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I'm building an 8x12 garden shed in Toronto Canada (cold winters, fairly hot & humid summers). 2x4 construction, exterior Tyvek wrap, hardie siding onto studs, roof is metal standing seam over OSB sheathing. Had some extra pink insulation from a house reno and figured I'd insulate the walls & roof, & throw in some 3/8 ply so I could heat it up a bit with a space heater when I'm working out there during random evenings - nothing fancy, 5-10C or so would be plenty warm to take the gloves off every couple weeks. Floor is 2x6 PT over limestone & patio stones and I didn't insulate or vapour barrier it, so permanent climate control was never a plan...for the most part it will be unheated with no plans for summer AC. It will just have a little workbench and some tools & garden storage.

I didn't think an interior vapour barrier would be necessary for this type of use, but people I've talked to are saying I should do one between insulation and ply. What do you think?

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It wouldn't be a bad idea. For short periods you wouldn't see substantial moisture buildup, but if you decided to heat for a few weeks or longer you'd start to see frost accumulating at the dew point (usually somewhere in the insulation). When that melts later you have decay or mold or both. Even if you're not in freezing weather you'll see moisture buildup.

4 mil poly is cheap. Might as well. Now, if your insulation already has a paper face, that will suffice. Be sure to unfold the flanges.

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