Spray Foam Calculator

Spray Foam formula

Area (sq ft) = Length (ft) x Height (ft) - Openings (sq ft)
Board feet = Area (sq ft) x Thickness (in)  (1 board foot = 1 sq ft at 1 in thick)
Order quantity = Board feet x (1 + waste %);  Kits = Order quantity / kit size (600 or 200)
R-value = Thickness (in) x R per inch  (closed-cell 6.5, open-cell 3.7)

Worked example

A 24 x 24 ft garage ceiling sprayed with open-cell foam 5.5 in thick to fill 2 x 6 rafters: 24 x 24 = 576 sq ft. 576 x 5.5 = 3,168 board feet. With 10% waste, 3,168 x 1.1 = 3,485 board feet, so 6 kits of 600 board feet or 18 kits of 200.

The R-value is 5.5 x 3.7 = R-20.35, about R-20. The cured foam is 3,168 / 12 = 264 cubic feet, which at 0.5 lb per cubic foot weighs 132 lb. A job this size is usually cheaper from a spray foam contractor, who prices by the board foot.

Quick reference

How to calculate spray foam

Spray foam is bought and priced by the board foot: one square foot at one inch thick. Measure the area you will spray, subtract doors and windows, and multiply by the thickness in inches. A 2 x 4 wall cavity is 3.5 in deep, a 2 x 6 cavity 5.5 in, and a 2 x 10 roof rafter 9.25 in, but closed-cell foam is rarely sprayed full depth because 2 to 3 in already gives R-13 to R-20 and an air seal.

Add at least 10 percent for waste. Kit yields are measured under ideal conditions, and cold cans, a cool substrate or a slow gun hand all reduce the foam's expansion. If the kit sits partly used, the foam left in the hose and nozzle is lost, so plan to empty each kit in one session.

Closed-cell vs open-cell

Closed-cell foam is dense, rigid and gives the most R-value per inch. It stops water vapor at 1.5 to 2 in, adds racking strength to a wall and is the choice for rim joists, crawlspace walls, basements and anywhere the cavity is shallow. Open-cell is soft, roughly half the R-value per inch and less per board foot, and it is used to fill full-depth wall and roof cavities where thickness is not the limit.

Both are combustible and must be covered with a thermal barrier such as 1/2 in drywall in living space. Ventilate during and after spraying, wear a respirator and full protection, and keep the area clear until the foam has cured. Large areas are usually cheaper from a contractor with a rig than from DIY kits.

Key facts

• A board foot of spray foam is one square foot sprayed one inch thick, so 12 board feet make a cubic foot.
• Closed-cell foam cures at about 2 lb per cubic foot and delivers R-6 to R-7 per inch; open-cell cures at about 0.5 lb per cubic foot and delivers R-3.5 to R-3.8 per inch.
• Two-component DIY kits are sold in 200 and 600 board foot sizes; the rated yield assumes a 1 in pass on a warm, clean surface.
• Closed-cell foam becomes a Class II vapor retarder at about 1.5 to 2 in, which is why it is used on crawlspace walls and rim joists without a separate vapor barrier.
• The IRC requires a 15 minute thermal barrier, normally 1/2 in gypsum board, between spray foam and any living space, and an ignition barrier in attics and crawlspaces.

Frequently asked questions

Sources and references

• •Froth-Pak Insulation Spray Foam product information sheet (R-value, density and board-foot kit yields) (DuPont)
• •Which Spray Foam Is Right For You? Open-cell and closed-cell SPF properties, Building America (U.S. Department of Energy)
• •Chapter 3 Building Planning, Section R316 Foam Plastic (thermal and ignition barriers), 2021 IRC (ICC)