What is the difference between open and closed cell spray foam?

Open cell foam (lambda 0.038) 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 (lambda 0.022) is rigid, acts as its own vapour barrier, and is more thermally efficient — less thickness needed. Closed cell is about twice the price but needs less depth.

How thick should spray foam be?

Thickness depends on the target U-value. For walls at 0.16 W/m²K: closed cell needs about 100-120mm, open cell needs about 170-200mm. For roofs at 0.18 W/m²K: closed cell needs about 90-100mm, open cell needs 150-170mm. These are approximate — the existing construction contributes some thermal resistance that reduces the foam thickness needed.

Does spray foam need a ventilation gap?

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.

Is spray foam insulation safe?

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).

Can spray foam cause problems with my mortgage?

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.

TradeCalculator

Spray Foam Calculator — Thickness, Coverage & U-Value

Calculate spray foam insulation thickness for target U-value. Compares open vs closed cell foam with coverage per kit and ventilation gap requirements.

Area to Insulate

Roof, wall, or floor area

Foam Type

Target U-Value

Building Regulations minimum

Existing R-Value

Thermal resistance of existing construction

Price Per m² Installed (optional)

Include ventilation gap (open cell roof)

50mm gap above open cell foam

How We Calculate This

This calculator determines spray foam thickness needed to achieve a target U-value, then calculates the volume and number of kits required for the job.

The formula

Required R-value = (1 ÷ Target U) - Existing R - Surface resistances

Foam thickness = Required R × Lambda × 1000 (mm)

Volume = Area × Thickness × Wastage

Kits = Foam weight ÷ Kit size

For example, 50 m² roof at target 0.18 W/m²K with closed cell (λ = 0.022): R needed = (1 ÷ 0.18) - 0.5 - 0.17 = 4.89. Thickness = 4.89 × 0.022 × 1000 = 108mm. Volume = 50 × 0.108 = 5.4 m³. Weight = 5.4 × 35 = 189 kg = 7 kits.

Frequently Asked Questions

Related Calculators

U-Value Calculator

Calculate the U-value of wall, floor and roof constructions.

Insulation Thickness Calculator

Determine the insulation thickness needed to meet UK Building Regulations.

Heat Loss Calculator

Calculate room-by-room heat loss for heating system sizing.

Cavity Wall Insulation Calculator

Estimate cavity wall insulation quantities and potential savings.

Loft Insulation Calculator

Calculate loft insulation rolls needed for your loft area.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.