Why are my windows wet in the morning?
Windows that are wet every morning are showing overnight condensation. While you sleep, the glass cools to its lowest and you add moisture to the air by breathing — so by dawn the air at the glass tips past its dew point.
Reviewed by George Sora, Certified Passive House DesignerUpdated June 2026Quick answer & key takeaways
5 min read- Morning window wetness is overnight condensation from cold glass plus the moisture you breathe out.
- It's most common in bedrooms, where moisture is added all night and ventilation is often poor.
- Closed windows and curtains overnight trap humidity and keep the glass cold.
- It's a strong warning sign of conditions that grow mould in corners and behind furniture.
- Biggest misconception: it's harmless drips. It signals a moisture-and-ventilation imbalance worth fixing.
- Retrofit IQ's approach: log overnight RH and surface temperature, then fix ventilation and cold surfaces.
What this usually means
Overnight, with the heating off, surfaces fall to their coldest, and the glass — the poorest insulator in the room — gets coldest of all. Meanwhile, two sleeping adults release a litre or more of moisture into the air through respiration. Relative humidity climbs through the night until the air at the cold glass reaches saturation and condenses, leaving wet windows by morning.
It is almost always a bedroom story, and almost always a combination of a cold surface, a high overnight moisture load and too little ventilation to clear it. The same overnight conditions quietly wet cold walls and corners too, which is where the mould appears.
Common causes
Overnight respiration moisture
Sleeping occupants add significant moisture to the air for hours, raising the dew point in a closed room.
Cold glass overnight
With the heating off, single or poorly insulated glazing reaches its lowest temperature, giving moisture a surface to condense on.
Closed-up room
Shut windows and drawn curtains trap humidity and keep the glass cold, concentrating condensation.
Inadequate ventilation
Without continuous or background ventilation, the overnight moisture has nowhere to go.
Drying laundry or other moisture sources
Damp clothes or unvented activities add further moisture that lingers overnight.
Signs and symptoms
Glass streaming with water first thing
Mainly on bedroom windows, where moisture is added all night and ventilation is poor.
Pools on the sill and damp reveals
Water runs down the pane and collects on the sill, leaving the reveals damp and prone to mould.
Worse in winter and after heating is off
Surfaces are coldest overnight, so the air at the glass tips past its dew point by dawn.
Musty smell and corner mould
The same overnight conditions wet cold walls and corners, where mould then grows.
What most people check first
- Whether it's mainly bedroom windows and worst in winter.
- Trickle vents and whether windows are ever opened.
- Drying laundry in the bedroom or nearby.
- Damp or mould appearing in corners or behind furniture.
What most people miss
- That the wet windows warn of the same condensation on cold walls and corners.
- That bedrooms need ventilation to clear the overnight moisture load.
- That warming the glass and reveals reduces where condensation can form.
- The overnight pattern itself, which only logging over time reveals.
The building physics
The bedroom at night is a near-closed box with a steady moisture input (respiration) and falling surface temperatures. As humidity rises and the glass cools, the dew point of the air meets the temperature of the coldest surface. The first place that happens is the glass, so it streams with water by morning; the next places are cold corners and the wall behind the headboard or wardrobe.
Two levers control it. Removing moisture — through continuous extract or background ventilation — keeps humidity below the level at which it can condense. Raising surface temperatures — warmer glazing, insulated reveals, slightly more even overnight heating — keeps cold surfaces above the dew point. Address one without the other and the problem only partly resolves.
Crucially, simply heating the bedroom more without ventilating can make wall condensation worse, because warmer air holds more moisture that then condenses on the coldest surfaces. Ventilation has to be part of the fix.
How to fix it — the right way
Morning window wetness is fixed by removing the overnight moisture and warming the glass — heating the room alone is not enough.
- 01
Ventilate the bedroom overnight
Keep trickle vents open, use background extract, or fit continuous or balanced ventilation to clear the overnight moisture.
- 02
Remove moisture sources from the room
Do not dry laundry in the bedroom, and address any other moisture being added overnight.
- 03
Warm the glazing and reveals
Better-insulated glazing and warmer reveals keep the surface above the dew point.
- 04
Keep a more even overnight temperature
Letting the room cool right down maximises condensation; a steadier temperature reduces it.
- 05
Pull furniture off cold external walls
Allow air and warmth to reach the wall so the same condensation does not form behind it.
How to prevent it coming back
- Keep trickle vents open overnight.
- Do not dry laundry in the bedroom.
- Use background or continuous ventilation while sleeping.
- Wipe and ventilate as a stop-gap until a permanent fix is in place.
How Retrofit IQ investigates this
We log the bedroom overnight rather than judging from a daytime visit, because the problem is, by definition, an overnight one.
Do not spend money fixing symptoms before you understand the cause — investigate first, then build with confidence.
Do I need a professional investigation?
Because the problem is, by definition, an overnight one, logging humidity and surface temperatures through the night will pinpoint when the dew point is crossed — so ventilation and surface temperature can be balanced reliably.
It is worth investigating if mould is appearing in corners or behind furniture, as the wet windows are warning of the same hidden condensation.
Where to go next
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Frequently asked questions
Why are my windows wet only in the morning?+
Overnight the glass is coldest and humidity is highest from your breathing, so by morning the air condenses on the cold glass.
Is morning condensation harmful?+
The drips are manageable, but they warn of a moisture-and-ventilation imbalance that also causes hidden mould in corners.
Why is it worse in the bedroom?+
Bedrooms have a high overnight moisture load from sleeping occupants and are often poorly ventilated and cool overnight.
Will opening the window help?+
Some ventilation helps clear moisture, but a controlled, continuous solution is more reliable and doesn't sacrifice warmth.
Should I stop drying clothes in the bedroom?+
Yes — drying laundry adds large amounts of moisture overnight and strongly worsens morning condensation.
Will better windows fix it?+
Warmer glazing reduces condensation on the glass but, without ventilation, moisture can shift to cold walls and corners.
How do you find the cause?+
We log humidity and surface temperatures overnight and analyse the dew point, so we can balance ventilation and surface temperature precisely.
Stop guessing — find the real cause
Do not spend money fixing symptoms before you understand the cause. Every home behaves differently, and the only reliable way to know what is happening in yours is professional building performance diagnostics. At RetrofitIQ we verify buildings using the right combination of investigations:
- Thermal imaging
- Blower door testing
- Moisture & dew point readings
- Ventilation review
- Building physics assessment
- Passive House methodology