How to Seal Ductwork Leaks

Ductwork is the circulatory system of your home, and like any system, it loses efficiency when it leaks. Most residential duct systems lose 20 to 30 percent of their conditioned air through disconnected joints, rusted seams, or holes near the air handler. When you lose that air into the attic or basement, you aren't just wasting money; you are forcing your furnace or air conditioner to work harder while struggling to keep your living spaces comfortable. Done well, sealing your ducts turns a drafty, inefficient system into a tight, responsive one. You aren't just slapping tape over holes; you are creating an airtight seal that holds up under the constant pressure and thermal expansion of a heating and cooling cycle. This is a straightforward task that relies on choosing the right materials and taking the time to reach the awkward corners where leaks love to hide.

  1. Find Every Leak First. Turn on your HVAC system to full fan mode. Feel around the joints and seams of your ductwork with your hand for moving air, or hold a piece of incense near suspected leaks to see the smoke pull or push away.
  2. Clean for Better Adhesion. Wipe down the metal surfaces around the leaks with a damp rag to remove dust, grease, and construction debris. Adhesive will not bond to a dusty or oily surface, so let the metal dry completely before proceeding.
  3. Coat with Mastic Generously. Apply a generous bead of water-based duct mastic over the joints, seams, and holes using a disposable paintbrush. Ensure the mastic overlaps the metal by at least an inch on both sides of the seam for a proper structural bond.
  4. Embed Mesh in Wide Gaps. For gaps wider than a quarter-inch, embed a strip of fiberglass mesh tape into the first layer of wet mastic. Once set, apply a second coat of mastic directly over the mesh to fully encapsulate the gap.
  5. Press Foil Tape Firmly. If you prefer using metal foil tape for tight seams, peel the backing and press it firmly over the joint. Use a plastic squeegee or the back of a screwdriver handle to burnish the tape until the texture of the duct underneath is visible.
  6. Confirm the Seal Holds. Wait for the mastic to cure, typically 24 hours, then turn the fan back on. Run your hand over the repaired areas to ensure no air is escaping through missed pinholes or edges.