If you have an uninsulated garage that you want to heat, adding insulation is a must. Without it, a large portion of your heating bill will seep through the walls. The process of insulating garage walls is the same as that for insulating a house wall. The insulation is installed between the studs, and drywall is installed directly over that. A standard garage will have 16-inch on-centre studs. If yours are different, see an insulation supplier about getting a non-standard width of insulation.
Measure the wall from top to bottom alongside one of the studs. Measure it from the top of the floor plate to the bottom of the ceiling plate.
Put on a long-sleeved shirt, gloves and particle mask. Unroll a strip of insulation on the floor to the length of the measurement you took of the wall. Cut it off at that length, using a level to guide the razor knife for a straight cut.
Hang the insulation between two of the studs, filling the space. The paper-covered side of the insulation should face out at you.
Affix the insulation to the studs, shooting staples through the paper edges of the insulation every 12 inches.
Cut away the insulation around any electrical boxes or other obstructions on the wall, using a razor knife.
Repeat for each space between the studs for each exterior wall of the garage.