Condensation on Windows UK: Why It Happens and How to Stop It

Why Condensation Forms on Windows

The temperature difference between indoor air and cold glass causes water vapour to condense.

Every window in a UK home is a cold surface relative to the warm indoor air. When moist indoor air contacts the glass, it cools rapidly. If the glass temperature is below the dew point of the indoor air, water condenses on the surface [S1].

The dew point is the temperature at which air can no longer hold all its water vapour and some of it condenses into liquid. In a typical UK home with 50% relative humidity at 20°C, the dew point is approximately 9.3°C, meaning condensation will form on any surface below this temperature [S1].

Single-glazed windows are particularly susceptible because the inner pane is close to the outdoor temperature. Double and triple glazing significantly reduce the problem by creating an insulating gap that keeps the inner pane warmer.

• Warm indoor air holds more moisture than cold outdoor air
• When warm air meets cold glass, it cools below its dew point
• Water vapour condenses into liquid droplets on the glass surface
• Single glazing is most affected because the inner pane is coldest
• Double glazing reduces but does not eliminate condensation

Why It Is Worse in Winter

Larger temperature differences between indoors and outdoors increase condensation on glass.

In winter, outdoor temperatures in the UK frequently drop below 5°C, while indoor heating maintains rooms at 18–21°C. This large temperature difference means window glass is significantly colder than indoor air, making condensation more likely [S1].

Winter also brings behavioural changes that increase indoor humidity: windows and doors are kept closed, heating is on for longer, and drying clothes indoors becomes more common. All of these factors push indoor moisture levels higher.

Morning condensation on windows is particularly common because overnight, when heating is reduced or off, the glass cools further while moisture from breathing and other sources continues to accumulate in the room.

• Outdoor temperatures below 5°C make window glass significantly colder
• Heated rooms at 18–21°C create a large temperature gradient
• Closed windows trap moisture indoors overnight
• Reduced overnight heating allows glass to cool further
• Morning condensation is the most common manifestation

Solutions to Stop Window Condensation

Ventilation, heating, and moisture control are the three pillars of condensation prevention.

The most effective long-term solution is to reduce indoor humidity through ventilation. Opening windows for 10 to 15 minutes each day, using extractor fans in kitchens and bathrooms, and ensuring trickle vents are open all help remove moist air [S1].

Maintaining consistent heating keeps window surfaces warmer, reducing the temperature difference that causes condensation. Avoiding large temperature swings between day and night helps maintain this balance [S1].

Reducing moisture sources at their origin is equally important. Using lids on saucepans, running extractor fans when cooking, and avoiding drying clothes indoors without ventilation all contribute to lower indoor humidity [S1].

• Open windows for 10–15 minutes daily, even in winter
• Use extractor fans when cooking, bathing, or showering
• Keep trickle vents open on windows
• Maintain consistent heating at 18–21°C
• Use lids on saucepans when cooking
• Avoid drying clothes indoors without adequate ventilation

The Role of Double and Triple Glazing

Better glazing reduces condensation but does not eliminate it if indoor humidity is high.

Double-glazed windows have two panes of glass with an insulating gap between them. This keeps the inner pane significantly warmer than a single-glazed window, reducing the likelihood of condensation forming [S1].

Triple glazing provides even greater insulation and further reduces condensation risk. However, neither double nor triple glazing will completely prevent condensation if indoor humidity is very high.

If condensation persists on double-glazed windows, it may indicate a problem with the unit itself. Failed seals allow the insulating gas to escape, reducing the window's thermal performance. This can be identified by condensation between the two panes rather than on the inner surface.

• Double glazing keeps the inner pane warmer, reducing condensation
• Triple glazing provides even greater insulation
• Neither eliminates condensation if indoor humidity is too high
• Condensation between panes indicates a failed seal
• Upgrading glazing is effective but should be combined with ventilation

When Condensation Indicates a Bigger Problem

Persistent condensation may signal poor ventilation, insulation issues, or other damp problems.

If condensation persists despite good ventilation and heating habits, it may indicate a broader issue with the property. Poor wall insulation, thermal bridging, or inadequate ventilation systems can all contribute to persistent condensation [S1].

Condensation on walls rather than windows is a sign that the problem extends beyond glazing. This can lead to mould growth, which poses health risks particularly for children, the elderly, and people with respiratory conditions [S1].

In severe cases, a building survey or damp assessment may be warranted to identify structural issues such as thermal bridging, inadequate ventilation, or defects in the building envelope.

• Persistent condensation despite good ventilation: seek professional advice
• Condensation on walls: may indicate insulation or ventilation issues
• Mould growth: health risk, especially for vulnerable groups
• Failed double-glazed units: condensation between panes

Frequently asked questions

Bottom line

Condensation on windows is caused by warm indoor air meeting cold glass, and it is most common in winter when the temperature difference is greatest. The most effective solutions are improving ventilation, maintaining consistent heating, and reducing indoor moisture sources. Double glazing helps but is not a complete solution on its own. Persistent condensation may warrant a professional assessment.