U-value of a mansard roof

A steep mansard face calculated as BR 443 does it, and whether it has to meet the roof value or the wall value.

A mansard’s steep face, with 150mm of PIR (λ 0.022) between 47 × 150mm rafters at 400mm centres and 52.5mm of insulated plasterboard under them, has a U-value of about 0.15 W/m²K (0.146). That meets both values it might be held to under Approved Document L for a loft conversion — 0.15 for a new roof and 0.18 for a new wall in an existing home — and which one applies isn’t settled: energy-modelling conventions treat a pitch of 70° or more as a wall, while Approved Document L doesn’t say, so ask building control. Framed as a stud wall instead, at BR 443’s 15% timber-frame default, the same build-up is 0.152 and needs a 62.5mm board for the roof value (0.141). 100mm between the rafters and a 62.5mm board gives 0.16: enough for a wall, not for a roof. The slates count for nothing: the space behind them is well ventilated, so BR 443 (§4.7.1) leaves them out.

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The build-up (outside → inside)

Calculated as a wall at 70°: heat flow within 30° of horizontal counts as horizontal (BR 443 §4.1), with the slate and its batten space disregarded (§4.7.1, §4.7.3). The timbers are rafters, at width ÷ spacing + 0.01 (§4.5); two rows show a stud-framed mansard at BR 443’s 15%. The insulated plasterboard is entered as its PIR core and a 12.5mm board. The link opens as a new element in an existing home, judged against the wall value, 0.18 (Approved Document L 2021, Table 4.2); the roof value for the same work is 0.15.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm PIR between 150mm rafters (50mm gap outside it) + 62.5mm insulated plasterboard≈ 0.16Meets 0.18 (wall), misses 0.15 (roof); the gap beside the PIR’s foil face counts as a low-emissivity cavity
100mm between + 52.5mm board≈ 0.17Meets 0.18 only
100mm between + 72.5mm board≈ 0.15Meets both
120mm between + 52.5mm board≈ 0.16
150mm between (full) + 37.5mm board≈ 0.16Misses 0.15 (0.165)
150mm between + 52.5mm board≈ 0.15As pre-filled: meets both (0.146)
150mm between + 62.5mm board≈ 0.14
150mm between, plain plasterboard≈ 0.22One layer: the rafters bridge it, and air-gap level 1
Rafters at 600mm (8.8%): 100mm between + 62.5mm board≈ 0.15Misses 0.15 (0.152)
Framed as a stud wall (BR 443’s timber-frame default, 15%): 150mm + 52.5mm board≈ 0.15Misses 0.15 (0.152); meets 0.18
Framed as a stud wall, 15%: 150mm + 62.5mm board≈ 0.14Meets both (0.141)
150mm + 52.5mm, taken as a pitched roof (heat flow upwards)≈ 0.150.148 against 0.146: BR 443 takes heat flow as horizontal above 60°

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

Two things set a mansard apart from a pitched roof. The heat flow: BR 443 takes heat flowing within 30° of horizontal as horizontal (§4.1) — any slope steeper than 60° — so the inside surface takes the wall value, 0.13, not the roof’s 0.10 — calculated as a pitched roof the pre-filled build-up reads 0.148 rather than 0.146. And the slates: the space behind slates or tiles on battens is well ventilated (BR 443 §4.7.1), so the slates, the battens and that space are disregarded and the outside surface resistance becomes 0.13 (§4.7.3). After that it is the insulation and the timber. BR 443 has no default for rafters, so their share is width ÷ spacing + 0.01 (§4.5): 12.8% at 400mm centres, 8.8% at 600mm. A mansard framed as a stud wall — sole and head plates, noggings, trimmers round the dormers — takes BR 443’s timber-frame default of 15% instead (§4.4.1), which puts the pre-filled build-up at 0.152. A continuous layer under the timbers breaks that bridge and, as a second layer, takes the air-gap correction to level 0. Planning sets the angle: Camden’s guidance describes the lower slope as “usually 60-70°”, and Kensington and Chelsea’s order for Redcliffe Road sets 72°.

Condensation risk in a mansard

U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled mansard, with each assumed vapour value tested across its range: the breather membrane outside is vapour-open, and the insulated plasterboard’s foil and taped joints hold vapour back on the warm side. The places to watch are the junctions — the box gutter behind the parapet, the floor and the dormer cheeks — where the insulation is easily thinner than on the plane.

Check this build-up for condensation — free

Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (fRsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer on the warm side, a more vapour-open layer on the cold side, more insulation to warm the inside surface — and applies the one you pick in a tap, with Undo.

Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.

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 mansard roof?

A mansard’s steep face, with 150mm of PIR (λ 0.022) between 47 × 150mm rafters at 400mm centres and 52.5mm of insulated plasterboard under them, has a U-value of about 0.15 W/m²K (0.146). That meets both values it might be held to under Approved Document L for a loft conversion — 0.15 for a new roof and 0.18 for a new wall in an existing home — and which one applies isn’t settled: energy-modelling conventions treat a pitch of 70° or more as a wall, while Approved Document L doesn’t say, so ask building control. Framed as a stud wall instead, at BR 443’s 15% timber-frame default, the same build-up is 0.152 and needs a 62.5mm board for the roof value (0.141). 100mm between the rafters and a 62.5mm board gives 0.16: enough for a wall, not for a roof. The slates count for nothing: the space behind them is well ventilated, so BR 443 (§4.7.1) leaves them out.

Is a mansard a roof or a wall for Building Regulations?

For energy, Approved Document L doesn’t draw the line. The National Calculation Methodology’s modelling guide for Scotland counts any part of a roof pitched at 70° or more as a wall, as the 2008 guide for England and Wales did, and RdSAP treats a mansard storey with slopes of at least 70° as a storey of its own; practice varies, so meeting the roof value, 0.15, avoids the argument. For fire it is defined: regulation 2(6) of the Building Regulations 2010 counts a roof pitched at more than 70° beside a space people use as part of the external wall. On a relevant building — one with a storey at least 18m above ground containing dwellings, an institution or rooms for residential purposes — regulation 7(2) then requires materials of class A2-s1, d0 or A1 there, which rules out PIR and a timber frame alike (membranes and some other components are exempt); and on a residential building with a storey over 11m, Approved Document B asks for insulation of class A2-s1, d0 or better in the external wall, or a BS 8414 test to BR 135. A slope of exactly 70° is a wall for energy modelling but not an external wall for regulation 7.

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

Roofs

Walls

Floors

Different build-up? See the full roof 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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