Flash And Batt. The first step in the flash and batt approach is to apply a thin layer 1 to 2 inches of a two-part polyurethane closed cell spray foam like Dow Froth Pak to seal the surface and provide the initial R value. Flash and Batt or flash and fill refer to the method of installing a 14 of spray foam to seal all of the air leaks and to prevent the warm interior from contacting the cold rim joist.
But whether the spray foam actually becomes a vapor retarder depends on the type of foam used. Another concern is that in a heating climate the flash-and-batt method creates a vapor retarder on the wrong side the cold-in-winter side of the fiberglass batt. It is cheap simple to install if not easy to install well and readily available.
Its a technique used by some insulation contractors where a flash of closed cell spray foam at 1- to 2-inches is applied in an effort to create an air seal and then puts fiberglass batt insulation over it.
Flash and batt begins with a thin flash coat of foam sprayed against the inside of the sheathing. The reason for the combination is that the spray. There are advantages to this strategy that provides two different insulation products spray polyurethane foam and then a traditional batt on the inside. It is cheap simple to install if not easy to install well and readily available.