Humidity control systems add or remove water vapor from indoor air to stay within proper humidity ranges.
Humidity control is important for three reasons:
It is a large factor in people's thermal comfort.
Excess moisture in a building can lead to mold and mildew, causing problems for indoor air quality.
It is a large energy user. Dehumidification alone can be a quarter to a third of cooling energy in humid climates or seasons. (However, humidifying does not tend to be a large energy load, even in dry climates or seasons.)
Humidity is most often measured by relative humidity, which is the ratio of moisture in the air to the maximum possible saturation of air at a given temperature. The warmer the air is, the more water vapor it can hold. Absolute humidity is the actual amount of water molecules in the air.
Psychrometric charts can help plot absolute and relative humidity data alongside temperature for your site to help you understand how much humidity control you might need in your design. The horizontal lines are absolute humidity, while relative humidity is shown in the curved lines, from 10% at the lowest to 100% (the dewpoint) at the upper left edge of the graph.