How to Repair Leaks in A/C Ductwork

Leaky ductworks are a problem because they waste energy and raise your home’s heating and cooling costs.

Fortunately, fixing this problem couldn’t be easier, and it requires minimal expense.

Here’s how to seal holes or tears in ductwork:

Locate Leaks: Turn on the HVAC fan and touch the ductwork. Feel it up and down and check for drafts that indicate escaping air.

Seal Leaks: Turn off the fan and apply special metallic foil tape over the hole or gap. Don’t use standard duct tape, since the adhesive will come loose over time. Purchase the metallic foil tape that’s specifically made for ductwork repairs.

Apply Duct Mastic: Use a brush to apply duct mastic over the tape and joints to improve the seal.

Watch this video to find out more.

Leaks in ductwork are often found where lines branch off or separate. With the furnace fan running, you can feel air escaping the duct. Once you’ve found the leak, use foil duct tape to cover the gap and seal the leak.

For an extra layer of protection, use a paintbrush to apply a coat of duct mastic. This thick, sticky material will dry to form an airtight seal that will keep the air in those ducts, where it belongs.

Further Information


  1. what is an RAPP boot? I have this notes on a set of specs.
    what do you recommend for lighting rods which penetrate a metal roof?

  2. Apartment was bug bombed in the trash room and inside of walls. I want to know if it can get into the AC or ducts causeit has a chemical odor I used bleach to clean coils can I get rid of it with water and brush?

  3. I have noticed that if my heat or AC is on it runs smoothly with no problem. However after few minutes of running (approximately 15 minutes) the system automatically stops running and there is a heavy banging sound that is immediately heard above the ceilings where the ducts are . this large banging sound can be heard through out the house. This banging sound does not come on when the system automatically starts running again but will do only when it is about to stop. What can I do to stop this heavy banging sound in my house while the AC/ Heat system is operating?

  4. In May of last year I had a new a/c unit installed in an older home in central Phoenix. On a property inspection tenants complained the two bedroom in that house were colder than the rest of the house. On an inspection in the attic two lengths of ductwork leading to bedrooms, spanning approximately 12 feet, have been partially crushed due to workers stepping on them. The ductwork is approximately 24 inches wide with a height of 18 inches. Is there a way to fix the damaged portion using some kind of inflatable bladder inside the duct work or does it have to be completely replaced. Any idea of cost would also be greatly appreciated. Thanks,Cw

    • Hi, Cw,
      Replacing just the crushed section and seaming it into the rest of the ductwork is your best bet.
      The cost will vary, based on location, but an HVAC tech could probably do it in an hour or so.
      Good luck with this project!

  5. We have a continuous tapping in our 2021 Mobile Home!! It is throughout the home but mainly in primary bath floor. Where do we start? Ductwork, plumbing, expanding or contracting? This started in the colder weather we are having! Who would we hire to help us? Know condescension in duct work… this is so irritating!! Please help.

    • Hi, Brenda! We’re always looking for homeowners to call into our radio show and ask questions directly to Danny and Joe. We’ve reviewed your question and shared it with our radio show’s producer.
      Thanks for writing! 🙂


Please enter your comment!
Please enter your name here