How to Control Humidity in a Greenhouse

When taking one’s urban gardening to the next level, investing in a greenhouse is the ideal next step. A greenhouse helps protect crops from excess cold and heat, as well as from undesirable pests, so that you can grow certain crops all year long in a controlled environment. However, understanding how exactly to control that environment, specifically the humidity levels, can be kind of tricky when first getting started.

How do you control the humidity in a greenhouse? There are a variety of methods that help to regulate the humidity in a greenhouse: draining condensation, providing proper circulation, using heat-and-vent systems, mechanical or chemical dehumidifiers,  as well as a variety of eco-friendly methods that involve using compost and thermal mass objects as heat conductors. 

The goal of all these humidity management techniques is to maintain the right balance between temperature and humidity so that the plants thrive in a moist environment but are not adversely affected by an excess of moisture. The preferred techniques used to control humidity will depend on how much money you are willing to spend, how much time you have to regularly check on your greenhouse environment, and the type of crop and amount of plants you have in your greenhouse.

Why is Controlling Humidity so Important?

While the proper amount of humidity in a greenhouse is relative, the goal is to achieve a percentage of humidity which does not adversely affect your plants.  As they say, too much of a good thing is still… too much

While you want your greenhouse to have ample amounts of humidity for your crops to thrive, too much humidity can ultimately wreak havoc on your plants. Fungal diseases like Botrytis and mildew love moist environments and are very commonly found in greenhouses due to the wet atmosphere. When the humidity is so high that the ceiling begins to drip, that water can splash potentially infected plant leaves, and that spray can then infect other plants, spreading fungus and disease at a much higher rate.

If water accumulates into puddles on your greenhouse floor, it could lead to algae growth as well as a breeding ground for mosquitos and other insects. These stray, excessive water droplets from humidity can also cause light reflections, which in turn reduce the amount of light in your greenhouse.  Plants need light, and without it, their growth will not be what you want. 

Too much humidity begins to interfere with a very important process called plant transpiration. Plant transpiration is when moisture is pulled through plants from their roots to their small spores under their leaves, where that moisture then turns to vapor and is released into the atmosphere. Basically, it is the evaporation of water from plant leaves. Humidity that is too intense can cause plants to transpire too much to where their roots can’t keep up.

Does my Greenhouse Need to be in Direct Sunlight?

Any crop you’re putting in a greenhouse is most likely going to require a full dose of 6 to 8 hours of direct sunlight each day. Most plants in a greenhouse are thirsty for the sun, so you’ll want your greenhouse to be in direct sunlight (not under some shady tree).

How to Determine the Right Amount of Humidity 

The ideal humidity in a greenhouse is contingent upon the amount of air present as compared with the total amount of water the air can hold. This is referred to as relative humidity. Relative humidity means that the ideal percentage of humidity in a greenhouse varies on a situational basis. Since warm air can hold more water than cool air can the temperature of a greenhouse is extremely important in measuring the correct amount of humidity. For example, if a greenhouse is at 68 degrees F, with 80% humidity, if that temperature were raised, the humidity would need to decrease. So, if the temperature were raised to 80 degrees F, the humidity would need to decrease to around 55%.

The key to regulating the ideal temperature and humidity are key in understanding this balance of amounts of air to amounts of moisture. Part of this balance is understanding that each crop has a Vapor Pressure Deficit (VDP) threshold. VDP in a greenhouse is essentially the difference between amounts of moisture in the air and how much moisture is on a plant’s leaves.

Why is it important to know about VDP? Because VDP drives transpiration, which affects a plant’s ability to intake nutrients successfully. If a greenhouse VDP is too high, it can lead to drought and stress on the plants. If the greenhouse VDP is too low, it can lead to diseased plants due to excess moisture.

How Do You Measure VDP?

Vapor Pressure Deficit is measured in kilopascals (kpa). Kilopascals is essentially the pressure of one thousand newtons of force occurring per square meter of an area’s surface. One thousand newtons equals about 225 pounds.

Optimal VDP is calculated by using temperature and relative humidity. Though generally speaking, the range for VDP inside a greenhouse should range between 0.45kPa to 1.25 kPa. 

