Thermal
Humidification
Adding moisture to indoor air. Useful in cold-climate winters, and capable of causing condensation and material damage when the added moisture exceeds what the enclosure and outdoor temperature will tolerate.
In more detail
Humidification adds water vapor to indoor air. Cold outdoor air holds little moisture, and heating it without adding any produces very low indoor relative humidity — the dryness behind static, dry skin and shrinking wood. Equipment ranges from portable evaporative and ultrasonic units to bypass, fan-powered and steam humidifiers integrated with the air handler.
The limit is not comfort but condensation. Adding moisture raises the dew point of the indoor air, and once that dew point exceeds the temperature of the coldest interior surface — usually a window, sometimes a thermally bridged corner or the inside of a wall — water forms there. That is why the humidity a house can carry in January depends on its windows and its enclosure rather than on a preference, and why the sustainable setpoint falls as it gets colder outside.
Equipment type matters for maintenance. Standing-water reservoirs support microbial growth when neglected, and ultrasonic units aerosolize whatever is dissolved in the water, including minerals, which settle as fine dust.
Why it matters
Winter humidification is one of the few comfort systems that can damage a house when it is set by feel. A humidifier held at a fixed setpoint through a cold snap will wet windows, sills and wall cavities. Getting it right means either an outdoor-temperature-compensated control or a willingness to turn it down when it gets cold — and it means knowing the coldest surface in the room, which is usually the glass.
The common misunderstanding
That a target relative humidity is a fixed number. Relative humidity is relative to temperature, and the same 45 percent that is unremarkable at 40°F outdoors will condense on many windows at 0°F. What determines whether condensation happens is the dew point compared with the surface temperature, not the humidity reading itself.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.