Natural ventilation works by supplying cooler air, such as through vents or an open-roof design, to allow the lighter, warmer air to be pushed out. All greenhouses built prior to the 1950s featured a system of louvers and vents to help allow excess heat to escape by pulling cooler air in.
More and more, new greenhouse construction is utilizing these same methods, but retrofitting an existing greenhouse is not significantly more than installing fans and shutters. In many cases, it can be about the same or even less.
Relying on natural ventilation alone will not work if the quantity of heat to be removed is too great. In that case, evaporative cooling systems are a simple and relatively inexpensive alternative.
Both, however, will require mechanical ventilation through the use of pads and fans, or a fogger system utilizing nozzles through the greenhouse. Mechanical systems may be employed in smaller greenhouses, but the larger structures will benefit from natural ventilation methods as much as possible.
Besides the energy efficiency and reduced costs it offers, natural ventilation allows for more even crop cooling. Crops can be spread evenly throughout the greenhouse, and the grower can adjust the vent openings as needed. This will result in a more consistent crop, which in turn can improve sales and the bottom line.
Naturally cooling large gutter-roof greenhouses has traditionally been a challenge in Southern climates, specifically due to lack of sidewall space and relying on roof vents alone can result in uneven cooling.
Open-roof greenhouse designs eliminate this issue, though to combat the heat on particularly hot, sunny days, a shade system can help control the amount of heat that can escape can be controlled.
A few additional considerations to keep in mind with new greenhouse construction or retrofits:
Size of Vents. The total combined area of roof and sidewall vents should be the same and should be at least 15 percent to 20 percent of the floor area. Gutter-connected designs will need larger roof vents or otherwise an open-roof cooling option.
Location of Vents. Sidewall vents are recommended to be located above bench height to allow outside and inside air to mix before it reaches plants. Again, for gutter-connected designs, roof vent location is more important.
Greenhouse Height. The trend toward taller greenhouses helps improve natural ventilation. Not only does this create the buoyancy effect, where lighter, hotter air rises, but also it helps keep this air well above plant level.
Greenhouse Orientation. If possible, greenhouses should be located where the summer wind follows along the sidewall. That helps utilize the natural flow of air. To make that more effective, ensure that trees, buildings and other greenhouses obstruct it as little as possible.
Have questions about your greenhouse’s ventilation system? We can help! Call us at 1-800-531-4769, and you’ll reach a friendly, knowledgeable representative every time.