During colder months, the air inside a home often feels noticeably dry, even when outdoor weather reports indicate high humidity. This contradiction—dry indoor air versus moist outdoor air—can be confusing. The experience of static electricity, dry skin, and irritated sinuses suggests a significant lack of moisture indoors. Understanding this difference requires examining how temperature interacts with two specific measurements of atmospheric moisture.
Understanding Absolute and Relative Humidity
To explain the disparity between indoor and outdoor moisture levels, it is necessary to distinguish between absolute humidity (AH) and relative humidity (RH). Absolute humidity is a precise measurement that indicates the actual mass of water vapor present in a specific volume of air, typically expressed in grams of water per cubic meter (g/m³). This value remains constant unless water vapor is physically added to or removed from the air.
Relative humidity, in contrast, is expressed as a percentage and represents the ratio of existing water vapor compared to the maximum amount the air could possibly hold at its current temperature. The capacity of air to hold moisture is directly tied to its temperature; warmer air can hold significantly more water vapor than colder air. If the air temperature changes but the actual amount of moisture stays the same, the relative humidity percentage will change dramatically.
How Heating Systems Cause Low Indoor Humidity
Low indoor humidity is largely a consequence of the relationship between temperature and relative humidity. During winter, cold outdoor air enters a home through ventilation or leaks. This air has a very low absolute humidity because cold air naturally holds very little water vapor. Even if this cold air is saturated with a high relative humidity of 90 percent, the total mass of moisture within it remains small.
When a home’s heating system warms this incoming cold air to a comfortable indoor temperature, the air’s moisture-holding capacity increases substantially. The small, fixed amount of water vapor (low absolute humidity) is now held within air that has a much greater capacity. This process is similar to pouring a fixed amount of water from a small cup into a much larger container; the amount of water has not changed, but the container is far less full.
Consequently, the relative humidity plunges because the air is holding a much smaller percentage of its total potential moisture. For example, outdoor air at 32°F and 80 percent RH contains only a fraction of the moisture of indoor air at 70°F. This causes the indoor relative humidity to drop well below the comfortable range of 30 to 50 percent, an effect exacerbated by forced-air systems that constantly circulate and heat air.
Other Environmental Factors That Dry Indoor Air
Beyond the effect of heating cold air, several other environmental and structural factors contribute to dry indoor conditions. A significant factor is the rate of air exchange, which is influenced by the building’s envelope—the barrier between conditioned and unconditioned space. A home with a loose envelope, featuring drafts and leaks, allows a constant stream of dry outdoor air to infiltrate.
Ventilation systems that constantly cycle air without humidity control also contribute by removing moisture-laden indoor air and replacing it with drier outdoor air. In new or recently renovated structures, porous building materials like wood framing, drywall, and plaster actively absorb moisture from the surrounding air. This natural absorption draws water vapor out of the air until the materials reach equilibrium with the low ambient humidity.
Methods for Increasing Indoor Humidity
There are several practical steps homeowners can take to mitigate the effects of low humidity and restore moisture to the indoor environment. The most direct mechanical solution is the use of humidifiers, ranging from small, portable units to whole-house systems integrated with the HVAC. These devices work by releasing water vapor directly into the air, raising the absolute humidity.
Passive methods can also be effective for a localized increase in moisture. These methods introduce evaporated water or steam into the air.
Passive Methods for Increasing Humidity
- Boiling water on a stovetop.
- Air-drying laundry and dishes instead of using a machine’s heated dry cycle.
- Placing open containers or bowls of water on or near heat sources, such as radiators or heating vents, to encourage evaporation.
- Grouping houseplants, which naturally release moisture into the air through transpiration.