Misted double glazing: why windows go cloudy and how it's fixed
A double glazed window that has gone permanently cloudy, foggy or streaky between the panes has not just got dirty. It has failed. The mist sits inside the sealed unit where no cloth can reach it, and it will not clear on its own. The good news is that fixing it almost never means buying a new window.
What a sealed unit actually is.
The glass in a double glazed window is a factory-made sandwich: two panes bonded to a spacer bar around the edge, with the cavity between them sealed and dry. That dry gap is what does the insulating. Around the edge, the seal keeps moisture out, and a drying agent inside the spacer bar mops up any damp air that was trapped at manufacture.
The unit is a single component, separate from the frame that holds it. That distinction is the whole story when it comes to repair.
Why units fail.
The edge seal lives a hard life. The glass expands and contracts with every warm day and cold night, flexing the seal season after season. Eventually it lets damp air creep in faster than the drying agent can absorb it, and once that drying agent is saturated, moisture condenses on the inside faces of the glass where you cannot touch it.
That is why misting starts as faint fogging in a corner on cold mornings, then spreads and eventually leaves permanent streaks and mineral marks. South-facing windows and units above radiators tend to go first, because they cycle through bigger temperature swings.
Why it matters beyond the view.
A misted unit has lost its insulating gap to damp air, so it no longer performs the way it did new. The room loses more heat through that window, and the fog itself tends to arrive exactly when you would like the light: cold, bright mornings.
It does not fix itself, and drilling the glass or so-called demisting treatments only hide the symptom: the failed seal and saturated spacer are still there, and the fog comes back.
How replacement actually works.
If the frame is sound, we measure the failed unit, have a new sealed unit made to that size and specification, and swap it into the existing frame. The frame, sill, handles and opening parts all stay. It is a fraction of the disruption of a window replacement, and the new unit brings current glass performance into the old frame.
This is also the moment to upgrade the glass itself if you want to: the replacement unit can carry modern low-emissivity coatings, acoustic glass or obscure patterns, whatever the room needs. If the frame has gone as well, warped, cracked or draughty around the edges, we will say so, because a new unit in a failed frame wastes the unit.
Cloudy glass?
Replace the failed unit, keep the frame.
If the mist is between the panes, the sealed unit needs replacing. We measure it, make it and fit it into your existing frame, and tell you honestly if the frame is past it.
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