U-value of a dormer cheek wall (loft conversion)

The timber side walls of a new dormer, and what reaches 0.18.

A tile-hung dormer cheek with 100mm PIR between the studs and a continuous 50mm PIR layer inside them reaches about 0.17 W/m²K, inside the 0.18 Approved Document L sets for a new wall in an existing home (a dormer’s cheeks count as walls, Table 4.2 note 2). 100mm between the studs alone gives only about 0.32, because the studs (15% of the wall, BR 443) bridge it; 25mm inside brings it to 0.22 and 40mm to 0.19, still short. 46mm is the least that clears it.

Open this build-up in U-Monkey — pre-filled, free

Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report

The build-up (outside → inside)

BR 443 counts nothing outside a well-ventilated air space, so the tiles and their batten space drop out and the breather membrane and plywood count. 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).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm PIR between studs only≈ 0.32The studs bridge it
100mm between + 25mm PIR inside≈ 0.22
100mm between + 40mm PIR inside≈ 0.19Misses 0.18 (0.1895)
100mm between + 50mm PIR inside≈ 0.17As pre-filled: meets 0.18
75mm between + 50mm PIR inside≈ 0.20Thinner studs, same lining: misses
100mm mineral wool between + 50mm PIR inside≈ 0.21

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 continuous layer inside the studs matters more than the depth between them, because it covers the 15% of the wall that is timber. Space is the constraint in a dormer, so the inside layer is usually an insulated plasterboard or PIR with taped joints that doubles as the vapour control. As pre-filled, U-Monkey’s condensation check finds no condensation in any month. Dormer cheeks are lightweight, with little to store heat, which is worth knowing for a loft bedroom’s summer comfort: the thermal-mass figures in the app show it.

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 dormer cheek wall (loft conversion)?

A tile-hung dormer cheek with 100mm PIR between the studs and a continuous 50mm PIR layer inside them reaches about 0.17 W/m²K, inside the 0.18 Approved Document L sets for a new wall in an existing home (a dormer’s cheeks count as walls, Table 4.2 note 2). 100mm between the studs alone gives only about 0.32, because the studs (15% of the wall, BR 443) bridge it; 25mm inside brings it to 0.22 and 40mm to 0.19, still short. 46mm is the least that clears it.

What U-values does a dormer need?

In England a new dormer is a new element in an existing home, so Approved Document L’s Table 4.2 applies: 0.18 W/m²K for the cheeks, which count as walls, and 0.15 for the dormer’s roof (Table 4.2, note 2); the window needs 1.4. A flat dormer roof is a small warm roof, so see the warm flat roof page; the main roof slopes being converted are retained elements, with 0.16 for the roof in Table 4.3.

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.

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