Duct leakage occurs when conditioned air escapes from supply ducts or unconditioned air enters through return duct leaks before reaching its intended destination. The EPA estimates the average home loses 20-30% of conditioned air to duct leakage — a massive energy waste.
Leaks commonly occur at duct joints, connections to registers, plenum connections, and anywhere ductwork passes through walls, floors, or ceilings. Ducts in unconditioned spaces like attics and crawlspaces lose the most energy because the temperature difference between the duct air and surrounding air is greatest.
Duct leakage testing uses a blower door and duct pressurization equipment (Duct Blaster) to measure total leakage. The proper seal for duct leaks is mastic sealant (a paste applied to joints) or UL 181-rated foil tape — never standard cloth duct tape, which fails within a few years. Professional duct sealing can reduce energy bills by 20% or more.