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    July 22, 2023 6 min read 0 Comments

    Greenhouse Ventilation

    Greenhouse ventilation is a crucial aspect of maintaining a healthy and productive growing environment. Proper ventilation helps to regulate temperature, humidity, and carbon dioxide to oxygen ratios, which are all essential for plant growth. In this post, we’ll discuss the importance of greenhouse ventilation and the different types of ventilation systems available.

    Why Do Greenhouses Need Ventilation?

    Greenhouse ventilation helps to optimize the environment of your structure throughout the year, extending your growing season and producing the highest yields at the best quality. A good greenhouse ventilation system will:

    1. Regulate temperature: excess heat is a major plant killer, and many plants are quite heat-sensitive.
    2. Control pests: air circulation will help ward off opportunistic pests that love to prey on sickly plants.
    3. Maintain fresh air levels: a good greenhouse ventilation system will let air come in freely and circulate it throughout the entire structure.
    4. Aid in pollination: consistent air movement will help your plants pollinate, especially with self-pollinators like greenhouse varieties of tomatoes.

    Types of Greenhouse Ventilation

    There are two primary types of greenhouse ventilation: passive and active. Each method has its advantages and drawbacks. The best choice for your greenhouse will depend on factors such as climate, budget, and specific growing needs.

    Passive Ventilation

    Passive ventilation relies on airflow through vents, windows, and doors to regulate temperature and humidity. This method is energy-efficient and cost-effective, as it doesn't require the use of fans or other electrical equipment. Natural greenhouse ventilation can be achieved through:

    1. Roof Vents: These are openings located at the top of the greenhouse structure, allowing hot air to escape and promoting natural convection.
    2. Roll Up Sidewalls: Sidewalls that roll up facilitating cross-ventilation and the exchange of fresh air.
    3. Windows and Doors: Opening windows and doors can also aid in natural ventilation, especially during periods of mild weather.

    sidewall ventilation

    Active Ventilation

    Active (or mechanical) ventilation systems use fans and other equipment to actively circulate air throughout the greenhouse. This method offers more precise control over airflow, temperature, and humidity, but it can be more expensive to install and operate. Mechanical ventilation options include:

    1. Exhaust Fans: These powerful fans pull stale air out of the greenhouse, creating negative pressure that draws fresh air in through vents and openings.
    2. Circulation Fans: Also known as horizontal airflow (HAF) fans, these devices move air within the greenhouse, promoting even temperature distribution and reducing humidity pockets.
    3. Air Exchange Systems: These systems consist of intake and exhaust fans working together to maintain constant airflow and a balanced growing environment.

    Active ventilation is great - especially when you can connect it to a thermostat which will automatically turn the fans on once a set temperature is reached. This allows you to create a hands-off control system and frees up your time to focus on other farm tasks.

    That said, it’s worth noting that active ventilation requires you to have electricity at your greenhouse site. It can also be expensive to install and operate.

    Assessing Your Greenhouse Ventilation Needs

    The right greenhouse ventilation system will depend on your specific growing requirements and environmental factors. Consider the following aspects when selecting a ventilation strategy:

    1. Greenhouse Size and Design: The size, shape, and layout of your greenhouse will impact airflow patterns and ventilation effectiveness. Larger greenhouses may require more powerful fans or additional vents to maintain proper air circulation.
    2. Climate: Consider your local climate when selecting a ventilation system. Greenhouses in hot or humid regions may require more aggressive mechanical ventilation methods, while those in cooler or drier climates may be better suited for natural ventilation.
    3. Plant Needs: Different plant species have varying temperature, humidity, and airflow preferences. Consider the needs of your specific crops when designing your ventilation strategy.
    4. Budget: Factor in the initial costs of equipment and installation, as well as the ongoing energy expenses associated with mechanical ventilation systems.

    Greenhouse Ventilation Calculator

    The University of Massachusetts recommends that your total fan capacity should be 2 times the floor area of your greenhouse. Fan capacity is measured in cubic feet per minute (CFM) of air moved. So in a 30' x 100' greenhouse, the fan capacity should be 30' x 100' x 2 = 6000 CFM.

    From there you’ll need to calculate the CFM of your fans. The number of fans can be calculated by dividing the capacity needed by the output of one fan.

    Greenhouse fans are generally available in 12", 24" and 36” diameter blades. Fan output and efficiency increase with increase in blade diameter. Here’s the capacity for our greenhouse fans:

    • 12” Fan - 900 High CFM
    • 24” Fan - 4450 High CFM
    • 36” Fan - 8860 High CFM

    Using the initial example, let’s assume we’re going to purchase 24” fans. To calculate the number of fans needed = 6000 CFM divided by 4450 CFM/fan = 1.35 fans. Use 2 fans.

