Duct Sealing Materials
Materials used to seal ducts should last as long as the ducts themselves. The most commonly used material, cloth duct tape, is not recommended because its adhesive dries out and the tape falls away. Many foil duct tapes also do not have quality adhesives.
The recommended duct sealing materials include duct sealing mastic, UL-181 aluminum tape, and high quality caulking or foam sealant. Mastic is the most effective material to seal all types of ductwork and can be purchased in tubs or caulk tubes. Mastic looks like drywall joint compound and dries in hours. It sticks well to most surfaces and requires little preparation.
For larger gaps, a fiberglass mesh tape serves as a backing, and is then covered with mastic. Mastic and better grades of duct tape can be purchased from heating and air conditioning supply houses which cater to commercial contractors. These better grade tapes cost more than standard products. However, cheap tape is no bargain as it can lose its seal within months.