U-value of a cellar wall when converting a cellar (basement conversion)

Insulating an old brick cellar wall behind a cavity drain membrane, with the ground counted.

A 215mm brick cellar wall lined with a cavity drain membrane and 62.5mm PIR insulated plasterboard (50mm of PIR) on dabs has a U-value of about 0.25 W/m²K, for a cellar 2m deep under a typical house whose existing floor is upgraded as well (70mm of PIR under a screed gives about 0.21, inside the 0.25 Approved Document L asks of a retained floor). That is inside the 0.30 Approved Document L asks of a retained wall insulated inside; 40mm of PIR gives about 0.28 and 25mm about 0.35, which misses. The bare wall is about 0.84 — far better than the same brick above ground, because the ground is part of the calculation (BS EN ISO 13370).

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The build-up (outside → inside)

Calculated to BS EN ISO 13370 for a heated basement: perimeter 30m, floor area 50m², floor 2.0m below ground, soil λ 2.0, with the floor’s resistance R 3.24 m²K/W (70mm of PIR under a 65mm screed laid over the existing floor, a floor U-value of about 0.21, inside the 0.25 for a retained floor) entered as the app’s floor R. A new floor, as where the cellar is dug out and relaid, should meet Table 4.2’s 0.18 instead: about 90mm of PIR here (0.17), which leaves the wall figures on this page unchanged. The pre-filled link opens as a retained wall being upgraded, judged against Approved Document L’s 0.30 (2021 edition, Table 4.3).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Bare cellar wall: 215mm brick, lime plaster≈ 0.84
25mm PIR board≈ 0.35Misses 0.30
40mm PIR≈ 0.28
50mm PIR≈ 0.25As pre-filled (0.246)
75mm PIR≈ 0.19
50mm PIR, cellar floor left uninsulated≈ 0.20BS EN ISO 13370 links wall and floor
50mm PIR, cellar 2.5m deep≈ 0.23

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

A converted cellar keeps water out by managing it: BS 8102’s Type C protection lets water through the old wall into the studded cavity drain membrane, which carries it to a drainage channel and pump, and the lining goes on the room side of the membrane — on a mesh-faced membrane, insulated plasterboard can be dabbed straight onto it. The ground does a lot of the work, so modest insulation goes a long way, and the result depends on the cellar’s depth, its plan and the floor: BS EN ISO 13370 links the wall and the floor, so insulating the floor raises the wall’s figure (0.20 with the floor left bare). The membrane is vapour-tight, so the room needs heating and ventilation that keep its humidity down, as the membrane makers stress.

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 cellar wall when converting a cellar (basement conversion)?

A 215mm brick cellar wall lined with a cavity drain membrane and 62.5mm PIR insulated plasterboard (50mm of PIR) on dabs has a U-value of about 0.25 W/m²K, for a cellar 2m deep under a typical house whose existing floor is upgraded as well (70mm of PIR under a screed gives about 0.21, inside the 0.25 Approved Document L asks of a retained floor). That is inside the 0.30 Approved Document L asks of a retained wall insulated inside; 40mm of PIR gives about 0.28 and 25mm about 0.35, which misses. The bare wall is about 0.84 — far better than the same brick above ground, because the ground is part of the calculation (BS EN ISO 13370).

What U-values does a cellar conversion need?

In England, Approved Document L treats a cellar’s existing walls and floor like those of a loft or garage conversion — retained elements (paragraph 4.12(b)) — so a wall worse than 0.70 should be upgraded to 0.30 if insulated inside, and the floor to 0.25 (Table 4.3), so far as that is technically and functionally feasible with a simple payback of 15 years or less (paragraph 4.13). New elements meet Table 4.2. Both wall and floor are calculated with the ground, to BS EN ISO 13370, which is why thin linings can meet them.

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.

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