Hello, I posted in general Q&A about a vapor barrier application but would like to expand on that. I have searched this site and others and can’t seem to figure it out.
House was was built in 1968

So I am doing a rehab in a bath, when i removed the drywall there was a plastic vapor barrier on both sides of the stud on the interior walls, I thought a vapor barrier only went on the interior side of an exterior side of a wall in the North (I an in Northern VT). I have no evidence 0f mold or moisture in the walls.

I also think there is also plastic on the ceiling behind the drywall and there is possible signs of mold on the side of the plastic facing the bottom of the shingles but not totally sure looks like it’s just dirty. the roof is a cathedral with minimal insulation.

The other question is do i insulate the interior walls  (and behind, below the new tiled walk in shower) and joist bays? The bathroom is gutted to the studs and I will be ripping up and replacing the subfloor and repairing a few joists where the plumber got creative.

Would plastic be installed on the inside walls on the bathroom side? 

So is the idea that you want to keep out as much water vapor as you can from getting into the joist bay and thats why you would add the plastic or no plastic to allow as much airflow as possible to dry the cavity…

thanks, for your help…sorry all over the place with this one…

Mark