In Mold, Housing and Wood, wood products experts at the Western Wood Products Association write:
Where is mold found in buildings?
The presence of molds in our everyday environment means they can grow anyplace under the proper conditions. In all cases, moisture is the essential element for mold growth in buildings. There are many potential sources for unwanted moisture in buildings. For example, improperly maintained air conditioning systems that create excessive condensation can be a breeding ground and distribution mechanism for mold particles.
Mold growth may be found in walls, ceilings and floor cavities when standing water is produced in a building or gets in and stays for more than a few days. Sources for water to support molds and other fungi in homes include plumbing leaks, gaps in roofs, siding or masonry, poorly sealed windows, porous slabs and foundations, inadequate drainage, and faulty roof drains and downspouts.