Insulation borrowed the lumber unit: one board foot of foam = one square foot of surface, one inch deep. Enter the area you're spraying and the depth you need — get board feet and how many DIY kits that is.
Two-part spray foam kits are rated by board-foot yield — the same area-times-depth unit lumber uses, detached from wood entirely. The common DIY sizes are 200 bf and 600 bf kits: a 600 bf kit does 300 ft² at 2 inches, or 200 ft² at 3 inches, in ideal conditions. Manufacturers rate yield at ideal temperature and technique; cold canisters, dusty substrates and first-timer trigger work all eat into it, which is what the waste percentage above is for — 10% is optimistic, 15–20% is honest for a first project.
Closed-cell foam runs roughly R-6 to R-7 per inch and adds structural stiffness and a vapor retarder at ~2 inches. Open-cell runs about R-3.7 per inch, is cheaper per board foot, and breathes. To size depth, divide your target R-value by the per-inch rating: an R-21 wall wants ~3″ of closed-cell or ~5.5″ of open-cell (which may not fit a 2×4 bay — check your stud depth). Codes vary by climate zone; confirm targets with your local inspector before buying kits.
R-values per manufacturer spec sheets (typical ranges); yields are manufacturer-rated at ideal conditions.
| Open-cell | Closed-cell | |
|---|---|---|
| R-value / inch | ~R-3.7 | ~R-6–7 |
| Cost / bf (kits) | lower | higher |
| Vapor retarder | no | yes, at ~2″ |
| Adds rigidity | little | noticeably |
| Best at | interior walls, sound, big volumes | rim joists, crawl spaces, thin bays |
Match the foam to the assembly, then size depth from the R-value you owe your climate zone — the U.S. Department of Energy's insulation guidance lists recommended R-values by zone and application. Then the board-foot math above turns that depth into kits. Pro jobs are quoted per board foot too, typically with minimums; getting your own bf number first keeps those quotes comparable.