How much spray foam do you need?

How much spray foam do I need?

Board feet = square feet × thickness in inches. Example: a 40 × 8 ft wall (320 ft²) and a 30 × 20 ft roof section (600 ft²) sprayed 3 inches thick take 920 × 3 = 2,760 board feet; with 10% extra, 3,036. With kits that yield 600 board feet, that is 5.06 — so 6 kits.

What is a board foot of spray foam?

One square foot, one inch thick — 144 cubic inches. Spray foam kits list their yield in board feet, so 600 board feet covers 600 ft² at 1 inch or 200 ft² at 3 inches. The 600 in the calculator is an example; use the yield printed on your kit.

How thick should spray foam be for my R-value?

Divide the R-value you want by the foam's R per inch. The U.S. Department of Energy's Building America guide puts open-cell foam at about R-3.6 per inch and typical closed-cell foam at R-6 per inch, so R-21 takes 3.5 inches of closed-cell or about 5.8 inches of open-cell. Your product's data sheet gives its exact figure — pick "Custom" to enter it.

Open-cell or closed-cell?

According to the same DOE guide, closed-cell foam is a vapor retarder and is usually sprayed in layers of 2–3 inches per pass, while open-cell can go on up to about 10 inches in one pass; a 5½-inch wall cavity filled with open-cell gives roughly R-20. Which one suits a wall or roof depends on your climate and the assembly — ask your installer or building department.

How much extra should I add?

Overspray, uneven layers and trimming mean some foam never ends up as insulation. The calculator adds 10% as an example — change it to suit your job and experience.

How many liters or cubic feet of foam is a board foot?

A board foot is 144 cubic inches, which is 2.36 liters, or 1/12 of a cubic foot. A kit that yields 600 board feet therefore makes 50 cubic feet of cured foam. Kit yields are nominal figures at a stated thickness; the manufacturer's data sheet explains the conditions.

Is the thickness the depth of the stud cavity?

Not necessarily. Foam is often sprayed to fill a cavity, but it can also be sprayed to a chosen depth, or over the framing. For a 2×4 wall the actual cavity is 3½ inches deep; 8 feet by 30 feet of wall filled to 3.5 inches is 840 board feet, 924 with 10% extra. Check what depth your product and the local code require.

The arithmetic

Spray foam is sold and priced by the board foot, one square foot of surface at 1 inch thickness. The calculator adds the areas of the sections you enter, multiplies by the thickness in inches to get board feet, adds your percentage for overspray and uneven coverage, and then divides by the yield of a kit, rounding up to whole kits. When you enter a target R-value instead of a thickness, the thickness is the target divided by the foam's R-value per inch. The calculator uses about 3.6 per inch for open-cell and 6 per inch for closed-cell foam, as given by the U.S. Department of Energy's Building America program, or the figure from your product's data sheet.

Worked example: a 1,200 ft² roof deck to R-30

With closed-cell foam at R-6 per inch, R-30 needs 30 ÷ 6 = 5.0 inches. The roof deck holds 1,200 × 5.0 = 6,000 board feet, 6,600 with 10% extra, which is 11 kits of 600 board feet. With open-cell foam at R-3.6 per inch, the same R-30 needs 8.33 inches, 10,000 board feet, 11,000 with 10% extra and 19 kits. If you add 20% instead of 10%, the closed-cell job grows to 7,200 board feet, 12 kits. The thickness needed for the same R-value is the main driver of cost, and the DOE guide notes that closed-cell is sprayed in layers of 2 to 3 inches per pass, so 5 inches is two passes.

Open-cell and closed-cell compared

Closed-cell foam is denser, has a higher R-value per inch, and acts as a vapor retarder. Open-cell foam is lighter, has a lower R-value per inch, and can be applied up to about 10 inches in one pass. Which is better depends on the climate, the assembly (wall, attic, crawl space) and the code in your area, and some jurisdictions require specific products or an ignition barrier over foam. This calculator only counts material; the building department or a qualified installer decides what is permitted. Also note that R-value per inch is a product figure that depends on the foam and its age, so the data sheet is the source to enter in the custom field.

Common mistakes

Entering the area of the floor when the foam goes on the roof deck, which is larger on a pitched roof, undercounts the job. Using the nominal cavity depth for deeper cavities, or ignoring the framing members that need to be covered, changes the board feet. Mixing the units is another: in metric mode the thickness is in centimeters and the area in square meters. Buying only the exact number of kits is risky because the yield of a kit varies with temperature and technique. And spray foam is not a do-it-yourself job for every product: the mixing, ventilation and re-entry times on the label must be followed.

Sources: U.S. Department of Energy, Building America, "Which Spray Foam Is Right For You?" (2009): open-cell foam about R-3.6 per inch, closed-cell typically R-6 per inch, layer thicknesses and the 5½ inch open-cell cavity; the board foot as 144 cubic inches; conversion 1 R = 0.1761102 RSI (NIST SP 811). Related: insulation calculator, attic ventilation calculator and roof calculator.

Sources: U.S. Department of Energy, Building America, “Which Spray Foam Is Right For You?” (2009); R to RSI — NIST Special Publication 811. Not sure what R-value you need? The insulation calculator gives it by climate zone. How we check our calculators.