You need a fan that is mounted as high as possible and blows the hot air out, an exhaust fan. You could open a door on the opposite side to allow cool air in or mount an inlet shutter opposite the fan.

The rule of thumb is one air exchange per minute to hold the temperature rise to 10-15F on a sunny day. Thus you need a fan rated about 12x16x10= about 2,000 CFM.

The cost of running the fan shouldn't be too much. I run two or three 8,000 CFM fans most of the year and that's only ~$50 per month.

if I install it in the south gable end, do I need a vent in the north gable end to exhaust the air?

Although in theory it could work both ways, I think most people use the blower as an exhaust. Your intake will be passive as air pressure forces in new air to replace what was exhausted. Typically it comes in around the door, but it will enter wherever there are gaps. If possible, it's better to mount the exhaust blower on the end opposite of your door and away from any obvious gaps.

Thanks for the help! I understand now that I have to find a fan that has a rating of about 2000 CFM in order to have a complete air exchange every minute. But, would it work to have the smaller fan and a roof vent or two? Or do they work against each other? I would have automatic openers on some of the side windows as well. I've been looking at the greenhouse supply sites and it seems as soon as you put "greenhouse" in front of fan the price goes up by about 100%, and you have to pay extra for thermostats and plug-ins. There must be something different in an attic exhaust fan to make it so much cheaper. I'm still tempted though.

as you put "greenhouse" in front of fan the price goes up by about 100%

Yep. Fans or anything else.

As long as you understand the basic principles of air flow, different designs can accomplish the same goal. Air will always flow from areas of high pressure into areas of lower pressure, and it will take the path of least resistance to get there.

If you open windows, it's best to have them on opposite sides and oriented with the prevailing wind. The cross-flow is free venting and great for plants. Some heat will then trap itself near your ceiling, thus the roof vent openers.

But when you introduce blowers and have several open gaps, it's hard to control where the air is flowing. If it were me, I would probably put a $40 250cfm wal-mart blower at about ground level, blowing inward, and then close all but the roof vents. That should exchange your air once about every five or six minutes, pushing cool air in and hot air out. The effectiveness of air exchange relates to the temperatures outside versus in. Vents and fans both stop working as a sole cooling method when the outside temp gets high enough. Then you need shade, too.

Hot air rises so it's by far best vented out or pushed out at the highest point. Cool air should be introduced opposite an exhaust fan and probably down low.

Don't mix an exhaust fan near a vent or door. The air will just travel in a circle and never cool most of the structure.

I looked up my area's average temperatures and found this: Daily temperatures rise above 85' on an average of only one day per year and fall below 23' on an average of only 2 nights per year.

During the winter, the average daily high and low temperatures are 47' and 38', respectively. The summer months are equally mild, with an average high temperature of 68' and low of 53'.

With this info, do I still need a complete air exchange once per minute? I've read both of your responses and now wonder if I have an exhaust fan in the upper north gable end set to come on at temperatures above 80', and have 4 automatic vents on the lower opposing side windows, as well as the screen door opened in the south side shouldn't that be sufficient airflow in my moderate climate? I'd like to stay away from the automatic roof vents because they seem to fail so quickly, much faster than the ones in the side windows. And, what temperature am I aiming for? Is about 80' good for my peppers and tomatoes and for my seedlings? I am still eating lettuce from last fall's outdoor sowing, so I don't expect to need much extra heat in the greenhouse in winter. (Sorry, I just love to brag about my winter lettuce!)
Again, thank you for your advice. I just want to do this right the first time.

Wind is your other variable. I tend to have windy spring weather, so most of my venting can be accomplished with just open doors on opposing sides. But even with roll-up sides, I give up on the greenhouse when the outside summer temp hits the mid-90's. It's too hot to be in there without shade.

Tomatoes and peppers like days in the 80's. Even the low 90's is ok as long as they cool off at night.

karin:

You do have a very mild climate and that does make a big difference. I don't think you need the automatic roof vents and you would never mix them with an exhaust fan. So I think your last plan is sufficient. If it still gets too warm consider shade cloth during the "heat" spells.

I'm excited to have a final plan, although half the fun is the planning. Now to get some nice days to start building!