I am working on solutions for a cathedral type ceiling rehab project in a century home in Northern Ohio. The challenge is that there is limited space for insulation and limited ceiling height. There is no viable ventilation in the application (timber frame construction). I am considering a layered application (see attached illustration) -- from the exterior sheathing down of 2" polyiso foam (r11), 3.5" of rock wool (r15) and then using either Prodex Total (r-16, 5mm Reflective Insulation) or Double Bubble as an insulating vapor barrier over the joists that will then be covered with drywall.
1) I could incorporate a 1/2"air channel between the polyiso and the roof deck, however, there is no means of venting it (I intend to seal the polyiso in place with spray foam). Is there any value in adding an airspace between the foam and the roof deck?
2) Is there any problem with using this combination of materials in this manner? Prodex's specifications show a R16 value of the product (with an airspace). This application would eliminate the airspace. What is a viable expectation of the r-value of such a product in this application?
3) Would another combination in this application be advisable?
Thanks for your input!