Yes, the 1/2" insulation will give you R-3, give or take. HOWEVER, if THAT is all the insulation you have, you'll want fresh insulation blown into your walls. And after you demo the outside wall, that would be a GREAT time to have it done.
Now, assuming you have good insulation in the walls now (which you didn't respond to from my reply), you only need a flat surface. Hence, I suggested 1/2" vs. anything thicker. THen, problem solved!
So, what do you have for wall insulation now???
Jay J -Moderator
PS: If you have R-19 or better, go w/1/2" insulation, and be done w/it. Yes, more is better but, obviously, it will give you problems AND possibly look like crap ...