Ideal indoor humidity in winter

Thirty to fifty percent relative humidity is the band nearly every authority converges on, and unusually for this category the agreement is real rather than copied. It has a hard upper bound with a specific reason behind it, a softer lower bound, and a cold-weather exception that most winter humidity advice omits entirely.

The upper bound is not a preference

The EPA's guidance on home humidifiers states it plainly: do not humidify to indoor relative humidity levels exceeding 50 percent, because higher levels encourage the growth of biological organisms. That is dust mites, mold and mildew, and it is the reason the ceiling is a ceiling rather than a comfort suggestion.

It is also why we size every recommendation on this site to hold a target rather than to maximize output. A humidifier that can push a room to 65 percent is not a better humidifier; it is a machine capable of causing the exact problem you would otherwise be paying it to prevent.

The lower bound is about you, not the building

Below roughly 30 percent is where the familiar winter complaints start: dry nasal passages, chapped lips, static shocks off door handles, and a scratchy throat in the morning. It is also where wooden furniture, floors and instruments begin to shrink and check.

The lower bound is softer than the upper one because the consequences are gradual and reversible. Nobody's house is damaged by an afternoon at 28 percent. Two months at 20 percent is a different matter, and it is what an unhumidified house in a cold climate genuinely does in midwinter.

The cold-weather caveat

Here is the part most winter humidity advice leaves out. In cold weather the practical ceiling is not 50 percent, it is whatever your coldest surface will tolerate before water condenses on it. That surface is almost always a window.

Warm indoor air holding 45 percent relative humidity meets a single-glazed pane at near-outdoor temperature, is chilled below its dew point, and drops its water onto the glass and the frame. Run it long enough and the water sits in the frame, and that is a mold problem you built deliberately. The colder it gets outside, the lower the indoor humidity you can hold without this happening, and the effect is strongest on the worst glazing.

The general shape of the relationship is well established in the HVAC engineering literature: the limiting indoor relative humidity for no condensation falls as outdoor temperature falls, and rises with better glazing. ASHRAE publishes tables for it. We have not reproduced specific figures here because we could not extract them from the source document we located, and we would rather point you at the principle you can verify on your own glass than at a number we could not check.

Why the number drifts down in a heated house

This is the mechanism behind the whole category and it is worth understanding once. Relative humidity is relative to temperature. Cold outdoor air at 20F and 70 percent relative humidity contains very little water in absolute terms. Bring that air inside, heat it to 70F, and the amount of water in it has not changed - but the amount it could hold has risen enormously, so the relative figure collapses.

That is why houses get dry in winter and why the dryness tracks the heating rather than the weather. It is also why the problem is worse in a well-heated house than a cool one, and why turning the thermostat down a couple of degrees genuinely does raise indoor relative humidity without adding a drop of water.

What this means for buying

• Size to hold 40 percent, not to reach 60. Use the sizing chart and buy the machine that meets your number, not the largest one in the aisle.
• Prefer a unit with a real humidistat that switches off at a target rather than running continuously. An evaporative unit is partly self-limiting by physics, which is a genuine safety margin.
• Buy a separate hygrometer and put it across the room from the humidifier.
• Watch the windows for the first week. They will tell you your house's real ceiling faster than any table can.