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?