Homes built before 1980 were rarely equipped with adequate insulation, according to the National Association of Realtors, and even modern homes may not have enough insulation to keep out the cold. Spray foam insulation represents one of the most effective ways to add insulation to an existing home, but spray foam is also used in new construction in place of traditional fiberglass. Depending on how much insulation you have now, adding spray foam can cut energy bills by 10 percent to 50 percent.
Understanding Spray Foam
Polyurethane spray foam insulation comes in two basic varieties, distinguished by their physical makeup. Open cell foam is the more common of the two; it's used in most dry areas such as attics and wall cavities. It has an R-value of 3.5 to 3.6 per inch. Closed cell foam, which has an R-value of 6.5 per inch, is safe to use in wet areas such as basements. Thanks to its higher R-value, it's also the better choice for maximizing thermal resistance in narrow or tight spaces.
Insulation and Board Feet
Unlike many other types of insulation, spray foam is typically measured in board feet rather than square feet. Understanding board feet makes it easier to compare prices and figure out how much your project will cost. One board foot equals an area measuring 12 inches by 12 inches by 1 inch. To find board feet, multiply the square footage of an area by the depth in inches. For example, a wall measuring 10 feet high by 50 feet high has square footage of 500. If the wall cavity is 3.5 inches thick, you need a total of 1,750 board feet of spray foam insulation.
Price Per Board Foot
Open cell spray foam cost between $0.35 and $0.55 per board foot on average, in 2014. Closed cell ranged from $1.00 to $1.70 per board foot in open spaces and as high as $2.00 per board foot in a confined space.
It may be helpful to know the price per square foot of spray foam to help you better compare this material to other types of insulation. Open cell insulation cost about $1 to $1.20 per square foot; closed cell insulation rang in at $1.75 to $3 per square foot as of 2014. Fiberglass batts cost between $0.70 and $1 per square foot, while cellulose ranged from $1.17 to $1.50 per square foot as of 2014.