Owens Corning R-20 EcoTouch
Your trying to fit a qt. of jello in a pint jar. R-20 is from 5-1/2 to 6" thick, adding the foam will back a double vaper barrier, a big no no. If you added 2 X 2's to the wall you could just use R-20 or 19 batts.
A 2 X 4 wall will only hold R-13.