Managing the humidity levels in your greenhouse can be challenging. If you’re worried about the high or low humidity levels in your greenhouse, you’re at the right place to start finding a solution for a cleaner and safer greenhouse. 

How to control and manage humidity levels in a greenhouse:

General Humidity Management	Adjusting High Humidity Levels	Adjusting Low Humidity Levels
Proper watering	Warming plants	Spraying water
Adequate spacing	Ensure proper air circulation	Drip irrigation systems
Removal of weeds	Correct ventilation	Shades and shade nets
Squeegees/Well-drained floors
Anti-drip plastics
Proper glaze selection
Greenhouse pitch

Depending on the season, there will be various fluctuations of humidity levels in your greenhouse that can eventually become harmful, causing diseases and bugs in your soil and crevices of your greenhouse. Read on to learn how to control the humidity levels in your greenhouse and other tips that will help your greenhouse flourish.

How to Control Humidity Levels in a Greenhouse

Humidity levels can have drastic effects on your plants when uncontrolled or forgotten. Both ends of the spectrum can have harmful effects. 

High moisture is harmful because it brings pathogens and diseases. The moisture promotes germination of fungal pathogens and powdery mildew, which can be harmful to your plants and also for you. Fungal molds are dangerous and need to be prevented and removed. Diseases can quickly spread from plant to plant by splashing debris. 

Many common sources of humidity and moisture dysfunction come from:

• Dripping water from the condensation covering the greenhouse 
• Dripping water from glazing 
• Dripping water that falls onto plant surfaces 
• Puddles of water in corners and behind shelves

Furthermore, if you remove all the humidity and lower your humidity level, this will also lead to negative implications for your plants. When the humidity is too low, your plants may die from heat or thirst, depending on the species. 

General Humidity Management

The following general humidity management options involve merely controlling the humidity level to provide ways of managing humidity levels in the greenhouse and methods on how to adjust high or low humidity levels, respectively. 

Regular and Proper Watering 

Whether your humidity is too low or high, always practice regular watering. In general, keep these three things in mind to avoid unwanted humidity from watering: 

1. Prevent excess watering
2. Avoid puddles 
3. Water early in the day 

Proper watering benefits plants, and adding just enough water prevents excess water from falling on the floor and forming puddles. Be careful not to leave large puddles of water in corners and areas that do not get direct sun. 

In warmer weather, watering early in the day is an easy trick to decrease unwanted humidity. If there are water droplets or water puddles, they will dry throughout the day, which will result in fewer bugs to search for nooks at night and help prevent fungi from forming. 

Adequate Spacing

Thick canopies are natural and a healthy feature of plants, but it is a culprit in locking down moist and humid air into the night. Mesh benches and proper spacing will help keep air circulating under plant canopies and let the transpired air to move out to the open and not stay trapped under the thick canopies. 

To avoid canopies from trapping moisture: 

1. Space out plants 
2. Keep canopies dry 
3. Keep canopies ventilated 
4. Don’t overlap canopies

Remove Weeds

Remove your weeds! Although often overlooked, weeds are also a culprit to adding humidity into the air. Weeds hold moisture in the leaf, then transfer it to the canopy and generate moisture through transpiration. 

Squeegees

This is a trick that many homeowners use for preventing mold in showers. Thankfully, this works well in any circumstances where you need to remove water from hard surfaces. 

A squeegee can remove more than 50% of the water from your greenhouse surface, especially in areas you see excessive condensation. Just run a squeegee and direct the runoffs to the drains! 

Well-Drained Floors

Speaking of runoffs, this leads us to our next suggestion: drain your floors thoroughly. To avoid puddles throughout your greenhouse, be sure to have well-draining floors and drainage pathways. 

This implies that you need to ensure that the water (from watering, plants, and leaks) has a flow path to follow to the drain. It’s easy to use elevation tricks or install drainage tile mats to handle runoff. 

Anti-Drip Plastic 

If you continuously find a stream of condensation on your glazing, try using anti-drip wetting agents on the hard surfaces. Anti-drip agents come in forms of sprays or, often, in the formulation of the glazing for greenhouses. 

Select the Right Type of Glazing 

When possible, use double glazing for your greenhouse rather than single glazing. Double glazing provides better insulation for temperature control and unwanted condensation. 

Double glazing also provides better temperature control inside the greenhouse via the glazing surface. When the glazing surface is warmer, the dew point will be harder to reach than single glazing, which will fall short in preventing the temperature drop. 

Greenhouse Pitch 

A greenhouse with a steep roof can also guide water droplets to follow the path along with the glazing rather than drop onto the plants below. A recommended roof pitch is 6:12, which will allow the droplets to run off on the surface. 

