Spray Foam Insulation Calculator — ft, sq ft, cost

How to Calculate Spray Foam Insulation

Set the foam type, enter wall length × height (or paste a known surface area), then pick a target thickness or a target R-value. A board foot is 1 ft² sprayed 1″ thick, so a 320 ft² wall at 3″ needs 960 bf before waste. Two-component kits are sold by yield (200, 600, 1,200 bf); allow 5–10% waste for trim loss, overspray and partial kits since opened kits cannot be stored.

R-value follows the IRC / ASHRAE basis: open-cell ~R-3.7 per inch, closed-cell ~R-6.5 per inch. The calculator caps applied thickness at the cavity depth so a 2×6 stud bay (5.5″) never reports more foam than will physically fit, and a framing-factor toggle derates cavity R to a whole-wall R that accounts for thermal bridging through studs. Pair the result with a wall insulation R-value calculator to confirm the assembly meets code, and a roof & attic insulation calculator for ceiling planes.

Formulas

Board feet = area (ft²) × thickness (in). From R-value: thickness = target R ÷ R-per-inch. Open-cell R/in = 3.7, density 0.5 lb/ft³; closed-cell R/in = 6.5, density 2.0 lb/ft³. Foam weight = area × (thickness ÷ 12) × density. Kits = ceil(board feet with waste ÷ kit yield). Whole-wall R = R_cavity × (1−ff) + R_wood × ff, with wood ≈ R-1.25/in.

FAQ

Open vs closed cell? Open-cell is cheaper, expands more and dampens sound but is vapor-open, so it needs a separate vapor barrier. Closed-cell is denser and structural, reaches its own Class II vapor retarder rating at ≥2″, and gives higher R per inch.

How thick should I spray? Walls: open-cell 3–5.5″ (fills a 2×4 or 2×6 cavity), closed-cell 1–3″. Attics: open-cell 5.5–10″, closed-cell 3–5″. Spray foam air-seals the building, but exposed sheathing and the rim joist may still need a house wrap and a slab-side waterproofing membrane.

On install day

DIY two-part kits run roughly $1.00–1.50/bf for open-cell and $1.50–2.50/bf for closed-cell; professional installs are typically $1.25–2.00/ft² open-cell and $2.50–4.50/ft² closed-cell in 2025. The number-one estimator mistake is sizing kits by area alone — coverage is yield in board feet, so doubling the thickness halves the area one kit covers, and partial kits at the end of a job almost always get wasted because hose-cured product cannot be re-used. Mind the IRC R316 ignition barrier rule: foam left exposed in attics and crawlspaces needs an intumescent paint or 1/2″ drywall over it, and substrate must be above 40 °F with humidity under 85% or the foam will not adhere. Plan ventilation for 24 hours of off-gassing after the pour.