YWindow sweating overnight is a timing problem as much as a humidity problem. Conditions that are fine during the day cross a threshold at night – glass gets colder, ventilation stops, and moisture accumulates in a sealed room for 7–8 hours with nowhere to go.
Fixing it means intervening before you go to sleep, not just wiping the glass in the morning.
Why overnight is when condensation is worst
During the day, windows warm up from sunlight and room activity, ventilation occurs naturally as people move through the house, and humidity from cooking and showering has time to disperse. At night all of that reverses simultaneously.
Outdoor temperatures drop, pulling window glass down to its coldest point of the day – typically reached just before dawn. Indoor humidity climbs as two people breathe and perspire in a closed room for 8 hours. The room cools slightly as heating cycles down. Curtains trap cold air against the glass.
By 5–6am, indoor humidity is at its overnight peak and window glass is at its coldest. Condensation is the inevitable result.
The mechanism is the dew point. Every combination of temperature and humidity has a temperature at which moisture starts condensing out of the air. In a typical bedroom at 20°C and 60% humidity, the dew point is around 12°C. Single-glazed windows in cold weather can drop to 5°C on the interior surface – well below that threshold. Double glazing raises the interior surface to 14–18°C, which is why condensation is far less common on double-glazed windows.
Step 1: Reduce moisture before bed
The most effective intervention happens in the two hours before sleep, not during the night.
Shower earlier in the evening. A 10-minute hot shower releases 2–3 litres of vapour. Run the bathroom fan during and for 20–30 minutes afterward. If you shower just before bed, that moisture is still dispersing through the house when you close the bedroom door.
Don’t dry clothes indoors overnight. A full load of laundry releases 2–3 litres of moisture as it dries. In a sealed room overnight, all of that goes directly into the air. If drying indoors is unavoidable, dry in a separate room with a window cracked and the door closed.
Ventilate the bedroom before closing it. Open the window for 10–15 minutes before bed. Cold outdoor air in winter holds very little moisture – once it warms up inside, relative humidity drops. This exchanges the moisture that has accumulated during the evening for drier air before you seal the room for the night. It’s the single most effective pre-sleep intervention.
Cook with extraction running. Kitchen steam from dinner travels through the house and settles in cooler rooms overnight. Running extraction while cooking and keeping the kitchen door closed contains the moisture before it spreads.
Step 2: Manage airflow around the glass overnight
Condensation forms heaviest where humid air sits still against cold glass. Curtains pulled fully closed trap a pocket of cold air between fabric and glass – this zone gets colder than the rest of the room and condensation concentrates there.
Leave a gap at the bottom and sides of curtains to allow room air to circulate across the glass surface. During the day, open curtains fully to expose the glass to room airflow and allow any condensation to evaporate.
Keep furniture away from window walls. A bed or sofa pushed against a window blocks airflow to the glass and creates a cold stagnant zone that stays damp.
A ceiling fan on its lowest setting, or a quiet desk fan aimed toward the window wall, keeps air moving across the glass overnight. Moving air prevents the local cooling that causes moisture to drop out at the glass surface – condensation forms significantly less readily when air is circulating.
Step 3: Raise window surface temperature
The warmer the glass, the less condensation forms. Several options at different cost levels:
Trickle vents – the small openable slots at the top of double-glazed window frames provide continuous background ventilation without significant heat loss. Many people close these in winter to reduce draughts, but they’re the one source of passive overnight ventilation that prevents moisture from accumulating. Keep them open.
Draught-proofing around frames – cold air leaking through gaps in the frame cools the surrounding glass and wall faster than room temperature can compensate. Foam draught-proofing tape on frame gaps is inexpensive and raises the frame temperature noticeably.
Temporary window insulation film – adheres to the frame and traps a layer of still air against the glass, raising surface temperature. Effective on single-glazed windows and older double glazing where the seal has degraded. Available in DIY stores and easy to apply and remove seasonally.
Secondary glazing – a second pane fitted inside the existing frame raises interior glass temperature significantly and is cheaper than full window replacement. Worth considering if condensation is severe and consistent across multiple windows.
Step 4: Wipe condensation daily until the cause is fixed
This doesn’t address the cause, but it prevents the progressive damage that makes condensation a structural problem over time.
Condensation that sits on sills and frames soaks into wood and drywall. Within weeks this creates persistently damp zones where mold establishes. Within months, paint blisters, wood softens, and drywall deteriorates.
Wipe the glass, sill, and frame dry each morning. A window squeegee or vacuum handles the glass quickly. A dry cloth for the sill and frame. If the sill paint is already peeling or the wood feels soft, the moisture has been sitting long enough to cause damage – the frame needs to dry out fully before repainting.
When overnight condensation won’t stop
If condensation persists despite consistent pre-sleep ventilation and curtain management, one of these is usually the cause:
Indoor humidity is consistently above 60%. Measure with a hygrometer — the morning reading before opening any windows shows the overnight peak. If it’s regularly above 60%, ventilation alone isn’t enough. A dehumidifier running for an hour before bed brings it down to a level where condensation is unlikely.
The glazing is single-pane or the double-glazed seal has failed. Single glazing gets cold enough to produce condensation even at moderate indoor humidity. Condensation between the panes of a double-glazed unit means the seal has failed — the only fix is replacing the unit.
Cold bridging in the frame or wall. If condensation consistently appears at the edges of the glass or on the wall beside the window rather than across the full glass surface, the frame or surrounding wall is conducting cold from outside. Draught-proofing and frame insulation help, but persistent cold bridging may require professional assessment.
Quick summary
- Overnight condensation peaks just before dawn – glass is coldest and bedroom moisture is highest after 8 hours in a sealed room
- The most effective fix happens before bed: ventilate for 10–15 minutes, shower earlier, don’t dry clothes in the room overnight
- Leave gaps in curtains and keep furniture away from windows to maintain airflow across the glass
- Keep trickle vents open – closing them in winter removes the only source of passive overnight ventilation
- If condensation persists, measure morning humidity – above 60% consistently means ventilation alone isn’t enough, use a dehumidifier before bed
- Wipe condensation daily to prevent mold on frames and sills while you work on the underlying cause
Related Articles
How to Reduce Humidity in a Bedroom at Night
Condensation on Windows in Winter: How to Stop It
Water on the Inside of Windows Every Morning: Why It Happens
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