U-value of spray foam insulation in a pitched roof

Open- and closed-cell foam sprayed between the rafters: the U-value it gives, and why it can make a house hard to sell.

Open-cell spray foam (λ 0.039) sprayed 150mm deep between the rafters, under a breathable underlay, with the air and vapour control layer its BBA certificate requires and a plasterboard ceiling under it, gives a pitched roof a U-value of about 0.31 W/m²K — well short of the 0.16 Approved Document L asks of a renovated roof. 200mm, the most its BBA certificate allows, gives about 0.24, and 150mm with 30mm more between battens under the rafters about 0.25, the figure the certificate gives for that build-up. Closed-cell foam is better per millimetre (λ about 0.025–0.028) but far less vapour-open. And before choosing either, know that surveyors and mortgage lenders treat spray foam with caution: the RICS says its presence could affect a surveyor’s opinion of value.

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The build-up (outside → inside)

Open-cell foam as Huntsman’s H2Foam Lite (λ 0.039, and μ 4.4 for the condensation check, BBA 23/7062), closed-cell as U-Monkey’s generic entry (λ 0.026, μ 60). Rafters at 12%, battens under them at 11.8%; the tiles and their batten space lie outside a ventilated layer and are not counted. The pre-filled link opens as an upgraded existing roof, judged against Approved Document L’s 0.16 (2021 edition, Table 4.3).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm open-cell between the rafters≈ 0.44
150mm open-cell between the rafters≈ 0.31As pre-filled
175mm open-cell (deeper rafters)≈ 0.27
200mm open-cell (deeper rafters)≈ 0.24The most BBA 23/7062 allows
150mm open-cell + 30mm between battens under the rafters≈ 0.25BBA 23/7062 Table 6: 0.25
100mm closed-cell (λ 0.026)≈ 0.35
125mm closed-cell≈ 0.29
150mm open-cell + 50mm PIR under the rafters≈ 0.18The foil-faced board, taped, is the AVCL
For comparison: 150mm PIR between the rafters + 50mm under≈ 0.14

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

Spray foam’s appeal is that it fills round the rafters and seals the roof in one pass, but it is not a high performer: open-cell foam is about as conductive as mineral wool (λ 0.039), and with the rafters bridging 12% of it, 150mm only reaches 0.31. A continuous layer under the rafters helps most, as in any rafter roof: 50mm of PIR takes 150mm of foam to about 0.18. The BBA certificate for Huntsman’s H2Foam Lite E (23/7062) limits it to roofs with a breathable underlay, requires an air and vapour control layer under it on the warm side, asks for a pre-installation survey that includes a condensation risk assessment and timber below 20% moisture content, and caps it at 200mm. Closed-cell foams such as BASF’s Walltite (NSAI 13/0377, λ 0.025–0.028) and Huntsman’s Heatlok HFO Pro (Kiwa, 0.025) insulate better but resist vapour far more, and the RICS warns that foam can seal in moisture unless adequate precautions are taken. Both kinds hide the timbers: the RICS consumer guide (2023) notes that a roof structure covered in foam “cannot be fully seen or inspected”, the Property Care Association’s inspection protocol (2023) treats foam sprayed onto the tiles or onto a high-resistance felt as higher risk, and LABC Warranty decided in 2022 not to accept spray foam in roofs.

Condensation risk with spray foam

Open-cell foam is vapour-open, so the layer under it decides the condensation risk. The BBA certificate requires an air and vapour control layer on the warm side, and with one, as pre-filled, U-Monkey’s free BS EN ISO 13788 check finds only a trace of seasonal condensation at the underlay (about 1 g/m² in December) and gives a conditional pass, depending on the underlay staying low-resistance (sd 0.05m or less).

Leave the AVCL out — as many installations have — and the check predicts about 1.2 kg/m² of condensation on the underside of the underlay by February, evaporating in summer, and rates the roof “not a pass”: BS 5250:2021 asks a warm pitched roof for an AVCL on the warm side, and the calculation can’t see the room air leaking into the roof, which is how most moisture gets there. Closed-cell foam resists vapour far more (U-Monkey takes μ 60, against 4.4 for this open-cell foam), and without an AVCL the check predicts only about 55 g/m² with 100mm of it, but still rates the roof “not a pass”. Foil-faced PIR under the rafters, taped at its joints, can do the AVCL’s job: with 50mm of it under 150mm of foam the check finds no condensation in any month.

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, a vapour-open underlay, a ventilated void, a warm deck — 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 spray foam insulation in a pitched roof?

Open-cell spray foam (λ 0.039) sprayed 150mm deep between the rafters, under a breathable underlay, with the air and vapour control layer its BBA certificate requires and a plasterboard ceiling under it, gives a pitched roof a U-value of about 0.31 W/m²K — well short of the 0.16 Approved Document L asks of a renovated roof. 200mm, the most its BBA certificate allows, gives about 0.24, and 150mm with 30mm more between battens under the rafters about 0.25, the figure the certificate gives for that build-up. Closed-cell foam is better per millimetre (λ about 0.025–0.028) but far less vapour-open. And before choosing either, know that surveyors and mortgage lenders treat spray foam with caution: the RICS says its presence could affect a surveyor’s opinion of value.

Will spray foam insulation stop me selling or remortgaging?

It can make it harder. The RICS’s consumer guide (2023) says spray foam is a factor that could affect a surveyor’s opinion of value and that a surveyor may ask for an independent specialist report; lenders that do accept it usually want one, made to the Property Care Association’s inspection protocol, and some won’t lend at all. Have the specialist report, the product’s BBA or Kiwa certificate and the installer’s paperwork ready. Removing the foam is possible but disruptive, and the timbers then need inspecting — the very thing the foam had prevented.

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