Duct Construction

Ductwork must be constructed in accordance with the International Mechanical Code. All joints, seams, and connections must be securely fastened with welds, 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.

• Exception:

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