The R-value will depend on the wall thickness, and the manufacture of the insulation.
3 1/2" R-11 to R-15
5 1/2" R-19 to R-21
You should use a vapor barrier, walls and ceiling.
When I built my garage I used 3 1/2" R15 in the walls and the first layer in my trusses was the same with 8" R-30 on top of it giving me R-45 in the ceiling. If at all possable you should vent the space above your ceiling, helps to prevent heat build-up.