Spray Foam Board Feet and Kits
Board feet and kit count for closed-cell and open-cell foam at your thickness and area, the R-value you get, the barrier the code needs, and what the job costs in material.
200 sq ft at 2 in closed-cell is 400 board feet of cured foam, which is 2 of the 200 bf kits and 3 with the 10 percent waste. Closed-cell at 2 in measures R-13, and an interior conditioned application needs the 15-minute thermal barrier.
How the math works
- Board feet: 1 board foot is 1 sq ft of surface at 1 in of finished thickness, so the cured volume is area times thickness and open-cell and closed-cell count the same way. Kits are sold on the cured yield: the 200 bf closed-cell kit covers 200 sq ft at 1 in and 100 sq ft at 2 in, and the 600 bf open-cell kit covers 600 sq ft at 1 in and 200 sq ft at 3 in.
- R per inch: closed-cell at the aged (5-year, L-2008.2) value is R-6.5 per inch, and the new-install value is about R-7.0; open-cell is R-3.6 per inch. Closed-cell at 2 in is R-13, and open-cell at 3.5 in is R-12.6.
- Barrier rules (IRC 2009.2 and 2009.3): foam in a conditioned space needs a 15-minute thermal barrier, commonly 1.5 in of intumescent paint or 1.5 in of gypsum board; an unconditioned attic needs an ignition barrier, and 1 in of foam with the right profile qualifies for the ignition-barrier exemption. Exterior roof applications take a coating, not a barrier.
- Kits: the 200 bf closed-cell kit (a 2.2 gal set) and the 600 bf open-cell kit are the DIY unit; split foam is by the pound for rigs. Two 200 bf kits plus a cleaner, the gun, the hoses, and the PPE is the standard weekend kit for a rim joist, a band, and a sill line.
- Placement: closed-cell is the vapor retarder and the structural adhesive at 2 in, and open-cell is the vapor-permeable, sound-absorbing wall fill at 5 in in a 2x6. In a cathedral assembly, closed-cell against the deck with the barrier under it is the unvented configuration.
How many board feet of spray foam do I need?
Area times thickness in inches: 1 board foot is 1 sq ft at 1 in. 200 sq ft at 2 in closed-cell is 400 bf of cured foam, which is 2 of the 200 bf kits, 3 with the waste allowance, because a 200 bf kit covers 100 sq ft at 2 in.
What is the difference between closed-cell and open-cell foam?
Closed-cell is 1.7 to 2.0 lb per cu ft, R-6.5 per inch aged, a Class II vapor retarder at 2 in, and adds about 65 psi of compressive strength and structural adhesion. Open-cell is 0.5 lb, R-3.6 per inch, vapor-permeable, quieter, and cheaper per R-value in a wall cavity.
Do I need paint over spray foam?
Yes, on the interior side of a conditioned space: IRC 2009.2 asks for a 15-minute thermal barrier, and the manufacturer intumescent paint at 1.5 in, or 1.5 in of gypsum, is how that gets done in a wall. Unconditioned attics take an ignition barrier, and 1 in of the right foam profile qualifies. Exterior foam takes a UV coating.
Is closed-cell or open-cell better for a rim joist?
Closed-cell at 2 in on a rim joist is the job: R-13 in a 2 in band, the air seal at the sill plate, and enough rigidity to hold it in place. Open-cell at 2 in gives R-7 and soaks up more product per inch. Rim joists are the highest-return foam job in most houses.