Green Building Library
Weatherization Tips
How to Seal Ducts with Mastic
- Mastic seals better and lasts longer than tape.
- For an experienced installer, mastic is often faster to apply than tape.
Step 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 cloth soaked in denatured alcohol.
Step 2
Apply Mastic to Joints With Less Than 1/4 in. Cracks
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 the duct—about 1/16 inch thick.
Step 3
Apply Mesh Tape to Large Joints Before Applying Mastic
If the crack in the duct connection is larger than 1/4 inch, use mesh reinforcing tape in addition to mastic. If the tape is sticky on one side, cut enough tape to cover the joint, press the tape into place, then cover with mastic. Apply enough mastic to completely cover the mesh tape. If the mesh tape does not have a sticky side, first apply a thin layer of mastic, press the tape into the mastic, then apply the finish layer of mastic.
Step 4
Wrap Ducts With Insulation
Most mastics dry to touch in 1 to 2 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
Tape 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 "hula skirt."
When a Caulking Gun Works Best
Sometimes spreading mastic with a brush does not provide good control over the amount of mastic applied. Where appearance and wasted mastic are a concern, a caulking gun works best. Most manufacturers offer mastic in gunnable tubes.
Tools & Supplies
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© Copyright 1993 Iris Communications, Inc.
