Hey all, I live in southern Ontario in climate zone 5.  We are doing a major overhaul of a house from the 50s. It has a cathedral ceiling and the rafters only allowed for closed cell spray foam as they are too shallow for anything else to be sufficient. The exterior of the roof was finished previously and is 3/4”board sheathing with 3/8” plywood and asphalt shingles. I don’t trust that an old house like this won’t shift or that the spray foam is perfect so I was thinking about putting intello on the rafters before drywall. I figured this would help ensure air tightness yet still allow moisture to escape should it get in the cavity. 

So, can I do this or is it a terrible idea. The inspector didn’t like the idea but he isn’t familiar with smart membranes and is used to just seeing poly. 

Any help would be much appreciated! Thanks in advance!