Are exposed on the other side to a ground temperature of about 50 degrees. So the amont of insulation is not critical. In your case, I would go with 1 inch (R 5) since with the air gap, your wall will have some "built in" insulation as well.
To attach, I would tack pieces of firring strips to the sides of the 2x4s, set in far enough to allow for the 1 inch of styrofoam.
Dodgeman (and still a Packer fan)