In almost all cases mold grows where there is excess moisture in a building or on building materials that contain cellulose or other organic material. This means that to find mold you must first “find the water.”
Mold grows by using moisture in the form of water and vapor. Catastrophic events such a roof leak, sewage back-up, pipe break, natural disaster, firefighting, or a backed-up sink, toilet, dish/clothes washer or bathtub, can result in the growth of mold if wet building materials are not dried promptly.
Water vapor from the atmosphere also causes mold growth. Mold can grow in buildings that have relative humidity between 60 and 75 percent or higher. A few examples are inadequately ventilated bathrooms, kitchens and laundry rooms. Water vapor condenses on building materials when the surface temperature of that material is below the dew point. That is to say that the ability of the air to “hold” moisture is limited by temperature. Warm, moist air loses its ability to hold moisture when it contacts a cold surface and water vapor condenses onto the cold surface. For example, water droplets form on the outside surface of a glass of iced tea from condensation.