I have a 3 car detached garage. It is unfinished inside and I love that (no sheetrock or ceiling). The only problem was there absolutely no ventilation. While it is 100 outside, the garage would get over 111degrees inside at the floor (the gauge didn't go any higher) and WELL over 120 above the rafters.
Nerve racking, that is the only way to explain cutting two 12 x 18 squares in the garage (for vents at the base) and a 14½" hole in the center of the roof. All in all, the installation went without a hitch and I even finished the vents off in a 1x4 frame. I caulked and sealed the powered attic fan really well. In fact, I think I may have over done it a little, but I know it was done well.
The end result: amazing difference. The fan is set to turn on at about 105. With the doors shut, you can really feel the air flowing in the vents and the fan really sucks the hot air out of the garage. This was WELL worth the $100 investment for all materials and parts. So if any of you are like me and have a HOT garage you love to work in, try installing some good ventilation.