I want to insulate the outer walls of my attached garage. I asked the city and they said if I plan to ever heat the space the walls need to be rated r20. The existing studs are 2x4s. Do you see any problems if I was to install 1" r5 rigid foam over regular r15 fiberglass bats? I would then finish it with 5/8" drywall. Thanks to any who can offer advice.