I recommend you take a somewhat different approach.

Install shutters in one end, and make sure they are covered with window screen to block the entry of insects. I have mine automated with solar-powered vent openers, so they open as the greenhouse warms to allow some passive ventilation.

In the opposite end of the greenhouse, you could install a swamp cooler to push the warm air out and blow cooler, humidified air in. I only have an exhaust fan in the rear, pulling the air in through the shutters. That fan shutter is screened, as well.

Ray, they're in Missouri. The summer air is already so wet you can't eat potato chips outside.

Backyard greenhouse
 

I had a swamp cooler on my greenhouse when I lived in Louisville KY, and even with all that heat and humidity trapped in the Ohio valley, having been blown east out of MO, it worked fine.

How big was your g/h and how big was the swamp cooler? My swamp cooler is 20' x 3' and when the humidity is above 50%, I can only keep the g/h around 82-85 and boy is it humid but not really cool.

Below 50% I can keep the temps in the 70's and the air is cool but not sticky humid. Lexington isn't as warm or humid as L'ville.

Brooke

There's minimal evaporative cooling when the dew point is over 55 F / 13C but the effluent air will be quite humid.

I know that is why I asked Ray the question. It has been too humid for the last two years.

Brooke

I saw a swamp cooler INSIDE of a very humid greenhouse in Houston once, and despite science suggesting otherwise, the air coming out was cooled.

The air is cooler Ray but it doesn't cool as much as I would like when the outside air is also humid. The air is cooler two feet but that doesn't help 30' away.

How did you get your g/h in L'ville to cool with a swamp cooler.

Brooke

thanks for all info on evaporative cooling. They are not real effictive around here. There are times that they do work (cool), but most of the time they are inefficent, do to our high humidity. My friend, who use to let us barrow space in his greenhouses, used one to cool his houses. Last year we kept our house as cool as his, with only running fans that blow air through the house. I am looking to add loovers and an exhaust fan and a thermosat soon