Thanks for the thoughtful response, Dillbert. Unfortunately for us, an "on the dirt" install of our greenhouse would have required heavy equipment for earth moving or someone way much younger than we are with strong backs, so we went with this application instead...speaking of budgets!
We had an "in the ground" greenhouse years ago, where the base of it was about 4 feet in the ground and it was all done with bent conduit and heavy duty poly, double layered. That was a fantastic experience, but work and time constraints eventually forced us to dismantle it after recovering the top for the third time, and ultimately sold the property it was on. So here we are, having another go but in an entirely different direction.
I think the misting would be very beneficial in cooling also, and perhaps raising the humidity to some extent. This area is not known for high humidity but there are moments; I think misting would be effective also. Thanks for all the info. This is a great forum!!
If you find yourself in a hole, stop digging.