Repairing Heating and Cooling Ducts

Unsealed or poorly sealed ducts can be one of the biggest energy and money losers in a home. Remember: insulation wrapped around ducts does not stop air leaks.

How to Seal Air Ducts with Mastic

Mastic seals better and lasts longer than tape. For an experienced installer, mastic is faster to apply than tape.

1. Clean Duct Surface. Use a cloth to wipe dust from the surface of the duct. If oily film or grease covers the duct, wipe clean with a damp cloth.

2. Joints with Less than 1/4 Inch Gaps. Load brush with mastic. Coat the entire joint with a continuous strip of mastic. Use end of the brush to work mastic into joint. Spread mastic at least one inch on each side of the joint. Mastic should be thick enough to hide the metal surface of duct -- about 1/16 inch thick.

3. Joints with Gaps Greater Than 1/4 Inch. If the gap in the duct connection is larger than 1/4 inch use fiberglass reinforcing membrane in addition to mastic. If the membrane is sticky on one side, cut enough membrane to cover the joint, press the membrane in place, then cover with mastic. Apply enough mastic to completely cover the membrane. If the membrane does not have a sticky side, first apply a thin layer of mastic, press the membrane into the mastic, then apply the finish layer of mastic.

4. Wrap Ducts with Insulation. RCD Masticsdry to touch in 2 to 4 hours. Insulation can be installed over wet mastic. But try not to move the ducts too much, because the mastic seal could be damaged. All duct support work should be done before applying mastic.

Round to Rectangular Transitions

Fiberglass reinforcing membrane wrinkles when it's run along a joint between round and rectangular ducts. Here's how to make it lie flat. With a utility knife, make a series of slices about every two inches. When wrapped around the round duct the slices will fan out like a "hoola skirt."

When a Caulking Gun Works Best

Sometimes spreading mastic with a brush does not provide good control over the mastic applied. Where appearance and wasted mastic are a concern, a caulking gun works best. See diagrams below to learn where to seal.


Use a UL 181A-M and UL 181 B-M Listed mastic designed for the type of duct being installed or repaired. Mastic can be applied by gloved hand, brush, trowel or caulking gun. Insulation can be installed over mastic that is still wet.

Fiberglass Reinforcing Membrane

This product is designed to be used with mastics. It reinforces the mastic when there is a gap of 1/4 inch or more. If the membrane has a sticky side, apply it over the cleaned metal surface, then apply the mastic. If the membrane does not have a sticky side, first apply a thin layer of mastic, then place the membrane in this layer of mastic and add another layer of mastic over the top so the membrane is completely covered.

Duct Supports

Flexible ducts can easily be pinched by wire or twine supports so air flow is restricted. The best support material is woven polypropylene strap that comes in various widths. Sometimes called "webbing strap," this material is wide enough so it won't bite into flex duct. For convenience, use it for both metal and flex duct support.

Tools & Supplies

  • 2 Inch Brush
  • 3 Inch Brush
  • Cloth or Towel
  • Fiberglass Membrane
  • Utility Knife
  • Inspection Mirror
  • Tin Snips

Content provided by RCD Corporation