Garden & Greenhouse


Promoting Greenhouse Ventilation and Dehumidification

Posted September 1st, 2017 by Robin Nichols in

A healthy amount of humidity is beneficial to plant life cycles, but humidity can be a touchy character. Effective ventilation is vital to any ideal greenhouse system and is one of the foundational puzzle pieces to optimizing plant production. Temperature and humidity inside a greenhouse can change drastically throughout a single day, let alone an entire season. High humidity can lead to the slowing of production, the rise of pests and the spread of disease. Giving plants a consistent breath of fresh air is important, and knowing how humidity works will allow growers to choose the ventilation methods that are best for their operation and avoid the mistakes that create unhealthy greenhouse environments for plants.

How Temperature and Relative Humidity Affect Each Other

Relative humidity is the amount of water vapor in the air. When the temperature increases, relative humidity decreases and the air is able to release moisture. Therefore, when temperatures are lowered and humidity grows, the air loses the capacity to release moisture, holding it in the greenhouse’s atmosphere. It may seem counterintuitive, but a great rule of thumb to remember, is that when temperature increases, relative humidity decreases and vise versa. For this reason, when and how much one waters their greenhouse crop is a decision that should be considerate of this rule. High humidity is correlated to a number of conditions that can negatively affect parts of the grow cycle, like photosynthesis and transpiration, so maintaining proper humidity levels is essential.

How High Temperatures and High Humidity Negatively Affect Plant Health

Humidity and air temperature affect many aspects of plant health, from leaf temperature, which is responsible for photosynthesis, to moisture in the greenhouse, which has an impact on the occurrence of disease and pests. With a lack of adequate air circulation, the plant’s ability to make water evaporate from its leaves and to draw nutrients from a growing medium are hindered. Even worse, high temperatures force the plants stomata to close in an effort to conserve water, which constricts the movement of CO2 and O molecules. The build-up of transpired gases and water vapor essentially suffocates the plant if the conditions persist for long enough. The ideal range of leaf temperature for photosynthesis is between 59 to 86 degrees, but greenhouse temperatures can quickly rise without ventilation, becoming too hot for basic plant functions.

Essentially, a lack of control over temperature and humidity create high-risk environments for plant health, opening up a potential Pandora’s Box of disease and pests. There are many factors that lead to dangerous humidity levels, including the combination of regulatory plant processes, like transpiration and the enclosed environment of an unventilated greenhouse.

Effects of Moisture on Greenhouse Pests and Disease

Plants transpire, or release moisture through their leaves, which evaporates into the air. Except, when humidity levels are too high, evaporation slows down and water sits on leaves, just waiting for trouble. Transpiration is basically the plant sweating, so between irrigation and plant transpiration, a greenhouse can become a really humid place without airflow. Overnight, as temperatures cool inside of the greenhouse, moisture also accumulates as condensation on the ceiling and drips onto foliage, creating a moist leaf canopy. Humid vegetation provides a breeding ground for many pests inside unventilated greenhouses, and pests inside the greenhouse can be more severe than those outdoors, vulnerable to predators and the elements. Pests like aphids, thrips and whiteflies can transmit diseases to plants that can quickly escalate into a serious problem, so the risk of a very humid environment is two-fold.

Some of the most common diseases associated with too much humidity are botrytis (blight), powdery mildew or other fungal issues. These problems arise quickly, spread even more quickly and can decimate an entire crop. Diseases like these can spread before a grower ever notices, which is why it is not only important to keep a watchful eye on plants, but equally important to adapt a greenhouse environment by using available technology to maintain a healthy environment. One of the most basic elements of a healthy greenhouse environment is a ventilation system.

Passive Ventilation Methods

Being able to maintain a comfortable temperature with sufficient passive ventilation can help growers to avoid dramatic temperature changes and instability in humidity levels. Passive ventilation provides low-tech, natural solutions to battling humidity levels in the greenhouse, by promoting air circulation and regulating air temperature. It includes roll-up sides, roof vents, doors or any design techniques that maximize airflow and considers building orientation. Wind-driven ventilation design takes into account the wind pressure surrounding a greenhouse, which is as easy as creating cross-ventilation by opening doors and windows on different sides.

Buoyancy-driven ventilation is another passive ventilation method, except instead of cross-ventilation, vents are placed vertically, one higher up on the sidewall and the other below it, closer to the ground. As mentioned, the density of air is affected by temperature and is greater in cooler weather and less dense as heat rises. In buoyancy-driven ventilation designs, warm, moist air is pushed through the top vent as cool air comes in through the bottom, making the heavier, cool indoor air become lighter.

