We have a block house (aac) and there is a wall of windows on the south side.  It was designed for block between windows, but contractor used wood studs, no insulation.  The wall is very cold where the studs are.  Someone suggested putting DOW styrofoam blueboard over the interior of studs to help R value...before putting up drywall. 

I was trying to keep house as nontoxic/green as possible.  Will doing the above help insulate wood at all? (the sides of the studs will still be open)  If it doesn't make a difference, I'd rather not do.

Also wondering what the least toxic rigid foam is?  I'd like to avoid any treated with flame retardant chemicals at the least...

High density EPS is water expanded so no off-gasing and high R.
Also much less expenssive than XPS, which is chemicaly expanded.

2x4s? 2x6s? doubled up or space between? Adequate sealing? A 2x4 stud on the long dimension (3.5 inches) is roughly R4, or better than the windows unless you spent big bucks on them. First test with a candle or smoke pencil for microleaks. Then filling the cavities and facing the studs with foam will definitely help, but bear in mind that the windows are a far larger area and probably have less R value than the bare studs. It is not a problem in the winter when the sun is shining. At night or on cloudy days you may be better off with insulating drapes.

I also have a south facing window wall in an aac house built with 4x6 post and beam. The wood isn't that much worse than the 8 inch block at R10. I am looking at Window Quilts to slow heat loss at night (roughly R6 with glass and shade.)