This was the first year that I used a greenhouse to grow herb as well as other vegetable plants. California's central coast is quite mild. If it's 80 degrees out, the greenhouse will approach 100 degrees. My greenhouse is well ventilated (passively), but if it's 80 degree outside, then a fan is necessary. To be honest, I would not grow anything in a greenhouse during the summer in Arizona or California's central valley. It's just too hot. You will definitely need a powerful exhaust fan. Also, a greenhouse seems to be an ideal environment for aphids and spider mite. Fortunately for me, a Neem oil and soap solution with application every 3 days for 2 weeks remedied the problem.