Duct Construction

Ductwork must be constructed in accordance with the 2003 International Mechanical Code. All joints, longitudinal and transverse seams, and connections must be securely fastened and sealed with welds, mechanical fasteners with seals, gaskets, mastics (adhesives), mastic-plus-embedded-fabric, or tapes.

"Duct tape" has a lot of great uses, but sealing ducts isn't one of them, so it is not permitted. Tapes and mastics used with rigid fibrous glass ducts must be labeled in accordance with UL 181A. Tapes and mastics used with flexible air ducts must be labeled in accordance with UL 181B.

Duct sealing in a commercial building. Flex duct is used for the last 2-4 feet for sound dampening.

Articulated duct joint sealed with a flexible, water-based mastic.

Exception:

Continuously-welded and locking-type longitudinal joints and seams on ducts operating at less than 2 in. w.g. (water gauge) (500 Pa).

Additional Resources

Aerosol Duct Sealing