Scott, like the others, I don't know the R value of drywall either. However, from the times I had to do load calculations on houses, I do know it is not much. Just as a guess, I'd say that since, 2 inches of stryofoam normally equal about 3 1/2 inches of fiberglass, that you'd be able to calculate it out and if my math is halfway correct, and assuming my reasoning is correct, you should end up with something like R-4.25 per 1/2 inch of foam. Probably way off base. Harold probably has those figures at his fingertips someplace, I'll have to find my books since I no longer do that kinda work. I do know that some local codes prohibit some foams on the interior of a house because of the toxic smoke ratings. The foam on a wall wouldn't have the structural strength that drywall would provide and would also dent and possibly break if hit or leaned against. Just a few thoughts from a confused mind. Good Luck.