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.