2x4 rafters, skip sheathing, no insulation, help with roof choices
The shingled roof on my 1940 farmhouse in Zone 3b is due to be replaced. My wife and I are considering the virtues of metal for our very hot, very dry climate where electricity often exceeds $0.40/kw hr.
I plan to remove what little insulation is presently on the attic floor. The hvac ducts are also in the attic with little insulation and are likely poorly sealed.
The roof has hips and gables and valleys, oh my. The rafters are healthy (nearly 1 3/4" thick) 2x4's on 24" spacing with every other one (every 48") comprising a site built truss. Sheathing is 1x3 with 2" spaces. There are separate, exposed rafter tails comprising a shallow 12" eave but no gable eaves.
I'd like to put an inch or more of polyiso on top of the existing skip sheathing with a radiant barrier/air gap somewhere above that supporting metal roofing. This sort of assembly makes sense to me as a means of reducing the insane solar absorption by the attic in my neck of the woods which I presently have very little given how well the cedar performs and leaks. I haven't seen a detail yet that includes skip sheathing as a base so if anybody has come across one, please share. If I can simply treat the skip sheathing as if it were plywood, I will need to find a reference to guide our building inspectors.
Does it make sense to complete the air barrier and insulation at the roof assembly in this case or drop back down to the attic floor to attempt to seal and insulate? If completing the air barrier and insulation at the roof assembly, how can I achieve code minimum insulation? I fear the 2x4's do not have enough depth to support the normal means and I fear the roofers around here won't have the slightest how to finish a roof deck with excessively thick insulation on top.
Suggestions or questions?
Posted Apr 20, 2017 7:51 PM ET
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