Window Condensation - Causes and Solutions


Condensation on windows is an issue which becomes much more common this time of year. Both old and new windows can have condensation forming, however the reasons vary depending on the location of the condensation, and when it forms.

What causes condensation?

Simply put, condensation forms on a surface when there is excess moisture in the air and a large temperature difference. When you are trying to reduce unwanted condensation there are two ways to do so.

  1. Reduce the humidity in the air
  2. Increase the temperature of the surface you are trying to eliminate the condensation from.

The humidity inside your home does vary, and can be increased by cooking, drying clothes indoors, and simply people being at home . The human body can produce half a litre of water vapour per night simply sleeping!

Lets have a look at the common areas for condensation to form and how to eliminate it.

Internal Condensation 

This is condensation that forms on the internal side of the glass (not the space between the two panes of glass). This forms for two reasons primarily:-

On older windows, with poor thermal efficiency, the glass temperature can drop low enough, that the humid air inside your house can condensate. There is little you can do to alleviate this. On newer windows, like our A-Rated windows, the glass temperature shouldn’t be an issue. However some people do experience internal condensation. This is normally due to lack of ventilation. Older windows are draughty, and normally allow a small exchange of air from outside to in. The air outside is generally drier than the air inside, and helps control your internal humidity. New windows are designed to be draught proof. As a result, the windows need to be vented regularly to allow the dry air inside, and keep the humidity under control. Triple glazed windows very rarely show internal condensation (as the glass temperature is much closer to the room temperature), however windows still need to vented to control your humidity.

All of our windows have a night vent locking facility to allow exchange of air, while keeping your home secure. Dry air is cheaper to heat then humid air, so you will save on energy bills by venting your windows regularly!

External Condensation

Something you may notice on new, A-Rated and Triple glazed windows, is that on cold, winter mornings, there will be condensation on the outside of the glass! This is due to the energy efficient glass - it does not allow the external pane to warm up (as all the heat is reflected inside), therefore the external glass can drop to very low temperatures, causing moisture to condensate on the glass. You see the same effect on car windows in the morning.

Therefore external condensation is a sign your A-Rated windows are working and you shouldn’t worry!

Inter-pane Condensation.

Double glazed units during manufacture, are hermetically sealed with argon gas or air, which has has almost all the moisture removed. The spacer bar is also packed with a material called desiccant, which draws any leftover moisture out and ensures that there is a no water vapour present at all internally, therefore eliminating any condensation inside.

Over time however, and especially on older units (which may have been poorly manufactured), these seals can sometimes breakdown, allowing moist air to enter the unit. Once this happens, the unit has to be replaced. Normally the seals break down due to inadequate drainage in the window itself, so sometimes to truly eliminate condensation, a new window will be required.

GGF released a very useful document, covering everything discussed today, and also other solutions to reduce condensation. Which can be found below.

GGF Condensation Guide