Humidity is a measure of the water vapor content in the air, but relative humidity (RH) is the most relevant reading for a home environment. RH is expressed as a percentage representing how much moisture the air currently holds compared to the maximum amount it could hold at that specific temperature. Because warm air holds significantly more water vapor than cold air, the temperature inside a house constantly affects the RH reading. Humidity almost always rises inside a house because the daily activities of the occupants continuously add moisture to a relatively sealed environment. This constant internal moisture generation means indoor humidity levels can quickly become problematic, often independently of outside weather conditions.
Daily Activities That Generate Indoor Moisture
The constant presence of people and their routine activities serve as the primary internal sources of moisture. An average adult generates approximately three pints of water vapor daily through respiration and skin transpiration alone. This continuous output adds a steady stream of moisture to the home’s air supply.
High-steam activities introduce large bursts of moisture in a short time. A five-minute shower can release about half a pint of water vapor into the air, and cooking a single meal contributes roughly one pint, especially through boiling and steaming. Five to seven houseplants can also add another pint of water per day via the natural process of evapotranspiration.
Structural issues can introduce far more significant amounts of water vapor from the ground itself. A basement or crawl space with a dirt floor or poor drainage can wick moisture from the soil, potentially releasing several gallons of water vapor into the lower levels of a home each day. Additionally, poorly sealed windows and doors allow the infiltration of external humid air, which further raises the internal moisture content during warmer months.
Consequences of Excess Moisture in the Home
Sustained high relative humidity creates a favorable environment for the growth of biological contaminants. The most immediate concern is the proliferation of mold, which can begin to colonize surfaces when the RH rises above 60%. Mold spores require only a moist surface and a food source, allowing growth to start within 24 to 48 hours on common building materials and settled dust.
Dust mites thrive in conditions with 70% to 80% RH, meaning that reducing the humidity below this threshold effectively controls their populations. High moisture content also causes general discomfort for occupants, often described as a sticky or clammy feeling. Since the body’s natural cooling mechanism of sweat evaporation slows down in saturated air, the temperature feels warmer than it actually is.
The structural elements of a home are negatively affected because wood is a hygroscopic material that naturally absorbs and releases moisture. When wood components like flooring, framing, and trim take on too much moisture, the fibers expand unevenly, resulting in warping, bowing, and twisting. This expansion and contraction can also compromise the adhesion of paint and wallpaper, causing them to peel or blister away from the surface. Condensation occurs when warm, humid air contacts a surface below the dew point, such as a cold windowpane, leading to continuous dampness and material degradation.
Practical Ways to Regulate Humidity
Monitoring the indoor environment requires a hygrometer and is the first step toward effective humidity control. Experts recommend maintaining a relative humidity level between 40% and 60% to maximize comfort and minimize the growth of contaminants. Keeping the RH below 50% is often the target to minimize biological growth.
Controlling moisture at the source is the most efficient method for immediate reduction. Utilizing exhaust fans in bathrooms during and for at least 20 minutes after a shower effectively vents water vapor directly outside. Similarly, using a ducted range hood when cooking and ensuring clothes dryers are vented to the exterior prevents significant moisture contribution. Promptly fixing any plumbing leaks, especially in basements or crawl spaces, eliminates major structural moisture sources.
Mechanical systems provide necessary support for managing ambient humidity. An air conditioning unit naturally acts as a dehumidifier, causing water vapor to condense into liquid and drain away as warm, humid air passes over cold evaporator coils. For persistent high humidity or in cooler conditions, a dedicated dehumidifier is beneficial. Compressor-based dehumidifiers are more efficient in warm, highly humid environments, while desiccant models, which use a chemical absorbent, perform better in cooler areas like unheated basements.