Air usually contains water vapor, the amount depending primarily on the temperature of the air. Warm air can hold more moisture than cold air, so as the air temperature falls, the maximum amount of water the air can hold also falls. The ratio of water vapor in the air to the maximum amount of water vapor the air can hold at a particular temperature is expressed as relative humidity (RH).

For example, a RH of 30% means that the air contains 30% of the moisture it can possibly hold at that particular temperature. As air temperature increases, its capacity to hold moisture also increases, so if air temperature rises and its moisture content remains the same, the RH decreases. Typically, a relative humidity of 40 to 60% is appropriate in many buildings.

Humidity affects the performance of buildings, causing condensation, mold growth, mildew, staining, slip hazards, damage to equipment and the corrosion and decay of the building fabric as well as poor performance of insulation. Too humid air is a risk to both human health and building materials, structures and building contents.

Humidity affects both thermal comfort and indoor air quality.

For example:

  • High RH (very moist air) will make people feel chilled in cold weather and hot and sticky in warm weather
  • Low RH (very dry air) can cause dryness and discomfort in the nose and make skin feel dry and itchy
  • Too humid air facilitates the growth of fungi (mold) and bacteria that can cause respiratory problems and/or allergic reactions and provides the conditions for dust mite populations to grow, which can affect asthma sufferers
  • Results in smells in poorly ventilated spaces because of fungal growth
  • Will result in condensation forming on windows, walls and ceilings that are colder than the air temperature and potentially damaging building materials