How spray foam is measured
Spray polyurethane foam is sold and priced in board feet: one board foot is one square foot of foam, one inch thick. To estimate a job, multiply the area by the thickness in inches, then add a margin for overspray, uneven application and trimming.
Board feet = area (ft²) × thickness (in) × (1 + waste)
DIY two-component kits are labeled by their theoretical yield, typically 200 or 600 board feet. Real yields are often lower in cold conditions, so many installers add 10 to 15%.
Open-cell or closed-cell?
| Closed-cell | Open-cell | |
|---|---|---|
| R-value per inch | about 6 to 7 | about 3.5 to 3.8 |
| Density | about 2 lb/ft³ | about 0.5 lb/ft³ |
| Air barrier | yes, from about 1 in | yes, from about 3.5 in |
| Vapor retarder | yes, from about 1.5 to 2 in | no |
| Typical installed price | about $1 to $1.50 per board foot | about $0.45 to $0.65 per board foot |
Closed-cell foam is denser, adds some racking strength to walls and resists water, which makes it suitable for basements, rim joists and exterior applications. Open-cell foam is cheaper per R-value in deep cavities such as rafters, and it absorbs sound better.
Example
500 ft² of wall at 3 in of closed-cell foam (R-6.5 per inch):
- 500 × 3 = 1,500 board feet, or 1,650 with 10% waste;
- three 600 board-foot kits;
- R-value: 3 × 6.5 = R-19.5, above a target of R-19;
- installed cost at $1.25 per board foot: about $1,875.
How much R-value do you need?
The 2021 IECC recommends, depending on climate zone, roughly R-13 to R-20 (or R-13 plus continuous insulation) for walls and R-30 to R-60 for attics. Enter your target R-value to get the foam thickness needed. Spray foam also stops air leaks, so it often performs better than its R-value suggests compared with batts.
Practical points
- Closed-cell foam is applied in passes of about 2 in at most to control heat buildup.
- Most codes require a thermal barrier, such as ½ in drywall, over foam in living spaces.
- Isocyanates in uncured foam are respiratory sensitizers: installers need supplied-air respirators, and occupants should stay out until the manufacturer’s reentry time has passed.
- Price estimates vary widely by region and job size; use local quotes for budgeting.