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Looking for the best insulation option for a new detached garage. I'll heat it to 45 most of the time and warmer when I'm working. I'll have steel on the ceiling. I'm thinking of sealing the perimeter j-channel and all steel sheets at the overlap, then insulating with blown cellulose. Most of the spray foam guys I've talked to about doing the walls want to spray foam on the roof sheathing and just say "trust me you'll like it better". The warmest part of the garage would be the 4700cu. ft. attic that I won't spend any time in or store anything in. The ceiling is at 12' so that's gotta be alot warmer near the 20' peak than where I'm working. My shingles are black. Would that shorten their life or void the warranty? Also I already have soffet and ridge vent. Is there any good reason to spray foam on the roof sheathing?
Thanks for your help