How to Seal Leaking Air Conditioning Duct


Leaking air conditioning ducts waste energy and cause high utility bills. Rather than spending an excessive amount on utilities or immediately calling a professional, consider sealing the leaky ducts yourself. If you do the work yourself, you will not only save on future cooling costs, you will also come out ahead financially on the labor side of the project.

Things You'll Need

  • Duct tape
  • Duct sealing mastic
  • Brush
  • Panduit straps
  • Pliers
  • Fab mesh
  • Examine the air conditioning duct system for leaks or loose flex connections. At the thermostat, turn the fan switch to the "On" position. Feel around each joint and every connection. Do not forget to check around the inside unit for leaks -- this is a common spot. Any leaks should be repaired for maximum efficiency.

  • Repair flex connections first. If the connection has completely fallen off, remove the old panduit strap. That is the large plastic zip tie that holds the inner part of the flex tube in place. If there is one on the outer layer, remove that also. Pull back the outer layer and insulation to reveal 8 to 12 inches of the inner layer. Slide the inner layer onto the boot connection. Use duct tape to tape the inner liner in place. Use a pair of pliers to install a new panduit strap. Pull the outer layer down and install a panduit strap. Paint the connection with a duct dealing mastic.

  • Repair ripped flex next. Pull the rip together. Use duct tape to hold it in place. Stretch fab completely over the tape. Large rips may require more than one piece of fab. Mastic all fab.

  • Repair leaking air conditioning boxes next. Stretch a piece of fab mesh across the leaking seam. Paint the fab with duct sealing mastic. A staple gun can be used on stubborn seams. Large gaps can be filled with insulation then use fab and mastic to seal.

  • Seal the air handler. The main cause for leakage here is that the tape that was used has become unglued due to moisture. Cut away the loose tape. Wipe everything clean. Stretch fab around the air conditioning unit. Apply mastic. If the unit sets on a platform, paint the platform and up the side of the unit about 3 inches. When painting the unit do not seal the panels, as they are needed for access.

Tips & Warnings

  • Use gloves
  • Be careful when in the attic so you don't fall through.

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  • Photo Credit Comstock/Comstock/Getty Images
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