At what relative humidity level do excessive condensation, corrosion, and mildew typically occur?

Excessive condensation, corrosion, and mildew typically occur at a relative humidity level of 80 percent or higher. At this level of humidity, the air can hold a significant amount of moisture, leading to conditions that promote the growth of mold and mildew, as well as potential damage from corrosion to various materials. When the relative humidity is high, surfaces can become cooler than the dew point of the air, leading to condensation. This moisture accumulation creates a hospitable environment for mold spores to germinate and thrive, contributing to indoor air quality issues and potential health problems.

Lower humidity levels, such as 20, 40, or even 60 percent, are generally considered to be within a range that is less conducive to such drastic issues, especially when considering typical building environments where humidity control is critical for both structural integrity and occupant health.