Wet windows: how much condensation is a problem?
Updated 2026-09-24
Short answer
Light fog on the glass in the morning is normal and harmless. Water running down to pool on the sill, ice forming between panes, or any damp appearing on an outside wall means the room's humidity is above what the building can take at that outdoor temperature. The fix is to lower the setting, not to add ventilation — and the colder it gets outside, the lower the safe setting goes.
Windows are the coldest surface, so they fail first
Condensation appears wherever a surface is below the dew point of the room's air, and a window is always the coldest surface in a room. That makes it a useful early warning rather than the problem itself: by the time glass is streaming, moisture is also condensing inside wall cavities where you cannot see it and where it does real damage.
The colder it is outside, the lower you have to run
A house that holds 45% comfortably in November will stream at the same setting when it drops below freezing, because the glass is colder. The widely-used rule of thumb steps the target down as outdoor temperature falls — from around 45% at 20°F above freezing to 30% or below when outside temperatures go well below zero. Anyone running a fixed setting all winter will meet this.
Whole-house units make this worse, not better
A furnace-mounted humidifier can push the whole building past its limit at once, and it is the only type with enough capacity to do real structural damage. That is why most whole-house controls cap out around 45% and why several will not let you set higher — a limit owners sometimes try to defeat, and should not.
What to do about it tonight
Drop the target five points and see whether the morning glass is dry. Repeat until it is. If you need more humidity than the windows will tolerate, the answer is better glazing or a warmer window surface, not a bigger humidifier.
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