Mastic is a sealant paste applied to joints and connection in ductwork, and can be used on cracks up to 1/4” wide. Use fiberglass mesh to help seal holes larger than 1/4” wide. Foil tape and butyl-backed foil tape can be used to seal holes and cracks as well. To achieve a good seal for a long time, it is important that ducts be sealed with mastic and fiberglass mesh where required.
