Sealing A/C Ducts at the Registers

Old 08-16-02, 03:15 PM
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Sealing A/C Ducts at the Registers

I just had an A/C service done and I was told that I need to have the gap where the ducting meets the register sealed. There is a 1/4" to 1/2" gap between the duct and the Sheetrock. Their estimate was $25 a vent which included a three step process where a heat resistant mortar is applied to seal the gap. Is there any alternative method for sealing this gap that I can do on my own? He mentioned that although Silicone would be heat resistant, it does breakdown and it's not EPA approved.
Old 08-16-02, 09:38 PM
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If you ever sheet rocked, they use a strip of paper to seal the joints. For plastering they use a plastic mesh, the same size as the strip of paper they use with sheet rock. You want to buy this inexpensive plastic mesh. It comes in a roll. Then you want to buy MASTIC. There are different kinds of mastic. You want to read the applications on the container. Usually it will state multi-purpose. Meaning to say that it adheres to metal and wall board.

Apply the plastic mesh so that it connects the sheet rock and the sheet metal and covers the gap between them. Then with a putty knife, apply the mastic to the plastic mesh until to covers the sheet rock, sheet metal and gap. Let dry and you're done.
Old 08-18-02, 04:59 PM
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Thanks for the advice. Is the mastic the same material as joint compound? Also, the mesh tape that I have (used for sheetrock) is not adhesive. How do I apply it? Is this the same process as sheetrocking?
Old 08-18-02, 05:55 PM
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You need to use a vapor barrier type mastic. Sheetrock joint compound won't give you a good vapor barrier. Go to and click on products. You'll see they have an HVAC products section that contains the type of mastic oyu're looking for. The ar several brands- fosters, hardcast, rectorseal, airseal, etc. I tried the Home Depot website and didn't see it so you'll probably have to go to an HVAC specialty house to buy. Get the water based stuff. It cleans up easier and won't stink up the house. I usually put down a layer of mastic and then lay the mesh over that pressing in on somewhat, and let it set up for a bit before covering with a top coat.

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