Adjusting High Humidity Levels

When you have high humidity levels, as mentioned previously, you will run into several unwanted problems. These are issues that generally arise at any time of the year, and, depending on your conditions, can be critical to your plants’ health and requires immediate adjustment. 

Keep in mind that moist places caused by leaky pipes and drippy corners attract unwanted pests such as cockroaches! Even the cleanest greenhouses are prone to terrible pests if the moisture level is too high. 

Warming up the Plants 

During spring and winter, most condensation happens at colder leaf surfaces and the air around it. By introducing heat from the bottom, you can improve the air with natural circulation. 

Most people know that “hot air rises.” This method of using hot air is a simple contribution to the air circulation method. A heated air introduced from the bottom carries out the trapped moisture under canopies out to the open, which can then be carried out with proper circulation and ventilation. 

Additionally, it warms up the plant leaves and surfaces that tend to be cooler than the surrounding air, which decreases its chances of condensation! 

Air Circulation 

Along with ventilation, air circulation, in general, will prevent the moisture from sitting on plant leaves as temperatures drop below the dew point. Thankfully, many hot air furnaces come with fans. This is to be used carefully in the winter as excess ventilation and circulation could also be harmful to your plants. 

Ventilation

Ventilation is a critical factor in releasing some of the moist air outside of the greenhouse. Ventilation and proper heating methods can significantly help in reducing moisture levels in a greenhouse. 

Heating and ventilation can work in conjunction. For example, with increased air movement, heat can be used to carry the densely moist air in the open-air direction. 

Adjusting Low Humidity Levels

For some plants in the summer, the hot pressure can make them suffer. With the help of moisture and humidity, they can experience a dampening down process, which helps them from overheating. Some heat-sensitive plants are tomatoes and cucumbers. 

Spraying Water

Just like the opposite of extracting moisture from the air, the simple alternative is spraying water to the surfaces of the greenhouse. 

By introducing water to the greenhouse’s flat surfaces in the early morning, the water on these surfaces will evaporate throughout the day and bring in moisture as it leaves with the heat. This provides a slow humidity addition for the air around the plants. 

Drip Irrigation 

For plants or conditions that require extra water and moisture, use drip irrigation systems or fine spray systems to deliver moisture directly to plants when they need a little pick-me-up. For a hands-free method, you may want to connect your humidity level indicator, such as a hygrometer like the one found on Amazon, to spray water in automated controls.

Shades and Shade Nets 

Additionally, to prevent direct heat on such plants, you can add shades or meshes to the greenhouse walls and ceilings to prevent rapid evaporation and redirect and dampen the sunlight. 

Shade or Sun cloths, like this type found on Amazon, come in various shading rates, are budget-friendly, and are highly effective against harsh sunlight. This will be a great addition to not only control humidity but also to control sun exposure for the more delicate plants.

Understanding Humidity Levels in your Greenhouse

Although it may feel overwhelming, don’t worry! By growing your plants in a greenhouse, you have the benefit of controlling your plants’ environment. With proper tools and an understanding of humidity, temperature, and light, you’ll be able to provide a suitable home for your plants to grow in. 

Whether it’s during low or high humidity, always practice good housekeeping and regular gardening practices. Regulating humidity levels can root from just the basics of keeping the greenhouse floors dry, no pun intended. 

Water puddles on the floor, leaves, the soil, or growing media can cause evaporation and increase the relative humidity, leading to condensation and a moist and humid environment. Additionally, evaporation adds humidity to the environment around the plants and takes away the energy needed to keep the greenhouse warm. 

Relative Humidity 

RH stands for Relative Humidity. The moisture in the air is often expressed by the relative humidity. This is a ratio of the weight of moisture present in the air and the total moisture holding capacity in that unit of air (at a specific temperature and pressure).

Humidity has an essential connection with temperature. If you think about how much hotter a hot sticky summer day is, you can relate. 

What about Temperature?

Temperature plays a significant role in the ratio of moisture for RH. The warmer the air is on the temperature scale, the higher the moisture capacity it will have. Therefore, warm air has a higher moisture-holding capacity than cold air.  

• Higher temperature = More moisture the air can hold
• Lower temperature = less moisture the air can hold

So, when the temperature rises inside a greenhouse, the RH will decrease, even though the same amount of moisture is held in the air.

To give you a better perspective: every 20 degrees Fahrenheit increase in a greenhouse, the RH decreases by one-half. This means that the 20 degrees increase doubles the moisture-holding capacity of the air. The air in a warm 80 degrees Fahrenheit greenhouse can hold double the amount of moisture compared to a cool 60 degrees Fahrenheit greenhouse. 

So, cooler air has less capacity to hold the water droplets, which is what can be harmful to your plants in the already humid air. 

