I don’t think a swamp cooler would work too well in a humid area. I don’t know at what point they become ineffective, but the higher the humidity the less effective they are. You might be better off using regular fans

Yah I was thinking the same thing about the cooler -that's why I asked. I have excellent air movement in the GH with the entire top half open and a strong central fan and two small ones running 24/7 but somehow that heat seems to be trapped inside. An orchidist friend suggested a powered (electric or solar) roof fan. Frustrated.

I had large, commercial greenhouses in SC, and in the summer, I had a large fan circulating air constantly. I had roll-up sides on the houses, and the big fan dragged in air from the outside and blew it back out the back of the house. You need something strong to keep that air moving, pulling it from one end to the other. Can you maybe post a picture of your fan setup so I could advise you better on their placement?

Thanks DG. I think I got it. The roof fan is doing a great job pulling the hot air up and out.