if the seal behind the FRP is so important it sounds like I should but vapor barrier on it too? But that goes against what I've always been told about totally sealing up a wall.
Vapor barrier goes on the warm side, in this case the outer side of the insulated wall. But isn't it the same as a house were you want the cool side able to breath a bit to remove what moisture leaks in the wall ?
I'm just speaking from what I have seen first hand. FRP holding pockets of water. Not pretty some of it coming down. If you glue it I would suggest rolling on the glue like laminate. Also many homes now use Tyvek on the outside.