What on earth does all this mean!? High VDP equals a drier greenhouse environment. Low VDP equals a wetter greenhouse environment. Which means you want your greenhouse to find a happy medium between the two.

Different Methods to Control Humidity 

Finding the correct amount of humidity and maintaining that humidity in your greenhouse is a delicate dance. You don’t want a totally dry nor a totally wet environment.  Ideally, you want the humidity to be somewhere between 40%-60%. Of course, any dehumidifying system you use will be affected by the amount of plant life in your greenhouse, and the crop type since individual plants have individual VDP levels.  If you would like to calculate VDP levels, check out this link.

The goal with any humidity management system is to keep the humid air circulating, to never allow dew to formulate, and to be patient and diligent in trying different methods that feel successful and also conducive to your lifestyle (as some methods are more time consuming than others).

Start with a Dry Greenhouse

Since too high of humidity is more likely to be the problem than too low of humidity, it is best to start out by having a dry greenhouse. A dry greenhouse entails only watering just enough to meet a crop’s needs, in proper increments of time. For example, you want to water at sunrise, not sunset, because the sitting water at night could be problematic.  However, during the day, the water can evaporate into the moist environment, vs. sitting stagnant in the cold.

Drain Condensation

Draining condensation off surfaces and any other collection points helps to create a higher VDP (i.e., drier atmosphere) to prevent fungal growth. The reason for draining the excess moisture is to ensure more light reaches the plants to encourage growth. 


It’s also important to keep the air in your greenhouse circulating so that the humidity is distributed evenly throughout the space.  That way, the moisture does not collect in one particular area. It’s also helpful to open the windows and doors to ventilate the greenhouse so that some of the moisture dissipates when levels get too high. You can use a circulating or exhaust fan like this one from Home Depot, electric ventilation system, or simply open windows or doors.

Use Heat

To monitor the humidity in your greenhouse, it is helpful to use heat as a tool to prevent the recollection of moisture during cooler weather or at night. Since we want to keep our greenhouse at a constant temperature at which dew cannot successfully form, you will need to use heat to bump down the humidity and dry the environment a bit. Heat can be derived from electric or gas power sources (like a heater) or it can be derived from the sun or thermal mass objects (like clay, rocks, metal.)

Test It

Test your humidity levels regularly using a psychrometer. Then, after some time, you will likely be able to discern the correct amount of humidity and temperature combination simply by feeling the environment yourself.

Heat & Vent Combo System
The most common form of dehumidifier found in greenhouses is a heating and ventilation system that works together to control and purge high humidity, to encourage dry air from outside to circulate in. Of course, when using a heat and vent system, be aware that its success will depend on the outside temperature and the outside humidity (since you are drawing in air from the outside to attempt to control the internal atmosphere of the greenhouse.)

This type of system will typically consume anywhere between 10%-30% of your greenhouse energy bill.

Ventless Dehumidifier Systems

Ventless dehumidifiers are mechanical or chemical, and unlike the heat-and-vent system, they do not draw air from the outdoors.  They are instead a closed system. Ventless dehumidifiers require less use of power and are therefore more energy efficient. Mechanical dehumidifiers use a traditional heat pump or refrigerator type system that fluctuates between heating and cooling the environment through a thermostat. Chemical dehumidifiers, on the other hand, use a hygroscopic solution saturated in salt that helps dry the air.

How do you Regulate Temperature and Humidity without Electricity?

Another slightly intimidating aspect of setting up a greenhouse is the potential to increase one’s power bills.  A greenhouse is, after all, a house for plants, and in order to regulate temperature and humidity, sometimes electricity is involved in its regulation. However, it is possible to design and run a reasonably warm greenhouse using zero electricity. Your greenhouse will not exceed a certain temperature (as in, it may not run at 75-80 degrees), but it will at least keep your plants from freezing in the winter and bruising in the wind.

Harnessing the power of the sun is paramount when attempting to run a greenhouse structure without power. The sun is free energy, after all! However, even the sunniest of climates encounter clouds, shorter days (thanks winter) and the occasional rainstorm. Fortunately, there are ways to work around the sunless hours.

