Zone 4A – Cathedral attic insulation at roof valley – GreenBuildingAdvisor

Hi all

I am in the process of building a two story, wood framed, L-shaped house in climate zone 4A. One part of the building will have the underside of the roof insulated to create a “tempered” cathedralized attic space within the thermal insulation envelope. The other parts of the building will have conventional unconditioned, ventilated attics. The roof has GAF Grand Manor shingles installed over Certainteed RoofRunner underlayment. The underlayment has a low vapor permeance of 0.06 (so a class 1 vapor retarder).

I am (stupidly) now realizing that the low end of the jack rafters in the cathedralized attic will have no vent due to the obstruction created by the valley rafter. I am apprehensive about creating a “hot roof” and sandwiching the plywood roof sheathing between the low-perm RoofRunner underlayment and closed cell spray foam. I also have concerns about using open cell in the rafters due to potential vapor diffusion and condensation on the underside of the sheathing. I thought about adding ventilation baffles in the jack rafters to at least provide an air gap and some airflow to the ridge vent, but am unsure if that would be effective.

Anyone have some insight or experience?

Thanks – Rob