Causes and Prevention of Condensation in Double Glazed Windows

Double Glazed Sash Windows

As we exit summer and the bleak British Winter is on the horizon, the weather becomes colder, and the central heating is turned on. Where the damp outside meets the warmth of our homes, condensation often occurs in our double glazed windows. Although it is to be expected under the circumstances, it is still frustrating nevertheless. 

In this article, we hope to defog the confusion surrounding condensation, its causes, preventative measures and its effects of your double glazing windows. 

Causes of Condensation in Double Glazing

The basic science behind condensation is when, in this instance, the surface temperature of the glass window is colder than the air temperature of the room; this is when you will observe condensation.

Condensation on the surface of your window isn’t necessarily a cause for concern. However, if the condensation is inside double glazing, this could be an indication of a fault. 

Condensation in double glazing usually occurs when a failure, such as a faulty seal, is present and the warmer air seeps between the panes of glass. This warmer air then merges with the colder outside elements and condenses in between the panes of double glazing.

 Effects of Condensation on Secondary Glazing

Condensation inside double glazing will drastically reduce the lifespan of your windows. Sitting condensation promotes mould growth, which will damage your window frames and will lead to additional problems. 

Aside from this, if mould grows in places where it cannot be removed, between panes of glass, for example, the fungus could spread further and onto the interior walls of your home. Interior black mould is a serious health hazard for the inhabitants of your home. There is also a high chance that the black mould can develop into wet rot or dry rot. As well as health issues, rectifying these problems can make quite a dent in your pocket. 

How to Prevent Condensation Inside Double Glazing

The best way to prevent condensation is first, ensuring that all window seals are in excellent condition and well looked after throughout the year. 

You can spot cracks in your window seals by running your finger around the perimeter. Similarly, if you feel any points of draught, this will tell you that there is a gap in your window seal and you should replace as necessary. 

If your double glazing window seals are okay, but you still have condensation, you might need to reduce your home’s humidity.

Your home should have a maximum humidity level of 50%. There are many ways in which you can reduce the humidity of your home or maintain a healthy level:

Use Extractor Fans and Ventilation

Ventilation is as simple as having openable windows in your home. Regardless of the temperatures inside or out, you should try to ventilate your home for at least 20 minutes each day. When cooking in the kitchen or showering without open windows, you should ensure you have an adequate extractor fan to remove excess moisture from the room. 

Don’t Hang Wet Laundry Indoors

Where possible, you should always try to dry your wet laundry outside, or at least in a room that is open to the elements or with high levels of ventilation. Drying laundry inside increases the water vapour in the room, which can lead to condensation and black mould.

Use a Dehumidifier  

If you have few windows, lack ventilation or live in a naturally humid climate, you should purchase a dehumidifier. Dehumidifiers remove any excess moisture from the air in the room, thus reducing your chances of condensation in your double glazing. 

Treating Condensation on Double Glazed Windows


If you notice a crack or gap in your window seals, but your UPVC double glazing is in otherwise excellent condition, you can restore the seals alone. It is often possible to extract any existing moisture between the window panes before resealing, which will help to keep your double glazed windows condensation-free for longer. 


If you have had your double glazed windows for a long time, sometimes replacing the entire units is more beneficial. Although this may seem costly, new UPVC double glazed windows can have a lifespan of up to 20 years. Therefore, it will provide better value for money in the long run, and you shouldn’t encounter any problems with condensation, mould, or wet rot within this lifespan.