Dave,
Ok, let's use a poly sheet or similar wrap for a moisture barrier. Where you gonna put it ?
On the inside to keep ambient moisture outside of the structure ? On the outside (like a house wrap) to contain interior moisture and keep any excess exterior from intruding ?
Next sub topic to consider: Metal roofing, what goes between the metal roofing and the sub roofing if anything ? Use the natural air flow of the formed "standing seam" channels topped with a vented ridge ?
deralte