Passive ventilation, often viewed as old fashion, has recently undergone a revival in the form of innovative greenhouse designs incorporating hinged roof panels, roll-up sides, and new vent designs. Saw-tooth style greenhouses are gaining popularity in the south because the design is applicable to natural ventilation. Cooler air from outside then rushes in through side vents to fill the void. Maximum cooling efficacy from forced air systems is usually achieved using one greenhouse volume, air exchange per minute to a height of eight feet. Several design criteria must be considered to achieve maximum efficiency including fan capacity and location and vent opening size.
Water absorbs a relative large amount of heat when moving from a liquid state to a gaseous state.
The most widely used evaporative cooling system in greenhouses consists of exhaust fans along one wall and cross-fluted cellulose pads along the other wall. Warm air from outside is drawn through the pads by the exhaust fans. The pads are kept constantly wet, and through the process of evaporation, heat is removed from the air passing through the pads into the greenhouse. A high-pressure pump delivers water into the greenhouse through overhead water lines. Specially designed fog nozzles generate fog containing water droplets with a size of less than ten microns when the system is activated.