Unvented roof design in hot/humid climate
I’ve been doing a lot of research but still have some questions. Hopefully some of the experts can chime in.
Im in climate zone 2. Hot and humid. A few years ago we converted the attic to living space. We converted the traditional vented roof to an unvented roof by installing open cell spray foam in the 2×6 rafter bays. The attic is now fully conditioned by connection to the central AC system.
We now plan on reroofing the house and installing an additional layer of 3” polyiso on the exterior since the current 5.5” of open cell spray foam does not offer the required R38.
So here is what I haven’t been able to figure out yet:
1.) should I put an air and or vapor barrier like some peel and stick membrane on the exterior side of the roof sheathing to keep humid outdoor air from entering through the roof? And if so, would I put that membrane on the existing sheathing underneath the polyiso or on the layer of osb that goes on top of the polyiso?
2.) I read that ventilation channels should be added with 2x4s between the polyiso and the shingles to prevent ice dams in cold climates. Since I’m in the hot humid zone, is there still another advantage those ventilation channels would provide? Would they keep my shingles cooler and therefore extend their life?
3.) the open cell spray foam is already in place so I can’t really change it anymore. I now read that article about open cell foam being risky in all climate zones because moist vapor can still travel through from the interior and reach the sheathing. How big of an issue is that really in my climate zone where the heat is on maybe three weeks a year? Am I in the clear if I put the 3” polyiso on the exterior and therefore keep the roof sheathing warm. Plus my attic is fully conditioned. So do I still have to worry about mold issues down the line because of the open cell foam?
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