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I have a rectangular hot air duct in my garage that is sheetrocked. A neighbor pulled down his sheetrock and it was filled with black mildew probably from the air conditioning. He left it plain metal as several other neighbors have a bare metal duct in their garage. It makes for a nice warm garage in the winter but I want to pull down my sheetrock and put 2" foam around the rectangular duct.

How should I fasten the foam board? Construction adhesive? Contact cement? Should I hold it with metal duct tape dealing the edges with the same? Should the ends be sealed with caulking? Type? Then paint the foam white?

Or should I build a wooden frame and use 1/4", 3/8", sheetrock?
 

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Not a pro with hvac, but the sheetrock might be required as a fire barrier between house and garage. Fumes from vehicles are carefully isolated from living space.

Bud
 

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Does it supply the garage or simply pass through the garage?
If it supplies, FYI: Section M1601.6 of the International Residential Code says, 鈥淔urnaces and air handling systems that supply air to living spaces shall not supply air to or return air from a garage鈥.
It was in the NY code far previous to the writing of the IRC. Not sure about your state, but they are usually similar.
If it just passes through, ok. However, most codes prohibit exposed foam and require at least covering it with gwb.
 

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You will be able to copy the frame work you have now or post pictures of what you have.

The drywall on the ceiling of the garage is 5/8" fire rated.
 
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