It is a common experience to step indoors and find the air feels heavier and more damp than the outside environment, even on a day when the outdoor air seems relatively dry. This stems from a complex interaction of atmospheric principles, human activities, and building characteristics that often result in higher humidity levels inside our homes. Understanding this phenomenon involves examining how moisture behaves in different temperatures and how our living spaces contribute to its accumulation.
Understanding Relative Humidity
Humidity refers to the amount of water vapor present in the air. Relative humidity (RH) measures the amount of moisture in the air compared to the maximum amount it can hold at a given temperature, expressed as a percentage. Air’s capacity to hold water vapor is directly influenced by its temperature; warmer air can hold more moisture than cooler air. If the temperature increases, the relative humidity tends to decrease, making the air feel drier, assuming the actual amount of water vapor remains constant. Conversely, as temperature decreases, its capacity to hold moisture lessens, increasing relative humidity.
The dew point is the temperature at which air becomes saturated with moisture, reaching 100% relative humidity, and further cooling causes water vapor to condense. This temperature is a direct indicator of the actual water vapor content in the air and is often considered a more reliable measure of how humid the air truly feels than relative humidity alone. While absolute humidity refers to the total mass of water vapor per unit volume of air, relative humidity provides a more practical understanding of comfort and the potential for condensation within indoor environments.
Sources of Indoor Moisture
Elevated indoor humidity stems from continuous moisture generation by human activities and household elements. An average family of four can produce about 20 gallons of water vapor weekly. Cooking, especially boiling water or steaming food, releases substantial amounts of water vapor. Showering and bathing introduce considerable moisture, and from breathing and perspiring.
Laundry activities, particularly air-drying clothes indoors, also add significant moisture. Houseplants release water vapor through transpiration, and pets contribute through their respiration. Moisture can also enter a home from the ground, wicking up through basements or foundations, or from plumbing leaks, adding to the internal moisture load.
Building Design and Air Exchange
Building design and construction significantly regulate indoor humidity levels. Modern homes, built to be highly insulated and tightly sealed for energy efficiency, can inadvertently trap moisture generated indoors. This contrasts with older, draftier homes that allowed more uncontrolled airflow, which permitted humid indoor air to escape and drier outdoor air to enter. Tightly sealed structures lack adequate air exchange, meaning internally produced moisture has fewer dissipation pathways, leading to accumulation.
Ventilation is important for managing indoor humidity, as insufficient air exchange prevents humid indoor air from being replaced by drier outdoor air. Air exchangers bring in fresh outdoor air while expelling stale, humid indoor air, reducing moisture levels. Insulation helps maintain consistent indoor temperatures, aiding condensation control. Vapor barriers prevent moisture from passing through building components. When installed correctly, these components work together to control moisture movement and protect the effectiveness of insulation, though improper placement can sometimes trap moisture.
Temperature’s Role in Indoor Humidity
Temperature influences why indoor spaces feel more humid. This means that if the absolute amount of water vapor inside and outside is the same, cooler indoor air will have a higher relative humidity because it has a lower capacity to hold moisture. For instance, air conditioning in summer cools indoor air, increasing its relative humidity even if no new moisture is added.
Conversely, warmer outdoor air can hold a greater amount of moisture, potentially resulting in a lower relative humidity outside, even if the absolute moisture content is higher than indoors. This temperature difference can make indoor spaces feel significantly more humid in relative terms, even when the outdoor environment might contain more total water vapor. Therefore, the perception and measurement of humidity are heavily influenced by the thermal conditions of the indoor and outdoor environments.