Green Building Library
Weatherization Tips

How to Seal 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.

Step 1
Reconnect Loose Ducts with Screws

If ducts are coming apart at connections or feel loose, push the pieces together
...then install three self-tapping sheet metal screws to secure the connection.

Connect flexible duct to metal fittings by pulling back the insulation and installing sheet metal screws or plastic straps so the spiral ribs of the duct liner are held against the fitting. Apply tape or mastic over the entire connection.


Step 2
Support Ducts to Wood Framing

Support Ducts to Wood Framing Support metal ducts by looping wire around the duct once and staple or nail the wire ends to wood framing.

Locate supports every 3 feet.

Don't use wire to support flexible ducts. Instead use a wide fabric strapping material. Staple the material to wood framing. Flex duct may require supports every two feet to keep it from sagging.

If it is impossible to secure the duct to framing in a tight crawlspace, support it with bricks or concrete blocks stacked on the ground.


Step 3
Seal Ducts with Mastic

Surface of duct must be clean.  If necessary, use solvent. Apply the mastic according to manufacturer specifications. For large gaps use both mesh tape and mastic. See the diagram on the sidebar to learn where to seal.



Use mastic designed for sheet metal. Select a mastic that has a consistency similar to mashed potatoes so it will be easy to apply by hand. Mastic can be applied by gloved hand, brush, trowel or caulking gun. Insulation can be installed over mastic that is still wet.

Mesh Tape

This tape is designed to be used with mastics. It reinforces the mastic when there is a gap of 1/4 in. or more. If the tape has a sticky side, apply it over the clean metal surface. Then apply the mastic.

If the mastic does not have a sticky side, first apply a thin layer of mastic, then place the tape in this layer of mastic and add another layer of mastic over the top so the tape 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.

Webbing strap is available from heating and air conditioning suppliers.

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© Copyright 1993 Iris Communications, Inc.

Where to Seal Duct Systems

Tubular Foam

Disconnected ducts


Building cavities used as duct

Plenums and air handlers

T's, Y's and L's


Straight joints