Revisions log for Answer #662

It's important to have an air level with circulation between the roof cladding and the insulation, otherwise the insulation will get wet in the summer. (Air outside is warm, inside the garage is cool -> condensation on the vapor barrier side of the insulation).

Also, don't use a simple vapor barrier, but a variable moisture vapor barrier. Simply put, this opens up the pores on the warm side (roof) in the summer and lets moisture in on the cold side, inside the garage. There are also flat roof products. In this case, you have to be on your guard.