This is something that worries me as I am setting up my huge garage too as a collection room. It appears to be rain proof but during a hot/sunny day, it can get very warm in there. I probably won't have my G1s there but I worry about my CHUGs/others.
Edit:
This is an idea to keep things cool:
http://www.ebay.com.au/itm/HPM-Digit...gAAOxy0zhTO-EB
I have a portable aircon unit, what I can do so it doesn't cost me a fortune is plug it in to a programmable power point like the one linked so it only turns on for a limited time a couple of times per day, enough to keep things cool during summer.