Polyurethane or spray foam insulation, like loose-fill insulation, can be sprayed into closed wall cavities and irregular spaces, and like rigid foam board insulation, it does not need a vapor barrier. Spray foam insulation has the additional advantage of being airtight; after it's sprayed into a space it expands to fill the space and adheres to all surfaces. This means no additional work has to be done to seal gaps where air could leak through. High-density polyurethane has a high R-value as well -- up to 7 per inch -- while low-density polyurethane has an R-value of 4 and is less rigid than the high-density foam. Despite the lower R-value, contractor Tom Silva of ThisOldHouse.com prefers low-density polyurethane foam because "it flexes enough to accommodate seasonal wood movement."
Spray foam insulation is expensive, in part because it must be installed professionally (although you can buy spray cans of polyurethane foam to seal around doors, windows and vents in your home). Experts say you should expect to pay about $0.07 per square foot per R-value for low-density foam and $0.15 for high-density foam, not including installation. The higher R-value of high-density polyurethane means you do not need to use as much of it. Another way to save money, according to construction pros, is to install a layer of spray foam along with a less expensive type of insulation; the foam will provide a vapor barrier and air sealing and the other material will add R-value.
Spray foam insulation is used in walls, crawlspaces and attic floors. Like loose-fill, it can be sprayed into closed wall cavities through small holes that are then sealed. Polyurethane spray foam is not flammable, though building codes usually require a layer of wallboard or plaster as a fire barrier, since it could release toxic gases during a fire. Polyurethane, a petroleum-derived product, is polyiso mixed with a resin. A soy-based resin is used to make a more environmentally friendly, low-density polyurethane foam with an R-value of about 4 per inch. Polyurethane itself is not recyclable, but it contains a small percentage of recycled material.
Need help figuring out how much insulation and what R-value you'll need? See our What to Look For page for details on those calculations, plus installation considerations and other factors to consider.