Teo2006 wrote:
I have an older home that I am remodeling. The stud walls are actual 2
x 4's. Currently, the exterior walls have about 2 inches of insulation
material consisting of blown in cellulose, wood slats(lathe) and then
plaster type material,then aproximately a 2 inch cavity then the wood
lathe and then the finish plaster. Apparently way back when the house
was built (1900) that was the type of insulation they used in the stud
cavities- a layer of lathe and plaster inside the stud cavities. I am
removing the plaster and lathe on the finished side of the wall and
will fill the 2 inch cavity with either fiberglass insulation or
polystyrene foam board. Do I use a vapor barrier before I install the
sheet rock ? Normally one would use vapor barrier but I am concerned
that the plaster and lathe that are inside the cavities are already
acting as a type of vapor barrier. If I install insulation in that 2
inch cavity and then a vapor barrier I may have a double vapor barrier
which would hold moisture in the wall. What do you think ? The walls
would end up like so....sheetrock,then 2 inches of insulation, then a
layer of plaster, a layer of lathe, cellulose insulation,then the
underlayment,then 2 layers of siding (wood covered by cement type tiles)

I would definitely add a vapor barrier. I can't imagine that the lathe
and"plaster" in your stud cavities is vapor impermeable. and even if
it was, I think you still need a vapor barrier on the warm side.
adding insulation to the setup will result in a colder stud cavity,
potentially leading to condensation, so you want to keep the moisture
out.

if it was mine, i'd gut everything out and insulate it properly.