Relative Humidity

Relative humidity levels can affect the release rate of many indoor air pollutants, their concentrations in the air, and the potential growth of mold organisms. Keeping the Relative Humidity (RH) in the ideal range will help indoor air pollution prevention efforts. The following chart shows you what happens at different levels of RH. Note that humidity levels from 0% to 100% are shown on the bottom horizontal line.


Relative humidity levels are often ignored in homes despite the fact that unsuitable levels can be unhealthy, increase heating and cooling costs, damage building components, and affect comfort levels.

Research has shown that high-relative humidity levels also support the growth of dust mites, mold count levels and bacteria levels that can lead to increased allergy symptoms and reduce indoor air quality (IAQ). However, little to no measured data is available on actual indoor humidity levels in homes across the United States.

The key to controlling relative humidity levels in a home is to maintain a moisture balance. Yes, some moisture in the air is necessary for human health and comfort, but too much moisture can cause problems. A moisture balance is maintained by making sure a home has exterior drainage away from the house in all directions, a properly sized heating and cooling system, vapor diffusion protection (through the installation of vapor retarders where needed), as well as adequately planned ventilation, especially at points where moisture is produced. This means that a kitchen stove needs an exhaust fan over it, which should be ducted to the outside. A circulating fan may remove some cooking odors from the air, but it will not remove moisture. A bathroom also needs an exhaust fan that leads to the outside. This fan should be used when anyone is taking a shower and for a short time afterwards, until moisture levels have decreased to the point that there is no condensation on windows and mirrors. In the laundry room, a clothes dryer should be vented to the outdoors. Having good quality windows is also important. For example, when the temperature inside is 700F and 1000F outside, condensation can occur on the interior surface of a single-pane window when the indoor relative humidity level is at only 15 percent RH. Condensation can occur on a double-pane window at 42 percent RH. By increasing the energy efficiency of your windows, therefore, you can lessen the incidence of condensation because the inside glass surface does not get as cold. For more information, please refer to the Indoor Air Quality page.

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