Mastic vs. Foil Tape: How Ducts Should Be Sealed

Despite the name, standard cloth-backed tape dries out and releases, usually within a few years in an unconditioned space.

Finding it on a system is a reliable sign the joints underneath need attention.

UL 181 rated foil tape is legitimate on clean, dry, accessible metal joints when applied to a properly prepared surface.

It is fast and neat, but it depends entirely on surface prep and adhesion.

Mastic is brushed on, fills irregular gaps, and cures into a flexible permanent seal that tolerates thermal movement.

On plenum joints, takeoffs and anything that will be buried behind insulation, it is the correct choice.

Clean the surface, mechanically fasten the connection, apply mastic with fiberglass mesh on gaps wider than about an eighth of an inch, then restore insulation.

A joint sealed this way should outlive the equipment it serves.