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