am redoing the plaster walls in a room in my house in New York (I am also opening up the wall cavities to redo the electrical wiring). I don't expect to find any insulation in my 2x4 walls (house built in the 1920's). I was thinking about furring out the walls to a true 2 x 6", then cutting three layers of the 2" tuff-r to fit within the wall cavities (between the now-2x6 beams), but leaving a 1/2 gap on the edges of the sheet foam, then using spray foam to seal the gaps. Follow with 5/8 water reistant gypsum, then two layers of plaster. The TUFF-R claims to have 12.2 R value for 2 inches. Will I get that x 3 if I put in three layers of Tuff-R? Or, since the 2x6's won't be insulated themselves, will they leak heat? Or, does it make more sense to leave the 2x4s as is, put two layers of 2" tuff-r in the cavity, then follow with a 2" sheet on the exterior of the studs? I'm trying to avoid losing too much floor space due to bumping the walls out. I'm pretty sure that this would be cheaper than sprayed in foam by a contractor. Any advice would be appreciated.