Excess moisture, whether in the form of water or high relative humidity, is typically the number one enemy of buildings and their contents. It can wreak all sorts of havoc. Moisture causes dry rot, or decay, that weakens structural components. It also invites structure-damaging insects, such as termites and carpenter ants.
Moisture causes metals to corrode, including valve handles, ducts, HVAC equipment, and tools. Stored paper goods and clothing can become mildewed and stained when the relative humidity rises about 70 percent. In summer, excess RH causes your AC to work harder than needed. High interior relative humidity can cause building materials, like siding, to have a higher than normal moisture content. That moisture can help lift paint from siding and exterior trim. In summer, humidity will make your AC equipment work harder that it should have to.
Moisture can also create unhealthy conditions in your home. High relative humidity allows mold to release mold spores (microscopic organisms that can bypass the normal filtering of the nose) and cause respiratory problems, such as asthma and allergies. It also supports the proliferation of dust mites, another allergen.
Moisture from surrounding soil can enter crawlspace and basements as water or water vapor. From there, water vapor can migrate throughout the house. In addition, below-grade spaces are like condensation machines. Warm moist air comes in contact with cool surfaces and remoisture content as water. Other sources of moisture include:
Source: Michigan State University Extension
Mold rotting away floor joists
Dehumidification: The Anti-Mold. Besides fixing a water leak, which is imperative to preventing the growth of mold in your home, removing moisture from the air is necessary to make your home mold and mildew free.