I am finishing the above-ground basement in my 1950's house, which has 2x4 framing, 16" o.c. I want to insulate as much as possible without breaking the bank (i.e. sprayfoam is out of the question). My current plan is to insulate with rockwool comfortbatt between the studs which should give me an r-value of ~14. I can't stuff any more insulation between the studs, so I'm wondering if there's any problem with me putting 2" rigid foam board (XPS) insulation over top of the studs on the interior side of the wall to get a higher r-value. I'd then drywall directly over top of the XPS foam board. Wondering if this is a recipe for disaster?
For what it's worth, I live in the pacific northwest where temperatures are very moderate in the winter.