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Using my nifty thermal camera, I discovered that my furnace closet's combustion air duct (visible in this question) is getting really hot when the attic heats up, and then (being sheet metal) it radiates that heat into the furnace closet.

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When this duct gets hot, it radiates heat into a drywalled, uninsulated stud wall and heats up the adjacent room:

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I am already planning to insulate the wall cavity, but it also seems like if there's any place where a radiant barrier might work, it would be here: the duct is hot, but not actually touching the drywall surface that it is heating up. Does this sound sensible? Should I also cover the drywall with a radiant barrier to reduce the amount of heat it's absorbing in the first place?

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You could tape a strip of aluminum foil to a portion of the duct facing the attic and test again with your camera. –  getterdun May 22 at 1:11
    
Radiant barriers need an air gap to work (to give the heat a place to radiate to), so it would have to be taped to the drywall, not the duct itself. But yeah, I should probably give it a shot. –  iLikeDirt May 22 at 1:57
    
Aluminum has quite a low emissivity and high reflectivity in IR, so the camera will not tell you it's actual surface temperature unless it is taking these into account. You can test it by putting a piece of aluminum tape on a glass window: the tape will quickly be the same temperature as the glass, but will look much cooler. –  ArgentoSapiens Jun 2 at 17:28

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The answer is no. In order for a radiant barrier to be effective, it needs to absorb the heat but NOT be in contact with anything else. There's not enough room between the duct and the drywall to suspend a radiant barrier between them but touching neither of them. Instead, because it would have to be mounted to the drywall, it doesn't work, since it just gets hot and then conducts the heat right into the drywall--which is exactly what's happening now.

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