I'm just getting my first greenhouse going, and using white greenhouse film that blocks 55% to help with the heat. I'm going to put my cement blocks on their side with screen over the holes to keep the bugs out as bottom ventilation. I also went with a wooden knee wall type setup with white paint to reflect a lot of the heat. I've taken my infrared thermometer out and tested that this solution indeed works very well. The air temp over the white paint is 114F right now while the air temp by my fence is 176F.
I plan on using shade cloth if necessary too, but I've read that the film I am using works better than almost all shade cloth. http://www.greenhousemegastore.com/product/white-greenhouse-film-6mil/uv-treated-plastic-film
For cooling I am trying to find a used aquafog machine, but haven't had great luck there. If I am successful with my small greenhouse I plan on expanding out with a larger hoop house. I'm already pretty heavily invested into my backyard vertical vegetable garden plans, so I hope I can figure out the cooling issues.
I am going for hydro / aquaponics and will be using a water reservoir that should act as a thermal mass to keep the temperatures more consistent as well. I have been freezing sealed bottles of water to drop in during the hot days to keep my water well oxygenated and the plants cooler.
Do any of you use evaporative cooling systems? How do they work out for you?