The 30x30 house addition is close to fifty years old by our estimation. It is, from interior to exterior: knotty pine verticals above wainscotting with horizontal below, followed by pink fiberglass insulation, followed by Celotex fiberboard, followed by vertical cedar board and batten siding. The insulation is very poor condition. The Celotex has been compromised, as has been the boards, by woodpeckers of the years. There are many, many air leaks.
The plan is to remove the board and batten, all of the messed up Celotex, and remove the poor insulation. This leaves only the knotty pine interior boards. New insulation is the first, of coarse. But that's where my dilemma is at. Go with 3.5" fiberglass? (not a fan of it, ... how do I secure it properly? Ideally it should be flange stapled to the studs. Can't do that in this situation.) 2nd option is to spray foam. I really, REALLY like that idea. But....the closed cell spray foam claims an R value of 7/inch. Preferred wall insulation in Michigan is minimum R19. Rolled fiberglass is R13, batts is R15. So....2" of spray foam should equal R14. will be airproof. So, should I fill the studs level to their surfaces, there by giving an R value of 24.5? Would one inch of foam at R7 be equivalent to ?x of any other insulation?
The remaining plan is to then cover with 7/16 OSB, followed a Tyvek type material, followed by vinyl siding.

Any opinions or suggestions?