U-value of an insulated raft foundation (EPS raft)

A concrete raft cast on 300mm of EPS — the low-energy ground floor, checked against a certificate.

An insulated raft foundation — a 150mm reinforced concrete raft cast on 300mm of EPS100 (λ 0.036) — has a U-value of about 0.11 W/m²K on a typical house footprint (perimeter 30m, area 50m², P/A 0.6), inside the 0.13 notional value. That matches the KORE insulated foundation’s NSAI Agrément certificate (20/0424), which gives 0.105 for the same build-up at P/A 0.6 and 0.076–0.103 for P/A 0.1–0.5: U-Monkey’s BS EN ISO 13370 calculation gives 0.106 and agrees to within about 0.001 across the table. 200mm of EPS gives about 0.15 and 400mm about 0.08.

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The build-up (outside → inside)

Calculated to BS EN ISO 13370: perimeter 30m, area 50m², walls 300mm, soil λ 2.0. EPS100 at λ 0.036 (KORE Floor EPS100 white in U-Monkey’s library); the comparison figures are Table 2 of NSAI Agrément certificate 20/0424, for 300mm of EPS100 on soil of λ 2.0.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
300mm EPS100, P/A 0.1≈ 0.08NSAI 20/0424: 0.076
300mm EPS100, P/A 0.2≈ 0.09NSAI: 0.091
300mm EPS100, P/A 0.3≈ 0.10NSAI: 0.097
300mm EPS100, P/A 0.4≈ 0.10NSAI: 0.101
300mm EPS100, P/A 0.5≈ 0.10NSAI: 0.103
300mm EPS100, P/A 0.6≈ 0.11As pre-filled; NSAI: 0.105
200mm EPS100, P/A 0.6≈ 0.15
250mm EPS100, P/A 0.6≈ 0.12
350mm EPS100, P/A 0.6≈ 0.09
400mm EPS100, P/A 0.6≈ 0.08
300mm graphite EPS (λ 0.031), P/A 0.6≈ 0.09

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

For the U-value a raft is simply a slab on insulation: BR 443 counts raft construction as slab-on-ground and leaves the dense slab out, so the EPS and the footprint (P/A) decide it. White EPS100 at λ 0.036 needs about 300mm for 0.11; graphite EPS (λ 0.031) gets there with about 250mm. What a raft does better than insulation under an ordinary slab is the edge: the insulation runs under the thickened edge and turns up round the perimeter in EPS edge profiles, so the wall-to-floor junction loses far less heat — a gain that shows in the junction’s psi-value rather than the floor’s U-value.

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 an insulated raft foundation (EPS raft)?

An insulated raft foundation — a 150mm reinforced concrete raft cast on 300mm of EPS100 (λ 0.036) — has a U-value of about 0.11 W/m²K on a typical house footprint (perimeter 30m, area 50m², P/A 0.6), inside the 0.13 notional value. That matches the KORE insulated foundation’s NSAI Agrément certificate (20/0424), which gives 0.105 for the same build-up at P/A 0.6 and 0.076–0.103 for P/A 0.1–0.5: U-Monkey’s BS EN ISO 13370 calculation gives 0.106 and agrees to within about 0.001 across the table. 200mm of EPS gives about 0.15 and 400mm about 0.08.

Is an insulated raft better than insulation under an ordinary slab?

For the floor’s U-value, no: 300mm of the same EPS under an ordinary ground-bearing slab gives the same 0.11, because BS EN ISO 13370 sees only the insulation and the footprint. The difference is at the edge. In a raft the insulation is continuous under the slab and up its perimeter, while a conventional wall often sits on a foundation that bypasses the floor insulation, so the wall-to-floor junction of a raft loses much less heat. That shows in the junction’s psi-value, which the free ΨMonkey calculator works out.

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.

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