Attic insulation – modified cut & cobble – cathedral ceiling – climate zone 5a – GreenBuildingAdvisor

Hello, we are in the middle of an attic remodel product, and I have decided to attempt the cut and cobble method of insulating our existing rafter bays with polyiso insulation boards. I would like some addvice or confirmation/critiques on my insulation strategy.

We have existing 2×6 rafters in our 1.5 story, 1941 brick home. We are located in climate zone 5a. I realize I still can’t get to the code-required insulation value with a 5.5″ depth. It’s a small space (see photo) so I really think we can only fur down and additional 1/2″. I have no interest in closed cell spray foam for a myriad of reasons. We also have no soffit/overhang on our roof. I will have a ridge vent coupled with a deck vent (smart vent) system to create ventilation channels in the rafter bays.

I can get my hands on recycled, paper-faced polyiso in 4″ and 1/2″ thicknesses for a great price. My thought was to stack 1/2″ foil faced with a 1″ air gap (facing the the roof sheathing) followed by 4″ friction-fit paper-faced, followed by 1/2″ paper-faced over the rafters to create an air barrier with taped seams. My thought process here is that I could skip a majority of the excessive “great stuff” spray foam-ing. I have atttached an insulation detail to show what I’m describing as well as a photo of the space. Am I missing something here? Any concerns with this assembly?

I should also add that the floor joists (ceiling joists of our main level) will be filled with R-30 rockwool insulation.

Any help or guidance is much appreciated. Thank you.