Humidity management for greenhouses

[BeyondTheGarden]

In the PNW, summers are dry, and humidity levels in my 8x12' greenhouse normally fall below 20%.  

My greenhouse is irrigated via a timer and misters; after the misters do their job (once daily) in the morning, humidity is above 90%.  But when the sun hits the greenhouse, the vents open and the exhaust fan scavenges the moist air out of the greenhouse, and humidity falls back below 20%.  Average daytime temps in the greenhouse are mid to upper 90's (with both vents open and the exhaust fan running full-throttle).  When ambient air temps are in the 90s, I have to prop the greenhouse door open to keep the greenhouse under 110.  I guess there's a converse relationship between shedding excess heat and retaining sufficient humidity.  

I have several containers of standing water where water-loving palms and plants soak their feet all day, but this doesn't seem to raise humidity.  

I know some commercial-scale greenhouses have ceiling mounted mist systems that kick on every 20(?) minutes or so, but I worry that the accumulated water droplets would cause leaf burn.  I also have seen many big greenhouses with shade cloth to filter the light, and I'm wondering if that's why they use shade cloth, to prevent leaf burn? 

Sorry for being long-winded, just want to paint a picture of my dilemma here.  

Edit.... I was just looking at the ambient humidity levels for this area, they normally don't seem to fall below 50%.... I wonder if my digital humidity gauge is wrong.... 

Edited September 3, 2021 by Jesse PNW

[Tyrone]

Hi Jesse. Very interesting question this. What switches your exhaust fans on. Is it a temp sensor? If so, rig up a misting system that gets turned on when the exhaust fans come on. If both are linked to temp that should do it.

I used to have a homemade greenhouse at my old property in Perth. I had a temp sensor set to 32C. Once the temp in there hit 32C the misters came on. Humidity was always kept high. Also when it was 42C outside it was in the mid thirties inside and about 90% RH. Just like Malaysia. Also the finer the mist the better. Shadecloth inside may also help moderate temps etc. Also keep your temp sensor out of direct sunlight. Under a bench out in the shadows is best.

[ShadyDan]

I too have a 8x12 greenhouse in the PNW, Located on Vancouver Island, without any humidity management. I have good ventilation and air circulation, but I have never had a issue with low humidity in my greenhouse. Most palms I grow are totally fine with humidity dropping during the day, especially here where the temps generally drop below 20C every night and humidity conversely rise back up. I also grow a few other plants (citrus, hot peppers) that thrive in my greenhouse without any humidity management.

The only time when I tried to manage humidity is during that crazy, record breaking heat wave earlier this year. I'm lucky in that I am less than 50m from the ocean in almost every direction to help moderate things, but still my temperatures got to 38C and 21% humidity OUTSIDE the greenhouse. I threw up some home made shade cloth and completely wetted the floor, wooden benches, etc... down 2-3 times a day to try and manage the heat/humidity. Everything pulled through with 0 issues.

I struggle more with excess humidity in the wintertime when my greenhouse only sees ~3 hours of sun a day (on the rare days when it is sunny) to try and keep the humidity lower and prevent fungal issues.