U-value of a basement wall (heated basement)

A concrete retaining wall insulated outside, calculated with the ground as BS EN ISO 13370 requires.

A heated basement’s 250mm concrete retaining wall with 150mm of XPS insulation outside and plasterboard on battens inside has a U-value of about 0.16 W/m²K, for a basement 2.5m deep under a typical house (30m perimeter, 50m² floor) with its floor insulated to match. The ground in front of the wall does part of the work: 100mm of XPS gives about 0.21 and 200mm about 0.12. The figure is an average over the buried depth, worked out with the floor by BS EN ISO 13370 — leave the basement floor uninsulated and the same wall comes out at about 0.12.

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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, wall 300mm thick above ground, soil λ 2.0, and the basement floor’s resistance R 4.6 m²K/W entered as the app’s floor R. XPS at λ 0.033 (Kingspan GreenGuard GG300 in U-Monkey’s library).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm XPS≈ 0.21
120mm XPS≈ 0.18
150mm XPS≈ 0.16As pre-filled
200mm XPS≈ 0.12
150mm XPS, basement floor not insulated≈ 0.12BS EN ISO 13370 links the two
150mm XPS, 2.0m deep≈ 0.160.162
150mm XPS, 3.0m deep≈ 0.15
150mm XPS, clay soil (λ 1.5)≈ 0.15BR 443: clay is the commonest UK ground
Uninsulated: concrete, plasterboard on dabs≈ 0.72

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

Depth, the floor and the ground move it as well as the insulation. The deeper the basement, the more ground lies between the wall and the outside air, so the average U-value falls with depth. BS EN ISO 13370 calculates the wall with the floor: an insulated floor leaves the ground beneath the basement colder, and the wall loses more heat into it, which is why U-Monkey asks for the floor’s resistance on the basement-wall element (R 4.6 here, the basement floor page’s 150mm of XPS). The soil matters too: U-Monkey uses 2.0 W/mK, sand or gravel, unless told otherwise, and BR 443 notes that clay (1.5) is the commonest UK ground. Insulating outside the wall keeps the concrete and the waterproofing warm and the room full size, and every layer of the wall counts, the concrete included (BR 443, 2006 edition, §10.1).

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 wall (heated basement)?

A heated basement’s 250mm concrete retaining wall with 150mm of XPS insulation outside and plasterboard on battens inside has a U-value of about 0.16 W/m²K, for a basement 2.5m deep under a typical house (30m perimeter, 50m² floor) with its floor insulated to match. The ground in front of the wall does part of the work: 100mm of XPS gives about 0.21 and 200mm about 0.12. The figure is an average over the buried depth, worked out with the floor by BS EN ISO 13370 — leave the basement floor uninsulated and the same wall comes out at about 0.12.

Does a basement wall need the same U-value as a wall above ground?

Yes: Approved Document L has no separate row for basements, so a basement wall is a wall — 0.26 W/m²K limiting in a new dwelling (Table 4.1) and 0.18 for a new wall in an existing one (Table 4.2). What differs is the calculation: the U-value includes the ground, by BS EN ISO 13370, as SAP 10.2 and BR 443 require, so less insulation reaches the same figure than above ground.

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

Walls

Roofs

Floors

Different build-up? See the full wall 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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