i'm working on my garage to studio conversion and on one out-facing
wall i'm having problems installing 2"-thick rigid polyiso foam board
insulation. I'm leaving the existing plywood-faced wall as is, so i'm
attaching the board to plywood. only, i can't get a flat fit against
the plywood. some of the 4'x8' insulation board is warped and i can't
screw it flat because screws go right through the foam. even the
flatter foam has some warp to it. is there a way to get it right solid
next to the ply? i mean, even if i glued it, how'm i going to keep
everything tight together while the glue dries?

second question: this stuff is backed with foil-kraft-foil on the heat
facing side and foil on the other side. isn't that the same as having
two vapor barriers? is this going to be a problem?

btw/ i am going to be covering the foam with 1/2" sheetrock.

anyway, i'd appreciate any help i can get!