How low to set your humidity when it is freezing outside
The right indoor humidity in winter is not one number — it falls as the outdoor temperature does, and your windows tell you when you have overshot.
Winter humidity advice usually stops at "aim for 40–45%". That is right in November and wrong in a cold snap, and the difference shows up as water running down your windows.
The reason is that your coldest surface sets the limit, and in winter that surface gets colder as the weather does.
Why the window sets your ceiling
Window glass is almost always the coldest interior surface in a heated room. Its temperature is pulled down by whatever is outside, so on a 0 °F night even a double-glazed pane sits well below room temperature.
Condensation happens when air touches a surface below its dew point. Raise indoor humidity and you raise the dew point; drop the outdoor temperature and you drop the glass temperature. Cross those two lines and the window is wet, no matter how comfortable the room feels.
A working table
These are conservative targets for a typical double-glazed house. Single glazing or metal frames need lower still; well-insulated triple glazing tolerates more.
Above 40 °F outdoors: 45% indoors. 20 to 40 °F: 40%. 10 to 20 °F: 35%. 0 to 10 °F: 30%. −10 to 0 °F: 25%. Below −10 °F: 20%.
These look uncomfortably low against the usual 30–50% guidance, and they are — that is the trade. In a hard freeze you accept slightly dry air rather than water pooling on sills, soaking into frames and growing mold in the seals.
Let the glass tell you
You do not need the table if you check the windows. Morning condensation on the glass means your setting is too high for the overnight temperature. Drop the humidifier 5% and check again the next morning.
Do this in the coldest room, not the warmest. The spare bedroom with the heating off is where you will see it first, and it sets the ceiling for the whole house if it shares air with the rest of it.
One caveat: a little condensation along the very bottom edge of the glass is normal and hard to eliminate, because the frame conducts cold most strongly there. Water across the middle of the pane is the signal that matters.
The takeaway. Start at 40–45% and drop 5% for every meaningful step down in outdoor temperature. If the glass is wet in the morning you are 5% too high, whatever any chart says.
Common questions
What should indoor humidity be at 0 °F outside?
Around 30% for a typical double-glazed house, lower if the windows are single-glazed or metal-framed. Holding 45% at that outdoor temperature will put water on the glass.
Is 30% humidity too dry for a house?
It is the bottom of the healthy band rather than below it. In a hard freeze it is the right trade — slightly dry air beats water soaking into window frames and growing mold in the seals.
Why do my windows fog up only in cold weather?
Because the glass temperature follows the outdoor temperature. The same indoor humidity that is fine at 40 °F outside puts the glass below the dew point at 10 °F.