Condensation forming on the outside of new windows

We have discussed this before, but new windows are normally a fantastic way to improve both the appearance and thermal comfort of your home. New double/triple glazing tends to retain far more heat within the home, minimising the time your heating needs to be on as well as keeping rooms at a more stable internal temperature. New windows also tend to help reduce condensation on the glass, because the internal pane of glass is that much warmer than single glazing, so the moisture in the air doesn’t condense on it, helping keep the windows mist-free!

Condensation forming between the panes of glass is common in slightly older double glazing – the reason for this is normally because the airtight seal on the double glazing unit has broken down, allowing air to enter the space between the two pains.

A problem we are beginning to hear quite a lot about though is condensation appearing on the outside of the window, and as you might expect people aren’t happy about it! Imagine forking out thousands of pounds for new windows only for condensation to form on the outside of windows, ruining the views out of your home.

Why does condensation form on the outside of new double glazing?

Unfortunately, condensation forming on the outside of new double glazing is a natural phenomenon and it arises because the window is working so well at preventing heat loss from your home.

Condensation is defined as the process by which a gas turns into a liquid. If the temperature of an object falls below what is known as the dew point temperature, then water vapour from the air will condense on the object’s surface. The dew point varies according the amount of water in the air (compare a shower room to a sitting room for example) and the temperature of the air. The warmer the air, the more water vapour it can hold – but it can only hold so much, so if this saturated air encounters a surface that is below the dew point temperature then it will condense.

The reason water condenses on the outside surface of the glass is the temperature of the glass drops below the external dew point temperature. The new double or triple glazing units tend to have inner panes made up of low emissivity glass and this prevents the movement of heat across the glazing unit, so the outer pane never gets warm.

As mentioned at the beginning of this article, condensation forming on the outside of your new windows is actually an indication that your new windows are performing very well, although we appreciate it is not ideal!

Unfortunately, there is nothing that can be done to reduce this phenomenon occurring, however the good news is that it only occurs in very specific circumstances – a combination of high relative humidity and clear cold conditions normally experienced in Spring and Autumn.

If you are experiencing this type of ‘external condensation’ the only real way to get rid of it is to take a towel to your external windows. Or, when the sun rises and heats the outer pane of glass, condensation will tend to evaporate.