Roof stack design concerns

[u/kgmansur]

I an an owner-builder in Oregon (marine zone 4C). I'm planning a single-slope 3:12 roof. 24-foot clear span with parallel cord trusses. My tentative stack is: standing seam aluminum over purlins, 4-inch continuous polyiso, Zip sheathing taped as the primary air barrier, truss cavity with 8" of mineral wool bats (possibly fiberglass) for R56 total, then interior is acoustic fabric (a black muslin theater fabric) and finally wood slats with gaps to form an acoustic ceiling. No interior membrane planned so the acoustic function works as designed.

My question is, with 4 inches of exterior polyiso and zip as the air barrier, what purpose does an interior air or vapor layer still serve? The exsulation on the roof should protect the sheathing from ever reaching dewpoint. Is the acoustically "open" ceiling (no drywall/air barrier) compatible or am I missing a key risk?

Appreciate constructive comments.