Q&A / 

Hot Garage Ventilation

DEAR TIM: The garage attached to my town home gets the full afternoon sun. The temperature in the garage has reached 110 degrees at times this summer. There is some attic space above the front half of the garage and two bedrooms over the rear half of the garage both of which get very warm during the summer. Would an insulated garage door help keep the garage cooler, or is there a way to vent the garage to remove some of the hot air? I have a roll-down screen, but in the summer it does not seem to offer much relief from the heat. Jan McM., Largo FL

DEAR JAN: No wonder your garage is an oven. Its orientation couldn't be worse. As the day progresses and temperatures climb, they often peak between 4 and 5 p.m. This is exactly when the direct rays of the sun are cooking everything they shine upon including your garage and everything inside it.

Add to this the heat from any cars that have been on the road and then are parked in the garage. All of the car's mechanical parts can be very hot after it has been driven for just 10 or 15 minutes in the summer months. Some of the parts, like the disc-brake rotors can easily be in excess of 250 F. The car will radiate heat into the garage for hours after it is parked. If you were to install an insulated garage door and close it, you would trap this heat inside the garage where you do not want it. An insulated garage door may be a great idea for other times of year, it just becomes problematic in the summer months.

This garage gets very hot in the summer and it is directly beneath a large shade tree. PHOTO CREDIT: Tim Carter

This garage gets very hot in the summer and it is directly beneath a large shade tree. PHOTO CREDIT: Tim Carter

Just as the metal parts in your car absorb and radiate heat, so do all of the objects in the garage. Think of how a dying campfire's embers and any rocks that surround it radiates heat for hours after the flames have stopped flickering. This low-level infrared heat can be felt inside a garage even early the next day. I know, as my own garage gets very hot in the summer months. It is very common for my interior-garage temperature to be 10 or 15 F warmer than the outside air temperature when I go into the garage in the morning. The ceiling and walls of my garage are well insulated.

The heat from the garage will absolutely contribute to high temperatures in the attic space above as well as the finished rooms. Hopefully the ceiling cavity between the garage and the finished spaces was insulted. If not, you may want to consider adding blown-in insulation.

To cool your garage as efficiently as possible, you probably will need to install one or two powerful sidewall ventilation fans. There are any number of fans designed to pull air through the garage and exhaust it back outdoors. But keep in mind they will rarely, if ever, be able to drop the temperature lower than the outdoor temperature. This means if it is 90F outdoors at 5 or 6 p.m., it will probably be in the upper 90's F inside the garage.

The fans must have plenty of incoming air to feed their voracious appetites. You will probably have to keep the garage door up off the floor about 4 inches to ensure plenty of air from outdoors is flowing through the garage. The fans will do the best job of cooling if they are located high on the wall directly opposite the garage door.

The air stream will seem very cool to you if you stand in it. But do not think the objects in the garage feel as cool as you do. You feel cool because the evaporating perspiration on your skin is rapidly extracting heat from your body. The inanimate objects in your garage do not sweat, so they will take a while to cool down. Consider putting the fans on a timer that allows them to run for several hours or use a thermostat to control when they go off. You can buy thermostats that will turn the fans on and off at predetermined settings.

To do any appreciable cooling you are looking for ventilation fans that move thousands of cubic feet of air per minute. These fans are not difficult to find, especially if you have access to the Internet. Always use price as a guideline. Fans with well-balanced blades, high-quality motors and excellent exterior weatherproof self-closing louvers will always cost more. I urge you to buy as much fan as you can possibly afford.

Be sure the electrical wiring that is installed to make the fans work is done according to all codes. There is a good chance you will have to install a new circuit or two if you buy large fans. They can draw a significant amount of electricity, and can overload existing circuits if you are not careful.

Author's Comments:

The following are comments from several emails between Norm Sippel and the author.

      "I faced a similar problem when I moved here three years ago. My hobby is racing vintage sports cars. I need to work on the cars year-round. Early in March of our first year here, I went to the garage (which faces east) at 10 a.m. I measured the steel door temperature with a pyrometer at 114 degrees. It was a radiator. No way I could work in those conditions in March, much less in August.

My solution - I insulated the garage doors (a double & a single) with 2" foam panels from a big box home store. I glued them to the panels between the ribs & under the beams. The few metal pieces that were still exposed were still hot, but closer to 100 degrees. The ambient temperature in the garage dropped into the low 90s. I added blown-in insulation after that. Mid-summer the ambient temperature in the garage was down into the high 80s. Running two 5" air conditioning inlets into the ceiling that I can open if necessary dropped the temp even further. I keep them closed when I am not working in the garage & there is no return duct for safety reasons. This dropped the temp in August to the low to mid 80's. That's a tolerable temperature.

You are correct that bringing a hot car into an insulated garage will just keep it hot in there. The brake rotors you note as a major heat source cool a lot faster than the radiator, exhaust & engine do. But if Jan leaves the car out until after dark when it cools down (to the high 70s/low 80s here) before bringing it in, it will be a much improved situation. And, the insulated garage door will do its job in the afternoon sun. That alone should lower the in-garage temperature about 15 degrees in the circumstances described."

(Author wrote: I am all for insulated garage doors, you just have to realize they block the heat transfer in both directions. The insulation holds in the heat overnight.)

      "That's true. But, managing the interior temp is a lot easier when you don't have so many square feet of metal radiating heat to the interior.

Before I moved south, people said to avoid west-facing back-yards. I had hoped to find something where I could watch the sun set. Well, that didn't happen. I'm a mile inland from the Gulf. And, the back of my house with big sliders faces north. BUT, I have even bigger sliders that do face west. So, we have to deal with that. The best solution has been Queen Palm trees. In three years, they are tall enough that they shade the sliders by 4 p.m. in mid-summer."

....

"You're welcome. Keep those educational columns coming. As a major DIYer, I love 'em."

....

"Here's a link to the magazine I work on: www.vintageracecar.com. But, in a previous business life, I worked for Fine Woodworking."

Norm Sippel
Palm Harbor, FL

Column 690

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One Response to Hot Garage Ventilation

  1. Saw your discussion on garage door insulation. I am about to replace my existing double-wide garage door (not insulated but has two vents in it) in order to meet Miami-Dade hurricane codes. Garage faces north- northwest and isn't a problem with heat overload that I have noticed. There are no vents in garage ceiling but there are vents in roof above. Several garage door vendors want to sell me an insulated door. Do I need an insulated garage door, and should it also have vents?

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