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Insulating a detached garage won't reduce your home's heating or cooling bills—and insulating an attached garage will only have a small effect on your home's energy efficiency. However, insulating an attached garage can reduce the transfer of heating and cooling to and from the outdoors, which can reduce the amount of energy your home needs to stay comfortable. Also, having an insulated garage is just more pleasant. Balance the cost of insulating your garage with how you'll use the space to determine if the project is worth it.

You do not always need to remove old insulation before adding new. If the existing insulation is in good condition—not wet, physically damaged, moldy, or infested with pests—you can add new insulation directly on top of it for a compounding effect. You can also mix different types, such as adding blown-in insulation around existing fiberglass batts.

However, removal is recommended if the old insulation is compromised by water damage or pests. You should also remove it if you plan to install a different type of insulation, like switching from roll or batt insulation to spray foam. If the existing insulation is very old or damaged, you will likely get more benefit from removing it and starting fresh.

If you plan on heating your garage, you must install a vapor barrier when drywalling the garage. A vapor barrier protects your garage from frost and mold that can occur through condensation. If your garage is unheated, you don't need to install a vapor barrier. The only exception is if you regularly crank the heat in your car before getting in it. In this case, you may still want to install a vapor barrier, as your car can heat your garage.

The life span of fiberglass insulation can be as long as 80 to 100 years, but because of changing building codes, it’s best to get your insulation checked every 10 to 15 years. Signs of degrading fiberglass insulation include bits of batts on the floor or batts detached from frames. Having optimal, healthy insulation in your home will improve its energy efficiency, lowering your monthly bills.