Cletis - I'd say the air space is between the sheets of foil, the bubble insulation, and that's why the double bubble type with 3 layers of foil is best ... I have it on the ceiling of daBus and just recently while working on adding the rock wool insulation up there I was quite pleased to notice the large difference in temp between the inside Reflectix surface and the metal roof as I pulled edges tight, must have been 10 degrees or more
if there's a place around you can buy a foot or 3, try it on your roof or just on a piece of metal in the sun
here's a pic of what I did
Bill
