Insulate once. Insulate right.

Spray foam insulation, in plain terms

Spray foam insulation is spray polyurethane foam (SPF): two liquids mixed at the spray gun that expand into foam and stick to studs, rafters, concrete and metal. Because it fills gaps as it expands, it both insulates and seals air leaks in one step, which is why it is used where air leakage is hard to stop with batts, such as rim joists, roof decks and crawl space walls.

Open-cell vs closed-cell

Open-cell foam is light and soft, at about R-3.6 per inch; closed-cell foam is denser and rigid, at about R-6 per inch^[1]. Closed-cell also resists water, and the Department of Energy says open-cell should not be used below ground level^[3]. See open-cell vs closed-cell spray foam.

What it costs

Contractors usually price by the board foot: one square foot, one inch thick. HomeAdvisor's national installed range is $0.60 to $1.60 per board foot for open-cell and $1.30 to $3.10 for closed-cell^[2]. Try the spray foam cost calculator or read the full cost guide.

Safety and codes

Most of the documented risk is during spraying and curing, when the chemicals can irritate the lungs, skin and eyes and cause sensitization^[5]. CPSC staff advise leaving the home during and for at least 24 hours after installation^[4]. Foam is combustible, so codes generally require a thermal barrier such as half-inch drywall over it^[3]. See is spray foam insulation safe? and spray foam building codes.