Insulating a stud bay that contains back plaster
I have an old house in New England (built in 1888). I’m gutting a small 10×15 entry and have removed tongue and groove boards from the inside to expose a stud bay that contains a back plaster layer. The exposed depth is 2″. I’d rather not remove the back plaster if I can help it. What is the best way of insulating the 2″ cavity to get the highest R-value? Spray foam? It looks like there is no real R-value benefit to going with closed versus open cell when considering the whole wall R-value and a gap on the closed cell due to tough trimming. If I use open cell, do I need a plastic vapor barrier on the inside before sheetrock? Any other suggestions?
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The 2" cavity won't do much for insulation, I guess it is better than nothing. Since the cavity is probably uneven, spray foam is probably your best bet, go with open cell so you can easily trim it flat. There is slight R value boost (R8 vs R11) if you can go with full 2" of closed cell SPF but that is a lot of work to trim flat.
With open cell, our code would call for an interior vapor barrier, I would check your local code.
Thanks Akos.
What do you mean by "back plaster"? Is it recessed in the back of the bay attached to the outer sheathing? Is this where the two inch gap you speak of is, from the front of the stud back towards the outer sheathing? If so, just buy some 2" foam insulation w or w/0 foil vapor barrier, cut strips and push into place and put the t&g back or wall board.