External condensation of double glazing


From time to time, we receive enquiries about the appearance of external condensation on the glass.

My windows are all misted up on the outside!!

What’s wrong with them?

The simple answer is nothing is wrong with them! In fact, external condensation can be proof that energy-efficient windows are performing well.

Condensation is a naturally occurring phenomenon that is more common, the more highly insulating your windows are and is not a deficiency in the glass or window. 

Moisture condenses out of the air onto a cold surface that is said to be below the dew point. The dew point varies with the air temperature and the amount of moisture it contains. Particularly in spring and autumn, the glass temperature can fall to a low level during the night, and the dew point can be comparatively high in these seasons. The glass temperature can be below the dew point under these conditions, and moisture can condense onto the surface.

If nothing is wrong, why is there condensation?

The longer answer is that the modern windows that FENSA Approved Installers (very much like ourselves) fit these days are so much more thermally efficient than those installed in decades past. As a result, the heat used to warm homes doesn’t escape through to the outer pane.

Therefore, the outer pane of glass is cooler, and when its temperature falls below the dew point, moisture in the air condenses on it.

Why does condensation only form at certain times of the year?

The reason window condensation can be worse during spring (and autumn) mornings is very simple. These times of the year, the glass’s temperature can really drop overnight, yet the dew point remains relatively high.

You may notice that not all of the panes are affected by early morning condensation, even in the same window. Subtle variances in orientation and the position of objects outside the window can change the glass’s surface temperature to the point that one pane suffers and another does not. Any object such as a canopy or tree blocking off the window to a clear night sky will also reduce the effect. Some windows can be affected more than others, and slight changes in orientation or shelter can mean that windows or even panes next to each other can react differently.

What can I do to stop external condensation from forming?

Regrettably, there is not much that can be done to stop this phenomenon. It ordinarily doesn’t last long and shows that the heat is used to warm your home is being saved rather than lost through your windows – thus possibly lowering your heating bills and reducing your home’s carbon emissions. A little heat from the sun warms the outer glass enough to evaporate the moisture, and a gentle breeze or wind will help.

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