Cooling a greenhouse....

Thought I'd throw in my experience for what its worth. I built a 10' x 14' 8mm double wall poly GH a few years ago.
I run my exhaust fan and my swamp cooler in unison with temp controls. I have a speed control hooked up to my exhaust fan so that I can keep a slight positive pressure going. When temp dictates @80 they both kick on, if temp rises much above this a secondary temp controls kicks in the water pump for the cooler. Even with these controls I need a shade cloth for 90+ days. I use a 50% aluminet cloth on a roller I built so I can easily roll it up and out. I never have issues with humidity until later in the season..NW Oregon that's when the dehumidifier comes in. A few pics of the build..

chemphlegm said:

yeah my greenhouse RH is significantly higher than the rh outdoors without any controls. Add a dirt floor, a plastic sheet and some watered plants to air blowing through a wet sponge....I suspect the rh then would be so high/ risky for cannabis flowers to flourish healthy

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Everything is relative. I use hail netting http://www.smart-net-systems.com/agricultural-nets/crop-hail-protection instead of plastic in the summer no problem with heat or rh it keeps the bugs out and diffuses the light.