Terms of the Trade: What Is Expanding Foam Insulation?

A construction worker applying spray foam insulation.

Spray foam insulation, also known as spray polyurethane foam, is an alternative to standard insulation, such as those made of fiberglass. The foam expands once its sprayed from the gun to be used to fill in wall cavities or where standard insulation would typically go, and then hardens. The R-value of spray foam varies, but it is generally favored to combat air leaks.