We are currently remodeling the master bedroom in our 50+ yr old house and found that the bat insulation (on the exterior walls) behind the old drywall was slightly damp and deteriorating - so much that it took off the black paper behind it when we removed it from the walls. (I'm assuming this is some sort of vapor barrier correct?) I plan on using normal paper-faced bat insulation to replace it, but should I replace the vapor barrier behind it as well? The old drywall appeared to have water damage throughout, which was probably from the damp insulation (no vapor barrier between them).
I've heard that using two vapor barriers can actually be a bad idea, as it will trap moisture between them and to just use one barrier, against the drywall.
NOTE: Not sure if any of this matters, but the old drywall was covered up with paneling by the previous owner, and the outer walls are horizontal 1x6's with wood siding (which the previous owners covered up with vinyl siding).
We just had our Basement finished. Contractor used rolls of paper insulation and for a vapor barrier they installed clear plastic sheeting over the insulating. I live in Northern Illinois, not sure if climate has anything to do with type of barrier. You might check with your county for local codes.