Spray foam board-feet, kits & sets (open vs closed cell)
Spray foam is planned in board-feet — one board-foot is 1 ft² at 1 inch thick — and bought by the kit or set. Get the board-feet, divide by the kit yield, round up. The wrinkle is open vs closed cell.
Board-feet: the unit that runs the whole job
Foam is quoted and sold by the board-foot: 1 ft² sprayed 1 inch deep. So board-feet = area × thickness (in). A 500 ft² wall at 2 inches is 500 × 2 = 1,000 board-feet. Then sets = ceil(board-feet ÷ kit yield). Work it in the spray-foam board-feet calculator or the set / kit calculator.
Worked example: 500 ft² at 2 inches
board-feet = 500 × 2 = 1,000 bd-ft. With a DIY kit rated at 600 board-feet: sets = ceil(1000 ÷ 600) = ceil(1.67) = 2 kits. With small 200 board-foot kits it'd be ceil(1000 ÷ 200) = 5 kits. Same foam, different packaging — check the yield printed on the kit you're actually buying.
Kit and set yields
| Kit / set | Board-feet | Covers at 2" |
|---|---|---|
| DIY small kit | ~200 bd-ft | ~100 ft² |
| DIY large kit | ~600 bd-ft | ~300 ft² |
Bigger pro rigs yield far more per set. Full list in the board-feet per set table.
Open vs closed cell changes the depth — and the board-feet
The two foams hit R differently: open-cell ~3.6 R/inch, closed-cell ~6.5 R/inch. For a target R, depth = target R ÷ R/inch, so open-cell needs almost twice the depth — and board-feet track depth. For a 500 ft² wall to R-13:
| Foam | R/inch | Depth for R-13 | Board-feet |
|---|---|---|---|
| Open-cell | 3.6 | ~3.6 in | ~1,806 bd-ft |
| Closed-cell | 6.5 | ~2.0 in | ~1,000 bd-ft |
Open-cell is cheaper per board-foot but you spray far more of it; closed-cell costs more per board-foot, needs less depth, adds structural rigidity and a low perm rating. Compare both in the open- vs closed-cell calculator. Which to use is a design call — not a verdict this tool makes for you.
Real yield is lower than the label
Kit yields are nominal — they assume ideal chemical temperature and near-zero waste. In the field, cold tanks, off-ratio passes, trigger-drool and overspray eat into it. Buy a margin: round sets up and keep the last kit in reserve rather than planning to scrape it empty. Running out mid-wall with a half-cured pass is a bad place to stop.
Field discipline (and safety)
- Warm the tanks to the label temperature — cold foam under-yields and cures wrong.
- Spray in lifts (thin passes for open-cell; closed-cell in ~2" max lifts) so it cures and doesn't scorch.
- PPE: supplied-air or proper respirator, suit, gloves — the isocyanates are no joke; ventilate and honor the re-entry time.
- Ignition/thermal barrier: exposed foam usually needs a code-required cover. That detail is set by the manufacturer and your local code — check it, don't improvise.
Worked example: an attic in closed-cell
Foaming a 1,000 ft² cathedral or roofline attic to a real R-38 in closed-cell (~6.5 R/inch) needs 38 ÷ 6.5 = ~5.85 inches. board-feet = 1,000 × 5.85 = 5,850 bd-ft. In 600 bd-ft DIY kits that's ceil(5,850 ÷ 600) = 10 kits — and at that scale the DIY-kit price per board-foot is usually higher than hiring a rig, which is the point where most people call a contractor. Cavity depth also caps you: 2×10 rafters give 9.25", so the most closed-cell you can fit is 9.25 × 6.5 ≈ R-60, and open-cell in the same bay tops out around 9.25 × 3.6 ≈ R-33. Check the cap in the cathedral / roofline calculator.
Yield reality: temperature, ratio and waste
The label yield is a lab number. Three things erode it on your job: chemical temperature (tanks below the spec — often 70–80°F — foam under-expands and you get fewer board-feet), off-ratio passes (an emptying A or B tank sprays sticky or brittle foam that yields poorly and may need cutting out), and overspray and trigger-drool (foam on the drop cloth is foam off the wall). Plan on real yield 10–20% under nominal, keep the tanks in a warm space overnight, and finish the smaller of your two tanks first so a mid-pass swap doesn't leave an off-ratio streak. Round your kit count up and keep one in reserve.
Passes, thickness limits and cure
Depth per pass matters. Closed-cell goes on in lifts of about 2" max — thicker traps exothermic heat and can scorch or even smolder in the core; let each lift firm up before the next. Open-cell expands ~100× and can go thicker per pass but will keep growing, so plan to trim it flush with a saw. Both cure by chemical reaction, not by drying, so cure time is short but the fumes are the gate: honor the manufacturer's re-occupancy time before anyone goes back in without supplied air.
Quick numbers to leave with
- Board-feet = area × thickness (1 bd-ft = 1 ft² at 1"). 500 ft² at 2" = 1,000 bd-ft.
- Sets = ceil(board-feet ÷ kit yield). 1,000 bd-ft ≈ 2 large (600) or 5 small (200) kits.
- R/inch: open-cell ~3.6, closed-cell ~6.5 — open needs ~1.8× the board-feet for the same R.
- Real yield 10–20% under label — warm the tanks, round sets up, keep one in reserve.
- Lifts: closed-cell ~2" max per pass; honor the re-occupancy time.
Measure the area, pick your depth from the target R, multiply for board-feet, divide by the real kit yield, round up — and warm the chemical before you pull the trigger.