As spring approaches, let us explain why external condensation can be a common problem for homeowners, especially at this time of year.

The beginning of March marks the end of winter. Although spring officially starts on the 20th of March, the days are already noticeably longer with patches of warm weather hinting that summer isn’t far away.

For the glass and glazing industry, the transition to spring often sees an influx of enquiries about the appearance of external condensation on windows, particularly from customers with high-performance low-emissivity glazing. While some homeowners may see it as a nuisance, they should know that external condensation is a tell-tale sign that the windows are doing their job! But why does it happen?

The dew point

External condensation occurs due to a natural phenomenon called the dew point, which is more likely to occur in spring and autumn.

Put simply, the dew point is the temperature at which a concentration of water vapour in the air will form dew. If the air cools down sufficiently to the dew point, it can no longer hold the moisture, forcing the water vapour to condense.

In spring and autumn in particular, the temperature of the outer pane of a window can fall to a low level during the night, and the dew point can be comparatively high. In these conditions, the glass is likely to be below the dew point, and so moisture condenses onto the surface.

The appearance of condensation is more common with modern, high performing low-emissivity (low-e) glazed windows, as the outer pane is colder due to heat being retained inside the home. With single or older double-glazed windows, more heat passes through, warming the outer pane and reducing the likelihood of external condensation. This can be compared to frost on roofs – roofs with good loft insulation can remain frosted for a long time, while roofs without insulation quickly defrost.

Stopping this natural occurrence

While the effect doesn’t usually last long, there is little that can be done to avoid condensation to the outside of the window entirely. Once the window pane warms from the sun, the moisture will evaporate. A gentle breeze or wind can also help to clear it.

However, some homeowners may notice that not all panes are affected by early morning condensation, even within the same window. Subtle differences in orientation and the position of objects outside, such as trees or nearby buildings, can alter the surface temperature of the glass to the extent that one pane is affected and another isn't.

Increasing the room temperature can also lessen the effect, but this is understandably counterproductive after installing glazing which helps to reduce energy use!

Pilkington Anti-condensation Glass

Pilkington Anti-condensation Glass offers a solution that prevents the onset of external condensation and improves the view through windows.

It is an online coated product with a pyrolytic coating that is durable enough to be used on the outside-facing surface, as well as being robust and easy to clean. It can also be laminated, toughened, bent and incorporated into insulating glass units (IGUs).

Those with Pilkington Activ™ installed in their windows will notice fewer occurrences of external condensation. Although it is no less prone to condensation than other types of glass, its properties mean that water does not form beads on the surface. This means the effect isn’t as noticeable as it would be on a window without self-cleaning glass.

Keeping the heat inside

The presence of external condensation on low-e products is a natural occurrence which proves to homeowners that the glass is effectively keeping heat inside.
The situation is different for condensation that appears on the inside, for example in rooms with a very high moisture content, or between the panes, which can result from a failed unit seal.

As uptake in high performance thermal insulating glazing increases, queries about condensation are also likely to rise. However, it’s important to inform homeowners that this proves that the glass is performing as designed, and that solutions are available with Pilkington Anti-condensation Glass if required.