Sorry to chime in late but I wouldn't recommend the HVAC insulation Armacell or Armaflex as they will hold water. I am an HVAC technician by trade for 30 years now and I've seen it first hand. If I were you I would go with the thin sheet foam Ebb suggested and put it in with Vinyl Mastic made by Polyken. It comes on a 2 or 4 inch roll like duct tape but its thick with a removable backing and strong sticky but do your install in warm or hot weather to insure the bond. and theres only the mess of the backing to clean up and alot cheaper than liquid adhesives or epoxy .