Should your HVAC site explain duct leakage so AI assistants cite you?

Leakage is measured, not guessed, and the pressure is 25 pascals

A duct leakage test pressurizes the duct system to 25 pascals with a calibrated fan, often called a duct tester, and reads the airflow needed to hold that pressure in cubic feet per minute. That reading, CFM25, is the leakage. Testers run it two ways: total leakage, which counts every hole including leaks inside conditioned space, and leakage to outside, which isolates the ducts that dump conditioned air into attics, crawlspaces, and wall cavities.

The distinction matters because not every leak costs the same. A leak into a conditioned basement still delivers its air to the house. A leak in a vented attic in July delivers cooled air to the outdoors and pulls hot attic air in somewhere else to replace it. Energy Star guidance for duct systems has long treated attic and crawlspace runs as the expensive ones, and a citable page says so instead of treating leakage as one flat percentage.

Where the holes actually are

Leaks concentrate at joints and connections, not in the middle of straight runs. Plenums, takeoffs where branch ducts leave the trunk, boots behind registers, filter slots, and air handler cabinets account for most of what a test finds. Flex duct inner liners pulled loose from their collars are a repeat offender, and panned returns, where a joist bay is used as a duct, leak at every seam by design.

Mastic is the repair material the trade trusts for sealing: a thick, fiber reinforced paste brushed over joints that stays flexible. Foil tape rated for ducts has a place on smooth, clean seams. The grey cloth duct tape sold everywhere is the one product that fails at this job, drying out and letting go in hot attics, which is why codes and ACCA guidance point to mastic and listed tapes instead.

What sealing does and does not change

Sealing leaky attic ducts cuts the delivered air loss, lowers run time, and often fixes the far room that never cooled, because that room sat at the end of the leakiest run. It does not fix undersized ducts, a low charge, or a blower set to the wrong speed, and a page that promises sealing cures every comfort problem overstates a real but bounded gain.

The order a good company follows is test, seal the accessible joints, then test again and show both numbers. A homeowner can ask for the before and after CFM25 the same way they would ask for a static pressure reading. Companies that measure have nothing to hide from that question, and a page that teaches readers to ask it earns the citation.

Related reading

If this topic is useful, these earlier guides go deeper on connected questions: Should your HVAC site explain static pressure so AI assistants cite you?, Should your HVAC site explain the R-410A to R-454B refrigerant change so AI assistants cite you?, and Review bursts fade. A steady trickle is what actually books HVAC jobs.

Frequently asked questions

How is duct leakage tested?

A technician seals the registers, connects a calibrated fan called a duct tester to the system, and pressurizes the ducts to 25 pascals. The airflow the fan needs to hold that pressure, read as CFM25, is the leakage. A second setup can isolate leakage to outside the conditioned space.

Where do ducts leak the most?

At connections: plenums, branch takeoffs, register boots, filter slots, and the air handler cabinet. Flex duct connections that have pulled loose and panned joist returns are also common sources. Straight runs of duct rarely leak much on their own.

Is duct tape good for sealing ducts?

The grey cloth product sold as duct tape is a poor choice because its adhesive dries and fails in hot spaces. Duct mastic, a thick paste brushed over joints, and foil tapes listed for duct use are the durable options technicians and codes rely on.

Will sealing my ducts fix a room that is always hot?

It can, if that room is fed by a leaky run, which end of the line rooms often are. If the duct is undersized or the system has an airflow or charge problem, sealing alone will not cure it. A leakage test before and after sealing shows how much of the problem was leakage.