I posted this on the heating & cooling forum, but I thought I might get a few more responses here.

Problem:

I have a register boot in the ceiling that has inaccessible attic space above it. In fact, the attic space is so tight that when I remove the boot and push it into the attic there is very little room before it hits a rafter. The boot was insulated with fiberglass insulation when it was installed but I think that there are air cavities that are letting the hot, humid outside air come in contact with the boot and condensate to the point of running down the outside of the boot and into the living space. The boot is connected to R8 flex duct which is not condensating. Because of the tight fit, I am not sure how to properly insulate it without ripping out the recently installed wood ceiling.

My idea:

I came up with a crazy idea of drilling a number of small holes in the boot and injecting foam insulation. I would then cover the holes with foil tape. My hope would be to create several inches of airtight insulation around the boot. Any opinions on whether this will work or not?