When colder outside air enters an enclosed space through forced or natural ventilation and is then heated to room temperature, the temperature increase significantly reduces the relative humidity inside — often to just a “fraction” of what it was outdoors.  In these cases, humidification may be necessary, if:

1. 1. 1. The outside relative humidity is low and the rate of air change is such that the relative humidity falls,
      2. There is a great amount of heat generated in the space which increases the temperature, or
      3. The relative humidity to be maintained is very high.

Humidity may also be required in tightly closed rooms, where the natural temperature rise on warm days lowers relative humidity.