Most residential duct systems have numerous leaks that waste energy and lead to room-to-room pressure imbalances. Unfortunately, though, few building inspectors outside of California bother to enforce existing code requirements that residential duct seams be sealed with mastic or high-quality duct tape.
Most model codes, including the International Residential Code (IRC), include duct tightness provisions:
To learn how to test residential duct systems for leaks, see Duct Leakage Testing.
All about mastic
Most energy-conscious builders seal duct joints with mastic. Mastic is a gooey, non-hardening material with a consistency between mayonnaise and smooth peanut butter. Duct joints should always be secured with #8 sheet-metal screws before seams are sealed with mastic.
Sealing duct seams is messy work, so wear old clothes. The mastic is spread over duct seams with a disposable paintbrush, putty knife, or your fingers. (If you spread mastic with your fingers, wear rubber gloves.)
Gaps in ductwork or plenums that are over 1/16 or 1/8 inch wide can be sealed with mastic as long as the gap is first reinforced with fiberglass mesh tape. If you’re using mastic to seal seams in fiberglass board ductwork, use fiberglass mesh tape for all joints.
Sources of mastic
Among the distributors of AirSeal #22 mastic is AM Conservation Group.
All about duct tape
Since common hardware-store duct tape — technically known as cloth-backed rubber-adhesive duct tape — fails quickly when used on ducts, most energy-conscious builders seal duct joints with mastic. Although mastic works well on galvanized steel ductwork, it has its disadvantages: it is messy to apply and awkward to use on clamped flex duct joints.
According to section 503.3.3.4.3 of the International Energy Conservation Code (IECC), any tape used on duct board…