Standing seam condensation issues over uninsulated roof in (occasionally) conditioned space?
I live in Seattle, Climate Zone 4C. I am designing a complete roof re-build for a detached garage that I share with my neighbor (CMU wall in the middle). The proposed roof is a simple gable (2.5/12), with Zip System 5/8″ roof sheathing, and standing seam metal on top. At the same time the new roof is put on, I will be converting my half of the garage (about 180s.f.) into a workshop with continuous rigid insulation at the existing concrete walls and floor. We are not planning any above-deck or cavity insulation in the roof system, as my neighbor’s half of the garage will be completely unconditioned and my half will have a small unit-heater that I will only use when I am actively working. In other words, the rafters and the bottom of the zip will be exposed on the inside.
My question is this: when I do turn on the heat in my half of the garage and (potentially) leave it on for stretches of time during a project, do I need to worry about moisture moving through the Zip and condensing on the underside of the cold metal roofing? If so, would I be better off simply doing 5/8″ CDX, taping the seams, and then installing a vapor impermeable membrane such as I&WS on the entire roof deck? (drying to the inside, nothing passing through)
I recognize I might be overthinking this, but with the current astronomical price of steel I want to do everything I can to ensure longevity for this roofing system.
Thanks in advance for anyone who chooses to weigh-in! Cheers,
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