What relative humidity level must be achieved through ventilation or dehumidification for effective moisture control?

Below 60%.

Effective, long-term mold prevention hinges on proactively controlling the environmental conditions that permit mold spores to absorb moisture and begin growing. The consensus among professionals points to a specific threshold regarding ambient moisture content. To successfully inhibit the germination and proliferation of mold spores on surfaces, the relative humidity must be actively managed and maintained below the 60 percent mark. Achieving this level, usually through improving ventilation systems or installing dedicated dehumidification equipment, starves the mold of the prerequisite moisture it needs to sustain its life cycle, thus making the environment inhospitable even if dormant spores are present.