Here are some tips on how to generate heat and energy using no additional power sources:

  • Compost – Compost puts out a natural amount of heat that can organically warm up a greenhouse. In fact, a healthy heap of compost can reach 100 degrees and maintain that temperature for a while, especially when fed with oxygen. Creating paths made from darker mulch will add heat through the energetic process of decomposition.
  • Thermal Mass Objects – objects with thermal mass are capable of absorbing, storing, and dispersing the sun’s heat. This can range from rocks, clay, bricks, metal, big drums of water. These objects hold and release heat for free! Place some rocks, bricks or big black drums of water inside your greenhouse to help generate natural heat.
  • Double-Pane Windows – There is a reason why it is mandatory for certain cold climate cities to ensure all their homes and apartment buildings have double-paned windows. Single-pane windows are incredibly inefficient at moderating temperature.  However, double-paned windows allow warmth from the sun to come in during the day and prevent the loss of heat when the temperature drops at night. Of course, getting double-paned windows for a greenhouse will cost a pretty penny… in which case you may consider just installing some form of power in the greenhouse. However, you can create a make-shift double pane by adding a thick layer of plastic to the window to help better insulate. 
  • Reflection – The ancient Egyptians used mirrors to direct the sun’s light into natural lamps. You can do the same thing by using reflective materials, like tin, on your south-facing interior so that the sun will bounce off the wall and hit the crop beds, which will generate more warmth and sun for photosynthesizing. 
  • Natural Shading – As we’ll discuss further down below, one eco-friendly and cost-effective method to naturally maintain temperature in your greenhouse is to provide shading during the summer month (using netting, shade paint, or blinds). 

If you’d like a more in-depth look at these non-electricity humidity regulating methods, watch this most charming lady break it down for you in this video.

How To Keep Greenhouse Cool in the Summer Months

While a lot of humidity management revolves around making sure there’s not a buildup in moisture, by keeping a constant amount of heat circulating, it’s as important to know how to cool the system down in the summer months.

There is a reason why greenhouses are also called ‘hothouses,’–they are meant to hold heat.  In the hot-hot summer months, this can be a real problem for your crops. While most plants in a greenhouse are sun lovers who need almost constant sunlight, they too have a limit when it comes to heat. Tomatoes, for example, start to show signs of damage at 90 degrees Fahrenheit. 


While you don’t want your greenhouse to be located in a shady spot, you do want to invest in a temporary shade for the squelching summer months to keep your precious plants alive. You can hoist up an actual shade (a dark mesh or heavy-duty fabric like that of an E-Zee-Up can work), or you can use shade paints. Shade paints are a much quicker and cost-effective option to cut some of the heat. Another shading option is to install blinds (this is the most effective heat filtering system, albeit, not the most cost-effective.) 

Damping Down

Desperate times call for desperate measures.  Just like the over-heated kids you might see walking around a theme park in the peak of July with those hand-held misting fans, your plants need a good dampening down themselves. Damping down helps raise the humidity (since extreme heat lowers humidity). You do this by wetting the surfaces inside the greenhouse (from hard surfaces to the walking paths to the leaves). Once the water evaporates, it will add moisture to the overall atmosphere which helps your crops cope better with the heat. You will want to do this rather often during the summer months – it’s hard to over-do this when it is truly blistering. However, since you probably don’t have all day, every day to stand around spritzing down your greenhouse surfaces, it is most conducive to wet down the greenhouse in the morning and the evening.

The Key Take Away

Owning your own greenhouse is incredibly rewarding, especially when the environment is successfully maintained, allowing your plants to truly thrive. Of course, it is not necessarily easy to control the humidity and temperature as this fluctuates daily unless you have a high-tech self-sustaining greenhouse system. However, most people do not and must control their humidity by hand, knowledge, experience and the occasional mechanical dehumidifier.

Now that you understand the science behind the ideal relative humidity for any given greenhouse environment and now that you are equipped with the different methods of maintenance, you will likely find controlling the humidity in your greenhouse less intimidating. If your concern is cost, you can attempt the electricity-free methods. If low-maintenance is your concern, you can install mechanical dehumidifiers or heat-and-vent systems to regulate moisture for you. 

When in doubt, win the lottery and hire contractors to build, regulate humidity, and maintain your greenhouse for you.  If only it were that easy, right?

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