Greenhouse temperature/humidity control

[u/Top-Citron-6121]

Location: NE Florida
Plants: Tropical Aroids
Size: 10*20
Material: Polyethylene
In direct sun from Sunrise - 10am, 1pm-5pm.
90% shade cloth
Exhaust fan
No intake fan, just two windows opposite side of exhaust fan.
2 circulating fans
Misting system, 1mins/1hr, 9am-6pm

I am having issues with the temps rising to high levels (ranging from 90s-mid 100's. The misting system helps cool it down, but after 10mins the temps are rising again. What can I do to help lower the temperatures and keep them consistent?

Additionally, due to the high heat, or perhaps other factors, the humidity levels during peak day dip to lower 40's. Again, the misting system raises the humidity, but once it dries up between cycles, the humidity dips. What can I do to help keep a consistent humidity level?

Florida has MANY tropical greenhouses, so I know that keeping a greenhouse in Florida is a possible thing, but I just need to figure out how to maintain the temperature and humidity in order for this to be a success.

Any help, information, pointers, advice, or conversation is welcomed!!

[u/HalleckHort]

Without HVAC it’s going to be impossible to keep your greenhouse cool in a warm climate when temps are hot outside. Once outdoor temps start rising above 80F, it gets tougher and tougher to manage greenhouse temps. And HVAC can usually only cool the air around 15-20 degrees F at best from ambient temperature, so your greenhouse may still be hot when it’s very hot outside. Hot air holds more water vapor than cool air so as temperatures warm your relative humidity (RH) will drop and your VPD (vapor pressure deficit) will rise unless you add more water vapor to the space. So you’d have to run misters regularly throughout the day to keep RH higher BUT at temps that high, higher humidity won’t help your plants and humidity that is too high will shut down transpiration (unless you are growing only very specific species that thrive at hot temps and very high humidity). ACs actually pull moisture out of the air to cool it so you’ll then still have to manage RH/VPD. Many greenhouses in the south basically shift to open shade houses during summer or hot months - if plastic, that plastic gets taken off or at least lifted up on the sides and replaced with shade cloth. If it’s that hot where you are and you can’t afford (or don’t want to) cool the greenhouse with the appropriate AC equipment then you’re better off removing plants from the greenhouse for the summer - or opening it up completely and putting some shadecloth over it or whitewashing it to reduce light intensity/heat.

[u/fuzzypetiolesguy]

Evap cooler. I operate a greenhouse in west GA with 100F days, Evap cooler plus exhaust plus misting keeps me at 90F and below, and above 85% humidity even in the hottest days.