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Glass:
Misting up
Double-glazing sealed units misting up outside or going misty inside the glass
Once you have 'misty' double-glazed sealed units:
In the trade we say that the double-glazed sealed unit has 'broken down'. This means that the perimeter edge seal has lost its integrity and that it is letting in moisture somewhere. The moisture itself is hard to spot at first, sometimes becoming visible only as the Sun or another heat source 'draws' the moisture up. Typically, the moisture will be drawn in as the atmospheric pressure changes, and it will build up, eventually condensing into water inside the sealed unit. This can sometimes look like a fish tank with several inches of water inside and no outlet, and is quite funny when it is in a door and the water sloshes about!
What you can do about misted-up double-glazed sealed units:
The only answer is to replace the double-glazed sealed units with new ones. That almost always means little or no alteration or disruption to the framework if it is sound as the glass sealed unit is designed to be removable and replaceable. Trying to separate the glass panes is not practical as the sealed unit is
factory made with the panes bonded together to last a lifetime.
 As I walk up to a house that I am visiting to give an estimate for repairs to double glazing, I scan any coloured leaded lights with my eyes, mainly looking for greens that have gone blue.
This picture shows an extreme fading and discoloration of the coloured double-glazed sealed units. If you have coloured glass roses that have blue leaves, the sealed unit has probably started to break down, even though it may not be showing signs of moisture inside as
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Why did the double-glazed sealed unit break down?
There are a number of different glazing techniques for fitting double-glazing sealed units into different frame materials such as wood, as opposed to either PVC-U or aluminium frames, and there are some important Do's and Don't’s to ensure a reasonable life expectancy for the sealed unit.
Although there many reasons why a double-glazed sealed unit may mist up inside when it has 'broken down' because of premature failure of the perimeter seal, the essential cause will lie in how the sealed unit was made, how the framework it is fitted into was made or how the double-glazed sealed unit was fitted into the framework. On top of this, environmental conditions may play a part, such as wind or sonic loads and possible exposure of the perimeter seal to aggressive chemicals such as those found in washing-up liquid. This may all sound a bit complicated, but the cause of a premature failure of a double-glazed sealed unit can usually be fairly easily traced once it has been removed from its frame.
 This is a photo I took of a doubleglazed window sealed unit that is filling up with water.
It looks funny when a door has water sealed inside it that splashes around as you open and close it!
The reason this happens is that moisture is getting in and condensing into water, which then cannot escape, because it is effectively in a 'tank'. |
Choose the frame material below to find out more specific causes of sealed unit failure:
Double-glazing sealed unit failure in hardwood and softwood
Double-glazing sealed unit failure in PVC-U and aluminium frames
Read up more about sealed units:
Do double-glazed sealed units have a vacuum?
Types of glass
About safety glass
Double-glazed sealed units - manufacturing techniques
Double glazing air gaps and insulation
Why did my double-glazed sealed unit explode?
Double glazing and Brewster's Fringes
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