Insulating a garage ceiling generally means insulating between the roof joists that run up along the angled underside of the roof (since most garages don't have finished second floors). The concept is the same as insulating any other wall, except the angle of the ceiling can complicate the process of hanging the pieces. Measure the space between the roof joists before you buy your insulation to make sure you're getting the proper width, which may be 16, 18 or 22 inches, depending on your garage.
Things You'll Need
Batt insulation with paper barrier on one side
Stand on the rafters of the garage to access the roof joists that run up the inside of the roof. Measure the length of each side, from the peak of the inside of the roof down to the tops of the walls.
Roll out your batt insulation. Mark out the measurement you took onto the paper side of the insulation. Cut it straight across with your razor knife.
Take the cut piece of insulation up to the rafters. Hold the top of the insulation piece to the top of the space between two roof joists, with the paper side facing the interior.
Staple the piece to the joists, shooting the staples through the paper tabs on the sides of the insulation into the sides of the joists. Let the bottom portion of the piece hang down from the peak.
Push the bottom portion of the insulation piece into the space between the joists. Staple along the sides, working your way down the joists. Place staples every 12 to 18 inches. The insulation will fill the space completely.
Repeat for each space between each pair of roof joists. Do the whole upper surface of the garage.