Well For those people who are considering building a greenhouse, I just wanted to share this bit of information. Depending on your location, your biggest temperature control problems may be economically controlling excess heat during summer, or cold during winter. My problem is extreme heat during summer. About 6-8 months ago someone in another forum mentioned they built a "Subterranean heating and cooling system" for their greenhouse, and it kept their greenhouse temps cool during summer (with very little electricity). So I looked up Subterranean heating and cooling systems, and found a good source of info on it. I printed it all out and put it into a 3 ring binder for easy reference. It will be the hart of my greenhouse cooling system.
Subterranean Heating and Cooling System Explained
Subterranean Heating and Cooling System (SHCS) and Greenhouse Design FAQ's
I will be using almost twice the recommended amount of tubing for the the greenhouse square footage (1.5 feet of tubing for each square foot of greenhouse). That would be 300 feet of tubing recommended, for 200 square feet of greenhouse. But I'll be using 500 feet of tubing (600 if I have enough space). I will also be splitting it into two separate systems instead of just one (250-300 feet of tubing each), and only cost me about $8 a month to run (total for both systems).