A major source of heat loss/gain are the side east and west facing plaster & lathe side walls.

I’ve measured the temperature of those walls; they stay hotter than outside temps well into the night, and there’s 5-10 degree increase as you go up the wall from floor to ceiling. They’re actually hotter than ceiling and kneewalls alot of the time. There’s also some moisture coming through; the plaster is rotted through under the window apron and there are cracks in the corners.

There’s foam (open cell) going in the attic, ceiling deck and kneewalls. The company doing the foam doesn’t think its worth the cost and effort to put put holes in the wall and fill the space with foam. But on the walls, there’s only a couple of inches between the plaster and the brick. There doesn’t seem to be any old insulation. I’m considering putting drywall over the plaster. There’s other remodeling going on and most of the window trim is coming out anyway, so it wouldn’t be too much additional work. The electrical outlets could be accommodated. I’m OK with losing an inch or so of room space. Taking out the plaster is not an option.

My questions:

– Is the foam company right that benefits of filling the walls is not worth the the cost and effort?
– One option is to just put up half-inch drywall straight over the plaster, but the r value is only about .5, which doesn’t seem much of a difference (though better foam around the windows will also help.)
– Would it make sense to put a layer of half-inch rigid sheathing insulation (like the R-3 rated Corning Foamular) between the wall and the new drywall?
– If I do that will there be issues with a moisture barrier on the plaster like the rigid sheathing? Or would it be better to just use 3/8 or even 1/2 drywall?
– What’s the best way to construct this? Do I need to “sister” studs to the studs under the plaster? Or put in some kind of furring? If so, how thin can they be to minimize the loss of space? Or is it OK to use a construction adhesive to attach insulation to the plaster and the longer screws to go through that and attach to the existing studs?
– Finally, the room has old radiators that need a gap between the wall and the radiator in order to get some convection going. Should I be worried about losing that gap? Should I do anything different behind the radiators to get a good seal there and make sure I’m not sucking air in from the outside in the process? (Doing much work back there will be hard anyway.)

Apologies for the long and detailed question. You help is appreciated. Thanks!