Humidity control: the part that keeps mold from coming back

Indoor relative humidity above roughly 60% supports growth on ordinary surfaces without any leak at all. In hot, humid climates the cooling system is also the dehumidifier, and an oversized unit that short-cycles never removes enough water.

Common fixes include correcting equipment sizing, extending run times, adding a dedicated dehumidifier, sealing crawlspaces, venting bath fans through the roof rather than into the attic, and correcting exterior grading and downspouts.

In cold climates the mechanism inverts: interior humidity condenses on cold uninsulated surfaces. There the fix is insulation, air sealing and ventilation rather than dehumidification.

Whatever the climate, a hygrometer in the affected space is the cheapest diagnostic tool you can own.