Spray Foam Insulation Calculator — m, m², cost
How to use this calculator
Pick a preset or enter wall length × height (m) with foam type, target thickness or R-value, and cavity depth. The output is coverage in m² at the chosen thickness, kits needed, thermal resistance (R) and U-value of the cavity, and whether a separate vapour barrier is still required by EN ISO 13788.
Open-cell — λ=0.038 W/mK, density 8 kg/m³. Needs separate vapour barrier.
Closed-cell — λ=0.022 W/mK, density 32 kg/m³. Acts as vapour barrier at ≥ 50 mm.
Saved Calculations
| Time | Type | Area m² | Thick | Kits | R-Value | Cost |
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How to Calculate Spray Foam Insulation (Metric)
Choose the foam type, enter wall length × height in metres (or a known area in m²), then set the spray thickness or a target R-value. Thermal resistance follows EN ISO 6946: R = thickness (m) ÷ λ. Open-cell PUR sprays around λ=0.038 W/mK, closed-cell around λ=0.022 W/mK, so a 100 mm closed-cell layer gives roughly R = 0.10 ÷ 0.022 ≈ 4.5 m²K/W. Applied thickness is capped at the cavity depth so a 140 mm timber bay never over-reports foam.
Kits are rated by coverage at a 50 mm reference depth; spraying thicker proportionally reduces the area one kit covers (coverage = rated × 50 ÷ thickness mm). Add 5–10% waste for overspray and unusable kit remainders. Cross-check the target with a metric wall insulation calculator and a roof insulation calculator for the whole envelope.
Formulas
R-value = thickness (m) ÷ λ. U-value = 1 ÷ R. Volume = area × thickness. Weight = volume × density. Open-cell λ=0.038 W/mK, ρ=8 kg/m³; closed-cell λ=0.022 W/mK, ρ=32 kg/m³. Kit coverage at target thickness = rated coverage × (50 ÷ target thickness mm).
FAQ
Open vs closed cell? Open-cell is cheaper and dampens sound but is vapour-open, so it needs a separate vapour barrier. Closed-cell is denser and structural, acts as its own vapour control at ≥50 mm, and delivers better thermal performance per mm.
What U-value should I target? Walls: U ≤ 0.30 W/m²K (well-insulated). Roofs: U ≤ 0.20. Floors: U ≤ 0.25. Confirm exposed sheathing with a house wrap calculator and below-grade faces with a waterproofing membrane calculator; check local building regulations for the governing limits.
On install day
Pro spray foam runs roughly €15–25/m² open-cell and €25–45/m² closed-cell at 50 mm in 2025; DIY two-part kits land near €8–14/m² for the chemistry alone. The number-one estimator mistake is sizing kits by area without normalising for thickness — kit ratings are at a 50 mm reference, so spraying 100 mm halves the m² a kit covers, and half-used kits cannot be re-pressurised once cured. Mind reaction-to-fire rules (BS 9991 / Bauaufsichtliche Zulassung): foam left exposed in roofs and crawlspaces needs an intumescent coating or a 12.5 mm gypsum cover. Substrate must be above 5 °C and below 85% RH or adhesion fails. Plan 24 hours of ventilation after the pour for VOC clearance.
