Feedback on unvented flat roof detail
I’m looking at putting an unvented, flat (2%) roof on an office shed I’m building in my back yard. I’ve read a lot of the articles about low slope roofs on this site, but would still love it if someone took a quick look at my plan:
– Climate zone 3 (California, SF Bay Area.)
– Flat 2×8 rafters directly on top of wall plate, with sloped furring strips on top to create 1/4:12 slope.
– 1/2″ OSB decking
– 2″ Polyiso panels (fiberglass face) on top of decking.
– 60 mil EPDM membrane.
– Open cell spray foam insulation on underside of decking (between rafters).
– 1/2″ painted drywall ceiling below that.
I’ve read some people advising closed cell foam on the underside. I understand that open cell foam lets the roof dry to the inside. However, doesn’t this also carry the risk of the roof deck absorbing interior moisture?
My other question is whether I need to ensure that the ceiling interior (between the drywall and spray foam insulation) is vented to the interior of the building, or if painted drywall is permeable enough on its own.
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