©2013 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
I'm working out the details for an addition I'll be building next summer. I'm in northern Minnesota. I'll have a cathedrial cieling at a 10/12 pitch and I'm planning on using 2x12 rafters wo have the room for insulation. The room is 29' wide the direction that the rafters run.
From what I've been reading I don't necissarily need to ventilate the roof as long as I make certain to seal the space very well. I have a couple ideas I'm looking at for that and I'm looking to see if either of my potential ideas have any red flags against them and perhaps pros and cons between them.
Idea one is to frame up the rafters. Sheet with 5/8 osb. Asphalt shingles on top. Between the rafters place tight fitting, maybe sealed with caulk, 2" foam sheets against the osb. Then put 2" fiberglass between the rafters. Finish with with poly and sheetrock. This would be unvented and I would place a sealing block between the rafter tails on top of the wall to prevent air movement.
Another idea is similar except I've read some that you can simply place the foam sheets between the rafters and the roof decking. It would basically be, working down, asphalt shingles, roofing felt (ice and water wear needed), 5/8 osb, 1 1/2 foam, with 12" fiberglass between the rafters finished with poly and taped sheetrock insides. My main concern here is if it's ok for the foam to sit drectly on the rafters with out any additional support. I would simply be using longer nails to nail through the osb, foam, and into the rafters.
Also I haven't yet decided for certain that I'll be using aspahlt shingles. I may use metal. If I switch to metal does that change any of this? Also, if I switch to metal would it be ok to eliminate the osb and replace with furring strips (thinking of the idea of having the foam above the rafters)? Or would elininating the osb be as bad of an idea as it sounds?