What Studies Say About Greenhouse Ventilation Methods

Managing temperatures in a greenhouse is of utmost importance to growers, particularly when they start to rise in the summer.Several greenhouse ventilation methods are available, but selecting the one—or the combination—that is best suited for your efforts is dependent upon local climate and even the type of crops you are growing.

With that in mind, these are the most commonly used ventilation methods and a summary of what research says about each one.

Natural Ventilation. This is the most economic option for cooling a greenhouse, but depending on your local climate, natural ventilation alone might not be the most effective. Vents and louvers help aid in allowing warmer air to naturally escape a greenhouse to keep the temperature at a desired level. Roof and side vents—at a recommended 15 to 20 percent of the floor area each—assist in drawing in cooler air and pushing warmer air out of the structure. To take advantage of this, the greenhouse orientation should be such that the normal summer wind direction passes over the ridge. Additional options to maximize natural ventilation include taller greenhouses that draw hot air higher above plants or even open-roof greenhouses that have the potential to save on energy and irrigation costs.

Mechanical Ventilation. Employing mechanical ventilation, such as through the addition of fans, can complement natural ventilation in a greenhouse. To maximize their effectiveness and energy efficiency, you must consider the volume of the greenhouse, the ventilating efficiency ratio (VER) and motor size of the fans. The VER is the ratio of the volumetric rate of air movement to the rate of energy consumption, and fans with a VER rating of 15 or higher are desirable. Slightly larger fans operating on smaller motors can also help save on energy costs. A thermostat will be an invaluable investment to help you monitor the temperature, and placing a thermostat near the center of the greenhouse at plant height is recommended to give the most accurate reading. Also consider alarms that will notify you of high temperature or power failure, as in hotter summer months just a few minutes without proper ventilation can result in excessive heat inside a greenhouse.

Evaporative Cooling. Two types of evaporative cooling systems are fan-and-pad systems and fog-mist systems. Fan-and-pad systems are more common and work by fans drawing air through wet pads to cool the environment. Approximately one square foot of pad is needed for 20 square feet of floor area. Clean water with low mineral content is recommended to prevent clogging and coating of the pads to yield optimal performance. Fan-and-pad systems tend to be preferred by growers in humid conditions, but a fogging system is another option. Some of the advantages of the latter option include that it uses less water, it not only cools, but corrects the vapor pressure deficit, and that it can be installed with little to no structural modifications.

Wondering what ventilation system might work for you or what accessories you’ll need to keep your greenhouse cool? We at Gothic Arch Greenhouses can help. Call us at 1-800-531-4769 or visit www.GothicArchGreenhouses.com to get started.