Benefits are lower heating / cooling bills and a comfortable indoor environment.
Spray foam can be used as both an insulating and air sealing product for wall and ceiling cavities. The foam is sprayed into cavities and expands to fill all the nooks and crannies. In open wall application, excess foam is scraped off the studs to form a uniform wall cavity.
This is primarily a new construction technique, however it also has good retrofit applications with attic conversions, in basements and crawlspaces, and in more intensive remodeling projects when your walls are opened for other reasons.
Spray foam insulation makes it easy to completely fill wall cavities with insulation and to perform air sealing in the same step, and eliminates the need for separate air-tightness detailing. Spray foam can also be used primarily as an air-sealing component, and followed by less expensive insulation such as cellulose to provide the needed R-value.
There are two types of spray foam: open-cell and closed cell. The closed cell foams typically have a higher R-value than open-cell foam, and closed cell foams are generally less vapor permeable, and thus serve as a vapor barrier as well as an air-barrier.