Air Conditioning Your Garage

Window air conditioners are usually the best method

window air conditioning unit

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Garages are rarely served by heating or cooling systems, even when they are attached to the home. Most people do not concern themselves about cooling a garage in summer. However, if you use your garage space as a workshop to work on cars or home maintenance projects, or if it serves as a center for working on gardening tasks, a garage in summer can be a pretty unpleasant place to work if it is not cooled. 

You have several options for cooling a hot garage, depending on your local conditions. Garages in areas with mild summers can get away with an exhaust fan that provides large air changes throughout the day, but if your summers are hot, muggy or both, you may be eyeing a full-blown window air conditioning unit as the best solution. 

Benefits to Air Conditioning the Garage

Not only will adding air conditioning to your garage make it a more pleasant place in the summer, but it can also help with your home air-conditioning expenses. When you cool your garage, it helps keep the rooms surrounding the garage cooler, too. For example, if you have an attic room over your garage that struggles to stay cool in the summer, removing the heat from the lower part of the garage will make a huge difference.

Other interior rooms also benefit, and it may ultimately cut your energy costs. In the same way that insulation keeps warm air from infiltrating your house, the pocket of air inside your garage can help slow the absorption of outdoor heat into the home. In an uncooled garage, hot air slowly creeps into your home through a shared wall or door, raising indoor temperatures and forcing your air conditioner to work harder.

But keep in mind a caution if you imagine that your home's central air-conditioning system can be used to cool your garage.

Caution Regarding Central Air Conditioning

Whatever you do, don’t tap into the house air conditioning system to cool your garage. This may seem like a logical solution, and many homeowners have attempted to extend central air conditioning into a garage simply by adding a length of ductwork through the side of the home and into the garage. 

It is a bad idea for several reasons:

  • Your garage typically will not have an air return back into the HVAC system, and when a central air duct is extended into a garage, it can create abnormal pressurization in the garage. This forces the rest of the home to become slightly depressurized because the air in the garage can’t get back to the air return. While this change in pressure can't be sensed, except perhaps by the slight breeze when the door to the garage is opened, it poses problems. Negative pressure in your home has to be relieved somehow, and what usually happens is that your home will draw air in from outside. Outdoor air bypasses the HVAC filter system, allowing pollutants and allergens to build up inside. And it also makes it harder to keep your home cool, since there is a constant flow of warm air coming in from outdoors.
  • The other enormous drawback to tying the whole house air conditioner to the garage is the potential for dangerous fumes to enter your home via the ductwork. Whether you’re working on the mower or just warming up the car, all kinds of fumes are in your garage at any given time. Some of them smell odd, but others, like carbon monoxide from the exhaust, can be deadly.

Sizing Garage Air Conditioners

Adding a simple space air conditioner to the garage is by far the best solution for cooling the space, but determining the proper size is critical. Many people add air conditioners to their garages and are disappointed when they discover that they don’t cool properly--simply because they didn’t know how to size their units. Any ductless air conditioner, be it a window unit or a portable unit, is only capable of cooling a certain amount of space based on its design. Air conditioners are rated in units called British Thermal Units (BTUs), which describe how much heat can be removed from a closed space.

Determining the proper air conditioner size starts with measuring the size of your garage. If you have a typical two-car garage, it should measure somewhere around 20 feet wide by 20 feet deep. A 400-square-foot space like this requires a 9,000 to 10,000 BTU air conditioner, according to Energy Star. Using a BTU calculator is an easy way to estimate how large a unit you need, though you may need to adapt these recommendations slightly. If your garage is heavily shaded by large trees, for example, you may be able to get by with a slightly smaller air conditioning unit; a garage that gets direct sun from dawn to dusk may require a slightly larger unit.

Carefully consider where you’ll put the unit. Ductless air conditioners work better if they’re placed away from corners and closer to the center of the area. For a window air conditioner, a window located near the center of the garage wall should do just fine. A portable air conditioner gives you more flexibility, but may still end up along a wall to save space (and that’s okay). 

Increasing Garage Air Conditioner Efficiency

Even though adding an air conditioner to your garage space will remove plenty of heat from your garage, you may still have some uncomfortable days if the garage is completely uninsulated. Insulation in the garage is just as important as it is in the house, even if you go with a very basic insulation package. Here are the most important areas to keep in mind:

  • Garage doors. Your doors absorb an amazing amount of heat during the day. By adding insulated garage doors, you can greatly reduce the heat that comes in. Replace the insulating strips around your door and check that all cracks are sealed. Many garages can get by with upgraded garage door insulation alone.
  • Ceiling. Like the rest of your home, your garage gets a lot of extra heat from the attic above it. Add some rolled insulation to the floor of the attic (or to the ceiling of the garage, if the attic is finished). You’ll stay cooler, and your garage air conditioner unit won't have to work as hard.
  • Walls. Adding insulation to your exposed garage walls will help keep it cool, but it’s a luxury if you’re on a budget. Start with the walls where the sun shines directly, then move to walls that are in the shade most of the time. (In an attached garage, the wall shared with your house is already insulated.) You’ll realize a small energy saving by adding wall insulation, but how much depends heavily on your local conditions.
  • Windows. Most garages have at most one or two windows if they have any at all. They represent a small percentage of the heat that comes into the garage, but if your windows face the sun most of the day, covering windows with sun-blocking curtains will keep your garage cooler. If you want the light those windows provides, a reflective film can also help cut the heat while allowing some light in.