Humidity Definition

What is Relative Humidity?

Relative humidity (RH) is the amount of water vapor in the air, expressed as a percentage of the maximum amount of water that the air can hold at a given temperature.

Because of temperature variations, RH can vary dramatically. In fact, it is not uncommon to find summertime RH levels ranging from 50% to 100% simultaneously in the same structure. Warmer air holds more water than cooler air and for every one degree change in temperature, the RH changes by about 2%.

For example, if your home has a basement, the basement will typically be cooler than the main floor. If the main floor is 77°F and a comfortable and safe 40% RH, the basement temperature may only be 67°F with a much higher humidity level. Even though there is only a 10° temperature difference, the RH would be over 60% in the basement of the same home. The basement will likely feel damp because the temperature is closer to the 62°F dew point.

Dew point is the temperature at which air is saturated with water and condensation occurs. A common way to measure dew point is to start with Relative Humidity (RH), which is the ratio of the amount of moisture in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage.

Homes built on a slab have a similar problem. Though the room temperature may be 77°F, the slab temperature is much cooler. As the slab temperature approaches the dew point temperature, relative humidity rises and condensation can form. When this occurs, mold mildew and other biological growth soon follows.

Symptoms of high humidity includes musty odour, allergic reactions to mold or dust mites, cupped wood floors, feeling sticky or clammy, visible condensation on windows and walls, presence of water stains, peeling wallpaper, blistering paint and presence of pests.