Spray foam kits are sold by board feet: one board foot is one square foot of foam one inch thick. Enter the area and thickness; the calculator gives the board feet, the number of kits and the R-value you will reach.
Spray foam formula
Board feet = area (sq ft) × thickness (in) Kits = board feet × (1 + loss %) ÷ kit yield, rounded up R-value = thickness × R-value per inch
Worked example: 200 sq ft of rim joist at 2 in
- Board feet: 200 × 2 = 400.
- Add 10% for overspray: 440 board feet.
- DAP’s Touch ‘n Foam System 200 yields up to 200 board feet, so buy 3 kits.
- Closed-cell foam at R-6.5 per inch gives about R-13.
Open-cell vs closed-cell
Oklahoma State University Extension gives open-cell foam about R-4.2 per inch and closed-cell about R-6.5 per inch. Closed-cell is denser, adds rigidity and resists moisture, which makes it the usual choice for rim joists and below grade. Open-cell is cheaper per board foot and expands more, so it is often used to fill wall and roof cavities.
Why allow for loss
Kit yields are measured under ideal conditions. Cold chemicals, uneven passes and overspray all reduce the real yield, so a 10–15% allowance is sensible. Follow the kit’s temperature and safety instructions; ventilation and protective equipment matter with spray foam.
Questions
How many board feet are in a 600 kit?
600 board feet: enough for 300 sq ft at 2 in, before loss.
How thick should spray foam be?
It depends on the R-value you need. Divide the target R-value by the foam’s R-value per inch.
Is spray foam a vapor barrier?
Closed-cell foam at sufficient thickness acts as a vapor retarder; open-cell does not. Check the product data sheet.
Sources
- Touch ‘n Foam Low GWP System 200, DAP.
Used for: Kit yield of up to 200 board feet. - BAE-1410: The Importance of Residential and Commercial Building Insulation, Oklahoma State University Extension.
Used for: R-value per inch for open- and closed-cell spray foam.