What should I set my humidifier to today?

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-glazed pane 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.

It estimates the temperature of the coldest point on your window from the indoor and outdoor temperatures and the glazing 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 glazing and triple glazing 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.

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. Secondary glazing or better units raise the ceiling; running a humidifier past it just moves the moisture onto the frames.