The purposes of ventilation are to exchange carbon dioxide and oxygen, to remove hot air, and to lower relative humidity. Hobby greenhouses can be vented by natural flow-through ventilation or by forced-air ventilation.
Flow-through ventilation relies on side and top vents that pull cool outside air into the greenhouse through the side vent as warm air rises and exits through the top vent. This is called the chimney effect. The combined side and top vent area should equal about 20 percent of the roof area.
Forced-air ventilation systems consist of a louvered intake vent or vents and electric fan(s) controlled by a thermostat. The fans pull cool air into the greenhouse from outside through an intake vent and push warm inside air out. Be sure that the fan capacity is large enough to exchange the air in the greenhouse at least once per minute. Recommendations for warm climates call for a fan capacity to remove 12 to 17 cubic feet per minute per square foot of floor area.
