Cooling Basics: Keeping a greenhouse, or sunroom, cool is a balance between ventilation, misting and shade cloth, in the hotter months. Ventilation via vents may “sometimes” provide proper ventilation in a manufactured greenhouse, but seldom will one achieve success in a site built structure. Add an exhaust fan and inlet shutters/dampers to effectively move air. A 2-speed fan starts slow, eliminating strong blasts of cold air. Place fan high and shutters low. For sunrooms a 110 Volt Fan-Tastic Vent Fan may be sufficient. For larger greenhouses see Industrial Fans.
Natural Ventilation: Some greenhouses can be ventilated using side and ridge vents, which run the full length of the house and can be opened as needed to provide the desired temperature. This method uses thermal gradients, creating circulation due to warm air rising.
Ridge vents should be about one-fourth the floor area and the side vents about the same size. The roof vents should open above the horizontal position to provide about a 60-degree angle to the roof.
Also add a misting system. Mist is good for your plants and will dramatically assist in the cooling process. Jaybird Mfg makes simple to install and high quality units.