U-value of a basement floor (heated basement)

A slab 2.5m below ground with XPS under it: the ground and the depth do much of the work.

A heated basement floor with 150mm of XPS under a 150mm concrete slab and a screed, 2.5m below ground in a typical house footprint (30m perimeter, 50m², P/A 0.6), has a U-value of about 0.16 W/m²K. With 100mm of XPS it is about 0.21, and uninsulated about 0.57 — against about 0.92 for the same slab and screed at ground level. Kingspan’s basement tables for 100mm of its GreenGuard XPS (2.5m deep, 300mm walls, sand or gravel) give 0.18 / 0.20 / 0.21 at P/A 0.3 / 0.5 / 0.8; U-Monkey gives 0.18 / 0.20 / 0.22.

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The build-up (outside → inside)

Calculated to BS EN ISO 13370 for a heated basement: perimeter 30m, floor area 50m² (P/A 0.6), floor 2.5m below ground, walls 300mm thick above ground, soil λ 2.0. Kingspan’s figures are from its GreenGuard basements brochure (2020), which assumes the same depth and wall thickness on sand or gravel, with the λ 0.034 the board was declared at then. XPS at λ 0.033 (Kingspan GreenGuard GG300 in U-Monkey’s library).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Uninsulated slab≈ 0.57
100mm XPS, P/A 0.3≈ 0.18Kingspan’s table: 0.18
100mm XPS, P/A 0.5≈ 0.20Kingspan: 0.20
100mm XPS, P/A 0.8≈ 0.22Kingspan: 0.21
100mm XPS, P/A 0.6≈ 0.21
150mm XPS, P/A 0.6≈ 0.16As pre-filled
200mm XPS, P/A 0.6≈ 0.13Meets the 0.13 notional value
150mm XPS, P/A 0.6, 2.0m deep≈ 0.160.161
150mm XPS, P/A 0.6, 3.0m deep≈ 0.15

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

BS EN ISO 13370 treats the ground below and around a basement as part of the floor: the bigger the floor for its perimeter (lower P/A) and the deeper it sits, the lower the U-value, so a basement floor reaches a given figure with less insulation than one at ground level. The dense slab’s own resistance is left out, as BR 443 does for ground floors; the screed above it counts. XPS goes under the slab because it takes the load and tolerates the wet; PIR can go above the slab under the screed instead, on the dry side of the waterproofing, with much the same result (100mm of PIR there gives about 0.16). The floor and the walls are calculated together, so the walls’ figure depends on this floor’s insulation as well.

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 basement floor (heated basement)?

A heated basement floor with 150mm of XPS under a 150mm concrete slab and a screed, 2.5m below ground in a typical house footprint (30m perimeter, 50m², P/A 0.6), has a U-value of about 0.16 W/m²K. With 100mm of XPS it is about 0.21, and uninsulated about 0.57 — against about 0.92 for the same slab and screed at ground level. Kingspan’s basement tables for 100mm of its GreenGuard XPS (2.5m deep, 300mm walls, sand or gravel) give 0.18 / 0.20 / 0.21 at P/A 0.3 / 0.5 / 0.8; U-Monkey gives 0.18 / 0.20 / 0.22.

How much insulation does a basement floor need?

Less than a floor at ground level, because the ground and the depth help: in a typical house footprint 2.5m down, 100mm of XPS gives about 0.21 W/m²K and 150mm about 0.16, against the 0.18 Approved Document L sets as the limiting value for a floor in a new dwelling (Table 4.1) and for a new floor in an existing one (Table 4.2). The notional dwelling’s 0.13 takes about 200mm here.

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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