The cost of spray foam insulation is usually expressed per board foot, which is a measure of volume (not linear feet). 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. Open-cell spray foam costs $.40 to $.70 per board foot. Closed-cell spray foam costs $.75 to $1 per board foot. Expressed in terms of square feet, spray foam installation cost is $1.50 to $3.00 per square foot (for an average-sized home between 2,000 to 2,500 square feet, that's a total cost of $3,000 to $7,500.