Dead Air Insulates

• You cannot recapture air that has moved into the attic of a garage. Once your home's cool air, created by your HVAC or another method, travels into your garage in any manner, it will gradually warm up, rise and dissipate via house vents or gaps in walls. Insulation added to the garage attic will stabilize air in the garage below -- which slows down the pull of any air from your home's living space around a common door. The purpose of insulation is to create dead air, or air that lacks movement.
  
  

Walk-Out Doors Are Drafty

• Garage areas typically have gaps around exterior doors and windows. The garage walk-out door or an interior door opening to kitchen space will not be airtight. When you open a kitchen door to walk into your garage, for example, you will lose less cool air from the house if the garage air is on the cool side. Insulation in the garage attic will help to keep the garage air cooler in summer months. Having no insulation in the garage attic will cause the garage to heat up 10 to 15 degrees hotter than the home interior.
  
  

Try to Stop Air Leaks

• Garage attic insulation and door weather stripping both help prevent air leaks. If you insulate your garage attic, but you fail to install weather stripping around a walk-out door to the outside, your garage air will pull air from your house. On a warm day, warm air rising from garage space will cause a siphoning effect. Air that is cool inside your house will follow that draft. That's why stable air in the garage, which is the result of insulation, will prevent a draft. Your air conditioner won't need to kick on as often if existing cool air stays in place inside your home.
  
  

Cool Garages Are a Plus in Summer

• On hot days, a cooler garage makes a pleasant work space. The cooler space, caused by attic insulation in the garage, may prevent wasting home energy as well. If you find the garage temperature way too warm, you'll be tempted to keep coming back into the house to cool off -- thus wasting energy.