U-value of a garage conversion wall (insulating single-skin brick)

A stud wall inside a single-skin garage wall, and what meets Part L.

A single-skin (102.5mm) brick garage wall lined with a stud wall — kept 25mm off the brick, 100mm PIR between the studs and plasterboard — comes out at about 0.28 W/m²K, inside the 0.30 Approved Document L asks of a retained wall insulated internally (Table 4.3; paragraph 4.12(b) names garage conversions). The bare brick is about 3.3. 75mm of PIR gives about 0.34 and misses, and so, by a whisker, does 89mm (0.301); 100mm PIR plus 25mm across the studs reaches about 0.20. A wall built in the old door opening is a new wall and needs 0.18.

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The build-up (outside → inside)

The pre-filled link opens as a retained wall being upgraded, judged against Approved Document L’s 0.30 for internal insulation (2021 edition, Table 4.3). Studs at 15% timber (BR 443).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Bare 102.5mm brick≈ 3.30The starting point
75mm PIR between studs≈ 0.34Misses 0.30
89mm PIR between CLS studs≈ 0.30Misses: 0.301 before rounding
100mm PIR between studs≈ 0.28As pre-filled: meets 0.30
100mm mineral wool between studs≈ 0.40Misses, and needs a vapour control layer
100mm PIR between studs + 25mm PIR across them≈ 0.20
100mm dense block instead of brick, same lining≈ 0.28The lining does the work

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

The lining does all the work; the brick is there to keep the weather out, and single-skin brick does not always manage that, which is why the studs usually stand clear of it on a gap. The 25mm gap counts as a low-emissivity air space because it faces the PIR’s foil, and the studs bridge 15% of the insulation, so a continuous layer across them gains more than deeper studs. With mineral wool between the studs and no vapour control, U-Monkey’s condensation check finds moisture building up year on year on the inside face of the brick. As pre-filled, with PIR, its ISO 13788 screening finds none, and it reports that as a screening result only, as PAS 2035 notes that an ISO 13788 check might not be sufficient for internal insulation on solid masonry.

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 garage conversion wall (insulating single-skin brick)?

A single-skin (102.5mm) brick garage wall lined with a stud wall — kept 25mm off the brick, 100mm PIR between the studs and plasterboard — comes out at about 0.28 W/m²K, inside the 0.30 Approved Document L asks of a retained wall insulated internally (Table 4.3; paragraph 4.12(b) names garage conversions). The bare brick is about 3.3. 75mm of PIR gives about 0.34 and misses, and so, by a whisker, does 89mm (0.301); 100mm PIR plus 25mm across the studs reaches about 0.20. A wall built in the old door opening is a new wall and needs 0.18.

What U-values do I need for a garage conversion?

In England, Approved Document L treats the garage’s existing walls, floor and roof as retained elements (paragraph 4.12(b)): anything worse than the Table 4.3 threshold should be upgraded to its “improved” value — 0.30 for a wall insulated internally (0.55 where a cavity can be filled), 0.25 for the floor and 0.16 for the roof — so far as that is technically and functionally feasible with a simple payback of 15 years or less. New elements meet Table 4.2: 0.18 for the wall in the old door opening and 1.4 for new windows and doors. Building Control will ask for the calculations.

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.

Open this build-up pre-filled or call 01202 623236