What is Dewpoint?

This comes down to the dew point. Dew point, defined by Oxford languages, is “the atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form.”

For your greenhouse, the dew point is the temperature in which the water in the moist air becomes droplets that fall onto our plants and soil. This could also happen directly on plant leaf surfaces. When the leaf surface temperature is below a dew point, condensation will occur and cause droplets, leading to the development of pathogens and diseases. 

This will happen when the RH is too high. Remember that condensation will occur first on surfaces that are the coolest. That’s why you may see condensation on metal surfaces, roofs, pipes, and plants.

Squeegees, proper glazing, and pitch angles can be used to control condensation and unwanted drips of water on plants and habitats for unwanted bacteria and diseases. 

Desired Humidity Level

If it’s harmful to be too humid, but your plants need the moisture to survive, what is the ideal humidity? Most indoor growers aim for 50-70% relative humidity, although 60-80% is usually tolerable and common. 

The specific humidity and moisture level will vary for all plants, but the following is the desired humidity for preventing fungal growth and diseases. Remember, for other plants, you can always section off your greenhouse to provide shading and scheduled spraying or set up drip irrigation systems. 

Use the following chart to achieve the optimal humidity level:

Temperature (in Fahrenheit)	Humidity Level
50	83%
61	89%
68	91%
86	95%

Hints and Tips that Indicate High or Low Humidity Levels 

The following are some tips or hints that you may find throughout your greenhouse that indicate whether your humidity level may be too high or low. 

Some indications of high humidity levels in your greenhouse are:

• Fogging or heavy condensation that accumulates on windows and window frames

Note: If that water accumulation is causing black mold and unwanted moisture on the plant surfaces or walls and ceilings, be sure to lower your humidity quickly. 

• Spring and Fall are common seasons for high humidity. Special attention to water evaporation is needed during this time to prevent excess condensation. The nights are dramatically cooler as the sun goes down during these seasons.
• Temperatures drop during and after sunset. When the moisture sits in the air, the warm temperatures hold the moisture in the vapor form. When the sun goes down and the cooler night air comes through, the vapors experience condensation and soon become water droplets on the cooler surfaces of leaves and glazing. 

Some signs of low humidity levels in your greenhouse include:

• Instances of static electricity
• Dried and cracking millwork
• Dried soil
• Dried and cracking paint (if applicable) 
• Increased transpiration from plants

Note: Signs of cracking millwork and paint may be more relevant to homes but is a great indicator that resembles how a dry atmosphere may be affecting your area, including your greenhouse. 

Tools for Measuring Humidity

To help you keep track of the humidity in your soil and air, you can try using the following items: 

1. Soil humidity sensor 
2. Digital hygrometer (Amazon Link) and thermometer
3. Sling hygrometer

Simple testers, such as this soil moisture sensor meter found on Amazon, are an excellent budget-friendly tool for measuring soil moisture, light, and pH of your plants. There is no need for batteries, and they are easily inserted into the soil for effortless reading and detection. 

Another tool for keeping track of humidity and temperature will be installing a simple hygrometer on your greenhouse wall. It helps to tell temperature and humidity at a fast glance. Make sure to look at meters that can track the humidity fluctuations in your greenhouse. 

Secondly, a traditional sling hygrometer (often called a sling psychrometer) is a great instrument for measuring relative humidity and dew point in an area. 

How do you Read A Sling Psychrometer?

A sling psychrometer consists of two bulbs and works by reading the wet bulb and dry bulb thermometer separately. To find the relative humidity, you wet the wick bulb with water and rotate the two thermometers in the air. When they are spun, evaporation occurs on the wet bulb, which inherently determines the humidity level. 

After spinning the psychrometer, take both measurements and find the difference. (Note: The wet-bulb temperature is never warmer than the dry bulb. If it is reading to be warmer, check that the meter is not broken). 

Example Given: 

Dry bulb temperature (20 °Celsius, 68°F) – Wet bulb temperature (14 °Celsius, 57.2°F) = The difference (6 °Celsius, 10.8°F)

Finally, take the difference and check for the humidity in the sling psychrometer chart, such as the one provided by an article by NASA (Table 11-1). 

With a 6-degree difference, the Relative Humidity level is 51. 

Please note: Most calculations for sling psychometry charts are in Celsius. It should be understood that the 6°C represents the difference and not temperature. Hence, if 6°C were a temperature measurement, it would not be 10.8°F, but 42.8°F. 10.8 is simply the difference between 68 and 57.2 (degrees Fahrenheit). 

