r20 insulation with 2x4 studs in Garage
 

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.

Why not just install an R20 insulation and finish it with 1/2" drywall, 5/8" drywall against the living space for fire code?

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.