Condensation forms on windows when warm, humid indoor air touches cold glass and the moisture it’s carrying drops out as water. In winter, window glass gets cold enough that even moderate indoor humidity produces visible condensation.
The fix is not wiping the glass. That removes the symptom for an hour. The fix is reducing the moisture in the air and keeping the glass surface warm enough that condensation doesn’t form in the first place.
Why winter makes condensation worse
In summer, windows are warmer and humidity escapes through open windows. In winter both of those change at once – the glass gets significantly colder while the house seals up and traps moisture indoors.
The temperature at which air releases its moisture is called the dew point. In a typical winter bedroom with indoor temperature around 20°C and humidity at 60%, the dew point is roughly 12°C. Single-glazed windows in cold weather can drop to 5–8°C on the interior surface. Any air that touches that glass releases moisture as condensation.
Double glazing raises the interior glass surface temperature significantly – typically to 14–18°C in the same conditions – which is why condensation is far less common on double-glazed windows. If you have double glazing and still get heavy condensation, indoor humidity is higher than it should be.
What’s producing the moisture
Before fixing condensation, identify what’s keeping indoor humidity elevated. The most common contributors in winter:
Showering without running the exhaust fan releases 2–3 liters of vapor per shower. Without extraction, that moisture spreads through the house within minutes.
Drying clothes indoors releases 2–3 liters per load directly into the air. In a sealed winter house this is one of the largest single contributors to high humidity.
Cooking – boiling water and frying produce significant steam. Without kitchen extraction running, this spreads to adjacent rooms.
Breathing and perspiration overnight – two people in a closed bedroom release roughly 1 liter of moisture while sleeping. Combined with closed windows and cold glass, this is why bedroom windows are typically worst in the morning.
Houseplants in large numbers transpire moisture continuously. A room with many large plants has measurably higher humidity.
No ventilation during the day – modern well-insulated homes have very little natural air exchange. Without deliberate ventilation, moisture accumulates all day.
How to stop condensation on windows in winter
Reduce indoor humidity first
This is the root fix. Everything else is secondary.
Target indoor humidity of 40–50% in winter. Measure it with a hygrometer – guessing based on symptoms is unreliable because other factors affect how condensation appears.
Run the bathroom exhaust fan during every shower and for 20–30 minutes afterward. Run kitchen extraction while cooking, especially when boiling. These two habits alone remove the majority of moisture at the point it’s generated, before it spreads.
Stop drying clothes indoors if possible. If it’s unavoidable, dry in one room with the door closed and a window slightly open – or use a dehumidifier in the same room to capture the moisture before it spreads.
Ventilate briefly every day
Opening windows for 5–10 minutes in the morning exchanges the moist overnight air for drier outdoor air. Cold outdoor air in winter holds very little moisture. Once it warms up inside, its relative humidity drops significantly – a net removal of moisture from the indoor air.
This feels counterintuitive in winter but it works. Short bursts of ventilation remove more moisture than leaving a window slightly open all day.
Trickle vents — the small openable slots in the top of double-glazed window frames – provide continuous background ventilation without significant heat loss. Check that they’re open. Many people close them in winter to reduce draughts, which eliminates the one source of passive moisture removal the room has.
Improve airflow around the windows
Condensation forms most heavily where warm humid air sits against cold glass without moving. Curtains pulled fully closed against the glass trap a pocket of cold air between the fabric and the window – this zone gets colder than the rest of the room and condensation is heaviest there.
Leave a gap at the bottom and sides of curtains to allow room air to circulate across the glass. During the day, open curtains fully to expose the glass to room airflow and allow any condensation to evaporate.
Avoid placing furniture directly in front of windows. Sofas and beds pushed up to window walls block airflow to the glass and create cold stagnant zones.
A ceiling fan on a low setting or a small fan aimed toward window walls keeps air moving across the glass surface and significantly reduces condensation – moving air prevents the local cooling that causes moisture to drop out.
Raise the window surface temperature
The warmer the glass, the less condensation forms. Secondary glazing – an additional pane fitted inside the existing frame – raises interior glass temperature significantly and is cheaper than full window replacement.
Temporary window insulation film (available in DIY stores) adheres to the frame and traps a layer of air against the glass, raising surface temperature. It’s a low-cost winter fix that noticeably reduces condensation on single-glazed or older double-glazed windows.
Seal gaps around window frames with draught-proofing tape. Cold air leaking through frame gaps cools the surrounding glass and wall faster than the room temperature can compensate.
Wipe down condensation when it appears
This doesn’t fix the cause but it matters for preventing damage. Condensation that sits on window sills and frames soaks into wood and drywall, creating damp patches that develop mold within weeks.
Wipe the glass, sill, and frame dry each morning. Use a window vacuum or squeegee for glass, and a dry cloth for frames and sills. If the sill paint is already peeling or the wood feels soft, that moisture has been sitting there long enough to cause damage – the frame may need repainting or repair before mold takes hold.
Reading your condensation pattern
Where condensation forms tells you something specific about the cause.
Condensation on all windows throughout the house – indoor humidity is too high overall. Ventilation and moisture source control is the fix.
Condensation only in the bedroom – overnight moisture accumulation from sleeping in a closed room. Ventilate before bed, keep a trickle vent open, use a fan overnight.
Condensation only in the kitchen or bathroom – extraction is inadequate or not being used. Run fans longer and check they’re actually venting outside rather than into a ceiling cavity.
Condensation at the edges of the glass only – the glass itself may be fine but the frame is cold. Frame insulation or draught sealing helps.
Condensation between the panes of double glazing – the seal has failed and the insulating gas has escaped. The unit needs replacing – no surface fix will solve this.
When condensation causes lasting damage
Daily condensation that isn’t wiped creates a cycle of wetting and partial drying in the surrounding materials. Within weeks this produces mold on window frames and the wall immediately around the window. Within months it causes paint to peel, wood to soften, and drywall to deteriorate.
Signs damage has already started: black speckling on the frame or wall, paint bubbling near the window, soft or discoloured drywall below the sill, a musty smell near the window even after wiping.
At this point surface cleaning isn’t enough – affected drywall and wood need to be dried fully, treated, and repaired before repainting. Cleaning mold off a soft, damp surface and repainting over it produces the same result within a few months.
Quick summary
- Condensation forms when indoor air hits glass that’s below the dew point – lower humidity and warmer glass both help
- Target 40–50% indoor humidity in winter; measure with a hygrometer
- Run bathroom and kitchen exhaust fans consistently – these remove moisture at the source before it spreads
- Ventilate for 5–10 minutes every morning to flush overnight moisture accumulation
- Leave gaps in curtains and keep furniture away from windows to maintain airflow across the glass
- Wipe condensation daily to prevent mold and frame damage
- Condensation between double-glazed panes means the sealed unit has failed and needs replacing
Leave a Reply