To determine the proper size of exhaust fan you may need you must calculate the volume of your greenhouse.
Fans are rated by CFM, which is the amount of air they can move in a period of time, hense, CFM or Cubic Feet per Minute.
For a simple estimation multiply the length x the width x the height of the greenhouse. This will slightly over-estimate you volume because of roof pitch.
For Example: The 8' x 12' x 8' Garden Oasis would need 768 CFM (8' x 12' x 8' = 760 CFM). The 12" standard exhaust fan would be sufficient at 760 CFM.
When you are taking air out of the greenhouse your going to need to let it be replaced with the same volume of air. This is where the need for motorized air intakes comes into play. These should also be sized so that you are letting in the same volume you are drawing out.
Proper Ventilation is a Key Ingredient for Healthy Plants
Your plants will love the protection and light they receive from a greenhouse, but they will miss one thing: fresh air! That's why it is so important to include good ventilation in a greenhouse.
Plants need a fresh supply of carbon dioxide so they can properly photosynthesize and make food. Good ventilation also reduces overheating in the greenhouse and will help deter pests.
For best results, air should circulate throughout the entire greenhouse. In smaller greenhouse the cross ventilation is easily achieved by positioning base bents low on one side and a large vent or exhaust fan placed high on the wall of the opposite side. This will help bring in fresh air and pull stale air out of the greenhouse. Circulation fans placed inside the greenhouse help increase cross-ventilation as well as encourage pollination with in the greenhouse.
Find the right sized exhaust fan
for your greenhouse.