Spray Foam Insulation Calculator: Board Feet and Kits

How to use the spray foam calculator

1. Enter the area to spray: wall or roof area with doors and windows removed. For rim joists, add up the length × height of each section.
2. Pick the framing. If you are filling stud or rafter bays, choose the spacing and the calculator takes out the 1½ in faces of the framing. For a continuous surface, such as the underside of a roof deck or a rim joist, choose Continuous.
3. Pick the foam type. The R-value per inch fills in automatically (R-6 closed-cell, R-3.6 open-cell). Change it to match your product’s label.
4. Size it. Enter a thickness, or switch to Target R and let the calculator work out the thickness.
5. Enter the kit size in board feet, which is printed on the box.

How to calculate board feet of spray foam

A board foot of foam is 1 sq ft at 1 in thick. Kit makers rate their kits that way. DuPont’s Froth-Pak sheet lists the 210 kit at 210 board feet and the 650 kit at 650 board feet, both “theoretical yield … calculated at 1” thick”. DuPont adds that theoretical yield is measured in perfect laboratory conditions, without loss of blowing agent or variations in how the foam is applied. Real jobs get less, so the calculator adds 15% by default. That is our allowance, and you can change it.

The math is simple: 100 sq ft at 2 in is 200 board feet, which is one 210 kit with almost nothing to spare. Add 15% and you need 230 board feet, so two 210 kits or one 650.

Open-cell vs. closed-cell spray foam

Sources: the DOE Building America guide “Which Spray Foam Is Right For You?” and the Spray Polyurethane Foam Alliance. Closed-cell gives about 1.7 times the R-value per inch, so it is the choice where depth is limited and where foam may get wet, such as below grade. Open-cell expands far more, roughly 150 times its liquid volume against 35 to 50 for closed-cell, so it fills deep cavities for less money per board foot. DOE’s Guide to Home Insulation lists open-cell at R-12.6 for 3.5 in (a 2×4 cavity) and closed-cell at R-6.5 per inch.

Worked examples

Rim joists in a basement. 120 ft of rim, 9¼ in tall, is 92.5 sq ft. At 2 in of closed-cell foam that is 185 board feet, or 213 with 15% extra. That is R-12 at R-6 per inch, and one 650 kit with plenty left, or two 210 kits.

A 2×4 wall, 320 sq ft, 16 in on center, filled with open-cell. The bays are about 290 sq ft once the studs are removed. At 3.5 in that is 1,015 board feet, or 1,167 with extra, which is R-12.6. Two 650 kits cover it.

Target R-13 in closed-cell. 13 ÷ 6 = 2.17 in, which takes two passes of about an inch each if your product limits passes to 2 in. Over 100 sq ft that is 217 board feet, or 249 with extra.

How thick should spray foam be?

Start from the R-value your climate needs. ENERGY STAR’s recommended levels by climate zone are built into our insulation calculator. Then divide by the foam’s R per inch. In walls, the cavity depth limits you: a 2×4 holds 3½ in and a 2×6 holds 5½ in.

Closed-cell foam is often sprayed as a thin layer with batts or blown insulation over it, called flash and batt. DOE’s guide calls for at least 1½ in of closed-cell in these hybrid walls, and 2 in where heating degree days exceed 6,000. That keeps the inner face of the foam above the dew point in most climates.

Safety and code

• Thermal barrier. Spray foam is combustible. The DOE guide notes that the IRC requires foam in living spaces, and in most other accessible cavities, to be covered by a 15-minute thermal barrier, such as ½ in drywall. Some attics and crawl spaces entered only for service can use an ignition barrier instead. Price the drywall with the drywall calculator.
• Ventilation. DuPont lists one-hour re-entry with proper ventilation after spraying Froth-Pak. Wear the protective equipment the kit calls for, and keep others away.
• Temperature. DuPont notes that cure time depends on temperature and foam thickness. Stay within the kit’s temperature limits.
• UV. DuPont says exposed foam outdoors needs a coating to protect it from sunlight.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Board feet = spray area × thickness in inches (1 board foot = 1 sq ft at 1 in thick, the basis of kit yields). In framed bays, 1½ in framing faces are taken out (1 − 1.5 ÷ spacing).
• R-value = thickness × R per inch. Defaults are R-3.6 per inch for open-cell and R-6 for closed-cell (DOE Building America). Products range from about R-3.6 to 4.5 (open) and R-5.8 to 6.9 (closed) per SPFA, so use your product’s label.
• Kit yields are theoretical board feet at 1 in thick in laboratory conditions (DuPont). The default 15% extra is our allowance for overspray, uneven depth and the gap to real-world yield.
• Closed-cell passes are shown at 2 in or less, the low end of DOE’s 2–3 in per pass; open-cell can go up to about 10 in in one pass. Always follow the product’s instructions.

Sources

1. U.S. Department of Energy, Building America, Which Spray Foam Is Right For You?. Open-cell ≈ R-3.6/in, closed-cell ≈ R-6/in; pass thicknesses; vapor permeance; 15-minute thermal barrier (½ in gypsum) under the IRC; 1½–2 in minimum ccSPF in hybrid walls.
2. Spray Polyurethane Foam Alliance, Spray foam FAQs. Open-cell R-3.6–4.5/in at 0.5–0.8 pcf; closed-cell R-5.8–6.9/in at 1.8–2.3 pcf; ccSPF under 1 perm at 1.5 in.
3. DuPont, Froth-Pak Foam Insulation product information sheet. Froth-Pak 210 (43 lb) and 650 (125 lb) kits; theoretical yield at 1 in; R-6.2 aged at 1 in; one-hour re-entry with ventilation.
4. U.S. Department of Energy, Guide to Home Insulation. Open-cell foam R-12.6 at 3.5 in; closed-cell R-6.5 at 1 in.

Last reviewed October 26, 2026. Product figures vary by brand; the label on what you buy wins.