How Does a Greenhouse Work?

Most people understand that a greenhouse not only provides protection from cold and wind, it also provides the heat and light plants require. Even if it’s not insulated, a greenhouse becomes warm not only by trapping the heat generated by the sun during the day, but also by absorbing it into the ground, which further helps to warm the air. A greenhouse also serves to protect plants from pests and predators as well as unexpected or extreme weather events.

So, for gardeners who live in all but the warmest zones of the country, a greenhouse is a great way to either extend your growing season or grow crops year-round. But a greenhouse isn’t much use—no matter where you live—without a proper ventilation system that allows fresh air to enter, circulate, and exhaust. While additional equipment such as heaters and grow lights may be necessary for growing plants in northern climates, all greenhouses, no matter where they’re located, must have a system to move fresh air around.

Greenhouse Ventilation

If you remember high school Biology class, you may recall that in addition to sunlight, warm air, water and soil nutrients, plants require carbon dioxide (CO2) in order to photosynthesize and grow. Plants ‘breathe’ by absorbing CO2 and releasing oxygen. For outdoor gardens, this isn’t an issue since the air normally circulates freely, assuming you haven’t planted your crops too close together. But given its enclosed nature, a greenhouse doesn’t allow for that natural circulation of fresh air, which would lead to ever-decreasing levels of CO2, limiting your plants’ growth. You must provide ventilation to allow your plants to thrive.

In addition to being critical for your plants to grow, fresh, moving air simulates outdoor winds to make plant cell walls stronger resulting in more vigorous plants that produce more fruit. Moving air facilitates pollination as well.

An unventilated greenhouse is also naturally prone to overheating which is not good for your plants, which are happiest in environments where the ambient air temperature is between 60° and 75° Fahrenheit. Most plants get stressed and can die when it’s too hot. Temperatures in a greenhouse can soar to sweltering levels during the day in a sunny climate, even in winter, without outdoor ventilation and mechanical air circulation.

Finally, a major reason for installing vents and fans is to control unnaturally high humidity levels in the greenhouse that can be detrimental to your plants’ health. Moisture builds up in the greenhouse when there are wide swings in daytime and nighttime outdoor temperatures—common in spring and fall—or when there’s a disparity between indoor and outdoor temperatures, as in winter. This moisture, in particular, can build up on the ceiling of the greenhouse and persistently drip down on the plants. Wet foliage, which doesn’t have an opportunity to dry out, is a magnet for mildew, fungi and pests.

Choosing Ventilation Equipment

Sizing and Placement of Vents

A properly ventilated greenhouse starts with a system of sidewall (base) vents that draw in fresh air in addition to vents positioned high on the walls near the ceiling or on the roof (ridge vents) that expel hot, humid air. In order to size these vents properly, you must calculate the floor area. A rule-of-thumb is the total combined ridge vent area and the total combined sidewall vent area should each be around 20% of the floor area.

So, if your greenhouse is 10 feet wide x 20 feet long, for example, that would make the floor area 200 square feet. 20% of 200 feet is 40 feet. Your greenhouse should have a total ridge vent and a total sidewall vent area of 40 feet each.

If positioned properly, this system of vents can go a long way toward providing a passive system for fresh air circulation, but adding vented exhaust fans will probably still be necessary, particularly if you plan on using the greenhouse during warm months. If that is the case, you will need a more robust, thermostatically-controlled, multi-speed fan system in order to mitigate heat and humidity.

Sizing and Placement of Fans—Seasonal Considerations

Fans are rated in terms of how many cubic feet of air they move per minute. To figure out how many and what size fans you need to circulate air in your greenhouse, you must first calculate its volume, in terms of cubic feet. This calculation is length times width times height (L x W x H). The height measurement can be tricky if your greenhouse has a pitched roof, as is commonplace, so you may need to estimate that measurement.

Using the example of our 10-foot-wide x 20-foot-long greenhouse, which has a 10-foot (or thereabouts) ceiling, we can calculate its volume as 10 x 20 x 10 = 2000 cubic feet.

Another piece of important information is how much air must circulate in your greenhouse, for best results, and that depends how warm it is outside. The rule-of-thumb for the rate of air circulation during summer months is at least one air change per minute. In the case of the greenhouse example used above, you would need a fan with a rating of 2000 cubic feet per minute or greater.

In winter months, outside ventilation isn’t so important for temperature control—when in fact supplemental heaters may be necessary—but is critical for controlling humidity levels. Humidity is a problem in winter particularly because the imbalance between indoor and outdoor temperatures creates damaging moisture buildup as we described above. To address this problem, you must calculate a rate of ventilation that will remove excess humidity but keep temperatures in balance to minimize heating requirements. In this case, the rule of thumb is three air changes per hour (or one air change every 20 minutes).

Ventilation rates for spring and fall will be somewhere in between the summer and winter rates described above, depending on your climate.

In terms of where the fans should be placed, since warm air rises, it makes sense to place an exhaust fan somewhere near the greenhouse ceiling, ideally on the opposite side of the greenhouse from where the base vents are, to create airflow. A shutter fan allows you to close it when the fan isn’t running to conserve heat, as necessary.

In addition to the intake and exhaust vents, oscillating fans can be used throughout the greenhouse to allow for maximum air circulation among your plants.

Greenhouses can be game changers for gardeners, particularly those of us who live in the northern half of the country with a frustratingly short growing season. But just having a greenhouse won’t guarantee that your plants will thrive; you must ensure they have a healthy growing environment with plenty of fresh air circulation in addition to their other, more obvious essentials for growth.

Do you do anything special with vents and fans in your greenhouse to ensure maximum air circulation? If so, we would love to hear from you in the comments section.