Enter your numbers
Hold 40% or below
At this outdoor temperature your glass stays clear up to about 40%.
- Maximum humidity
- 40%
- Coldest point on the window
- 46 °F
- Your dew point at that setting
- 44 °F
Why the number moves
Condensation is decided by one comparison: the dew point of your indoor air against the temperature of the coldest surface it touches. In a heated house that surface is almost always a window.
Glass temperature follows the weather. On a mild day a double-pane window might sit only a few degrees below room temperature; at 0 °F outside the same pane can be in the forties, and the very edge of the unit — where the spacer bridges the two panes — is colder still. Raise your humidity and the dew point climbs to meet it.
That is the whole mechanism, and it is why a fixed humidifier setting that works in November puts water on your sills in January.
What this calculator does
It estimates the temperature of the coldest point on your window from the indoor and outdoor temperatures and the window type, then works out the highest indoor humidity whose dew point stays below that point, with a small safety margin. The result is rounded down to the nearest 5% so you are not aiming at the exact edge.
Glazing matters more than most people expect. At 10 °F outside, single-pane glass and triple-pane glass differ by more than twenty percentage points of allowable indoor humidity — which is to say, the same house with better windows can be substantially more comfortable in winter without any risk.
Using it
Set it in the evening for the overnight low rather than the afternoon high, because the coldest hour is what wets the glass. Then check the windows in the morning: if there is water across the middle of the pane, drop the humidifier 5% and check again the next day.
A little condensation along the very bottom edge is normal and hard to eliminate — the frame conducts cold most strongly there. Water across the glass itself is the signal that matters.
If the calculator tells you 25% and that feels unpleasantly dry, the honest answer is that your windows are the constraint. Storm windows or better units raise the ceiling; running a humidifier past it just moves the moisture onto the frames.
A worked example
A worked example for the question this calculator exists to answer: how high can I safely run the humidity tonight?
Look up the outdoor temperature
A forecast low of 10 °F. The colder it gets outside, the colder the inner face of your glass, and the lower the humidity you can hold without condensing on it.
Identify the window
Standard double-pane glass. This is the single biggest variable — the difference between single and triple-pane glass is worth 30 or 40 percentage points of allowable indoor humidity at the same outdoor temperature.
Read the ceiling
About 35% at 10 °F with double-pane glass. That is the highest humidity the coldest pane in the house will tolerate without water forming on it overnight.
Check it in the morning
Dry glass means you had margin and could go a little higher tomorrow. Water across the middle of the pane means you were above the ceiling regardless of what the calculator or the hygrometer said, and the setting needs to come down.
How to read the result
This calculator produces a ceiling rather than a target. It is telling you the maximum, not the ideal — if 35% is comfortable for the people in the house, there is no benefit in pushing toward the limit. Dry-air symptoms mostly appear below 30%, so a ceiling in the mid thirties is a comfortable place to sit.
The number moves with the weather, which is the part most people miss. A setting that was correct in November is wrong during a hard freeze in January, and a humidifier left on the same setting all winter will put water on the glass every time the temperature drops. Anything with a humidistat should be adjusted at least when a cold snap is forecast.
The glazing figures assume a reasonable installation. An old timber sash with a failing seal, a metal frame without a thermal break, or a window behind a heavy closed curtain will all run colder than the calculation expects — which is why the glass itself is the final authority rather than the number.
Common questions
What should I set my humidifier to in winter?
It depends on how cold it is outside and what your windows are. Roughly 40% when it is 20–40 °F out, 35% at 10–20 °F, 30% at 0–10 °F and lower below that — with single-pane glass needing less and triple-pane glass tolerating more.
Why do my windows fog up even at 35% humidity?
Either the glass is colder than typical — single-pane glass, a metal frame without a thermal break, or a failed sealed unit — or the reading is taken in a warmer room than the one with the wet window. Measure in the room that fogs.
Is condensation at the bottom edge of the window normal?
Largely yes. The spacer and frame conduct cold most strongly there, so the bottom inch runs several degrees colder than the center of the pane. Water across the middle of the glass is the sign your setting is too high.
Why is my ceiling so much lower than the recommended 30–50% range?
Because the general range assumes surfaces are warm enough to tolerate it, and during a hard freeze they are not. The 30 to 50% band is about comfort and health in ordinary conditions; the winter ceiling is about the coldest surface in your house on that particular night. When they conflict, the ceiling wins, because condensation and mold do more damage than a few dry days.
Should I lower the humidity at night or during the day?
At night, if you have to choose. Surfaces reach their coldest just before dawn, and bedrooms rise ten to twenty points overnight with people in them, so the worst combination happens while everyone is asleep. Running a humidifier during the evening and switching it off at bedtime is a reasonable compromise in a house with cold windows.
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