Conditioned attic roof detail
I am in the process of fine tuning the details of a one and a half story house I would like to build next year. It will be in Northern New York on the border of the 5A and 6A climate zones The upstairs has a knee wall and a couple access doors to the attic space. I think I have decided to just run the zip-system up the walls and around the top of the roof rafters to simplify the air sealing. The roof rafters are engineered 12" I joists, so I will fasten 2X4's for an 18" roof overhang from the wall, but I plan on running the air barrier straight up the wall to the roof by cutting and taping the wall sheathing around the protruding rafters. Then I would fasten the last 18" of roof decking down to the rafters after the transition from the wall to roof transition is taped. This overhang will be vented. I would like to use 3" of used craft faced polyiso board on the roof deck held down with fern strips for most of the roof. On the bottom 3' I would like to use plywood covered with ice and water shield, over top of the foam. I would put a rib style(not standing seam) metal roof on top of everything. The metal roof would not have seal strips at the bottom and I would use a vented cap so that it would vent and release heat in the summer time. Any water that entered the bottom would then drain down the ice and water layer. The 12" roof rafters would then be filled in with cellulose resulting in a R-56 or so roof assembly.
I hope I have been fairly clear and haven't confused to many people. What does anybody think of this system? Do you see any foreseeable problems that I may run into. This assembly seems like it would work to me but those have been famous last words. Maybe I am coming up with an even more complicated solution to an only complicated problem.
Posted Jul 23, 2014 9:45 AM ET
Edited Jul 23, 2014 1:54 PM ET
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