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Condensation Inside Double Glazing

Condensation or ‘misting’ inside a window is a sure sign that the sealed unit has failed. Somewhere, a point in the edge seal of the window lost its integrity and is allowing the ingress of water.

Condensation or ‘misting’ inside a double glazed window

Condensation in Double Glazed unit

At first the condensation can be hard to spot; it might only be visible when a heat source, such as the sun, causes the moisture to rise.

But if left unrepaired for a long period, it can lead to a volume of water sitting in the base of the window between the frames, unable to escape.

Once a double glazed sealed unit has ‘broken down’ it will continue to draw moisture in, typically when there are changes in the atmospheric pressure.

The panes are held apart by a spacer bar which incorporates hundreds of silica balls to soak up moisture.

These balls keep moisture out of the air or gas between the panes, but when the silica gets to the point that it is saturated, moisture is able to enter the gap between the panes and will begin to condensate on the internal surfaces.


Do any types of window suffer more than others?

There are various reasons that a sealed unit can break down, whatever the type of window, or material of the frame. Sometimes it is a result of how the unit was manufactured, how the framework was made, or how the unit and the framework were fitted together.

Cheaper double glazed units are unlikely to be sealed as well as a more expensive unit, but assuming that the quality of construction is the same, there are some frames which are more susceptible to condensation than others.

Timber frames tend to suffer from rot and therefore condensation, and need to be well maintained and re-painted regularly.

Aluminium framed windows are generally considered the strongest and most durable, but can also be prone to condensation. UPVC double glazing is less likely to suffer, and is usually cheaper than aluminium.

Restrictions may apply in conservation areas or on listed buildings requiring timber frames, and there are hardwood options available which are considered more robust than standard timber frames.


How can it be prevented?

Condensation cannot be prevented by a course of maintenance; it is something that will happen naturally to any window, although it should not become a problem for at least the first five years after installation.

Purchasing well made windows in the first place will ensure longer life and lessen the likelihood of repair or replacement.

Most companies offer a long guarantee on their windows and there is no reason a double glazed unit can’t last in excess of ten or fifteen years.


Can the window be repaired?

Double glazed windows can be repaired and the entire window does not necessarily need to be completely replaced. The sealed unit is designed to be able to be removed from the frame, and therefore replaced.

This is quite a difficult process as the failed unit must be removed without damaging the frame when removing the beads, which can become brittle with age.

It is probably not a DIY job, and it is more sensible to contact a reputable and experienced window fitter to carry out the repair.

Once windows have started to condensate, they will lose some of their thermal efficiency and sound proofing; if you are replacing the sealed units, it may be possible to upgrade to a more efficient product.

Another type of repair available for a sealed unit which has failed, involves drilling a small hole in the glass from outside and installing a small vent into the window.

This does require specialist tools and is not something that can be undertaken yourself, but companies offering this service claim it results in savings of up to 50% over replacing the sealed unit.


What about guarantees?

The majority of companies offer at least a ten year guarantee on their products, with some even offering a lifetime guarantee.

A breakdown of a sealed unit should be covered under the guarantee and this includes the failure of a seal and the resultant condensation.

Make sure that ‘breakdown’ is included within the guarantee, and learn the difference between a guarantee and a warranty, which may require an annual payment to continue it.

If a sealed unit fails during the guarantee period, the supplier should replace it, or entire window if necessary, free of charge without any quibbles.

Make sure you keep your receipts and any paperwork related to the original installation to show that you are still within the guarantee period if need be.

You can also refer to the ‘Your Legal Rights’ section of this website for more detailed information.



Condensation is a natural problem within today’s modern heated homes, and double glazing is no different. Eventually any sealed unit will suffer condensation between the panes, simply due to age.

Condensation within double glazing

Condensation within faulty double glazing

Well made units, which are professionally fitted into a well made frame, will undoubtedly last longer. While it may be tempting to opt for cheaper windows upon installation, in the long run they will prove to be more expensive in terms of repair or replacement.

At a minimum, no sealed unit should fail within five years of being fitted, and it is reassuring that most manufacturers offer at least a ten year guarantee against failure.

When double glazed units do fail, whether down to poor quality or simply being at the end of their lifetime, it is possible to replace the unit within the frame rather than replacing the entire window itself.

With the advancement in the quality of double glazing units, their U values, their thermal efficiency, and their sound proofing, this can present an ideal opportunity to upgrade your windows to reduce heat loss, and improve on the overall efficiency of your home.

However, if the sealed units fail within your guarantee period make sure you know your rights, and get them replaced free of charge by the supplier.

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