U-value of insulation over the rafters (warm pitched roof)

PIR boards over the rafters, mineral wool between — the re-roofing warm roof.

A warm pitched roof with 100mm of PIR laid over the rafters and 100mm of mineral wool between them achieves a U-value of about 0.14 W/m²K, inside the 0.15 for a new roof on an extension and the 0.16 limiting value. 120mm over the rafters reaches about 0.13, and 150mm meets the 0.11 notional value. Because the boards run over the rafters, the timber bridges only the wool between them. The boards go on from outside, so this is usually done on a new roof or when a roof is stripped and re-covered.

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)

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm PIR over, rafter void left empty≈ 0.20The boards alone fall short of the 0.16 limit
80mm PIR over + 100mm wool between≈ 0.16Just misses the 0.16 limiting value before rounding (0.164)
100mm PIR over + 100mm wool between≈ 0.14Inside the 0.15 for a new roof on an extension
120mm PIR over + 100mm wool between≈ 0.13Comfortable pass
150mm PIR over + 100mm wool between≈ 0.11Meets the 0.11 notional value (0.108)

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

Thickness over the rafters is worth more than depth between them, because the layer is continuous. It also earns BR 443’s level 0 for air gaps — insulation in two layers, one covering the rafters and any gaps in the other — which U-Monkey applies automatically. With foil-faced PIR above the wool, the vapour control layer on the warm side is essential: take it out and U-Monkey’s free condensation check will not pass the roof, because BS 5250 expects an air and vapour control layer on the warm side of a warm pitched roof. As pre-filled it finds no condensation in any month, a conditional pass that relies on the breather membrane being vapour-open, so enter your underlay’s declared value. The counter-battens are fixed through the boards into the rafters, so check the fixing lengths against the thickness you choose.

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 insulation over the rafters (warm pitched roof)?

A warm pitched roof with 100mm of PIR laid over the rafters and 100mm of mineral wool between them achieves a U-value of about 0.14 W/m²K, inside the 0.15 for a new roof on an extension and the 0.16 limiting value. 120mm over the rafters reaches about 0.13, and 150mm meets the 0.11 notional value. Because the boards run over the rafters, the timber bridges only the wool between them. The boards go on from outside, so this is usually done on a new roof or when a roof is stripped and re-covered.

Is insulation over the rafters better than between them?

Per millimetre, yes: it is continuous, so the rafters bridge only what sits between them, and it costs no headroom inside. The catch is access — it goes on from outside, so it suits a new roof or one being re-covered, and it raises the roof line, which changes the eaves and verge details. Where the roof covering is staying put, insulation between and under the rafters is the usual route; see the rafter-level insulation page and compare the two in U-Monkey.

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.

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