Spray Foam Calculator — Thickness, Coverage & U-Value

Open cell foam (lambda ~0.038 W/mK) is softer, breathable, and allows moisture vapour to pass through — suitable for timber frame walls and between rafters with a ventilation gap. Closed cell foam is rigid, acts as its own vapour barrier, and is more thermally efficient — less thickness needed. This calculator uses a premium closed-cell lambda of 0.022 W/mK; many UK BBA-certified closed-cell foams (for example Enverge Nexseal LE, BBA 09/4610) are 0.026–0.027 W/mK, so real thickness can be around 20% greater — always work to the declared lambda on the product's BBA certificate. Closed cell is about twice the price but needs less depth.

Thickness depends on the target U-value. For new-build walls at 0.26 W/m²K (AD L 2021 Table 4.1 limiting): closed cell needs about 60-80mm, open cell needs about 100-120mm. For roofs at 0.16 W/m²K (AD L 2021 Table 4.1 new-build and Table 4.3 existing-roof upgrade — a single value for all roof types): closed cell needs about 120-140mm, open cell needs 200-220mm. AD L 2021 Table 4.2 sets a slightly tighter 0.15 W/m²K for roofs in extensions/new elements. These are approximate — the existing construction contributes some thermal resistance that reduces the foam thickness needed. Note AD L 2021 does not set a separate flat-roof limit; the same roof U-value applies to flat and pitched roofs.

Open cell spray foam between rafters should have a 50mm ventilation gap above it to allow moisture to escape. Closed cell foam does not require a ventilation gap as it acts as its own vapour barrier. However, BBA certification and manufacturer guidance should always be followed. Some lenders and surveyors have concerns about spray foam in roofs — check before proceeding.

Professional-grade spray foam is safe once fully cured (24-72 hours). During application, the area must be evacuated and installers must wear full respiratory protection. Concerns exist about off-gassing during curing and potential issues with mortgage valuations. Always use a BBA-certified installer who is a member of the National Insulation Association (NIA).

Some lenders and surveyors are cautious about spray foam, particularly in roof spaces. Concerns include: difficulty inspecting roof timbers after installation, potential moisture trapping, and removal costs if problems arise. RICS guidance recommends that spray foam be installed by a certified contractor with a 25-year guarantee. Always check with your lender before installation.