Recommended Wet-Bulb Dry-Bulb Sling Psychrometers

Many farmers do enjoy the sling psychrometer over other forms of hygrometers. The following are the top-rated sling psychrometers in the market. All have Amazon Links:

Sling Psychrometer	Characteristics	Rating
Bacharach Sling Psychrometer	Well-rated model for its easy-to-use and compact size	5°C – 50°C (41°F – 122°F)
Brannan Masons Hygrometer	Budget-friendly and very easy to read	5°C – 50°C (41°F – 122°F)
Lancoon Digital Psychrometer	Digital and portable hygrometer offering a fast 10s response	0 to 100% RH, Temperature: -22°F to 199°F, Dewpoint Temperature: -22°F to 199°F, Wet Bulb Temperature: 32°F – 176°F

A Few More Tips on Ventilation, Heating, and Air Circulation

Ventilation, heating, and air circulation are critical components to keeping clean and well-leveled air in your greenhouse. 

Ventilation and Heating

Ventilation and proper heating methods can significantly help in reducing moisture levels in a greenhouse. Ventilation can be the answer for the stale and trapped air in greenhouses that may be a problem during hot or cold seasons. 

In colder seasons, specifically, heating and ventilation can work in conjunction. With increased movement, heat can be used to carry the densely packed (moist) air in the direction out from the canopies, and with proper ventilation, into the open-air movement. 

Although they work well in conjunction, either of them individually may be harmful to a greenhouse. A well-ventilated greenhouse during the cooler days will make the greenhouse too cold for the plants, causing them to freeze or even suffer from the dry air. 

As for the heater, the opposite effect will occur but cause similar problems. With too much heat, you will also evaporate air out of the plants, which could have a similar dry impact on plants. 

The best time to practice the heat and ventilation combo is during the evening as the sun goes down and in the early morning at sunrise. It is advised to provide this ventilation and heating process roughly two to three times per hour during the suggested times to ensure proper ventilation. 

The length of the process will depend on how efficiently and quickly (or slowly) your greenhouse ventilates. 

Recommended Ventilation Systems

Heaters can range from unit heaters to radiant heaters to convection heaters; therefore, you will want to select an efficient shutter system for proper ventilation according to your greenhouse specifications.

You can either prioritize the size (to match the size of your greenhouse wall) or the efficiency (CFM). CFM is the cubic feet per minute a ventilation fan draws out and is related to the area being covered. 

Some recommendations for shutter ventilation with exhaust fans are found below, with Amazon links.

Vent Fan	CFM (at 0.0 static pressure)	Area	Size
Wall Mounted Exhaust Fan – 10 inches	600 CFM	900 SQF	10 inches
Wall Mounted Exhaust Fan – 16 Inches	1200 CFM	1800 SQF	16 inches
Wall Mounted Exhaust Fan – 20 Inches	3368 CFM	2000 SQF +	20 inches

Some recommendations for shutter ventilations without fans are:

Shutter	Size
Broan 433 Automatic Shutter	16.75″ x 2.75″ x 16.75″
Fan Shutter	12” x 12”

Air Movement

As important as it is for there to be heated air movement in the greenhouse and proper ventilation and circulation of the air will ensure the efficiency of removing moisture and preventing air entrapment in corners of your greenhouse or under thick canopies. 

Heat Source/Furnace with Air 

Along with ventilation, air movement will prevent the moisture from sitting on plant leaves as temperatures drop below the dew point. Thankfully, many hot air furnaces come with fans. 

Fan-Jet System

Additional options include a fan-jet system. A fan-jet system is easy to set up. 

Follow these directions:

1. Get a plastic tubing, often called “poly convection tubing,” for the length of the greenhouse.
2. Punch staggering holes throughout the length of the tubing, if not provided already.
3. Install the plastic tube below the ridge (throughout the length of the greenhouse).
4. Connect a fan jet to one of the ends of the tubing.

This can run continuously and draw in fresh outside air or recirculate air within the greenhouse. This kind of system will not be ideal near plants that are hanging close to the air source. This could cause your plants to dry out too quickly!

Horizontal Air Flow

The final option for air circulation is providing horizontal airflow. Simple setups, such as fans placed along the side walls, will help push the air in directed flows. Even though it might not be as thorough, some air movement will help initiate air circulation. 

Even though air circulation is important, try to control the times of air circulation and ventilation. Maintain the air circulation scheduled separately from the ventilation and heating time. 

Conclusion

Although understanding whether your greenhouse is too humid or not humid enough may seem overwhelming, the overall benefit of growing your plants in a greenhouse is that it is in a more controlled environment. Thankfully, it is under your control! By using the correct tools, you can have plants growing strong throughout all four seasons. 

Keep in mind that clean housekeeping and thorough cultural practice of removing additional humidity contribution and water puddles throughout the greenhouse is critical regardless of whether your humidity level is too high or too low. Happy Gardening!

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