Board foot (spray foam)
Also called: board feet, bd ft, bf
The unit spray foam is priced and sold in: one square foot of surface covered one inch deep. Multiply the area in square feet by the depth in inches. A 600 board-foot kit covers 300 sq ft at 2 inches; an attic roof deck at 7 inches is 7 board feet per square foot.
Contractors quote spray foam per board foot — $0.60–$1.20 for open-cell, $1.30–$2.50 for closed-cell installed — because the cost is in the volume of foam, not the area it covers. That’s why “how much per square foot” has no single answer: 2 inches of closed-cell on a garage wall is 2 board feet per square foot ($2.60–$5.00), while 7 inches of open-cell on a roof deck is 7 ($4.20–$8.40).
To size a job: area × depth. A rim joist is the basement perimeter in feet × the joist depth in feet (a 2×10 is 0.79 ft) × inches of foam. A wall is perimeter × height minus openings. An attic roof deck is the floor area × 1.15–1.3 for pitch. Then add 15–30% on a DIY kit, because rated yield assumes ideal temperature and technique.
Not to be confused with the lumber board foot, which is 144 cubic inches of wood — the same volume, a different trade.