Mastic duct sealant is a thick, nonhardening substance used to seal seams in duct systems. Its use helps prevent leaks that lead to energy waste and unequal heating among rooms.
Mastic duct sealant is applied to seams using a paintbrush, putty knife or gloved fingers. Any gaps up to 1/8-inch wide are coated only with mastic. Larger openings must first be covered with fiberglass mesh tape. Screws are used to fix most joints firmly before spreading mastic. If the ducts run through fiberglass board, those joints are first secured with fiberglass mesh tape. In most cases, mastic provides longer-lasting seals than duct tape.