How to Seal Leaking Air Ducts

Ductwork acts as the circulatory system of your home, yet most of it is hidden away in basements or attics where leaks go unnoticed. When your joints and seams aren't sealed, you are essentially paying to heat or cool your crawlspaces, wasting up to 30 percent of your system's energy output. Properly sealed ducts don't just lower your utility bills; they improve indoor air quality by preventing the system from pulling in dust, insulation fibers, and basement mustiness. A job done well means a pressurized system that delivers air exactly where it belongs, ensuring your thermostat settings actually match the comfort level in your living spaces.

1. Find Every Leak First. Turn on your HVAC system to full blower speed. Use a stick of incense or a light piece of tissue paper to trace along the duct joints; the air movement will cause the smoke or paper to flutter at breach points.
2. Clean Metal, Not Ducts. Wipe down the area around every seam with a damp cloth to remove dust and grease. Sealants will not adhere to dirty metal, leading to premature failure.
3. Brush Mastic Over Joints. Use a stiff brush to apply a thick layer of duct mastic over the joints where two pieces of metal connect. Ensure the mastic overlaps both pieces of metal by at least one inch.
4. Mesh Bridges The Gaps. For gaps wider than a quarter-inch, embed a piece of fiberglass mesh tape into the wet mastic. Apply a second coat of mastic over the mesh to create a permanent bridge.
5. UL-181 Tape The Tight Spots. Use UL-181 rated foil-backed tape for smaller, hard-to-reach connections where a brush cannot fit. Press the tape firmly using a squeegee to ensure a bond across the entire surface.
6. Verify The Seal Holds. Allow the mastic to cure for 24 hours before turning your system back to its full settings. Re-check your marked spots with the tissue test to confirm no air movement is present.