Open cell foam insulation is a type of foam where the tiny cells are not completely closed. It is a very good air barrier but does not provide any type of water vapor barrier. It is much more sponge-like in appearance. It is often used for interior walls because it provides sound reduction by damping the movement of existing insulation. It is not recommended for outdoor applications or where moisture may be present.

Closed cell foam insulation is much denser than open cell. It has a smaller, more compact cell structure. It is a very good air barrier as well as a water vapor barrier. It is often used in roofing projects or other outdoor applications, but can be used anywhere in the home.