Condensation on the outside of windows

Last week we looked at why condensation occurs on the inside of windows, which is usually a result of cold weather outside and warm, moist air inside. But condensation forming on the exterior of windows is a different matter. So what is causing exterior window condensation, and can it be prevented?

External condensation – the facts

You will be pleased to hear that external condensation on glass is not a sign of faulty or poor quality double glazing – on the contrary, in fact condensation on the outer layer of glass actually shows that your double glazing is very thermally efficient.

Moisture condenses out of the air on to cold surfaces that are below what is known as the ‘dew point’ – the temperature at which water vapour becomes liquid. Condensation on exterior windows shows that your outer layer of glass is cold.

While the fogged-up pains might be an annoyance to look at when you open your curtains in the morning, the condensation shows that your double glazing is preventing heat from your home escaping outside. The more thermally insulating your double (or triple) glazing is, the lower the outer pane temperature is likely to be, and the greater the likelihood of condensation collecting there.

External condensation usually happens during autumn, early winter and early spring. During warmer months the glass won’t dip below the dew point, and in midwinter the air is often too cold to hold water vapour. Other weather factors can influence the condensation, such as wind and sunshine. Some windows will also be affected worse than others, depending on their location and surroundings.

Condensation appearing on double glazed windows is a relatively modern phenomenon – when single glazing was the norm, warmth from the home would heat the outside of the glass enough so that water would not condense there. As more efficient thermally insulating double glazing and triple has been introduced, exterior condensation has become more common.

What can be done to prevent external condensation?

Unfortunately there is not a lot that can be done to prevent water vapour from condensing on exterior windows completely. The lower the u-value of windows, the cooler the external surface will be, and the more likely they are to have a condensation issue. One possible solution is self-cleaning glass, which works to prevent moisture from beading on the surface.

Keeping your windows squeaky clean can help, as the water droplets will have less dust particles to cling to, and so will run down the glass much more easily.  Take a look at our blog on eco-friendly window cleaners to help get the job done.

In many cases exterior condensation does not last for long. While the temperature drops overnight the water will condense, but once the sun shines on the glass in the morning it will soon warm the surface up, causing the water to evaporate once again. If you’re

Hopefully this post has gone some way to reassure you that exterior condensation is not a sign of faulty windows. That pesky morning fogginess is simply a sign that your double glazing is doing its job, keeping your home warm and your heating bills down.