Exterior roof sheathing ventilation
I am designing a 2,000 sf [pretty good passivehouse] house in Colorado. The site is in Zone 7B, 10,000 HDD; -16 Design Temperature @ 8500′ elevation, in other words, damn cold.
We are implementing most, best building practices where practical for this budget [current energy model at 21,000 BTU/hr]. We will have interior and exterior air barriers.
My question/observation is concerning our wood roof construction. We are designing a cold roof [to reduce or eliminate ice damning] with a standing seam metal roofing, in a cathedral ceiling condition with 16″ TJI’s. It is a 24’x48′ shed roof. We will dense pack this with cellulose insulation with a polyethylene interior air barrier enclosure.
I would like to put the roof sheathing ventilation above the exterior air barrier roof sheathing because:
1: maximize and protect the roof insulation;
2: protect this insulation from cold air washing which reduces the effectiveness of the insulation;
3: we will have an above exterior roof batten system on top of the sheathing because of the metal roof application [needs ventilation] and applied roof battens for the roof overhangs that are not part of the exterior air barrier. So I am already need the exterior roof batten system to cool the metal roof and extend my roof overhangs.
Why would I not install as roof sheathing a permeable Zip roof sheathing system or a permeable roofing wrap on OSB and let the above sheathing ventilation do the job of the under sheathing ventilation? I cannot find good literature on this approach.
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