I have 1" of Styrofoam SM on the outside of my exterior walls, and R12 (3 1/2'' inches) of Fiberglass insulation in-between the stubs.
Can I add another 1" layer of Styrofoam SM on the inside of the wall (1" Styrofoam / 3 1/2" Fiberglass / 1" Styrofoam) or will this trap moisture between the 2 layers of styrofoam.
Also will I need to still use a plastic vapor barrier with this.

Thanks Ralph Hackert