How to Ventilate a Greenhouse

greenhouse ventilationProper ventilation is essential if you want to grow plants in a greenhouse. Ventilation removes excess heat, controls humidity levels, allows plants to get the carbon dioxide they need for photosynthesis, and lets air circulate to create a consistent environment and support pollination. A greenhouse can use a natural (passive) or mechanical (powered) ventilation system.

A natural ventilation system allows air to flow through wind and thermal buoyancy (air’s natural tendency to rise). In some cases, it might be possible to roll up a part of a wall to allow air to circulate throughout a greenhouse. If that is not possible, ridge and sidewall vents will be necessary for natural ventilation.

When the temperature in the greenhouse rises, the hot air rises and escapes through ridge vents. This creates a vacuum that draws cooler air in through the sidewall vents, which are located near the ground. Wind passing over the ridge vents also draws air into the greenhouse. The total ridge vent area and total sidewall vent area should each be equal to 20 percent of the greenhouse’s floor area.

A mechanical ventilation system uses fans or other mechanical devices to promote airflow and circulation. A mechanical ventilation system can produce more consistent conditions than a natural system and can create a sealed environment that helps to keep pests out of the greenhouse.

If you are going to use a mechanical ventilation system in your greenhouse, you will first need to know when you plan to use it. More ventilation is needed in the summer than in the winter. If you plan to use your greenhouse year-round, base the size of your fans on the amount of ventilation you will need during the summer. Buy fans that can be controlled by a thermostat or that have variable speeds.

Fans are rated for the number of cubic feet of air they can remove per minute. To choose the right fan size, calculate the volume of your greenhouse by multiplying the length, width, and height.

During the summer, the minimal rate for ventilation is one air change per minute. If the greenhouse has a volume of 3,000 cubic feet, you will need a fan with a rating of at least 3,000 cubic feet per minute. The ventilation requirement is lower during the winter because humidity is more of a concern than temperature control. The recommended rate of air changes during the winter is three per hour. Your needs in the spring and fall will be somewhere in between. Thermostatically-controlled fans or automated atmospheric controllers are recommended.

Since heat rises, exhaust fans should be placed near the top of the greenhouse. Intake vents or fans should be placed closer to the ground on the opposite side of the greenhouse to promote air circulation. Oscillating fans can help create uniform conditions within the greenhouse. Perforated polyethylene tubes can be connected to intake fans to help distribute fresh air.

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