Windows September 25th2015

How to stop condensation on windows

How to stop condensation on windows

As weather conditions change around Autumn with temperatures dropping and wetter weather, window condensation becomes a concern with many homeowners assuming it’s their windows at fault.  Condensation more often than not is a transitory effect that clears away in a couple of hours but it can be uncomfortable, looks unpleasant and can indicate issues that may cause ill health. 

Condensation comes in three forms and it’s not always bad news. Firstly, it can form on the outside of the window, secondly it can form inside the room on the window and thirdly it can form in between the panes of glass.  If it’s on the outside of the house this is good and a demonstration of quality windows. If it’s inside the house this is caused by a lack of ventilation and lower quality double glazing – for health reasons (caused by dampness and potential mould growth) we would recommend you change your windows but there is lots you can do to reduce the issue first. The last is a demonstration that your double glazing unit has failed and is letting in water and the double glazing unit should probably be replaced before you get water marks and reduced visibility. 

Single glazed windows with external condensation.

What is condensation and how is it caused?


The Glazing and Glass Federation (GGF) defines condensation as the "physical process by which a gas or vapour changes into a liquid". Its caused by a temperature inversion (an imbalance in temperatures) where one surface is at a different temperature to another for example when you take a bottle of milk from a fridge the glass surface falls below what is known as the "Dew Point". At this point water vapour from the atmosphere condenses into water droplets on its surface. 

The creation of clouds is a result of the process of condensation.

In our homes, the air surrounding us contains water vapour which is invisible to the human eye. The warmer the air, the more water vapour it can hold. However, there is a limit to the amount it can hold for a given temperature – think of your glass of beer on a hot holiday and how quickly the glass becomes wet. Once the limit has been reached the air then becomes “saturated.” Once this saturated air comes into contact with a surface at a lower temperature surplus water vapour forms – initially in the form of mist and, if excessive, in the form of droplets of moisture.

Bathroom mirrors often experience condensation because of the warm, moist air created when running baths and showers hits the colder surface of the mirror. 

Two sleeping adults produce approximately 1 litre of moisture in 8 hours, which is absorbed as water vapour into the atmosphere.

With this knowledge lets revisit how the location indicates the cause and therefore the actions that we recommend should be taken.

1. External Condensation – this is good


Condensation can form on the outside surface of glass when it’s temperature drops below the outdoor dew point. This frequently occurs on high efficiency double and triple glazed units. It indicates the internal temperature of the house is higher than that outside and that the windows are performing well and are thermally efficient. There is no action to take.

2. Internal Condensation – this is not so good 


If condensation forms internally (on the room side surface of the inner glass) this means that the temperature of the glass surface is lower compared to that in the room and is possibly at the outside temperature. Internal condensation is most likely to occur on single glazed windows and lower performing double glazed units with no gas between the panes as they more easily conduct cold between one pane to the other. To fix this issue firstly look at reducing the amount of water in your home and most importantly increase ventilation and air circulation. If condensation continues to be an issue you should install new double glazed units ideally with those that have Argon gas between the panes to reduce conductivity in the window.

Double Glazed window with internal condensation.

Ways to improve air circulation or removing water in the atmosphere

Ventilate – open some windows

Try allowing some air in to the room maybe open a window and don’t close doors in the house to allow for air movement around your property. Newer windows come with a multi-locking ventilation systems so you can fix your window into a position that allows air to circulate without becoming a security risk.

Use extractor fans

Ensure you turn on extractor fans and open doors, when cooking. This will allow excess heat to circulate through your home and not remain trapped in the kitchen. If you are able to open the kitchen window, even just a little, then it will help to prevent the build-up of excess moisture until the room returns to a normal temperature. 

Keep the lid on!

Keeping the lids on pans will prevent excess amounts of steam and prevent one of the main causes of condensation. Note: you don’t have to see steam for the moisture to work its way into the atmosphere. If the water is boiling, the lid should be on. Plus, in doing so, you will not only cook quicker but also save about 3% in energy costs per pan.

Drying your washing

Ideally, it is best to dry your washing outside. However, British weather is not sympathetic to this aim so whilst drying your clothes be sure to open the windows and doors to an enclosed room. If using a tumble dryer, ensure the pipes lead all the way outside.

Leave a gap

Pushing items of furniture right against walls can trap air around skirting boards, which will likely create black mould. To prevent this from happening, leave a little gap so the air can circulate.

Clear blocked airways

Blocked chimneys and vents can disrupt airflow and cause great problems in the long run with damp ingress into the property.

External maintenance

The purpose of drain pipes and gutters is to carry water away from your house. Broken materials will leak and blockages can lead to your external walls getting soaked. Keep an eye on the external surroundings of your home to spot any issues before they become a serious problem. 

3. Condensation within the window Cavity – really not good at all


If you find that the condensation on your windows appears within the two panes of glass it indicates broken there is a failure within the seal. In older or poor quality units the sealant used to create the seal may become loose over time or be of a low grade. The seal around the panes will start to degrade and crack, allowing condensation to form in between the two panes of glass. Condensation can also occur if there is a fault with the “spacer” bar. A good quality double glazed window will feature a spacer bar which contains desiccant (a substance used as a drying agent), which will absorb any moisture within the air gap and prevent condensation from forming.

Condensation within the window cavity leading to damp and mould.

What to look for in a good window to reduce condensation


If you do decide to replace your windows here’s what to look for:

  1. Windows that offer Argon gas between the panes
  2. A wide gap between the double glazing a minimum 14mm
  3. A high energy rating of A+ or higher

For more detailed information on windows, check our Windows Buying Guide.

'How to stop condensation on windows'

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