How to properly seal ductwork


All forced-air heating and cooling HVAC systems require a vast ductwork system to carry the cold or warm air, depending on the season, throughout the house. How well the ductwork is sealed dictates how efficient the entire system really is. A high-efficiency furnace or air conditioning air handler will not perform as well if the ductwork is leaking at every seam. Properly sealing ductwork optimizes energy efficiency with forced air systems.

Things You'll Need

  • Metallic duct tape
  • Plastic glide
  • Duct mastic
  • Rubber/latex gloves
  • Mesh duct tape
  • Utility knife
  • Turn off the forced-air heating or cooling system. Give the ductwork about 10 minutes to get back to room temperature as this is the optimal condition when applying the tape.

  • Tape around all of the accessible seams of the round ductwork with metallic-faced duct tape. Make sure that the tape overlaps on the ends by at least 1 inch. Push the air bubbles out from underneath the tape with a plastic guide by running the edge of the glide along the face of the tape. Repeat this process a few times to remove all of the bubbles.

  • Tape the seams of all accessible square or rectangular ductwork with mesh tape. Press any creases or ripples out of the tape with the glide. This is very important as the next step will not work if the mesh tape is not fully attached along the seams. Tape around the supply plenum just above the furnace/air handler. Cut the corners at the top with a utility knife so that they can be folded flat.

  • Put on rubber or latex gloves and open the premixed bucket of duct mastic. Dip one of your hands into the mastic and spread it over the mesh tape. Don't be shy as there is no such thing as too much mastic. Repeat this process on all of the mesh-taped seams. Allow the mastic to dry for 8 hours before using the HVAC system.

Tips & Warnings

  • Hold a piece of cardboard under the area you are working with when you are spreading duct mastic. This will minimize clean up.
  • Be careful of the metal ductwork edges as they can be razor-sharp. Always apply duct mastic against the grain of the metal ductwork to avoid cutting.

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  • Photo Credit air conditioner, conditioning image by Greg Pickens from
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