A third and important method used for passive ventilation is called night cooling, and it is exactly what it sounds like. Greenhouses are opened up at night to allow cooler air into the structure, resulting in improved air quality and thermal comfort, while allowing air exchange that helps water evaporate. Night cooling can help to decrease greenhouse humidity and temperature throughout the following day by a few percentage points, which can make a significant difference in the high heat of summer. During different times of the season, night cooling might have to be more closely regulated to avoid losing heat that would then need to be replaced mechanically.

Passive ventilation techniques have been utilized for thousands of years, so the technology is proven and so are its limitations. But, to be sure, passive ventilation can be efficient throughout many climates, during different parts of the year. The greatest benefit to passive ventilation is that it is also a huge money saver for those working on a tight budget or with many buildings, as it depends on natural forces and requires no maintenance, saving on electricity and repairs. For those looking to optimize accuracy and precision in their greenhouse environment, passive ventilation is highly variable, so a combination of passive and mechanical ventilation options might be the best way to go. The best time to address ventilation is during the design process, because it is easiest to integrate many of these tools, like roll-up sides and ridge vents. By preparing systems during the design process and anticipating probabilities of challenges, growers avoid drastic mid-season fixes or poor harvests.

But, because passive ventilation alone has its limitations due to changing wind patterns and high heat, growers need to revise their structures to achieve greater performance. When passive ventilation methods just don’t cut it, many growers wisely choose to supplement the deficits of passive ventilation by adding mechanical ventilation tools to their greenhouse system.

Mechanical Ventilation Methods

Combining both passive and mechanical ventilation to create the ideal greenhouse system is the answer for many growers. Complimenting a passive ventilation system with a mechanical system can help maintain greenhouse environments with a cost-effective approach, and the best part about mechanical ventilation is that it picks up where passive ventilation falls short in fluctuating climates. Mechanical ventilation solutions can help provide growers with significantly more control over their greenhouse environment when the ventilation flow rate is too low.

Mechanical ventilation methods include vents, fans, foggers, exhaust systems, cooling systems and dehumidifiers. These tools can more effectively improve air exchange rate and circulation in comparison to relying only on passive ventilation, which is dependable upon air pressure and wind. Mechanical ventilation helps to create a year-round, fully controllable growing environment no matter what weather conditions a grower faces.  Increasing production while simultaneously controlling humidity and temperature can necessitate more labor, but automated greenhouse technology these days is pretty impressive and has been shown to help out a lot of farmers facing a diminishing labor force.

Automation is particularly helpful as ventilation design mistakes are often made due to miscalculations. Automated sensors help to create a uniform atmosphere inside the greenhouse. Climate controllers act as a main command center that responds to sensor-acquired data by triggering the necessary ventilation tools depending on the data received.

Many of these ventilation options can be automated and computerized to create a virtually hands-off operation, which many farmers find provides them with more accuracy and helps them save energy and time for other vital pieces of their operation. Automating mechanical greenhouse tools still requires the human eye to over-see the farm’s robotic minions. Nevertheless, automation can drastically reduce the time growers spend calculating atmospheric details and adjusting manual ventilation tools, which is where even experienced growers can make mistakes.

Tips to Keep Humidity Levels Comfortable

Regardless of how one chooses to ventilate, make sure to sufficiently equip the greenhouse with enough ventilation to be effective. The size of a ventilation system relies on the size of a greenhouse, so speak to greenhouse design specialists to calculate how much ventilation is necessary for individual businesses. When designing a building, pay attention to air flow and areas of resistance by taking advantage of wind pressure outside of the building and making sure fans are sufficient for pushing around cool air. Miscalculating air flow capacity can be a costly mistake when designing a competent system, and failing to maintain vents or fogger nozzles can affect the performance of an otherwise ample system.

To minimize excess water released during the plant’s transpiration process, avoid watering right before or during sundown, as temperature levels drop, increasing relative humidity and the risk of overnight moisture accumulation in a closed greenhouse.

High humidity levels can be a huge problem for many growers, but there are dozens of affordable, easily managed options that incorporate the latest technology. If an operation can optimize control and air circulation within their greenhouse ventilation system, they will end up with a healthier greenhouse environment and final product. All of these tools can help growers maintain a comfortable environment that improves plant life cycles and increases harvests.

Amanda Williams is a content writer for GrowSpan a company that specializes in greenhouses and growing solutions. Visit their website at She is an experienced grower and owner of Town Farm in Ledyard, Connecticut.

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