What Is Duct Leakage?

Duct leakage is conditioned air escaping from supply ducts, or unconditioned air being drawn into returns, through gaps at joints, boots and plenums. It is measured as leakage at a test pressure and is addressed by sealing with mastic or approved tape, not cloth duct tape.

Also called: leaky ducts · duct loss · duct sealing

Why it matters to you

Where the ducts run determines how much this costs you. In coastal construction, runs in vented crawl spaces and hot attics mean leaking supply air is lost outright, while leaking returns pull humid crawl-space or attic air directly into the system — importing exactly the moisture the system is working to remove.

What to do about it

Rooms that never get comfortable, or a system that runs constantly without keeping up, are classic symptoms. Sealing accessible joints is worthwhile, and testing tells you whether the remaining leakage justifies more work.