Indoor humidity refers to the amount of water vapor suspended in the air within a home. When this level becomes elevated, typically above 60% relative humidity, it creates an environment conducive to several problems. High moisture content can lead to physical discomfort, promote the growth of mold and mildew on surfaces, and cause structural damage like wood rot or the warping of materials. Understanding the source of this excess moisture is the first step toward effective control. This article explores the primary ways water vapor enters and accumulates inside residential structures.
Moisture from Daily Household Activities
Daily household activities continuously generate water vapor, increasing the indoor humidity level. Hot showers are a significant contributor, as the heated water rapidly evaporates and saturates the surrounding air. A typical 15-minute shower can release approximately half a pound of water vapor directly into the home’s atmosphere. This moisture quickly disperses from the bathroom.
The process of preparing meals also contributes substantial moisture, particularly when boiling water or simmering liquids on the stovetop. Cooking activities can add between one and five gallons of water vapor to the indoor air per day, depending on the frequency and type of food preparation.
The occupants themselves are continuous sources of moisture through normal breathing and skin perspiration. An average family of four can collectively release up to half a gallon of water vapor into the air every 24 hours. This effect is often most noticeable in bedrooms overnight, where the concentration of people and lack of air movement allows humidity to build up.
Drying clothes indoors introduces water directly into the air through evaporation. Minor sources, such as houseplants or open aquariums, also contribute to the overall moisture load as water evaporates from soil, leaves, and open surfaces.
Structural and Foundation Sources
Air infiltration, the uncontrolled entry of warm, moist air from the outside, is a significant source of indoor humidity. This occurs through countless small gaps and cracks in the building envelope, including unsealed utility penetrations, electrical outlets, and the perimeter of windows and doors. When this humid outdoor air cools upon entering the conditioned space, its relative humidity rises, often leading to condensation on cooler surfaces.
Moisture originating from the ground can migrate into the home through the foundation, especially in basements and crawl spaces. Concrete is a porous material that allows liquid water to wick upward from the soil through capillary action, a process accelerated by poor exterior drainage. Once inside the crawl space, this ground moisture evaporates into the air, which then rises into the main living areas through stack effect.
The condition of the exterior grading and gutter system directly influences foundation moisture levels. If rainwater is not directed several feet away from the house, the soil surrounding the foundation becomes saturated. This hydrostatic pressure forces water against and through the foundation walls, contributing significantly to the overall moisture load in the lower levels of the structure.
Hidden plumbing leaks represent a sudden and often substantial introduction of liquid water into the structure. A slow drip from a supply line or a faulty drainpipe inside a wall cavity can saturate building materials like drywall and insulation. This saturated material then acts as a large, continuous source of evaporation, releasing high volumes of water vapor into the indoor air over an extended period.
Failures in the roof or exterior siding allow liquid water to penetrate the building envelope during rain events. Even small, intermittent leaks can introduce enough water to soak framing members and sheathing. This trapped moisture slowly evaporates into the interior space, often remaining undetected until mold growth or material damage becomes visible.
HVAC and Ventilation System Failures
The air conditioning system cools and dehumidifies air by condensing water vapor on its cold evaporator coil. An air conditioner that is too large for the space will cool the air quickly, satisfying the thermostat before it has run long enough to effectively remove sufficient moisture. This phenomenon, known as short cycling, results in cool but clammy air because the system is not operating for the necessary duration to fully dehumidify the space.
The water removed from the air by the AC coil must be properly drained away from the unit via the condensate line. If this drain line becomes clogged with algae or debris, or if it is improperly sloped, the water can back up into the drain pan. This standing water may then re-evaporate back into the ductwork or the surrounding air, negating the dehumidification process the system just performed.
The failure to properly vent moisture-generating activities contributes to high humidity. If a bathroom fan is undersized or if the ductwork is improperly installed, it cannot move the humid air outside efficiently. If occupants neglect to use exhaust fans during showering or cooking, the water vapor remains trapped inside the home.
Modern, tightly sealed homes require controlled mechanical ventilation to exchange stale indoor air with fresh outdoor air. Systems like Energy Recovery Ventilators (ERVs) are designed to manage this exchange while minimizing the introduction of excess moisture. If these systems are absent, improperly maintained, or if the home is too tightly sealed without any fresh air exchange, the internally generated moisture becomes trapped and accumulates.