Thermal
Relative Humidity
RH
The amount of water vapor in air expressed as a percentage of the maximum that air could hold at its current temperature. Because the denominator moves with temperature, the same quantity of moisture reads differently in different rooms.
In more detail
Warm air holds substantially more water vapor than cold air, so relative humidity is a ratio with a moving denominator. Air at 21°C and 50 percent RH, cooled to 10°C without gaining or losing any moisture, reaches saturation and begins to condense. Nothing was added; only the temperature changed. This is why a single whole-house reading describes the thermostat's location rather than the cold corner behind the sofa, the window reveal, or the closet on an exterior wall.
The absolute moisture content is what actually moves around a building, and dew point expresses it in a form that does not shift with temperature. When a moisture problem is being diagnosed across rooms or through an assembly, dew point is the more useful number.
Comfort, health, and durability all respond to humidity, and they do not share an optimum. Human comfort tolerates a fairly wide band. Dust mites and mold do better as humidity rises. Wood flooring, millwork, and instruments move with it seasonally. Very dry air dries mucous membranes and raises static. The commonly cited 30 to 50 percent band is a reasonable compromise across those, not a physiological threshold, and its upper end is often too high for a cold climate where interior surface temperatures make condensation the binding constraint.
Why it matters
Humidity is the variable that connects comfort to building durability, which is why it deserves more attention than the thermostat gets. Run a house at 50 percent through a cold snap and condensation appears on the coldest surfaces — window frames, the corner of an under-insulated wall, the closet on the north side — and moisture sitting on a cold surface long enough is a mold problem waiting for a substrate. Run it too dry and the floors gap and the air gets uncomfortable. The correct target moves with the season and with the enclosure.
The common misunderstanding
That there is one correct indoor humidity number, and that a hygrometer on the wall reports it for the whole house. Neither holds. The right target changes with outdoor temperature, because what limits winter humidity is the coldest interior surface rather than comfort. And relative humidity varies room to room with temperature, so a reading in a warm living room says little about the cold closet where the moisture will actually condense.
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.