Vapor Open Roof Assembly in Zone 6 – GreenBuildingAdvisor

We want to make a vapor-open, vented cathedral ceiling assembly (so we can add a loft when we have money sometime in the future) for a ~1100sf home in zone 6 (western NY on top of a hill, so plenty of wind and cold winters). We are trying to keep costs down so will opt for a shed roof. Our builder suggests i joists. We would particularly like to avoid spray foam. We need to be at least R49 to meet code, and would go higher if it means a better, more moisture-resilient assembly.

Our builder likes to use fiberglass in roof assemblies and wants to avoid blown cellulose in particular. We will have vents at top and bottom of the assembly, but I’m stuck on how to create site-built ventilation channels. I wonder if we could use purlins instead of sheathing and integrate the venting that way? Also, will we have issues with thermal bridging? I would welcome ideas and criticisms!

Potential Assembly:

Metal roof

Plywood sheathing

Venting – somehow!

Dupont Tyvek Commercial vapor-open membrane (OR Solitex Mento)

I Joists with approximately 14″-17″ Corning EcoBatt Fiberglass

Siga Myrex continuous air barrier (or Intello)

Service cavity/2x4s

Sheetrock