Spray foam is unmatched in its insulating qualities. The closed cell foam is best as it has trapped inert gases in it that resist most types of energy movement including radiant, convection and conduction. This means it saves a lot of heat energy. The greatest advantage to spray foam is its ability to expand into tight crevices where it would be difficult to insulate using conventional methods. The disadvantage of spray foam is the initial cost of application. Of course you need to research all the benefits of spray foam and decide if the benefits are worth the costs. Spray foam is best in areas such as the rim joists and other hard to reach areas. Often the bottom side of cantilevers is also spray foamed. Call a local foam contractor for more details.