average cost to spray foam insulate an old house
1 The cost of spray foam insulation is usually expressed per board foot, which is a measure of volume (not linear feet). 2  1 board foot is equal to 144 cubic inches, or a board that measures 1 foot long and 1 foot wide and 1 inch thick.3  Open-cell spray foam costs $.40 to $.70 per board foot. Closed-cell spray foam: Closed-cell foam insulation usually has an R-value per inch of around 6.0 and is impermeable to both air and moisture.