what insulation to use for garage walls
If your garage walls are framed with 2X4 you could opt for fiberglass batts of R-11 or R-13. If they are 2X6 you could use a R-19 rated fiberglass batt. Most garage walls are 16 on center, so you'll want to buy 15 wide batts. Check your local building code to determine if you need unfaced insulation or batts with a vapor barrier attached.