I live in climate 5b, but I am wondering about any cold climate. If the assembly is vented, there is drying potential to the exterior. But with foam board on the inside, drying to the interior becomes limited or non-existent. I know that if foam board is applied, the seams should be taped or otherwise air sealed.
In the BSC research report on high R-value roof assemblies, they list one type of vented cathedral roof with interior foam board insulation, but the roof has just 1 inch of XPS, which has a permeability of about 1.1. (roof 3C in the report)
http://www.buildingscience.com/documents/reports/rr-1006-ba-high-r-roofs-case-study-analysis
With more rigid insulation, or with a more vapor closed option, such as polyiso, does this place the assembly at a higher risk? Even with the venting- can the inward vapor drive be a concern here when the roof is hot during the summer or other sunny days?
A similar question- Roofs 3A and 3B in the report use foam board towards the exterior, but below the venting, in this case using the top rail of an engineered I-joist to place the foam board. Wouldn’t this create a condensation plane that is cold enough to allow for condensation (especially in roof 3A- with 1 inch of xps) The first condensation plane in 3B might have a well controlled temperature, but all layers of the foam would need to be well air sealed to prevent convection currents and condensation further up wouldn’t they?
I know that vented cathedral assemblies can work, but it seems that each of the 3 scenarios laid out in the BSC report are risky in one way or another. (an in the report, the durability ratings were only 2 or 3 out of 5)