For best growing conditions you should establish the preferred humidity levels of the plants in your greenhouse and then control the amount of moisture in the atmosphere to suit them. Humidity may be increased by various techniques using humidifiers and may be reduced by ventilation.
In the greenhouse the humidity level is very important as a very humid atmosphere may be harmful to some plants because the rate of transpiration and evaporation is reduced in a very humid conditions, and these plants my suffer damage from overheating unless you use the ventilation to bring into the greenhouse cooler and drier air. On the other hand, many tropical plants from native humid climates require high levels of humidity for a healthy growth and will not survive in a dry atmosphere.
If the air is too dry and the humidity level is low than plants will transpire more rapidly, often losing a great amount of moisture. In this case, plants that are not adapted to cope with low humidity levels will wilt unless you will offer them additional water to the roots. Dry climate pants have specific anatomical feature that reduce the transpiration rate in drought conditions.
The humidity levels in your greenhouse depend to the temperature as warm air is capable to hold more moisture than cold air before it becomes saturated. Relative humidity is a measure of the amount of water vapor in the air expressed as a percentage of saturation point at the same temperature. A humid atmosphere is defined as having a relative humidity of about 75 per cent, while a dry atmosphere has a relative humidity of about 35 per cent.
For measuring humidity in your greenhouse you can use wet and dry bulb thermometers in conjunction with hygrometric tables. An easier way is to use hygrometers which have a dial that gives readings for both the temperature and the humidity. Most greenhouse plants will develop well during the growing season in a medium with humidity between 40 and 75 per cent. If the humidity levels are above 80 per cent then diseases like grey mould and mildew may become a problem. In winter you should reduce the humidity level but the exact level required will depend on the type of plants you grow and the temperature from your greenhouse.
During summer you can increase the atmospheric humidity level by damping down your greenhouse: splash water on the floor and on the staging using a watering can or a hose. For plants that require very high humidity you can use an automatic spray system to control he humidity levels. In a small greenhouse you can mist-spray by hand or provide a trey filled with water that slowly evaporate into the air.