Vented Roof Assembly with Spray Foam
Every time I think I’ve come to some conclusion, I read a forum, and I fall back into a rabbit hole.
I have a 1911 “transitional” – mostly an American Foursquare Prairie – in Zone 6… but Detroit, so wet and can get 5-ish sometimes. I have a full attic space which I want to make into living space. It has 3 gable windows. Based on as much as I could process, I had decided on a roof assembly which maximized both internal and external insulation. I am having the roof completely redone, stripped to the old 1x decking. I had planned to do rigid poly, sheathing, shingle on the outside and closed foam with baffle on the inside.
However, MI is ironically not too forward thinking with its trades, and so I finally found a roofer who would do it, but the added costs of both labor and materials now puts it out of reach. Plus, it will apparently void the shingle warranty.
The shingle issue, I already knew about. Apparently, using closed foam directly on the decking can cause issues which make the shingles peel and buckle, because the temp delta is so high both in both winter and summer here. But the 2 layer rigid over the decking, under the sheathing seemed to eliminate that concern.
Now that’s scrapped. So, somewhere I read (though now can’t find the reference) that another solution is to fur the sheathing, creating ventilation channels on the roof, instead of inside. This is particularly pertinent to cathedral ceillings, which is what I will have in the end with the conversion. But then we have the spray foam inside, shingle curling on the outside issue.
Based on info I think I have collected, my idea is now this. Put a full layer of rubber water and ice over the original decking, then fur, then sheathing and the rest. Spray closed on the underside, then baffle, then rigid under my drywall to break the rafter thermal bridge. Cost is a factor, but the internal stuff I can do myself. The additional rubber layer will add a cost, as will labor for the furring. But it seems that sort of addresses all caveats while getting max R value in my budget.
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