What Is Relative Humidity?

Relative humidity (RH) is the measurement of how much moisture the air contains relative to the maximum it could hold at that temperature, expressed as a percentage. At 100% RH, air is saturated — it cannot hold more moisture and condensation begins on surfaces. Mold does not require 100% RH to grow, but sustained levels above 60% provide enough moisture for many mold species to colonize porous building materials.

The "relative" part of relative humidity is crucial: warm air holds more moisture than cool air. A room at 70°F and 50% RH contains significantly more moisture than the same room cooled to 50°F — where that same absolute moisture content would correspond to roughly 85% RH. This is why surfaces near cold exterior walls, windows, and in crawl spaces (where air is cooled) are often the first places mold appears even when overall RH seems acceptable.

The EPA recommends maintaining indoor RH between 30% and 60% to limit mold growth and dust mite proliferation. In practice, keeping RH below 50–55% provides a meaningful safety margin. Measuring RH with a digital hygrometer ($10–$25 at hardware stores) is a simple, inexpensive way to monitor areas at risk. Readings above 60% sustained for days or weeks indicate a ventilation or moisture intrusion problem that needs addressing.

Practical RH control strategies include: running exhaust fans during and after showers and cooking (high humidity activities), ensuring clothes dryers exhaust to the exterior, sealing crawlspace floors with vapor barriers, using dehumidifiers in basements and other damp areas, and improving attic ventilation. Air conditioning inherently dehumidifies as well as cools, making properly functioning AC an important mold prevention tool in humid climates.