Indoor humidity in winter

Indoor humidity in winter: why it falls when air is heated

Warm air can hold more water vapor than cold air. When outdoor air comes into a home and warms up, no vapor is added, yet the relative humidity falls: the same amount of vapor now sits in air that could hold more.

An example from the calculation itself: −10 °C and 80% outside, 22 °C in the room. That comes out at about 9%. Outside the air was close to saturated; after heating it is very dry on the relative scale.

Relative humidity is the share of the vapor the air could hold at its current temperature. Absolute humidity is the vapor itself, in grams per cubic metre. The calculator shows both. It computes saturation vapor pressure with the Magnus form and the Alduchov and Eskridge (1996) coefficients.

Indoor relative humidity calculator: what it leaves out

The result describes outdoor air that was simply heated, with no moisture added. In a real home, breathing, cooking, drying laundry and plants add vapor, so the actual humidity can be higher than the calculation. Read the number as a lower bound.

The calculator cannot know the real humidity in your room. For that, it has to be measured there.

What the EPA says about indoor humidity

In its brief guide to mold, moisture and your home, the US Environmental Protection Agency (EPA) writes: "If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity" (EPA). The context there is mold and dampness. It is an EPA recommendation, not a medical norm, and we do not present it as a health limit. The label under the result only compares your number with the EPA range.

How dry air is linked to winter illness is covered in Why we get sick more in winter: cold, dry air, and indoor time. The article shows what those conclusions rest on: animal experiments and models, not studies of people. The calculator itself makes no claim about health.