When looking for greenhouse ventilation options to keep greenhouses cool, it’s important to first understand why ventilation is essential. Ventilation is the exchange of inside air for outer air. The act of ventilation can control temperature, remove moisture or replenish carbon dioxide (CO2). If you’re trying to grow the healthiest plants possible in your greenhouse, establishing a proper ventilation system is essential.

Things to remember about greenhouse ventilation:
• Ventilation systems can be comprised of several different methods. However, always be careful if mixing parts of two specific systems.
• Roof vents with side inlet vents are great forms of natural ventilation. When the warm air rises on convective currents, it can escape through the top and draw cool air in via the sides.
• Most mechanical greenhouse ventilation options use exhaust fans to move the air out of one end of the greenhouse. While this occurs, the outside air comes through the other end of the house, via motorized inlet slots. For effective control, exhaust fans should be the right size to exchange the total air volume in one minute.
• To estimate the total volume of air (for medium and large greenhouses) a gardeners can multiply the floor area times 8.0 (which is the average height). A smaller greenhouse can be estimated by multiplying the floor area by 12.
• Requirements of ventilation will change from season to season. In warm summer months, the air volume changes should be 1:1, per minute. In the colder winter months, 20 to 30 percent of air volume exchanges per minute are adequate.
Greenhouse Ventilation Options
• Most often a single-speed fan won’t be able to meet the air exchange criteria needed. Instead, two single-speed fans are best, with a single-speed fan and a two-speed fan offering the best combination of ventilation rates for year-round tending.
• If a greenhouse is sold with a manual ridge vent, you’ll still need another ventilation system. The manual vent is only meant for backup purposes and shouldn’t be the main system for ventilation.
• Even with ventilation, an evaporative cooling or shade cloth might be necessary.
• Evaporative coolers with fans and an evaporative pad in one box may be good options. They evaporate water, cool the air and increase humidity. Heat is removed from the air to change the water from liquid to a vapor. Many commercial greenhouses place these pads on the air inlets (located at one greenhouse end) and use exhaust fans at the other end. This allows them to pull air through the greenhouse with minimal effort.
Overall, before choosing what greenhouse ventilation options will work best for you, it may be a great idea to do research on the type of greenhouse you utilize. Talk with local gardening experts to see what works best in your regional climate, as well.