Proper airflow is essential for healthy, thriving plants. Damp, crowded conditions cause fungal pathogens and insect pests to thrive. This can even result in rodents setting up shop in your greenhouse. One of the most common scourges of greenhouses is the whitefly. Air circulation eliminates their populations more safely and effectively than chemical controls.
Along with making sure that the plants you're growing have at least six inches of space between their outermost leaves, your greenhouse will need proper ventilation. Since carbon dioxide is essential for the process of photosynthesis, your greenhouse needs to have some simple vents evenly placed throughout the structure to allow fresh air inside. You should have at least one large vent positioned on or near the roof of your greenhouse in order to allow rising heat to escape.
There are many plant varieties that languish or die as the result of being exposed to temperatures that are too high. If you live in a warm climate, you can keep your greenhouse cool by having an evaporative cooler installed and using it in conjunction with an exhaust fan. Exhaust fans also push stale air out of your greenhouse and make room for fresh air to enter through the vents. Likewise, circulation fans keep air moving and ensure it reaches all parts of your structure.