My house is a 150 year old wooden structure in Michigan. When I gutted the upstairs bathroom I found that behind the plaster walls there was only some cellulose insulation that was blown in some time in the past, and then the outside beveled siding. There was not even any tar paper.
I put some panels of OSB in between the balloon framing studs so I can have a flat surface. On the exterior I will attach tar paper and put in new cedar beveled siding that needed replacing.
How do I insulate the bathroom interior for maximum insulation and to protect the exterior walls from the bathroom moisture? Yes, I have put in a good exhaust fan that now goes to the roof. I was thinking unfaced insulation covered with a whole sheet of clear plastic as the moisture barrier.
Someone has recommended I put foiled panels of insulation against the OSB panels and insulation over that. Do I also need to put tar paper on the inside over the OSB and studs?
If you're going to reside, I would apply plywood to the exterior surface and cover it with tarpaper or house wrap. As far as I'm concerned, OSB has no place in house construction, ever.
As for the bath, Unfaced batt insulation and a 6 mil vapor barrier works for me.