Greenhouse Ventilation and Cooling
Seasonal Ventilation Requirements for Greenhouses
Greenhouse ventilation involves removing air from inside the greenhouse and replacing it with outside air. Greenhouse ventilation is essential for both good temperature and humidity management and provide uniform airflow throughout the greenhouse. Ventilation is also important for air circulation and replenishing carbon dioxide. Poor air circulation reduces plant activity and can lead to problems with humidity and disease management. Ventilation is about air exchange. The ventilation may be natural (caused by wind and temperature forces) or mechanical (e.g., exhaust fans). Ventilation systems for greenhouses must be considered for three climatic conditions that typically occur during the year—winter, summer, and spring–fall.
Winter Ventilation
A heating system with adequate capacity is needed in the winter to maintain conditions conducive to plant growth and development inside the greenhouse. Even during the coldest part of winter when the heating system is running at full capacity, some ventilation is required in the greenhouse. Fresh, outside air must be ventilated into the greenhouse to remove the warm, moisture-laden air from within. Condensation occurs when warm, humid air in the greenhouse comes into contact with a cold surface, such as glass, fiberglass, plastic, or structural components.
Summer Ventilation
The main purpose of greenhouse ventilation during the summer is to prevent the inside air temperature from rising too high above the outside air temperature. This would occur due to the large influx of solar radiation through the greenhouse glazing material. The ventilation system must effectively move air directly through the crop and over the soil to prevent excessive temperature buildups around the plants. A generally accepted minimum ventilation rate for temperature control in the summer is one air change per minute.
Spring–Fall Ventilation
The recommended ventilation rates for the spring–fall seasonal periods will fall between rates required for summer temperature control and those required for winter humidity control. The spring–fall periods are characterized by days that are relatively cool and cloudy and other days that are warm and sunny. No special provisions are necessary for maintaining ventilation rates during this period except for the temperature and humidity controls that will determine the amount of ventilation necessary.
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