Indoor sources for moisture, condensation, or water leaks include the obvious (bathing, cooking, etc.), and the often forgotten sources like plants, unvented gas space heaters, breathing people and animals. A great deal of moisture can enter through building materials (especially the foundation), by "natural ventilation," and air leaks in humid weather. Left uncontrolled, these common sources of moisture, condensation, and water can contribute to the growth and spread of mold in the home.