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Ventilation in a greenhouse is definitely a concern that should be addressed before building.
Tubular greenhouses need to have an opening at both ends; the longer the greenhouse, the more necessary this becomes. Keeping a front and back door open allows air to circulate through.
Greenhouses with frames made to look like small houses often have necessary windows in the roof. Two is preferable to one, even in smaller, hobby greenhouses. Two or more openings allow for cross-breezes to blow through.
Automatic vent openers can be purchased and attached to greenhouse windows. Some greenhouse kits come complete with automatic vent openers for each window. These help maintain the temperature in the greenhouse by opening when it is hot and automatically closing as the day wears on and the weather becomes cooler. -
In addition to designing a greenhouse for cross-breezes, fans for air circulation help control both heat and humidity. Fans designed for circulation blow air into the greenhouse, keep it moving around and prevent stagnation.
Place fans in front of the back door to a greenhouse, or hang smaller fans near windows for circulation. Fans need energy to work. One option is to run an industrial strength cord from your home out to the greenhouse. -
Exhaust fans are different from circulation fans; be sure to know what you are buying before completing a purchase. Instead of blowing air into the greenhouse and moving the air around, exhaust fans pull air out of it.
Exhaust fans must be placed in a window; many greenhouse builders install it on the back wall. Exhaust fans prevent stagnation and mildew by preventing too much buildup of humid air.
Like any electrical appliance, exhaust fans must be plugged in. Again, running an industrial strength extension cord out to the greenhouse is a good option. When setting up the electrical connections, make sure that any plug or outlet that is outside has a protective, waterproof cover.
