How to Seal Leaky Ductwork: 7 Steps

Most ductwork is surrounded by fiberglass and located in dusty, dirty areas. Before working on your ducts, protect yourself with long sleeves, pants, gloves, safety glasses, and a dust mask.

Before starting your project, identify problem areas in your ductwork. Inspect for holes, damage, disconnections, and poor sealing. Pay special attention to areas with excessive dust around the vents, as this can indicate a leak.

Duct sealing products will not adhere properly to dust and dirt. To prep ducts for repairs, remove the insulation, then use a rag dipped in a mild cleaning solution to clean the area around the leaks. Scrub a few inches past the gap in every direction.

Mastic is a thick substance that dries to create a strong but flexible seal over holes, seams, and joints. Use a paintbrush or putty knife to apply a thick layer of mastic over small leaks, ensuring the coverage extends a few inches over the leak in all directions to create a strong seal.

Use foil tape along longitudinal seams or areas where the ducts connect. Cut a piece of tape large enough to cover the entire seam. Extend it past the edges of seams or overlap the tape on joints. Push the tape onto the duct, working to ensure there are no wrinkles. Firmly smooth the tape out to remove bubbles and ensure maximum adhesion.

Avoid using your heating or cooling system for 24 to 48 hours to allow the mastic to cure. Once the mastic has dried completely, run your HVAC to test it. Use a smoke pencil or incense at nearby vents to check if your seals worked properly. If the smoke escapes, apply more mastic or tape, allow it to dry, then test again.

Measure your ducts and cut fiberglass insulation to wrap around them. Secure the insulation with small pieces of foil tape before applying a long strip along the seam.