How to Seal Air Conditioning Ducts

Sealing a ripped duct reduces the amount of energy needed to heat or cool a house.

A duct system delivers cooled or heated air from the furnace to each room. Many homes have leaking or disconnected ducts that rob as much as 20 percent of the air-conditioning system's efficiency. Tears in the duct's outer liner promote heat exchange in problem areas of the house, most commonly the attic and basement. Other problem areas include the furnace to plenum joint and where the ducts terminate at the registers. Cloth-backed duct tape alone will not seal a leaking air-conditioning duct.

Things You'll Need

• Duct straps
• Stapler
• Rag
• Metal-backed duct tape
• Duct mesh
• Knife
• Duct sealing mastic
• Brush

Instructions

Seal Your Ducts

1. Reconnect all loose duct connections. Secure a round duct's inner liner to its collar with a duct strap, then pull the insulation and outer liner over the connection. Push the fiberboard duct seam together, then secure it with a stapler.

2. Clean the leaking area with a rag. The duct-sealing materials will not bond permanently to a dirty or oily duct.

3. Apply a layer of metal-backed duct tape to each leaking duct joint or tear in the outer liner. Press the duct tape onto the duct.

4. Cover the metal-backed duct tape with two layers of duct mesh. Cut the duct mesh to size with a knife. Press both layers of duct mesh against the duct's surface.

5. Apply a one-eighth-inch layer of duct-sealing mastic to the duct mesh with a brush. Extend the mastic one inch past all sides of the mesh. Use fiberglass-approved mastic, usually white, on flexible ducts, fibered duct board seams and duct insulation. Use sheet metal-approved mastic, usually gray, on sheet metal ducts.

References

• Energy Star; Duct Sealing; February 2009
• The Encyclopedia of Alternative Energy and Sustainable Living: Duct Sealing
• Thermal Energy Distribution: An Introduction to Residential Duct Systems

Resources

• California Energy Commission: Ducts and Vents