Can I re-insulate my cathedral ceiling using the stack-of-pancakes approach with a vent?
I have a gable roof with a cathedral ceiling built with 2×8 rafters 24” on center. There is a ridge vent already installed and the roof assembly to the north of the ridge has soffit vents while the assembly to the south ends in the main attic space which is also ridge and soffit vented. I have spent several hours reading through the “musings” and other online resources and decided to move forward with the stack-of-pancakes method. I have not, however, come across an example of a vented stack-of-pancakes and wondered if that had something to do with the permeability problems that kind of an assembly might present. Is there some other reason? Would this plan still have the potential for foam to pull away from the rafters and create condensation problems? Here is my assembly plan from the top down, please critique:
• asphalt shingles (existing)
• 1/2” plywood (existing)
• 1-1/4” vent (adding) [built on-site]
• 4” Polyiso (adding)
• 2” Polyiso (adding)
• 1/2” Polyiso (adding) installed to underside of rafters to minimize thermal bridging
• 1/2” drywall
I live in Jefferson County CO and according to Energycode.pnl.gov I am zone 5 which requires a minimum of R-38. My calculations would put this ceiling at R-40 Should I proceed?