Unvented roof with painted T&G deck
I’m planning out a reroof of my midcentury home here in the SF bay area (climate zone 3C). Given the current T&G roof deck is exposed to the interior (so no interior insulation) my plan is to keep the roof unvented but add insulation above.
Using the GBA detail for an unvented cathedral ceiling with asphalt shingles, this is my initial thoughts for the stackup:
T&G Deck > 5/8″ OSB + taped seams for air sealing > 2x 1.5″ Polyiso > 1/2″ OSB > Grade D Felt > Asphalt 3-Tab
Does this look reasonable? Do I need to worry about the fact that inside the house the T&G is painted and all seams are caulked? Seems like that could be problematic for inward drying, but maybe my 3C climate is mild enough that it won’t matter. Not sure.
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Not sure if the bottom layer of OSB is doing anything I think taping the foam is just as effective
On my T&G roofing I drilled a 1/4 inch hole from the top with an electricians long drill bit at every seam in the T&G directly above the exterior wall. I injected canned spray foam into every hole with the can hose all the way in the hole and while withdrawing it. Idea being to try an air seal that really badly though out roof system
My reasoning for the bottom OSB layer is really just to create an easier surface to air seal (since otherwise the T&G would be a pain and probably not reliable). But that's a great idea for sealing between the T&G and the insulation - if I end up skipping that first OSB layer I'll definitely be looking into that.
The OSB or not has nothing to to with the gaps between the T&G. Gap every 5 inches
Right, I just realized what you meant, which would be the case no matter what with the T&G.
you could put some peel & stick on the T+G as air barrier - even 10 mil PE would work. Tape the seams. You will have air tightness and moisture control. If you expect to many penetrations later (screws through the T+G) then put 1" of like EPS on the T+G, then the film/membran - to have a bit of safety margin.
??
I’m a retired contractor in the SF Bay Area. You need a building permit to do anything more than replace the underlayment and comp shingles so 3” of polyiso isn’t going to cut it. There is no concern with venting an exterior foam roof. Just air seal the T&G deck with peel and stick.
I had a similar job and we had to fill the cavity and put 4” of EPS above for R-38. The client wasn’t happy about losing their cabin style interior but it allowed for required structural improvements.