Glass · diagnosis
Misted double glazing: which surface is the water on?
Condensation on a sealed unit turns up in three different places, and only one of them means the window has failed. Telling them apart takes ten seconds and a finger, and it decides whether you are buying a new unit or opening a window.
The four surfaces
Counted from outside in. It is the trade convention, and every answer below depends on it.
The ten-second test
Do it in this order. The first one that answers, answers.
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Wipe it from inside the room
If the glass goes clear, the water is on surface 4 and the unit is fine. This is room air meeting the coldest surface in the room, which is nearly always the glass.
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If that did nothing, go outside and wipe it
If it clears from out there, it is on surface 1. Nothing is wrong — read on, because this one is a good sign and almost everybody reads it as a fault.
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If neither wipe touches it, it is inside the cavity
Surfaces 2 and 3 are sealed in. You cannot reach them, and neither can a cloth. The perimeter seal has failed and that unit is finished.
What each answer means
Surface 4 — inside the room
Not a window fault. Warm damp air is meeting the coldest surface it can find. The cure is ventilation, heating and extraction at the source: kitchens, bathrooms, laundry dried indoors. A better window moves the problem rather than removing it, because the glass stays the cold spot.
Surface 1 — outside
The window is working. On a clear night the outer pane radiates heat to the sky and drops below the temperature of the air around it, and because the cavity insulates so well, no warmth arrives from the house to stop it. Dew forms on the outside for the same reason it forms on a car roof, and it burns off with the sun. A poorly insulating window never does this: it is leaking enough heat to keep its own outer face warm.
Surfaces 2 and 3 — inside the cavity
The edge seal has let air in, and the desiccant in the spacer bar has taken up all the moisture it can hold. Once it is saturated it does not recover. From then on every warm afternoon and cold night drives water in and out of the cavity, and you watch it happen.
Why a misted unit cannot be repaired
This is the part customers argue about, so here is the mechanism rather than the slogan.
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The desiccant does not dry out again
The silica or molecular sieve inside the spacer is spent. Warming the unit does not regenerate it in place, and there is no way to change it without taking the unit apart.
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Drilling and draining it destroys what was paid for
A hole vents the cavity to outside air. It also lets the argon out, permanently. What is left looks clear on a dry day and has lost the insulation, the acoustic performance and any warranty. It will mist again the first humid week.
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The unit is the product, not the pane
The sealed unit is replaced whole — two panes, spacer and seal — and the frame stays where it is, provided the frame and its drainage are sound. If water sits in the rebate, the new unit fails the same way, so that gets fixed first.
The relevant standard is EN 1279, the European standard for insulating glass units. Its part 2 is the long-term moisture penetration test, which is precisely the property that has been lost here.
Before you order the replacement
A sealed unit is made to order and cannot be trimmed afterwards. Measure before you ring.
Take the sight size and the rebate rather than the hole in the wall, and take the diagonals: a unit ordered square for an opening that is not square will not glaze. The build-up matters as much as the size — 4-16-4 and 6-12-33.1 fit the same rebate and do not do the same job.
How to measure an opening · What the new unit will weigh · Which build-up gets you which U-value
What this page cannot do
It cannot tell you whether the unit is still under warranty, and that is usually the question worth money. Sealed units are typically warranted for a fixed number of years against exactly this failure, and a claim turns on the date and the maker mark stamped into the spacer, not on how bad it looks. Find that mark before you accept a quote for a new one.