Hybrid interior wall insulation system – GreenBuildingAdvisor

My background is custom home construction/historic preservation and restoration. I had a pre-fab 2 story garage/studio erected (Amish company out of PA.) on my property. 28 x 32 slab, 2 courses of concrete block perimeter. 1st floor framing is 2 x 6, 2nd floor is attic trusses w/14 ‘ open space (7’ ceiling height) the length of the building. I furred out the gable truss triangles on each end to create 2 x 6 bays for insulation.

At this point with all my “stuff” moved in I can’t bring a foam contractor in to shoot 2″ of polyiso in the walls. It would be a nightmare to empty the building at this point. My plan is to use 2″ of fiber-faced (not foil) polyiso foam board in the walls initially, so I can at least work comfortably in the building this winter, and then run all the wiring and then fill the remaining 3 1/2″ with Ruxol or hemp insulation or fiberglass. I’d cut the foam sheets 1/2″ light on all 4 sides and spray foam that 1/2″ gap for an airtight seal. The exterior is sheathed/sided with a one layer T-111 type product which came pre-painted. I’m in zone 6 or 6a in zipcode 12461. These are my questions:

* Is 2 ” of foam correct to abate the dewpoint inside the wall cavity once I have the entire 5 1/2″ filled?

*Is fiber-faced and not foil faced polyiso the correct choice?

*Should I treat the interior side of the sheathing with a vapor retarder or barrier?

* I’m thinking no polyethylene between the wallboard and the insulation to allow for moisture to migrate to the interior. Correct?

* Best technique/product for attaching the polyiso boards to the interior side of the sheathing?

*The top chords of the roof trusses are 2 x 8s. Best way to insulate that area? Pack it with polyiso foam boards (fiber-faced) tight up against the roof sheathing? There is a ridge vent, and ventilated soffits.