How to Seal Air Ducts (Stop Leaks, Save Money, Breathe Easier)
If your home has hot and cold spots, stubborn dust, or rising energy bills, the problem may be duct leakage. When an air duct has gaps, your HVAC system can push conditioned air into the attic, basement, or crawlspace instead of your rooms. Leaks often show up at joints and seams, plus where ductwork meets the plenum and air handler.
This guide explains how to seal air ducts with a simple DIY sealing process, and the clear signs it’s time to hire an HVAC pro.
Find duct leaks first (where air escapes most)
Most homeowners find duct leaks in unconditioned areas: attic runs, basement ceilings, and crawlspace trunks. Start at the air handler cabinet and the plenum, then follow the duct system outward. Check elbows, takeoffs, and connections on both metal duct and flex duct. Look for loose collars, crushed flex, and old tape that’s peeling.
Quick checks help you spot air leaks fast:
- Hold a tissue strip near seams while the air conditioning runs; fluttering shows airflow escaping.
- Use incense or a smoke pencil to trace leaks around joints.
- On a metal duct, brush on soapy water; bubbling can point to leaks.
Leaks reduce airflow at registers, waste conditioned air, and can pull in dust, insulation fibers, and allergens that hurt indoor air quality.
Quick safety checklist before you start
Turn off the power to the HVAC system at the breaker, then wait for the cooling equipment to stop. Wear gloves and safety glasses, and use an N95 if you may touch fiberglass insulation. Only seal ductwork, not combustion vents, flues, or anything that is not part of the duct system.
DIY duct sealing: mastic, foil tape, and the right sealant
Good duct sealing is a basic home improvement, but it needs the right materials. Here’s a simple sealing process that works.
First, clean surfaces so the sealant sticks. Wipe off dust and oil. Remove failing duct tape, because cloth duct tape fails on ducts when heat and pressure cycles loosen the adhesive.
Next, seal seams and joints with mastic sealant (duct sealant). Use a disposable paintbrush to coat every joint, seam, and connection, especially at the plenum and near the air handler. For bigger gaps, press fiberglass mesh into the wet mastic, then cover it with another layer of mastic. This makes sealing ductwork durable.
Use foil tape (also called metal tape) on flat seams, sharp edges, or to help hold insulation in place. Choose foil tape that is Underwriters Laboratories listed for HVAC use. For small gaps at register trim, silicone or caulk can help, but mastic is the main duct sealant for the air duct itself.
Seal, then insulate ducts in unconditioned spaces
Seal first, then add insulation to boost energy efficiency. In an attic or crawlspace, wrap ducts with foil-faced fiberglass insulation, then tape seams with foil tape. You’ll often notice steadier thermostat control and lower energy bills, especially on long runs (see our blog on attic HVAC tips).
Test your results and avoid common duct sealing mistakes
After the mastic cures, re-test with smoke or the tissue strip.
Common mistakes include skipping cleaning, using cloth duct tape, missing plenum connections, sealing only supplies, forgetting returns, and running the system before mastic fully cures. When leaky ducts are fixed, many homeowners notice more even room temperatures, stronger airflow, and less dust and allergens.
When DIY is not enough
Call a pro for leaky ductwork you can’t reach (behind walls), large disconnects, or comfort problems that stay after DIY, especially near gas appliances. Pros can run duct leakage testing and may use an aerosol method like Aeroseal to seal hidden leaks from inside the duct system.
ENERGY STAR explains why sealing matters and what to expect.
Scott – HVAC Project Manager
Reviewed by Scott, Bryant-Certified HVAC Project Manager – 17 Years of Experience
Scott brings 17 years of HVAC experience to his role as Project Manager at Superior Service. He is Bryant Certified and specializes in designing and overseeing heating and cooling solutions that keep homes efficient and comfortable year-round. Customers appreciate Scott’s ability to guide projects smoothly from start to finish.
Contact Superior Service
The best way to seal air ducts is straightforward: find duct leaks, clean the surfaces, apply mastic and UL-listed foil tape, then insulate and test. Done right, duct sealing helps your HVAC system deliver conditioned air where it belongs, improving comfort and indoor air quality.
If you’re in the St. Louis region and want expert help with sealing ductwork, testing, or professional sealing for hard-to-reach ductwork, contact Superior Service for HVAC ductwork service and duct sealing.
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