Insulated cathedral ceiling question
I reviewed your article on this topic (https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling) but am hoping to get your thoughts about my specific variation in mind.
I’m constructing a small (12×24) guest cabin with a “modern” post and beam frame. The plan for the (cathedral ceiling) roof assembly is v-groove over the rafters then 2 layers of rigid polyiso sheets staggered and taped, house wrap then Advantech screwed to the rafters followed by asphalt or fiberglass shingles over felt paper or whatever material is appropriate for the product.
Our larger cabin was constructed with metal roofing with, from the bottom, v-groove, foam, house wrap, horizontal 1/2″ strapping screwed to the rafters (so an air space) then metal screwed to the strapping. This has performed well thermally for 2 years including 2 NH winters and once the structure dried out there have been no apparent moisture issues.
The specific question – is there any reason the air gap used with the metal construction is needed with the asphalt or FG shingle construction? Any critique of either is welcome. Thanks, MF
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