What Causes External Condensation?

External condensation is a natural weather-related phenomenon, not a manufacturing fault. It occurs when the outer surface of the glass becomes colder than the surrounding air - often overnight - and moisture in the air condenses onto it, just as dew forms on grass or car roofs in the morning.

This effect is more noticeable with modern, energy-efficient glazing, such as low-emissivity (low-e) glass. Unlike older single or double-glazed windows that allowed more heat to pass through, today’s advanced glazing keeps warmth inside your home. Because the inner pane stays warmer and less heat reaches the outer pane, that outer surface remains cooler - making condensation more likely to form when conditions are right.

The Science Behind It

Condensation forms when a surface’s temperature drops below the dew point - the temperature at which moisture in the air begins to condense.

In spring and autumn, the air can hold more moisture while nighttime temperatures remain low. When the outer glass cools down, it often dips below the dew point, leading to condensation.

Once the sun rises or a gentle breeze passes over the glass, this light misting starts to evaporate.

Why Some Windows Show Condensation and Others Don’t

You may notice that not every pane of glass mists up in the same way. Small variations in window orientation, shading from trees or canopies, and exposure to clear night skies can all affect the glass temperature. A pane that’s slightly shielded may stay just warm enough to avoid condensation, while another remains cool enough for moisture to form.

Proof of Efficiency - Not a Problem

In fact, external condensation is a positive indication that your windows are performing exactly as they should. It shows that the insulating glass units are keeping heat inside your home, preventing energy loss and maintaining comfort.

How to Reduce Condensation

Indoor Condensation

While it can’t be completely eliminated, you can reduce condensation through good ventilation. Allowing fresh air to circulate helps control humidity levels and prevents excess moisture indoors.

Use trickle vents if fitted, or open windows slightly to let moist air escape and cooler, drier air enter. Interestingly, dry air is cheaper to heat than damp air, so this simple step can also improve your home’s overall energy efficiency.

External Condensation

Because external condensation is caused by natural atmospheric conditions, there is little that can be done to prevent it entirely at certain times of the year. In most cases, it is short-lived - the sun quickly warms the outer glass, causing the moisture to evaporate naturally. If you’d prefer to remove it sooner, you can simply wipe the glass with a soft cloth or use a squeegee to clear the surface.

In Summary

• External condensation is not a fault - it’s a sign of high-performance, thermally efficient glazing

• It’s caused by natural environmental conditions and usually disappears quickly.

• Your windows are doing their job: keeping the heat in and the cold out.

We’re proud to supply products that not only meet but exceed modern Building Regulations for energy performance - helping you enjoy a warmer, more efficient home all year round.

If you’d like more information on our windows click here or give us a call on 01484 519677.