Duct leakage test: what it measures, and why the real number is usually 20–30%

A duct blaster — a calibrated fan connected directly to the duct system with the vents sealed off — pressurizes the ducts to a standard test pressure and measures how much air has to keep flowing in to hold that pressure. Whatever air the fan has to keep supplying is air escaping somewhere the ducts weren't supposed to let it out. The result gets reported as a leakage rate, either as a percentage of total system airflow or in cubic feet per minute at the standard test pressure.

A duct leakage test can also distinguish leakage to the outside — into an attic, crawlspace, or exterior wall, the version that actually wastes conditioned air — from leakage between conditioned spaces inside the house, which affects comfort and balance but doesn't waste energy the same way.

Duct systems are built from sections of metal or flex duct joined together, and every joint, seam, and connection is a potential leak point — a house with dozens of runs branching to different rooms accumulates dozens of these joints, and even a well-installed system rarely seals all of them perfectly with tape and mastic alone. Ductwork run through an unconditioned attic or crawlspace is the worst case, since air lost there is heated or cooled air paid for and then lost to outside temperatures entirely, not just redistributed inside the house.

Older duct tape — the actual adhesive tape, not the fabric-backed cloth kind that shares the name — is a common failure point specifically, since it dries out and loses adhesion over years in a way mastic sealant and mesh tape don't.

Leakage doesn't just waste energy in the abstract — it directly explains two common complaints homeowners bring to a service call without connecting them to the ductwork: a system that runs longer than it should to hit the same temperature, and rooms that never quite match the thermostat's reading no matter how the system is adjusted. Both are duct leakage doing exactly what it does, just showing up as a comfort problem instead of a number on an energy bill.

A rating on a spec sheet is not a rating in your duct system — even a correctly rated high-efficiency furnace or AC still loses whatever share of its output leaks out before reaching a room, which is why sealing ducts is sometimes the higher-value fix compared to replacing equipment that's already working correctly.

Duct sealing typically runs $500 to $1,500, depending on how much of the system needs access and how extensive the leakage is. That's a real range, not a flat number — a single accessible run with a few loose joints is a different job than a whole attic system sealed room by room.

The return shows up two ways: lower utility bills from air that's no longer being paid for and lost, and more even temperatures room to room, since leakage upstream of a specific vent is air that room was supposed to get and didn't. Sealing existing ducts is also almost always cheaper than replacing them, and worth doing before assuming a comfort problem requires new ductwork entirely.

None of these prove duct leakage on their own — several have other causes — but any of them is a reasonable reason to test rather than guess:

Duct leakage is the efficiency problem nobody quotes on, because sealing ducts doesn't sell equipment the way a new furnace or AC does. ENERGY STAR's 20-to-30-percent figure for a typical home is a real, checkable number, not a scare tactic — and sealing is often a lower-cost fix than the equipment upgrade a related comfort problem gets blamed on first.