Fresh air is needed in greenhouses for a plethora of reasons. Fresh air provides carbon dioxide for photosynthesis and oxygen for root growth. Air movement improves transpiration and cell wall strength. Moving air blends and balances temperature and humidity to create a uniform environment across the greenhouse.
Because greenhouses store solar radiation, temperatures can quickly rise, especially during summer months. Heat and humidity are byproducts of healthy growing plants, but they can quickly build up in the greenhouse and will need to be ventilated out to control the atmosphere. High heat limits the growth of many plant varieties, and therefore greenhouses need a way to remove the excess heat. In a mechanically ventilated greenhouse, a series of side wall exhaust fans operate to remove excess hot air and replace it with fresh, often cooler air.