    Also, remember that fans and vents go together. For every fan you install, you need an equal number of vents.  So if you’re using 2 fans, be sure to also install 2 vents.

    Optimizing Greenhouse Ventilation

    To achieve the best results, it's essential to optimize your greenhouse ventilation system. Here are some tips to maximize airflow and create the ideal growing environment:

    Passive Ventilation Tips

    1. Maximize Vent Openings: Ensure your greenhouse has a sufficient number of vents, ideally covering at least 15-20% of the floor area. This will facilitate efficient airflow and temperature regulation.
    2. Strategic Vent Placement: Place roof vents on opposing sides of the greenhouse to promote cross-ventilation. Side vents should be situated near the base of the walls to encourage cool air intake and natural convection.
    3. Monitor Weather Conditions: Be prepared to adjust your ventilation strategy in response to changing weather conditions. During hot or sunny days, open vents and windows to release excess heat. On cooler days, close vents to conserve warmth and maintain a stable temperature.

    Active Ventilation Tips

    1. Proper Fan Sizing: Select fans with the appropriate capacity to ensure adequate airflow throughout your greenhouse. Calculate the required cubic feet per minute (CFM) based on your greenhouse size and plant needs.
    2. Fan Placement: Position exhaust fans at the highest point of the greenhouse to efficiently remove hot air. Circulation fans should be spaced evenly and directed toward the opposite end wall to promote optimal airflow.
    3. Regular Maintenance: Keep your fans and ventilation equipment clean and well-maintained to ensure efficient operation. Dust and debris buildup can reduce airflow and increase energy consumption.

    Monitoring and Adjusting Greenhouse Ventilation

    It's crucial to monitor your greenhouse conditions regularly and adjust your ventilation strategy as needed. Use a combination of thermometers, hygrometers, and CO2 sensors to track temperature, humidity, and carbon dioxide levels. Observe plant health and growth patterns, and make modifications to your ventilation system to address any issues that arise.

    Greenhouse Ventilation Best Practices

    Adopting best practices for greenhouse ventilation can help you achieve a thriving growing environment for your plants. Keep the following guidelines in mind:

    1. Operate Circulation 24/7: Except when using exhaust fans or open vents, maintain constant airflow in your greenhouse to promote uniform temperature and humidity levels.
    2. Avoid Obstructions: Ensure your greenhouse layout allows for unimpeded airflow. Avoid overcrowding plants or placing large objects in the path of circulating air.
    3. Insulate and Seal: Proper insulation and sealing of your greenhouse can help maintain temperature stability and reduce the need for excessive ventilation. Check for drafts and seal any gaps in walls, doors, and windows. In colder climates installing an air inflation kit with a second layer of plastic on your high tunnel or greenhouse can help a lot with this practice. 

    Upgrading Your Greenhouse Ventilation System

    If your current ventilation system isn't meeting your needs, consider upgrading to a more efficient and effective solution. Investing in upgraded ventilation can save you time and money in the long run.

    One of the best upgrades you can make is installing a thermostat that connects to your ventilation system to automate on and off features according to ambient temperature.


    Greenhouse Ventilation FAQ

    What is the best way to ventilate a greenhouse?

    The best way to ventilate is through a multi-prong approach. Rolling up the sides of your hoop house or opening windows and doors on the greenhouse are good ways to start. Passive ventilation methods have distinct limitations. They take time each day to roll up and roll down. You’ll have to always be on hand to manually adjust airflow depending on the weather and time of day.

    This is where fans - especially ones connected to a temperature control unit - come into play. Fans not only maximize greenhouse ventilation, but they allow for a more hands-off approach. Any bit of time saved during busy farm days is worth it!

    Don’t forget the value of shade cloth. While not a ventilation mechanism, shade cloth is very important to maintaining proper temperatures. To understand the best shade cloth for your growing situation be sure to read our article on Monofilament vs. Tape Shade Cloth.

    At what temperature should I vent my greenhouse?

    The ideal temperature for a greenhouse in the summer is 75-85 during the day and 60-70 at night. During the winter this generally decreases to 60-70 during the day and around 45 at night. Anytime your greenhouse is exceeding these recommendations, you’ll need to vent it. 


    Adequate greenhouse ventilation is crucial for maintaining a healthy and productive growing environment. By understanding the importance of airflow, selecting the right ventilation method, and optimizing your system, you can create the ideal conditions for your plants to thrive. Keep in mind that regular monitoring and adjustments may be necessary to ensure ongoing success. With the right ventilation strategy in place, you'll be well on your way to a flourishing greenhouse.

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