Vapor Retarder or not?
First time poster but long time reader. I’m currently insulating a detached workshop here in the arid SW (Zone 5B, zip is 87048). This building is 2×6 stud construction with a flat roof. At this point in time, I don’t plan to heat or cool the space unless I am present. I’ve installed a mini-split system (21 SEER) for the times I’m present. To date, I’ve sealed all the exterior cavities between stud and sheathing, filled all my holes in the studs/top plates, sealed the bottom plate to the stem wall, and am now working on insulation. The exterior has 2 layers of tar paper, Tyvek, then stucco. Insulation in the walls is Roxul (R23) and the ceiling is getting the same (R60) but installed in two layers perpendicular to each other and covering the bottom chords of the trusses. For roofing, I have a TPO membrane over the standard rigid insulation they use (about an inch worth IIRC). The underside of the roof is vented with parapet vents on all sides. Insulation will leave approx. 9″ between the insulation and the deck at a minimum increasing to approx. 12″ at it’s “peak”. When insulation is complete, I plan to drywall and paint both walls and ceiling to finish it all up.
Over the years it seems that the philosophy of using vapor barriers has morphed quite a bit. Moving more into vapor retarders determined by geography/climate and now I’m starting to hear of more folks recommending not installing a separate retarder at all, at least in my climate. I did an addition on my home a few years ago with very similar construction and used Membrain under the drywall. It’s only been a few years but I haven’t seen any signs of issues. I’m just wondering what the experts think would be appropriate in my situation. Would drywall/latex be enough especially given this building will only be used on occasion and not fully habited? I certainly don’t mind putting up Membrain again but it would be nice not to incur the cost of that if not necessary.
Thank you very much for your time and inputs!
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