im in climate zone 7 I'm renovating a house the walls detail currently from inside out is. Cement/Lime plaster over wood lath on 3/4" furing strips,tar paper, 3/4" ship lap boards, 2x4 stud cavity with cellulose, 3/4" ship lap, typar, then insulated vinyl siding. I was going to go over the old plaster walls which are in bad shape with new drywall and the ceilings I was going to strap with R/C channel and then drywall do you think i should add a 6mil poly vapour barrier to the inside of the plaster on walls and ceiling?

I would add unfaced rigid foam, tape it well and then add drywall.

i should have mentioned the house will probably be torn down for condos within 5 years so energy efficiency is not in the budget just a quick rehab. Just don't want the drywall turning to mush or if the house gets sold someone to have an inferior place with moisture issues

Then I would use Intello or MemBrain, also taped well.

i have used membrain before in a place that had 1.5" inches of foam on the outside of the house which was not enough for my climate. what would i want to use a vapour retarder instead of vapour barrier when i have no exterior foam?

Because drying to both sides is better than only drying to one side. But you can use poly with your wall in zone 7.

jonr> thanks for the advice, would you acoustical sealant right over the existing plaster and stick the vapour retarder right to it?