All of the conditions needed for mold growth (food sources and appropriate temperatures) are present in the indoor environment with the exception of adequate moisture. Prevention of mold growth indoors can not be achieved without proper moisture control. The following are some of the moisture problems that cause indoor mold growth.
Rainwater can enter a building through leaks in walls, windows or the roof. Surface or groundwater may enter when there is poor foundation drainage. Flooding can, of course, cause catastrophic intrusion. In buildings that have slab construction, water can seep or wick up through the cement floor causing mold to grow on carpet pads or carpet backing. The building envelope (walls, windows, floors, roof, etc.) must be well maintained to prevent water from coming in, both to prevent mold growth and to maintain the structural integrity of the building.
When relative humidity (a temperature-dependent measure of water vapor in air) becomes elevated indoors, building materials and furnishings absorb the moisture. Those damp materials can then provide a good place for mold to grow. If there are no cold-condensing surfaces and the relative humidity (RH) is maintained below 60 percent indoors, there will not be enough water in those materials for mold to grow. However, if the RH stays above 70 percent indoors for extended periods of time, mold will almost certainly grow.
In the summer, air conditioning can de-humidify indoor space. But if the system is too large or too small for the space it serves, the cooling system can create high humidity by cooling without removing water vapor. A properly sized and maintained system will dehumidify and cool a building. Learn more about the role of heating, ventilation and air conditioning (HVAC) systems in indoor environmental quality.
When there are cold surfaces in a building, water vapor can condense on those surfaces, just as water condenses on the outside of a glass of ice water. Insulation of exterior walls can prevent condensation and mold growth during the winter.
You should always be mindful of indoor sources of water vapor that can be problematic. Clothes dryers must be vented to the outdoors. Unvented gas or kerosene space heaters can generate enormous amounts of water vapor (as well as other air contaminants), and should be used sparingly and never as a primary heat source. Always run the bathroom exhaust fan when showering or bathing, and make sure the vent is exhausted to outdoors. A properly vented kitchen exhaust fan can remove steam created during cooking.
Learn more about the roles of moisture, mold and biologicals in indoor environmental quality.
For Additional Information
- California Department of Public Health: Building Dampness, Mold and Health
- Centers for Disease Control and Prevention (CDC): Facts About Mold and Dampness
- Environmental Protection Agency (EPA): The Key to Mold Control is Moisture Control
- University of Nebraska—Lincoln Extension, Institute of Agriculture and Natural Resources: Identifying Moisture and Mold Sources in Houses (PDF)
Produced and published by the NC Department of Health and Human Services; NC DPH: Occupational and Environmental Epidemiology. Last Updated: February 20, 2020