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Cross section of a double glazed sealed unit


Misted or foggy glass is the result of excess moisture in the cavity of the sealed unit. It is usually caused by the failure of the bitumen seal around the perimeter of the glass edge.


During manufacture of a sealed double glazed unit, all edges are hermetically "sealed" to trap the air inside the unit. The air "trapped" inside the unit at the point of manufacture will contain a certain amount of moisture, and as the temperature of this air falls with the weather conditions the desiccant will absorb the droplets of water.


If the bitumen seal breaks down, additional moisture in the air enters the cavity of the sealed unit, and the desiccant is unable to absorb all this moisture. Under certain conditions the water droplets will land on the internal surface of the glass. This results in the unattractive effect of condensation within the sealed unit.

New sealed units can usually be installed relatively easily and the integrity of your double glazed windows can be restored! We are happy to provide a quote completely free of charge and without obligation.


We can also upgrade your glass to new energy efficient units on request. Just contact us for a friendly consultation.

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Care and Maintenance

Other useful information can be found in our Care and Maintenance document, including regular checks and lubrication, how to tighten window handles (page 26), and tips on how to avoid condensation (page 29).


There is also a reference to the "Visual Standards" of glass (page 31). This section defines what can be expected with regard to the visual standards of glass, and imperfections and marks that will occur, but are not deemed to be faulty. These are Industry Standards to which we all have to adhere!

Download GGF advice on Condensation

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