How Aerosol Duct Sealing Works

What is aerosol duct sealing?

Aerosol duct sealing is a contractor process that seals a duct system from the inside by blowing a fog of sealant particles through it. Lawrence Berkeley National Laboratory built it for the leaks hand sealing cannot reach: ducts inside walls, between floors, or buried in a low attic corner. The LBNL research summary credits Mark Modera's group and says the project moved out of the laboratory in 1995.

The sealant is not a liquid. LBNL explains that liquids run, so the team chose a commercially available vinyl acetate delivered as sticky solid particles. The method is now sold commercially; Valley Home Ducts names no company.

How does sealant find the leaks?

The sealant finds leaks because leaks are the only way out. Before injection, the floor, wall and ceiling registers are covered, so air pushed into the ducts can escape only through gaps. Particles riding that air try to leave through each gap, stick first to its edges, then to one another, and the hole closes from the rim inward.

LBNL describes injecting for about an hour, with a computer calculating air loss and the sealing progress in real time. That running number is the useful part for a homeowner: the process starts and ends with a measured leakage figure, so you get a before-and-after reading instead of a promise.

By LBNL's account, the buildup happens where air is escaping, so the sealant collects at the leaks rather than lining every inch of sound duct.

What did the field tests show?

LBNL's field tests sealed some 80 percent of the leakage in more than 100 single-family houses, from New England to the Gulf Coast to the Nevada desert, and register flows increased up to 7 percent. Those are the lab's 1998 figures, reported in its own research review, not a forecast for any one house.

The lab's later slide deck, Remote Duct Sealing in Residential and Commercial Buildings (hosted by DOE, file dated 2008), adds three claims: the method seals holes up to about 1/2 inch across, the seal lasts 10 or more years, and the technique was patented in 1994. Read those as the developer's claims, since the slides also describe licensing the method to a company.

The same slides explain why attic ducts are the target. LBNL puts attic and crawlspace duct leakage at roughly 15 percent on each side of the fan and says that for insulated ducts outside the house, about 90 percent of the lost energy is not recovered. Air that leaks into a Valley attic in July is simply gone.

What can't aerosol duct sealing fix?

Aerosol duct sealing cannot fix disconnected ducts, large holes, crushed or kinked flex, poor support or a duct system that is the wrong size. DOE Building America states plainly that it cannot seal large holes or disconnects. A branch lying loose in the insulation will simply pour fog into the attic.

That is why a careful job starts with an attic inspection. A contractor reconnects fallen runs, patches big openings with sheet metal, and straightens crushed flex before injection; aerosol then handles the many small leaks left behind. If the system is undersized or the return is starved, sealing will not correct that; the Manual D sizing page covers design.

Asbestos is a stop sign, not a variable. The process pressurizes the ducts and measures leakage, and the EPA's Energy Savings Plus Health guidelines (2021) say not to run air leakage tests on ductwork that contains asbestos insulation. Suspect wrap or tape needs an accredited asbestos professional first.

Is aerosol duct sealing worth it for an attic system?

It is worth pricing when much of the leakage is out of reach, and less so when most joints sit in an open attic a contractor can crawl. DOE Building America's scoping advice says that if very few sections of duct are accessible, an aerosol sealant may be worth considering. In a single-story Valley ranch house with ducts strung across an open attic, hand sealing with mastic can reach most joints for less.

Many jobs combine the two: hand repairs for disconnects and big gaps, aerosol for the rest. Leakage limits still apply either way. When the air handler, outdoor unit or coil is replaced, Title 24 requires the connected ducts to test at 10 percent or less total leakage, or 7 percent or less to outside, per the 2025 code text on Energy Code Ace. A state-certified ECC-Rater, the title that replaced HERS Rater in the 2025 code, verifies the result; see the changeout page.

What should you ask before aerosol sealing?

Ask for numbers and for the safety steps by name. Useful questions:

1. What is the measured leakage now, in CFM, and what was the reading at the end?
2. Will disconnects, crushed runs and large holes be fixed by hand first?
3. Will someone run a combustion safety test before and after, if the house has a gas furnace or water heater?
4. If this is part of an equipment changeout, who arranges the ECC-Rater verification?

The third question matters. Tightening ducts changes air pressures in a house, and the ENERGY STAR duct sealing flyer tells contractors to check for backdrafting and run a combustion safety test after ducts are sealed. If you're unsure, call a licensed technician. Questions for any duct contractor are collected on the hiring page.

• 80%of leakage sealed in LBNL field testsLBNL
• 7%rise in register airflow, at mostLBNL
• 1/2 in.largest hole the method seals, LBNL's figureLBNL slides
• 90%of attic duct energy losses not recoveredLBNL slides

The hub for sealing and insulation compares aerosol with every other option, and the leakage test page explains the readings a contractor should hand you.

Questions from the field