Relative humidity is the ratio of water vapor pressure in air to saturation pressure at the same temperature, expressed as a percentage from 0 to 100 percent. HVAC systems must control relative humidity to maintain occupant comfort and prevent building damage. Comfortable indoor environments typically maintain 40 to 60 percent relative humidity at 72°F to 78°F.
Humidity Effects and Comfort
At 80°F and 80 percent relative humidity, occupants experience discomfort despite adequate temperature control because the high moisture content prevents skin cooling through perspiration evaporation. Conversely, air below 30 percent relative humidity causes dry skin, sore throat, and respiratory irritation. Building materials experience damage at humidity levels above 60 to 70 percent, promoting mold growth and wood deterioration.
System Design Requirements
Load calculations must account for both sensible and latent cooling to achieve target humidity levels. A standard residential system sized for 95°F and 75 percent relative humidity outdoor air must reduce humidity from approximately 95 grains per pound to 55 grains per pound indoors. Properly charged and airflow-balanced systems maintain indoor humidity within the 40 to 60 percent range. Oversized equipment cycles insufficiently, failing to dehumidify adequately.