Everything You Need To Know About How To Keep Your Garage Cool

If you’ve ever gone out to your garage to find melted candles or dead drill batteries from the intense Phoenix heat, you’re not alone.  You may be shocked to learn that Phoenix garages come uninsulated from the builder, leaving them to get hotter than the outside during the summertime often reaching temperatures of 120 degrees or more.  Many customers want a way to cool their garage so we’ve complied a list of best ways to cool a garage in Phoenix.  

1. Additional vent ducted off your main HVAC system. If you AC system is oversized (like many are), this may seem like a good option to add another register and duct from the HVAC system into the garage.  However even a standard two car garage requires 18,000 - 24,000 BTU of cooling capacity, and something that no AC system has the additional capacity to do.  In our experience, adding an additional register in the garage is barely noticeable and hardly makes it more comfortable, a two garage is just too big.  Even when homeowners enclose their garage to make it livable space, insulate the walls, ceiling and garage door panels, an additional two ducts never cool the space down enough.

2. Window AC unit.  Window AC units just aren’t big enough to cool a two car, uninsulated garage in Phoenix.  The largest window AC units are 15,000 BTUs and can cool 400 sq ft if the space is properly insulated to 78 degrees if left running all day long.  If your garage has wall and ceiling insulation, a 15,000 BTU window AC unit would need to run from 8 am continuously all day to keep it comfortable.  We don’t recommend using a window AC unit to cool a garage because it will cost about $80 - $140 each month to run during the heat of the summer assuming a well insulated garage ceiling and walls.

3. Swamp cooler.  Swamp coolers are an inexpensive way to cool an area however evaporator cooling doesn’t work well during temperatures above 110 degrees or in the humid weather of the monsoon season.  In the heat of the summer, a swamp cooler would move air through the garage but it wouldn’t help to cool it much.  Some homeowners will have us re-duct the swamp cooler duct from the main house to the garage, which can make good use of the swamp cooler.  We don’t recommend installing a new swamp cooler to cool a garage however because it wouldn’t be able to cool the garage well during the middle of the summer or monsoon months.

4. Ductless mini split system.  Ductless mini split systems are the best way to cool a garage because of the increased size available, superior efficiency ratings and ability to cool a garage much quicker than a window AC unit, swamp cooler or adding a separate register.  Ductless mini splits provide greater temperature control than tapping a new supply duct and register from an existing AC system.  Ductless systems are like its name implies and have no ductwork to spread allergens or contribute to ductwork leakage.  The indoor evaporative coil is mounted on the garage wall so no windows are required for installing mini split systems.  Ductless systems are also super efficient, with the lowest efficiency rating starting at SEER 17 system. 
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A ductless mini split system is the best and most cost effective way to cool a hot garage. Whether you have a workshop, car or hobby collection a mini split system is cheaper than a central AC system, cools the garage better than adding another register and duct off your existing AC system, works superior to a swamp cooler and room air conditioner.  See how much a ductless mini split system costs in a table we’ve created here.

How Much Will Insulating My Garage Help To Cool It?

There’s no doubt if you want to keep your garage cooler, it needs to be insulated. By insulating your garage you can expect to cool it from 5-15 degrees.  The easiest way to do this is to insulate the garage door panels with insulated foam board.  A 1.5” sheet of foam board can be cut to fit into each panel. If your garage door panel gets hit with direct sunlight, that aluminum will radiate lots of heat into the garage.

The next step is to insulate the ceiling of the garage with blown in insulation.  At least 10” of cellulose insulation should be added above the garage for an R-value of R30.  If there is no attic access in the garage, a new attic hatch can be cut in. 
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The final step to insulate your garage is the walls.  The shared wall between the garage and the house is already insulated.  The other exterior walls of the garage have zero insulation in them.  We use a method called drill and fill to drill at least one hole in each stud bay cavity, then dense pack cellulose insulation into each stud cavity.  A foam plug is used to close the holes leaving them ready for  texture and finishing.

What About Venting My Garage?

Some homeowners will open their attic hatch to let the warm garage air rise up into the vented attic space.  Homeowners have also asked about adding another high-low vent in the side wall of their garage to help circulate more airflow.  Both these work but won’t have a noticeable impact on the hot garage temperature.  A better ventilation solution is to install a ventilation fan (must be larger than a bath vent fan) in the garage ceiling to actively bring out the hot garage air and vent it to the attic.