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. The dew point is the temperature at which air is saturated with water and condensation occurs.

Because of the temperature variations inside your home, 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.

For example, if your home has a basement it 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 (RH levels will increase 2% for every one degree temperature drop). To prevent mold growth the EPA recommends levels of 30-50%RH. Mold spores germinate in high moisture levels after 12 hours and can grow within 24-48 hours. The basement of this home would clearly be at risk for mold and mildew growth, odors and structure damage, if the humidity issues are not resolved.

Homes built on a slab have a similar problem. Though the room temperature may be 77°, the slab temperature is much cooler. As the slab temperature approaches the dew point  (or point of saturation), relative humidity rises and condensation can form.

High RH contributes to many indoor air quality issues, including the following:

MOLD & MILDEW: Molds are fungi. They only require high humidity and an organic-based material on which to feed. It is no surprise that they are plentiful in humid environments. Cooler surfaces create a perfect breeding area for mold because the relative humidity at the surface will be near 100%. The surface may even be wet with condensation.

ODOR: Mold and Mildew are not only damaging and can create health issues, but can also produce a very unpleasant odor.  High humidity often penetrates organic materials causing smelly odors that cannot be removed without reducing and controlling relative humidity levels.

DISCOMFORT: High relative humidity levels leave warm areas feeling sweaty and sticky, and colder areas feeling cold and damp.