If you're redoing the roof pull off a couple sheets for access and get the ceiling done with spray foam. it is its own vapour barrier and gets in all the crevices. Normally they spray the actual roof and the attic becomes a warm space but in your case the ceiling might be a better option.
You could do the same with the exterior walls but you'd have to take all the sheeting off and that would be a massive pita. I did that for an addition on a clients house but that was two walls at about 10' each, not a whole building.
Thats what I'd try. but a spray foam guy might have different ideas.
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