How to use the spray foam calculator
- Measure the area. For walls, multiply the wall length by the height and subtract windows and doors. For an attic roof deck, measure the underside of the roof, not the floor. Not sure of the roof area? Use the roofing calculator.
- Choose the foam. Open cell foam is soft, cheaper and good for sound and air sealing inside walls and roof decks. Closed cell foam is rigid, gives almost twice the R-value per inch and also blocks moisture.
- Set the thickness or R-value. Pick the depth you plan to spray, or enter the R-value your local code asks for and the calculator works out the depth.
- Check DIY kits. DIY kits are rated in board feet. Real yield is often 10% to 20% lower than the label, so keep a little extra.
Spray foam formulas
R-value = thickness × R per inch
Thickness for a target = target R ÷ R per inch
Cost = board feet × price per board foot
Open cell vs closed cell
| Open cell | Closed cell | |
|---|---|---|
| R-value per inch | about 3.5 - 3.8 | about 6.0 - 7.0 |
| Installed price per board foot | $0.45 - $0.65 | $1.00 - $1.50 |
| Typical thickness | 3.5 in walls, 5.5 - 7 in roof decks | 2 - 3 in walls, rim joists, crawl spaces |
| Vapor barrier | No | Yes, at about 1.5 in or more |
| Best for | Interior walls, attics, sound control | Basements, crawl spaces, metal buildings |
Worked example
Spraying 1,000 sq ft of attic roof deck with 5.5 in of open cell foam takes 5,500 board feet and gives about R-20. At $0.45 to $0.65 per board foot, a contractor would charge about $2,475 to $3,575. With 600 board foot DIY kits you would need 10 kits. Reaching the same R-20 with closed cell foam takes about 3.1 in, or 3,077 board feet, which costs about $3,077 to $4,615 installed.
R-value guide
- 2 × 4 walls: 3.5 in of open cell is about R-13; 2 in of closed cell is about R-13.
- 2 × 6 walls: 5.5 in of open cell is about R-20.
- Attic roof deck: many codes ask for R-30 to R-60 in attics. Spray foam is often combined with other insulation to reach that.
- Safety: spray foam must be covered by an approved thermal barrier (such as drywall) in living spaces, and DIY spraying needs a respirator, goggles and full skin protection.
Lowering your energy bill? See how much you could save with the electricity cost calculator.
Frequently asked questions
How many board feet of spray foam do I need?
Multiply the area in square feet by the thickness in inches. 1,000 sq ft at 3 inches thick is 3,000 board feet.
How much does spray foam insulation cost?
Typical installed prices are about $0.45 to $0.65 per board foot for open cell and $1.00 to $1.50 for closed cell. A 1,000 sq ft attic roof deck with 5.5 in of open cell costs about $2,475 to $3,575.
What R-value is 2 inches of closed cell foam?
About R-13, at roughly R-6.5 per inch. Open cell foam needs about 3.5 inches to reach the same R-value.
How much does a 600 board foot spray foam kit cover?
A 600 board foot kit covers 600 sq ft at 1 inch, 300 sq ft at 2 inches or 200 sq ft at 3 inches. Expect real-world yield to be 10% to 20% less.
Is open cell or closed cell spray foam better?
Closed cell gives more R-value per inch and blocks moisture, so it is better for basements, crawl spaces and tight spaces. Open cell is cheaper per R-value in deep cavities and absorbs sound better.
More home project calculators
Last updated October 2026.