What Humidity Should a House Be in Winter? 30–40%, Lower on Cold Nights

Keep your house at about 30–40% relative humidity in winter, and lower on the coldest nights. EPA’s ideal indoor range is 30–50%,¹ but in January your windows set the real ceiling. In a 68°F room, by our estimate, clear double-pane glass starts to fog above 40% once it drops to about 7°F outside, and single-pane glass fogs above 30% at about 26°F.

Below: what EPA and CDC actually say, the dew-point math, a January ceiling for 20 US cities from NOAA normals, and how to tell whether your air is too dry or too damp.

What EPA, CDC and the research say

EPA’s mold guidance is the source of the range most sites repeat:

keep indoor humidity below 60 percent (ideally between 30 and 50 percent)

Its indoor air guide gives the same 30–50%,³ and its mold course adds that low humidity also discourages dust mites and cockroaches.² CDC is stricter:

as low as you can — no higher than 50% — all day long

No agency sets a winter floor below 30%, but one lab study points upward. In a CDC NIOSH test with coughing manikins in a 68°F room, flu virus kept 71–77% of its infectivity at 23% humidity or less, but only 15–22% at 43% or more.⁵

Maintaining indoor relative humidity >40% will significantly reduce the infectivity of aerosolized virus.

That was one simulated room, not a study of homes. Treat 40% as a target to reach only when your windows allow it.

Why cold windows set your winter limit

Water condenses on any surface colder than the air’s dew point. In a 68°F room, the dew point is 35.5°F at 30% humidity, 42.8°F at 40% and 48.7°F at 50%. Window glass runs far colder than the room. A DOE Building America window guide notes that single glazing runs only 10–15°F warmer than outdoors. Its Lawrence Berkeley National Laboratory chart gives the inside-glass temperature at 0°F and 20°F outdoors for each window type.⁸ We read those bars and turned them into a ceiling:

Glass temperature = room − f × (room − outdoor), where f = 0.77 for single pane, 0.42 for clear double pane and 0.20 for low-E double pane with argon (from the LBNL chart, 70°F room).

Humidity ceiling = e_s(glass) ÷ e_s(room) × 100, with the National Weather Service vapor-pressure formula e_s = 6.11 × 10^{(7.5T ÷ (237.3 + T))}, T in °C.⁷

These are center-of-glass figures. The DOE guide says the bottom edge of a window carries the highest condensation risk, and frames vary,⁸ so fog usually shows at the edges first. Below freezing the water forms frost, which starts slightly sooner than this formula shows.

Your city’s ceiling on an average January night

We ran the model at each city’s January average low from NOAA’s 1991–2020 normals, the same stations as our winter thermostat guide.¹¹

Across the 40,906 ZIP codes in our NOAA dataset, the median January low is 23°F. On that night, clear double-pane glass tolerates 51% in the median ZIP and low-E glass 73%, but single pane only 28%. In 57.5% of ZIP codes, a single-pane window fogs before the room reaches EPA’s 30% floor. On clear double-pane glass, each 10°F colder costs about 6–7 points.

Is the outdoor-temperature humidity chart right?

The stepped chart that Google’s AI Overview repeats (below 40% at 20–40°F outside, stepping down to under 15% at −20°F and colder) is published by Minnesota’s Center for Energy and Environment, which cites no origin.¹⁰ A Canadian builder’s metric version labels it for double-pane windows in a 68°F room. Our check: from −20°F to 20°F it runs 5–10 points below the clear double-pane estimate, roughly the margin colder edges need. It is too generous for single pane (11–17 points too high below 30°F) and much stricter than low-E glass needs.

NDSU Extension’s rule of thumb is that, at 40% humidity, double-pane windows start to show condensation below 0°F outside.⁹ That falls between our clear (7°F) and low-E estimates.

Too dry: measure first, then add moisture carefully

NDSU Extension warns that air that is too dry can dry out skin and nasal passages.⁹ A hygrometer, which EPA notes costs about $10–$50, tells you where you stand.¹ Your thermostat moves the reading too: air at 40% in a 68°F room reads about 35% at 72°F and 53% at 60°F. A night setback raises humidity just as the glass gets coldest. A space heater adds no water; it lowers the reading by warming the air.

If you add moisture, EPA advises using a humidifier only when conditions require it, at the correct setting, and cleaning portable units every third day. It suggests low-mineral water such as distilled water, because ultrasonic and impeller (“cool mist”) units can spray minerals and microorganisms into the air. Steam vaporizers and evaporative units are not expected to disperse substantial amounts of minerals.⁶

Too humid: signs and what to do

Condensation is the first sign. EPA says to dry the wet surface quickly and reduce the moisture source.¹ Its mold course lists humidifiers, steam radiators, dryers, combustion appliances, cooking and showers as sources, and notes that mold often grows on walls in cold corners behind furniture.²

EPA’s fixes: lower the humidity, vent clothes dryers and stoves outside where possible, use exhaust fans or open windows when cooking or running the dishwasher, add air movement, and raise the air temperature.¹ If condensation forms behind drapes, NDSU suggests leaving them open at night, at some cost in heat loss.⁹ The same dew-point physics is why a heat pump’s outdoor coil frosts in cold, humid weather.

Next step

Put a hygrometer in your coldest room, set any humidifier at or below your city’s row for your window type, and turn it down before an arctic night. That same night is when pipes freeze; if you plan backup power for the furnace, the generator size calculator adds up the watts.