Our GH is 16 x 10 so prety close to your planned size and we are zone 4-5. We have two large automatic roof vents and two large automatic front wall vents. Each roof vent has a double exhaust fan also. We also use a thrmostatically controlled circulation fan to move the air around to avoid mildew/mold issues. I looked into the solar fans and they simply didn't move enough air for either application.

Do yourself a favor while you're in the construction phase and run power and water to your GH. In the long run you'll end up wanting and needing both anyway.

I looked into exhaust fans but the concensus on here was that with roof vents (aI already had), they probbaly would not work as well because they'd draw air from the vents so I put the fans under my roof vents instead. Your CFM calculation seems pretty close if you do go that route.

We only have one normal door with a screen on it so I can't help much with the dutch doors. We don't really use the door for airflow or cooling since we're usually not hope to open it and we don't want the associated bugs and critters to get in. You will find that on a really HOT day there is no such thing as getting too much heat out.

We also use shade-cloth and a misting system for cooling and a propane and electric heat setup for heating.

Owning a GH is a process so don't expect to learn everything in the first year...although since you are still building, I'd read everything you can. In the end it will take experimentation to make it work on your site for the way you want to use it.

Northwestern Ontario?

Too hot? I would think polar bear protection would be the main concern!

I usually have my greenhouse (6' X 12') pretty well emptied out shortly after the threat of frost has passed. My only ventilation is automatic roof vents and a screen combination "storm" entrance door.

My old greenhouse had a window in the end-wall at floor level. I placed a "window" box fan just inside, to pull in air from the outside. It circulated the air inside the greenhouse, pulled in outside air, and forced warmer upper air out the roof vents.

That worked very well for me. But, nothing was automated. (Could have been if I had been willing to pay for it.) It worked well enough for me through the times when it was not too hot outside and I only had to deal with solar heat inside, and not hot ambient temperatures.

When my vents can no longer handle the heat, my plants are moved outside. I do heat it through the winter, however. I only heat it to the level of a "cool" greenhouse. At this time, I am glad I do not have many more doors and windows. My greenhouse is very well sealed and insulated. (5 wall poly.)

I keep some ornamental plants alive for my neighbors, and grow some cool-season vegetables for myself. (Cabbage, Brussels sprouts, cauliflower, lettuce, spinach, carrots, radishes, and beets last winter.) and provide extra space for tomatoes and peppers I start in the spring inside my house. I only heat it to a minimum of about 45 degrees F.

More doors and windows will probably not be a problem, because letting heat out will not hurt unless you are going to heat the greenhouse. If unheated, you will not retain enough heat to help through the night, anyway.

lol the polar bears better stay away! Actually we are having a problem with three black bears right now but that's another story. I think the way your greenhouse is set up sounds more 'doable' for us at this point. I'm thinking two roof vents with automatic openers and then I have a large old stand up fan to move the air around. With the dutch doors at either end I think we should get a cross breeze. I am thinking of building a large composter in one corner to attempt elevate or maintain the temperature on those cold nights but I don't expect to extend my growing season much more than a few weeks.