I agree with Joe. Trust me you do not want to insulate without a vapor barrior.
We just bought a reposed home that needs alot of work. The previous owners were renovating the garage. However, they did not create a vapor barrior. As Joe said, they might as well have put a lighter to their money.
Thus, the garage walls and insulation were soaking wet, molded, and rotten. When we began tearing all that out, we dicovered tons of insects. Actually, fireants fell on my husband's head! So please, do not put insulation up without a vapor barrior.
Granted our case was extreme because we found some leaks, but this illustrates what can happen if there are no vapor barriors.