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 it’s integrity and that it is letting in moisture somewhere. The moisture itself is hard to spot at first, sometimes becoming only visible 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 that cannot escape, and 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 unit 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 is not practical as the sealed unit is factory made with the glass bonded together to (hopefully) last a lifetime.
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