Humidity is the most complicated climatic parameter to control in the horticulture industry: relative humidity varies with air temperature, and plants continuously release moisture to the air. However, humidity control is crucial for crop health and prevention of diseases. Too dry an environment can reduced stem lengths and leaf sizes, whereas excessive moisture causes plant diseases such as botrytis and fungal diseases, limit nutrient uptake due to reduced transpiration, and cause condensation on the cover, reducing the solar radiation entering the greenhouse. Generally, it is suggested that greenhouse air RH should be kept in the range of 60–80% for healthy growth. Crop transpiration and evaporation of water from soil (combined - ‘evapotranspiration’) are the main sources of moisture in greenhouses.
The simplest approach to control humidity in greenhouses is through natural ventilation: on opening the vents, warm, humid air from the greenhouse flows out and is replaced by cold, dry air from outside. Removal of humidity by natural ventilation, however, unavoidably results in loss of heat, both sensible and latent heat. Some alternate approached to limit the heat loss while reducing humidity are dehumidification using forced ventilation and heat exchangers, heat pump dehumidifiers, and hygroscopic chemicals.
Higher relative humidity set points and reducing crop transpiration can further lower the energy use of dehumidification.