Window glass keeps losing heat to the cold outdoors for hours after sunset, so it grows colder through the night and crosses your indoor air’s dew point at its coldest point, usually just before dawn. Nothing in the room has to change for this to happen — the wipe-down each morning is the tail end of that overnight fall, not a new event.
Why the glass keeps cooling long after the sun goes down
Glass has almost no insulating value and very little thermal mass, so its surface temperature tracks the cold on the other side of it far faster than a wall does. As soon as the sun sets, the pane starts losing heat to the outdoor air and, on a clear night, to the open sky above it, and it keeps losing heat for as long as it stays dark.
Indoor humidity, meanwhile, does not fall at night — if anything it rises, because a sleeping adult releases close to a pint of water into the room over eight hours and the house is sealed up tighter than it is during the day. So the room side of the equation stays flat or climbs while the glass side keeps dropping, and relative humidity at the glass surface climbs with it until it hits 100% and water starts to condense.
That crossing point is rarely at bedtime. It typically arrives in the coldest hour of the night, in the run-up to dawn, which is exactly when most people are asleep and not around to see it start.
The morning wipe-down is a symptom, not the diagnosis
Drying the sill each morning removes the water but does nothing about the two conditions that produced it — an indoor dew point that is too high, and a glass temperature that gets too low. Both conditions repeat the next clear, cold night, so the puddle repeats with them.
How much water collects is roughly proportional to how far below the dew point the glass fell and for how long, which is why the same window can be dry after a mild, cloudy night and soaked after a cold, clear one. Cloud cover works like a blanket, radiating some heat back down at the glass; a clear sky lets that heat escape straight out, so the pane runs noticeably colder on the nights it matters least to be outside.
The indoor humidity ceiling, by how cold the night gets
| Overnight low outside | Indoor RH ceiling | What happens above it |
|---|---|---|
| 20–40 °F | 40% | Fogs in the coldest pre-dawn hour |
| 10–20 °F | 35% | Fogs for more of the night; water reaches the sill |
| 0–10 °F | 30% | Heavy condensation; ice becomes likely at the frame edge |
| Below 0 °F | 25% or lower | Even careful control may not be enough on single-pane glass |
Why one window fogs while another in the same house stays clear
Single-pane glass and older double-pane units without a low-emissivity coating run several degrees colder than a modern insulated unit under the same conditions, so they cross the dew point first. A room with the heat turned down overnight, or a door kept shut, loses the benefit of the rest of the house’s warmer air and its glass runs colder still.
Closed curtains and blinds make it worse rather than better: they trap a pocket of still, cold air right against the glass and cut it off from the warmer room air that would otherwise raise its temperature a few degrees. A window with the coverings drawn back overnight is measurably less likely to fog than the same window shut in behind them.
Tonight: what actually changes the outcome
Crack the bedroom windows for 5–10 minutes before bed rather than after you wake up to condensation — it flushes out the moisture built up during the evening before the glass has fallen to its coldest point. Leave curtains and blinds open, or at least not sealed tight to the sill, so room air keeps reaching the pane.
Run the bathroom fan through and after an evening shower, and skip drying laundry indoors overnight — both add water straight into the air that then sits, undisturbed, against the coldest surface in the house for eight hours. If a humidifier is running, turn it down before bed rather than leaving it at a daytime setting the glass cannot support after dark.
This week: track the pattern instead of guessing
Put a hygrometer in the room and check it against tonight’s forecast low using the dew point calculator — it will tell you how close your evening humidity sits to the point that overnight cooling will cross. The winter humidity setting calculator goes a step further and estimates the ceiling for your specific windows and outdoor low, rather than the rough bands above.
If the room runs humid before bed regardless of what you do that evening, the source is somewhere else in the house — a bathroom, a kitchen, an unvented dryer, or simply the volume of moisture a family of sleepers adds every night. Read the full condensation-on-windows guide for how to trace that source room by room.
When it is ice, not water, in the morning
On the coldest nights the same process can carry past the freezing point: the water that would have condensed instead forms as frost or ice directly on the glass, or condenses and freezes within minutes. That is a further sign of how far the glass fell below your indoor dew point, not a separate problem — see ice on the inside of windows for what it means and the deadline attached to thawing it.
When to stop chasing the window and treat the room instead
If holding your indoor humidity at the ceiling the table above suggests still leaves the house uncomfortably dry, the glass — not your habits — is the limiting factor. Warming the surface with storm windows, better weatherstripping or a full replacement unit raises the temperature the glass can tolerate before it condenses, which is the only way to keep both comfort and a dry sill through a hard winter.
Improving airflow past the glass helps in the meantime. See ventilation for how to move air across cold surfaces without just opening a window and losing the heat, and the bedroom page for the moisture sources specific to a room people sleep in.
Common questions
Why is condensation on windows worse at night than during the day?
The glass keeps losing heat to the outdoors for as long as it stays dark, so it grows colder through the night while indoor humidity stays flat or rises. It typically crosses the dew point in the coldest hour before dawn, hours after anyone last touched the thermostat.
Why is nighttime condensation worse on clear nights than cloudy ones?
A clear sky lets the glass radiate heat straight out to space, so it runs colder than on a cloudy night, when cloud cover radiates some warmth back down and slows the cooling. The same indoor humidity produces heavy condensation on one kind of night and almost none on the other.
Does closing the curtains make overnight condensation worse?
Yes. Closed curtains trap a pocket of still, cold air against the glass and cut off the warmer room air that would otherwise keep its surface a few degrees higher. Leaving curtains open, or the sill uncovered, is one of the few free changes that measurably helps.
What indoor humidity stops overnight window condensation in winter?
It depends on how cold the night gets and your glazing: roughly 40% when the low is 20–40 °F, 35% at 10–20 °F, and 30% or lower below that, with single-pane glass needing a lower ceiling and triple-pane tolerating a higher one.
Will cracking a window before bed stop condensation by morning?
A 5–10 minute crack before bed flushes out the evening’s buildup and can be enough on a mild night. On a hard-freeze night it only buys a few hours — the glass keeps cooling long after the window is shut again, so pair it with a lower evening humidifier setting.
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
- NOAA National Weather Service — Why Dewpoint is a Better Measure of Humidity — the dew point definition and how a falling surface temperature crosses it