Some people don't think of insulating the garage when they insulate their house, but doing so goes a long way toward reducing energy costs for the entire house as it provides an extra buffer. When you have a room built over a garage, it becomes important to insulate the garage's ceiling to prevent the heated space above the garage from losing its warmth. When you insulate a garage, use insulation with the same rated R-value as that used for your home's walls and ceiling.
Insulating the Garage
While insulating a garage won't make it substantially warmer unless you add a heater, it will help to provide an additional insulated space for the home. Many people enter and exit the home through the garage, and the added insulation helps make the car easier to start in the morning. Use the same insulation to insulate the garage as that used for the home's exterior walls. If the walls and ceiling are closed in, consider using loose fill insulation, as this form of insulation uses a machine to get the insulation where it needs to go through a hole in the sheetrock. Insulate the garage's ceiling by using the recommended R-value for your area as advised by the Department of Energy's Oak Ridge Laboratory.
If your exterior walls are constructed from 2-by-4 inch studs, the most insulation that can go into the wall while retaining its R-value is R-13 or R-15 as these are both made to fit a cavity 3 1/2 inches deep. Because insulation fits the width and depth of the cavity between studs, when you compress insulation to fit the space, you remove its insulating properties — the air trapped between fibers.
When you have a room above the garage, the floor rests on boards supported by joists. If the joists are 10 inches deep, the cavity can take high-density R-30 insulation, which is 8 1/2 inches thick, more than enough for floor insulation. If the garage is open to the rafters, the boards that support the roof, use R-38 insulation between the rafters at the roof, if they are 10 inches deep. To get more insulation in the garage install a ceiling and provide access to the space between the ceiling and the roof. This will allow you to add the needed insulation. While some ceilings in the southwestern part of the United States require a minimum of R-19 insulation, most attics or ceiling areas require a minimum of R-38 in the ceiling.
Garage Door Insulation
Use a special foil-backed blanket insulation to fit over the inside of the garage door. Most garage-door insulation has an R-value of between R-8 and R-12, because it is so thin to fit the space. This adds insulation to the garage door without impeding its functionality. The other option is purchasing a pre-insulated garage door that may have a higher R-value. Insulating the garage without insulating the garage door allows any heat in the room to escape through the door.