How to vent a PVC low-slope roof and prevent condensation
I’ve owned for 3 years a 1952 house in No. Calif. near SF Bay. It has a low-sloped flat-top roof (almost flat/slight slope). It has fascia-venting in every other bay, fiberglass insulation in the bays, and the slightly sloped flattop ties into a pitched composition shingle on the other end. When I go into the attic of the pitched composition I can see inside the flattop slightly so I believe the roof vents from the fascia vents into the pitched composition and out the o’hagan’s of the pitched roof.
The flattop is NOT leaking nor does it have any alarming issues as I have used a moisture meter on the interior gypsum ceiling and it’s all dry, except, in 2 small areas the gypsum registers moist because the bathroom fans don’t exhaust out the roof (they currently exhaust into the flattop, however the moisture from the bathroom fans is only affecting about a 1 foot round area of the gypsum near the fan locations). I know this is not good to vent bath fans into a flattop which is one of the reasons I’m going to get the entire roof replaced so I can add roof exhausts to the bathrooms, plus the roof is old and I want to be proactive.
I’m going to replace it with a IB PVC 60mil roof, I’m also going to replace roof deck with all new 5/8″ T/G CDX plywood, and roofer says 1/4″ non-prime dens deck over the plywood. My concern is I read that some PVC flat and low-sloped roofs may condensate. That is the LAST thing I want is to put on a brand new roof and create a different problem. I want to be pro-active and educate myself to figure out what is best to prevent eliminate that from happening.
My certified IB roofer says he can add some wood to create slightly more space between the fiberglass insulation and the roof deck and add couple of roof vents (in addition to what I already have). IB says to choose a darker color (not white). IB says I could add 1″ polyisocyanurate on TOP of the plywood roof deck to keep the plywood warmer. I’m worried that the polyisocyanurate may not allow my house underneath it to breath. Isn’t a breathing house important? Also if I do use polyisocyanurate, what kind? what thickness? I read low-perm and I read if it’s faced that that can cause problems, so I assume it must come un-faced? Also I suppose it needs to be sealed with tape.
While my roof is open, should my roofer seal around canned lights? fan openings? etc? Is dens deck mold resistant or is there something better to use? Please advise on my many questions, I don’t want to spend $14k on a roof that will cause a condensation issue, I know PVC won’t leak for 30+ years but last thing I want is a new roof with a new issue.
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