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-pane window 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-pane house. Single-pane glass or metal frames need lower still; well-insulated triple-pane glass 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.
Check it in your own house
The winter ceiling is not a rule of thumb you have to take on trust — your own windows will tell you where it sits in your house, in one night.
Note the forecast overnight low
The colder it is outside, the colder the inner face of your glass, and the lower the humidity you can hold. Pick a genuinely cold night rather than a mild one.
Set the humidity deliberately and record it
Whatever the humidifier or the household produces, write down the reading last thing at night in the room you are testing.
Look at the coldest pane first thing
Before opening curtains or doors. Dry glass means you were under the ceiling. Misting along the bottom inch is borderline and normal on the coldest nights. Water across the middle of the pane means you were over it.
Repeat two points lower or higher
Two or three nights of this gives you your own ceiling for your own windows, which is worth more than any table — a table cannot know that one window has a failing seal or sits behind a heavy curtain.
Common questions
What should indoor humidity be at 0 °F outside?
Around 30% for a typical double-pane house, lower if the windows are single-pane 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.
Is it bad for the house to run at 30% humidity all winter?
For the building, no — dry air does not damage masonry, plaster or brickwork. For solid timber it is the low end of comfortable: floors will gap and panel doors may open at the joints, and both close again in spring. For people, 30% is around the threshold where dry airways, static and cracked skin start. If a hard freeze forces you to 30%, that is the right trade; sitting there all winter by choice is not.
Do I need to lower the humidity in every room during a freeze?
In practice you are lowering it in the rooms that have cold glass, which is most of them, but the room that matters most is the one with the coldest window and the most moisture — usually an occupied bedroom with the door shut. Fix that one and the rest of the house is generally well inside its own ceiling.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions