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.