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Why Does Condensation Form on Windows

Why Does Condensation Form on Windows

The sight of beads of condensation on a window in the morning is a common sight in homes around the UK.  As the weather cools in the autumn, the presence of condensation on windows can become a continuous morning presence and if not treated, it can lead to problems with wooden frames including rot as well as providing the ideal environment for dangerous black mould to develop.

Preventing condensation from developing on your windows is possible, but it is useful to understand what causes it in the first place.

What Causes Condensation?

Condensation is caused when warm moist air meets a cold surface.  The change in temperature reduces the amount of moisture that the air can hold, and the excess condenses on the cool surface.  On a non-permeable material like a glass window, that moisture remains on the surface, usually dripping down onto the window ledge.  On porous surfaces, the moisture soaks into the material so it is less obvious – although it is on porous surfaces where condensation is more likely to lead to damp.

Inside a typical house there are plenty of sources for water vapour including cooking and cleaning.

On a bedroom window, the most likely culprit for creating condensation is breathing.

How Much Water Vapour do People Breathe Out?

According to studies by the Institute of Specialist Surveyors and Engineers (ISSE), an average person exhales around 40g of water vapour per hour when sleeping.  This means that if you have two average people sleeping in a bedroom for eight hours, they will exhale the equivalent of three glasses of water into the air.

While not all of this water will find its way onto the windowpanes, as the coldest part of any room, the windows are the place where condensation is most likely to form.

How to Reduce Condensation on Windows

The key to reducing the amount of condensation that will form on your windows in the morning is to improve air flow through your home so that the humid air is better dispersed.

If you take a shower or bath before going to bed, make sure that you use a bathroom extractor fan and close the bathroom door.  This helps to prevent excess moisture from escaping from the room into the rest of your home.

On warmer nights, leaving a window open will help to equalise the temperature and allow better air flow away from windows, however on cold nights this can cause heat loss from your home and it is not always practical if you live in a town or suffer from allergies.

The only reliable way of preventing condensation on windows at night is to improve the ventilation in your home.

Home Ventilation Can Prevent Condensation

An extractor fan is not a practical ventilation method for bedrooms, as the noise from the fan may affect your sleep.  Whole house solutions such as Positive Input Ventilation Systems (PIV) are more suitable.

PIV systems draw filtered fresh air from outside the house into a central pump which then disperses it throughout the house through carefully placed vents.

These systems are highly efficient and also, because the fans are usually located in the loft away from living space, there is almost no noise. 

The improved air flow through the house prevents cold spots where condensation can form from developing, and in turn this stops condensation causing damp.

Find Out More

If you are having trouble with condensation on your windows in the morning and want to prevent a damp problem from developing, contact us today to arrange a free survey with one of our local ventilation specialists who can assess your home and recommend the best course of action to remove condensation and damp forever.


Need help with condensation, mould or damp problems?

One of our local experts will contact you to learn more about your problems, offer free expert advice and make recommendations for a permanent solution.

During the free survey we will

  • check Assess any condensation, damp or mould problems in your property
  • check Take readings of the relative humidity levels
  • check Identify any underlying problems and make recommendations for a permanent solution

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