passes the limit; short of the notional
250mm EPS below the slab≈ 0.13Just meets the 0.13 notional (0.1299)

Figures computed to BS EN ISO 13370 with BR 443 conventions, exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up and the footprint and depth shown: your products, thicknesses, perimeter, area and ground will move the number, and that's what the calculator is for.

What changes the number

Perimeter-to-area ratio is the hidden variable, as on every ground floor: the same 150mm of PIR gives about 0.11 on a 100m² floor with 30m of exposed perimeter, but 0.131 — just over the notional — on a 40m² floor with 40m of it. Below a slab the boards carry the slab and everything on it, so they need the compressive strength for the load and a flat, blinded bed, and the slab edge needs its own insulation where it meets the wall — a junction for ΨMonkey. Keeping the slab above the insulation puts its mass inside the heated envelope, so the floor warms and cools more slowly than one with a screed laid over the boards.

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 a junction):

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 ground floor with insulation below the slab?

A ground-bearing concrete slab cast on 150mm of PIR achieves a U-value of about 0.13 W/m²K on a typical semi footprint (perimeter 30m, area 50m²), meeting the 0.13 notional value; 100mm of PIR gives about 0.18, inside the limiting value. With EPS, often used below slabs, the same 0.13 takes about 250mm. Above or below the slab makes almost no difference to the U-value, because BR 443 and BS EN ISO 13370 leave the dense slab’s own resistance out — its conductivity is close to the ground’s. U-Monkey does the full 13370 calculation from your perimeter and area.

Is it better to put floor insulation above or below the slab?

For the U-value it hardly matters: BS EN ISO 13370 leaves the dense slab out of the floor’s resistance, so 150mm of PIR comes out at about 0.13 either way on the same footprint (0.125 above the slab under a 65mm screed, 0.126 below it), the small difference being the screed. The choice is practical. Below the slab, the boards must take the load and go down early in the build; above it, they need a screed or a floating deck over them, which adds height but keeps the slab’s mass outside the heated zone. Model your actual floor in U-Monkey with its real perimeter and area.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) 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, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Floors

Walls

Roofs

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, and why two calculators can give different answers for the same wall.

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.

Open this build-up pre-filled or call 01202 623236