Insulating a garage makes sense if you're planning to heat the space. When it comes to choosing materials, you can use the same types of insulation used on the rest of the house, but some are better than others, depending on whether the garage is finished or not. You also want to look at insulating the garage door, which has different installation requirements than walls or ceilings.
Garage Insulation Basics
It pays to insulate your garage if you're adding heat, whether on a permanent or as-needed basis. If you're not adding heat, there's little point in insulating. It's a popular misconception that insulation adds warmth. In reality, insulation merely slows the transfer of heat through the insulated barrier (wall, ceiling, floor, etc.).
It's also important to realize the value of air-sealing in conjunction with insulation. Garages typically aren't built to be airtight and have lots of air gaps to the outdoors. You can insulate the walls, ceiling, and door of the garage to the highest R-value possible, but if you fail to fill those air gaps, you'll be wasting a lot of heat. So before insulating, go around the garage with a can of low-expanding spray foam and seal all gaps and cracks that let in daylight. (Of course, your garage door is essentially a gigantic air gap when it's open, but that's another matter.)
Fiberglass is the most commonly used type of insulation in garages (just as it's the most popular type in homes). It's sold in pre-cut batts and long blankets that fit between wall studs and ceiling joists. You can also get loose-fill fiberglass, which it suitable for blowing into a garage attic space above a finished ceiling.
If the walls and ceiling will remain open (not covered with drywall or plywood), it's a good idea to use paper-faced or encapsulated fiberglass bats, which are wrapped in a plastic film. These will give the walls a slightly more finished look, and you won't have the itchy fibers of the insulation exposed and ready to catch dust at all times.
Cellulose is a loose-fill insulation that is growing in popularity. Made primarily from recycled newspapers and treated with a fire retardant, cellulose is usually blown into wall and ceiling cavities with a special blowing machine. Blowers can be rented at many tool rental stores, and home centers will sometimes loan you a free one if you buy your cellulose from them.
Because it's loose-fill, cellulose is suitable only for finished garage walls and ceilings. If the garage is already finished (but uninsulated), you can install cellulose by cutting strategic holes in the wall material, spraying the insulating into the cavities between framing members, then patching the holes.
Rigid Foam Insulation
Rigid foam comes in 4 x 8-foot sheets and thicknesses of 1/2 inch to 4 inches. The most common materials include expanded polystyrene (similar to Stryofoam), extruded polystyrene, and polyisocyanurate. Rigid foam offers a high R-value per inch of thickness and can be cut to fit almost any space. It's a good choice for thin walls and for insulating garage doors. If you're turning the garage into living space or a full-time workspace and want to insulate the floor, one option is to use rigid foam covered in plywood or other subfloor material.
Note: Check the fire rating on rigid foam; some types are not fire-resistant and are not suitable for exposed applications.&
Spray Foam Insulation
Spray foam is excellent for both R-value and for air-sealing. As a high-end material typically used for energy-efficient construction, spray foam is overkill for most garage projects. But it might make sense if you're converting the garage to living space.
Garage Door Insulation
Don't insulate your garage walls and ceiling without insulating the big garage door, too. You can buy insulation kits for standard metal garage doors, or you can cut pieces of rigid foam insulation to fit each door panel/section. Keep in mind that the structural metal ribbing of garage doors is an excellent conductor of heat, and this typically doesn't get insulated. As a result, the overall thermal performance of the door will be well below the rated performance of the insulation itself.
Air-sealing is particularly important with garage doors. Create a seal along sides and top of the door with special garage door trim with an integrated weatherseal strip. Seal along the bottom of the door with a new rubber gasket, or "bottom seal." It's available in different sizes to cover small or large gaps between the door and the garage floor.