Hi, I seen the below Q&A from 2015. I have question about this and asked there but not sure it will get read’.

https://www.greenbuildingadvisor.com/question/question-on-cut-and-cobble-rigid-foam-in-a-vaulted-ceiling#comment-192365?utm_medium=email&utm_source=notification&utm_campaign=comment_notification&utm_content=view

Hi, I have vented shed/mono ‘low’ slope cathedral ceiling with 10″ roof joists (91/2″). Externally is plywood, ice & water and Ondura asphalt corrugate sheets (it allows air underneath it through the corrugate channels (picture attached)
Similar to above, could I put 31/2 inch EPS (seal it well) then add 51/2″ or 31/2″ inch of rock wool and then 4×8 sheets of polyiso or eps right underneath? Spray foam unless DYI is not an option due to access to location. DYI would be very expensive and not likely great results.
Questions then are, is half inch air space between first layer of EPS and sheathing enough and if not, is 21/2 ok if I use 31/2″ eps and 31/2 in rock wool? Also, is 1″ enough of the 4×8 sheets on last (bottom most) to cover joist bottoms (thermal bridge). The other part of this is I will be using tongue and grove pine and will attach through the innermost/inside layer of foam (eps/polyiso) so will this prevent the air permability of the T&G?

To add to above in case relevant, off grid cabin, 4 season use, Ontario (zone 6 I believe), wood stove heat for occasional
winter use and no AC in summer (perhaps window unit in july for a few hours a day off generator to remove humidity for comfort/sleep

I do have enough available ‘very inexpensive’ 31/2 EPS to double up / sandwich, but wondering if the eps/rockwool will save the second layer of cobble time (forgoing a couple hundred more $$ for the 416 sq/ft of roofing (16×26 cabin).

I have also read how to build a cathedral ceiling but it doesn’t address my questions specifically.

Final question for same cabin is for the floor. It is 3feet off ground as water floods under in spring up to one foot max. I have sprayed about 2″ of ccDYI, 2  component foam under. I may add to this after the winter, but as I complete the inside, the question is can I put a dricore floor (osb/foam R3) on top of the current plywood sub floor. Final inside surface will be some form of vinyl. Just wondering if the sandwich created from these layers is an issue: underside of spray foam, plywood, foam on back of OSB and then OSB…..thanks