U-value of an extension cavity wall (what reaches 0.18)

Matching the house’s 100mm cavity and still meeting 0.18 — or widening the cavity.

An extension wall built with the house’s usual 100mm cavity — brick, 100mm of λ 0.032 cavity batts, 100mm aircrete blocks and plasterboard on dabs — comes out at about 0.25 W/m²K, well short of the 0.18 Approved Document L sets for a new wall in an existing home. Lining it with 52.5mm PIR insulated plasterboard (40mm of PIR) brings it to about 0.17, which meets; a 37.5mm board gives 0.19 and misses. The alternative is a wider cavity: 150mm of the same batts meets 0.18 with thermal-grade aircrete (0.176) but not quite with standard aircrete (0.181).

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The build-up (outside → inside)

The pre-filled link opens as a new element in an existing home, judged against Approved Document L’s 0.18 (2021 edition, Table 4.2). Batts at λ 0.032 (Knauf DriTherm Cavity Slab 32 in U-Monkey’s library).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm cavity, λ 0.032 batts, plasterboard on dabs≈ 0.25Misses 0.18
+ 25mm PIR insulated plasterboard (37.5mm board)≈ 0.19Misses: 0.1949
+ 40mm PIR insulated plasterboard (52.5mm board)≈ 0.17As pre-filled: meets 0.18
100mm cavity, dense block inner leaf, + 40mm PIR lining≈ 0.19Dense blocks undo it: 0.189
150mm cavity, λ 0.032 batts, standard aircrete≈ 0.18Misses by a whisker: 0.181
150mm cavity, λ 0.032 batts, thermal aircrete (λ 0.11)≈ 0.18Meets: 0.176
150mm cavity, 100mm PIR + 50mm clear≈ 0.17

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and the air-gap correction included — 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 shown: your products, thicknesses and (for floors) footprint will move the number, and that's what the calculator is for.

What changes the number

Matching the house usually means keeping its brick line and a 100mm cavity, and full fill can’t reach 0.18 there on its own: the lining has to do the rest, at the cost of about 40mm more room than plain plasterboard on each outside wall. Widening the cavity to 150mm puts the extra insulation where it costs no floor space, at the price of a wider wall and foundation, and the result then hangs on the inner leaf: standard aircrete 0.181, thermal-grade 0.176, dense block about 0.20. Mortar joints in the blockwork, the dabs behind the plasterboard and the wall ties all nibble at it; U-Monkey counts the mortar and the dabs for you, as BR 443 requires, and offers to add the ties.

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 extension cavity wall (what reaches 0.18)?

An extension wall built with the house’s usual 100mm cavity — brick, 100mm of λ 0.032 cavity batts, 100mm aircrete blocks and plasterboard on dabs — comes out at about 0.25 W/m²K, well short of the 0.18 Approved Document L sets for a new wall in an existing home. Lining it with 52.5mm PIR insulated plasterboard (40mm of PIR) brings it to about 0.17, which meets; a 37.5mm board gives 0.19 and misses. The alternative is a wider cavity: 150mm of the same batts meets 0.18 with thermal-grade aircrete (0.176) but not quite with standard aircrete (0.181).

What U-value does an extension wall need?

In England, 0.18 W/m²K: an extension’s walls are new elements in an existing dwelling, so Approved Document L’s Table 4.2 applies (2021 edition; the 2026 edition, which takes effect on 24 March 2027, keeps 0.18 in its Table 3.2). The same table asks 0.15 of the roof, 0.18 of the floor and 1.4 of new windows and doors. A lesser standard may be appropriate for a wall where meeting it would reduce the floor area of the room by more than 5% (Table 4.2, note 3).

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.

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