Insulation board for my unfinish garage ceiling
Really any insulation will do enough to make your air conditioner do the job. Obviously the more the better, but only to a point. I'd guess that anything more than about R-15 won't improve efficiency much given the small temperature differential. The key is to reduce overall conditioned space volume and prevent mixing with outdoor air. If you use foil-faced I might install it foil-up, since its purpose is to reflect radiant heat. 

Your garage should have vents down low to allow a full circulation path. Soffit vents are typical. without that there's very little movement in the attic, and moisture buildup could be a concern.