Hi everyone, I posed a scenario a few days ago and got good feedback. However, there is one part of this setup I need some good advice on. That is the third paragraph here, this next paragraph is just some additional context.

I’m in northern New England, Zone 6. I have an addition on my house with a shed style roof, soffit vents, no ridge vent and unsure if a high vent point could work. I have it down to the studs. I will be using 4″ closed cell spray foam in ceiling with an aged R value of 30 for the full 4″. Next layer will be  R23 rockwool for added insulation and a thermal barrier. I plan on a T&G ceiling. (As an aside: While I may opt to keep the wood look on the interior, any feedback if priming the “back” of the board would be useful, perhaps with a vapor barrier primer?). I was then planning on having 3″ of closed cell spray foam sprayed in the walls until I recently read more on this site about better, cost effective, environmentally suitable options for the walls.

My real question is about an 8 foot shared wall with the partially insulated, unheated garage that is often 10 degrees warmer than outside, but can regularly be below freezing in deep winter. This wall currently has 5/8″ drywall on the addition side followed by what I think is very old asphalt impregnated fiberboard followed by the old house’s sheathing (around 1/2″ plywood). THEN there’s the 2×4 stud garage wall with fiberglass faced kraft. The kraft faces the garage. Then there’s OSB on top of this on the garage side, but hung in a very leaky manner. I don’t have plans anytime soon to heat the garage. So, I’m hoping for some feedback, given all these layers, what best practice should I aim for here? I’m willing to replace the fiberglass, I haven’t fully taken the OSB down to determine what kind of shape it’s in. Otherwise, leave as is? 

Thank you for helping me clarify how to handle this wall.