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I'm deciding to build a green house to grow some of my favorite plants. My place is quite hot and dry, and water is very precious. My priority is to keep humidity inside. But I couldn't find a solution about that. The normal green houses use fans and open windows to replace hot air with cool air from outside to control the temperature. But I guess ventilation like that means losing humidity which makes it require more water insertion into the green house.

Is there any design or solution for this? Or maybe I'm not considering something here?

update

I guess I should have explained more that I'm more interested not to lose water instead of having a very humid green house. I don't want to face a dried green house that needs watering several times a day.

I thought that a humid air will keep the pots from drying off, but now that it introduces the danger of fungus, I think an AC will be the best Idea here; Both to cool the air and converting humidity into water for later use.

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  • 2
    put AC in the greenhouse
    – black thumb
    Jun 25, 2018 at 3:29
  • Good idea, I should calc if that electricity consumption does worth it or not.
    – azerafati
    Jun 25, 2018 at 5:17
  • you want constant humidity, so recirculating the air would make it keep constant moisture
    – black thumb
    Jun 25, 2018 at 14:56

3 Answers 3

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One solution I've seen to balance the temperature in a green house without requiring you to vent is using an earth-air recirculating system or geothermal heat pump.

A very simple DIY version if you are building the greenhouse from scratch: * start by digging troughs underneath the green house 1-2 meters deep. * run plastic pipe/conduit through the troughs and connect more pipe to those so one end is an intake that brings are in at the top of the greenhouse and one air is the exhaust pipe that releases are near the bottom of the greenhouse. * install a fan to blow air through these pipes

During the day time, hot air in the green house runs through the pipes, gets colder, stores heat in the ground, and comes back into the greenhouse at a lower temperature. During night time, cool air in the green house runs through the pipes, gets warmer, cools off the ground, and comes back into the green house at a warmer temperature. It's a way to increase the "thermal inertia" of the system so that the temperature extremes are more moderate.

You maintain the option of opening vents to decrease humidity if you want.

My description is from memory and not experience. If you explore this path you should do more research about the specifics.

I will caution you and agree with other answers: too much humidity can be bad. Keeping a greenhouse entirely isolated creates additional problems with pests and diseases that require management.

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  • I just like the idea and will give more thought and research to it, it works like an AC but much better for a green house.
    – azerafati
    Jun 27, 2018 at 13:52
  • I also realized that I can have an AC installed to be turned on automatically when the humidity goes up more than what it should. It condenses water vapor from the processed air which I can store in a small tank to use later.
    – azerafati
    Jun 27, 2018 at 14:15
  • Unless you live somewhere with excess electrical energy (like right next to a hydroelectric dam, or giant wind turbine farm), an AC is unlikely to be worthwhile from a cost perspective.
    – greggles
    Jun 27, 2018 at 15:30
  • sticking to the geothermal heat pump, I thought of AC as an emergency solution if the humidity ever goes over the limit.
    – azerafati
    Jun 27, 2018 at 15:36
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Humidity is NOT a necessity for most plants. In fact, humidity causes huge fungus among us in green houses!

Blowing fans 24/7 is critical. It is no big deal in electricity but humidity will not at all be beneficial to your crops.

If you have a greenhouse you need to make sure you have a few things such as: thermometers for temperature. A way to open the bottom of the skirts of your green house both long sides to get air flow. Heat rises. Vents at the peak are very nice to have, depends on greenhouse design.

I always have big fans blowing, again, 24/7, even at night when the temperatures dip to freezing. My tender plants are wrapped in reemay at night, opened up during the day.

Are you growing in pots or are you growing in the soil covered by your greenhouse. I hope the latter if you are worried about water. The temperatures should always stay between 55 and 85...lower or higher starts compromising plant health and production. Oh...Fahrenheit. sorry.

Plants simply need water to survive. Too much or too little will kill. The tilth of your soil is important, organic matter holds onto water and chemistry making a soil...work better for you and plants.

Automation in your green house means your presence is going to be less and that is a problem. We just, this last 15 minutes now have a watering system. But it requires one of us to plug the hose into the piping and monitor. We will never put water on a timer. Not good. Plants are so different in their needs someone needs to be managing the amount of water and dispersion on a daily basis while observing the soil, the plants...the environment.

I will always have these two huge fans blowing 24/7 while plants are growing. Another point is as plants photosynthesize, they give OFF O2. If that O2 just hangs around the leaves of a plant that plant won't be able to get vital CO2. Blowing the air allows more availability of CO2 to your plants.

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Hot and dry are ideal conditions for evaporative cooling. When water evaporates, it: 1) increases humidity, and 2) decreases temperature. People sweat to get cool. So do plants.

If you rig your greenhouse so that you supply external dry cool air (e.g., from a shaded area) at the bottom, then a top window vent will naturally allow plants to control their own temperature by evaporation. Arranging for a bottom-up airflow in the way will protect plants from diseases related to excess humidity.

Another way to control heat is by selective cooling, with priority on roots. Pots buried in the ground will stay cooler. Larger pots will stay cooler than smaller pots. You can also try Kratky hydroponics in drought areas (I live in CA) and keep the nutrient solution cool. Hydroponics can save you water.

Lastly, use shade cloth to protect against excessive solar heat.

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