Home>Discussions>INSULATION & HVAC>Garage is hot as an oven all summer!
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Bob in Austin
Garage is hot as an oven all summer!

My house in Austin TX has an attached, windowless, unventilated 2-car garage within its roof footprint. The garage is an uninhabitable hotbox all summer - ranging from 95 to 120 degrees. Above the ceiling the attic area over the garage is open to the entire house attic. The rest of the house attic has about 15 inches of blown-in fiberglass insulation but the area over the garage is totally un-insulated. I added 6" of fiberglass batt insulation over the garage (between the ceiling joists) but this had no detectable affect upon the heat situation.

How can I safely (1) ventilate the garage to purge it of hot car fumes (at least for an hour or so after parking a hot car inside), and (2) cool the garage down to 80 degrees to render it inhabitable? The heat also quickly ruins all paints, caulking tubes, and the many other typical items that people normally store in their garages.

I'm thinking that the solution might include a packaged air unit, plus some kind of passive or fan-powered ceiling ventilation system that would run for an hour or so, ducting garage fumes up through the ceiling and out of a rooftop duct vent, and then (maybe?) automatically turns itself off.

Any solution also needs to be fire-safe as well as human safety-safe, since there are potential CO2, gasoline, and stored petrochemical product fumes in my garage. Has anyone successfully done this? Hundreds of thousands, if not millions, of southern homes have this same problem.

A. Spruce
Re: Garage is hot as an oven all summer!

It is interesting that the garage is open to the house attic. This should be a code required firewall. If it were me, I'd install a firewall to close off the house from the attic. This will not only prevent fire from rapidly spreading from garage to house, it will also slow down heat transfer and other issues. Frame the partition on 24" centers, insulate it, and install at least one layer of 5/8 drywall, preferably two layers for added burn through protection. Joints and fasteners just need one coat of joint compound and tape.

As for venting the garage, do you have skirt vents around the perimeter? Again, this should be a code required thing. If so, then you can install a power vent either through the roof or the gable end, which has a thermostatic switch to operate it, I would personally include a master override switch so you can turn it off if you need to. The thing is, though, you've got to have the perimeter venting in order to have supply air for the power vent, otherwise you'll create big problems with back drafting appliances housed in the garage.

Other options would be to leave the roll-up door up 3" and the side door to the yard open to allow air flow and fume exhausting.

Re: Garage is hot as an oven all summer!
A. Spruce wrote:

It is interesting that the garage is open to the house attic. This should be a code required firewall.

Not if the wall to the house & ceiling have 5/8" on it.

Adding a attic fan with a built in thermostat will take some heat out.

Re: Garage is hot as an oven all summer!

Vents, fans, firewall and attic insulation may help a little, but they won't decrease the temperature in the garage to 80 when it's 120 outside (in the shade).

Let me add this idea: a wall a/c unit rated for the size of your garage (400 sf min. for a 2 car garage), and while you are installing that, finish the exterior walls with insulation and drywall. On days when it's 120, even 100 will feel like a welcome relief.

Re: Garage is hot as an oven all summer!

You don't want any penetrations from the garage to other parts of the house. If you vent, do it to the outside, not the attic. I would first insulate the attic above the garage to the standard that the rest of the house is. You should also insulate all of the walls of the garage. If that doesn't work, go the route of a wall a/c unit or fan.

Re: Garage is hot as an oven all summer!

I'm not sure how much this helps, or if it would be allowed in your area, but a white or reflective roof may help some. I have seen here in CO that you can get a reflective paint that you can put on a roofing surface. Some of the products make pretty wild percentage heat reduction claims, but I imagine every little helps. Personally I would like to use that mylar foil looking stuff to reflect tons of heat, but that would look very space age and would probably not be tolerated in the neighborhood!

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