One of the best ways to correct overheating is to ensure optimal ventilation. Where possible, high vents should be placed on the side of your greenhouse opposite your prevailing summer winds. The high vents should be about 15 percent larger than your low vents. A good rule of thumb for freestanding greenhouses is 20 percent of the total floor area for upper vents and 10 percent of the floor area for lower fresh-air intakes.
6.1 Ventilation may be:
Natural – caused by wind and temperature forces.
Mechanical – using fans.
Even during the coldest time of the winter, when the heating system is running at full capacity, some ventilation is still required for the greenhouse. Fresh, outside air must be ventilated into the greenhouse to remove the warm, moisture-laden air from within the greenhouse. If moist air within the greenhouse is not removed, high humidities and excessive condensation will occur. Studies have shown that humidities over 90 percent foster rapid development of leaf mold as well as fruit and stem rot. Problems with leaf mold infection of tomatoes start occurring when humidities exceed 80 percent; below 70 percent, problems with infection are slight.
6.2 Mechanical ventilation – fans
Exhaust fans are used to create pressure differences in greenhouses, thus causing the air to circulate. These mechanical ventilation systems come in negative, positive and neutral pressure varieties. In positive pressure systems fans blow air into the building, resulting in positive pressure in which air naturally escapes through vents. With negative pressure systems fans blow air out of the building, causing negative pressure withing the building that allows air to enter through vents. Neutral pressure systems are a mix of both.
Negative pressure ventilation is the most common and efficient way to exchange air in a building. Optimally, fans will provide ample airflow with a low-pressure difference so that the air entering the building mixes properly with the air inside rather than coming in too fast (i.e. cold air will fall).