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galvanized fumes are a problem when welding, and there are even mixed stories about the dangers of those besides feeling nausious, but you won't have a problem with an oven. i used 20ga (i think) galvanized. i think i paid $26 a sheet for 4x8 from an hvac company. the company i called before that one wanted double, so it's definitely worth it to call around. i used R15 fiberglass insulation and it seems to be working great. one note: remove the kraft paper backing before installing. i used 1/8" aluminum rivets, which also seem to be holding fine after about 6 months.