Humidity is the presence of water vapor in the air, and outdoor conditions exert a strong influence on the moisture levels inside a structure. The relationship between outside and inside humidity is direct, meaning that when the air outdoors is saturated with moisture, that moisture will naturally attempt to equalize with the drier indoor environment. The degree of this influence depends heavily on the construction quality and mechanical systems of the building. To understand the dynamics fully, it is helpful to distinguish between two common measurements: absolute and relative humidity. Relative humidity, which is the figure most often cited, is the ratio of the current water vapor in the air compared to the maximum amount the air can hold at that specific temperature, expressed as a percentage.
The Physics of Moisture Transfer
Moisture from the external environment moves into a building through two primary physical mechanisms: air infiltration and vapor diffusion. Air infiltration, or air leakage, occurs when humid outdoor air is physically drawn into the building through cracks, gaps, and unintended openings in the structure due to pressure differences. This process is the most significant contributor to indoor moisture problems, transporting far more water vapor than diffusion. Vapor diffusion is the slow movement of water vapor directly through permeable building materials like drywall, wood, and concrete. This occurs when there is a difference in vapor pressure, causing the moisture to flow from the area of higher concentration to lower concentration. Controlling air movement is far more effective for managing moisture than controlling material permeability alone.
Building Factors That Influence the Connection
The structural components of a home act as a barrier, moderating the link between outside and inside humidity. Air sealing is primary, as it addresses air leakage, the largest source of moisture transfer. Sealing gaps around windows, doors, electrical penetrations, and foundation joints prevents the bulk flow of humid air into the living space. Insulation helps keep interior surface temperatures above the dew point, the temperature at which water vapor condenses into liquid water. Without adequate insulation, warm, humid indoor air contacting a cold surface will condense, leading to localized moisture issues. Vapor barriers or retarders are important to slow down vapor diffusion. In hot and humid climates, the vapor retarder is placed on the exterior side of the wall assembly to block incoming moisture, while in cold climates it is placed on the interior.
Consequences of Uncontrolled Indoor Humidity
Failing to control the influx of moisture from outside compromises both the building and the health of its occupants. When indoor relative humidity levels consistently exceed 60%, the environment supports the growth of mold and mildew. These biological growths can trigger allergic reactions, exacerbate asthma symptoms, and cause respiratory irritation. Excessive moisture also causes structural damage, including wood rot in framing and the warping of wooden floors and furniture. High humidity degrades finishes, causing paint to peel and wallpaper to lift from walls. Furthermore, high humidity impairs the body’s ability to cool itself through sweat evaporation, leading to clamminess and decreased thermal comfort.
Practical Strategies for Managing Indoor Humidity
Active moisture management begins with the proper use of mechanical systems. A dehumidifier removes excess moisture directly from the air. For homes with chronic humidity issues, a whole-house dehumidifier tied into the HVAC system is often the most effective solution, but portable units can be used for targeted control in damp areas like basements. Sizing the dehumidifier correctly for the space is important, as an undersized unit will not keep up with the moisture load.
The HVAC system is another powerful tool, since air conditioning naturally dehumidifies the air as it cools. Ensuring the HVAC system is correctly sized and maintained helps maximize this dehumidification benefit. Proper ventilation is also necessary to exhaust moisture generated by everyday activities, such as running the bathroom fan for at least 15 minutes after a shower or using the kitchen range hood while cooking.
Occupant behavior can further assist in reducing the internal moisture load. Simple actions like covering pots with lids when boiling water and ensuring clothes dryers are vented directly to the outside help limit the amount of water vapor released into the indoor air. Allowing for good air circulation by using ceiling fans and keeping interior doors open can also prevent isolated pockets of high humidity from developing.