Batt is insulation in the form of a sheet, rather than loose filling. Batts are pieces of blanket insulation pre-cut into 4 ft. or 8 ft. lengths. The choice between batts or blankets balances convenience against costs. Blankets are clumsy to install, but if many batts must be trimmed to fit, greater waste results. Batts or blankets are available faced or unfaced meaning they come with or without backing materials made of foil or paper. Backing materials can serve as vapor barriers. Foil backing works well as a vapor barrier, paper not as well unless it has been treated to reduce moisture permeability. For more details, see blanket insulation.