Where this starts
A sealed double-glazed unit is two sheets of glass bonded to a spacer bar around the edge, with the cavity between them filled with dry air or argon and sealed. The spacer is packed with desiccant, which absorbs the small amount of moisture sealed in during manufacture and keeps the cavity dry for as long as the seal holds.
THE WIPE TEST IS THE WHOLE DIAGNOSIS. Mist on the room side wipes off, and it is condensation on cold glass — a question about humidity and ventilation, not about the window. Mist on the outside face also wipes off, and on a cold clear morning it is a sign the glazing is working WELL, because the outer pane is staying cold. Mist that will not wipe off from either side is inside the unit, and it got there because the seal failed.
WHY IT CANNOT BE DRIED OUT. Once the seal is breached the cavity breathes with the outside air, and the desiccant absorbs moisture until it is saturated — which takes anywhere from months to years. After that, every temperature swing drives condensation onto the inner faces, and the desiccant has nothing left to give. Drilling and venting a unit is sold as a repair; it converts a sealed cavity into a vented one, which stops the visible misting and ends whatever insulating value the gas fill was providing.
THE PATTERN OVER A HOUSE IS INFORMATIVE. Units fail one at a time over years if it is ordinary ageing. Several failing within a season, or all on one elevation, points at something common — a batch, an installation detail, or a drainage problem in the frames that is standing water against the seals.
One thing worth settling before ordering: the unit is replaceable independently of the frame. A failed sealed unit does not require a new window, and a quote that assumes it does is quoting a different job.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
The perimeter seal has failed with age
Most commonMist that will not wipe off from either side, often with a faint tidemark or residue on the glass inside, on units of a certain age and worsening over time.
How to check: Try to wipe both faces. Water that cannot be reached is between the panes, and the seal is the only route in.
Room-side condensation on cold glass
Most commonMist that wipes off on the inside, worst on cold mornings, worst in bedrooms, kitchens and bathrooms, and clearing as the room warms.
How to check: Whether it wipes. If it does, the glass is intact and the question is humidity and ventilation in that room.
Water standing against the unit in the frame
CommonSeveral units failing on the same elevation or at the same age, often with blocked drainage slots in the frame bottom, or a cill that holds water.
How to check: Whether the frame's drainage slots are clear. A unit sitting in standing water loses its seal far sooner than one that drains.
Outside-face condensation, which is not a fault
CommonMist on the OUTER face on a clear cold morning, clearing as the sun reaches it, on modern well-insulating units.
How to check: Which face it is on. Outside misting means the outer pane is staying cold, which is the unit working rather than failing.
The unit was damaged during installation or transport
Less commonFailure within the first year or two, sometimes with a visible edge chip or a distorted spacer, and often only one unit among many.
How to check: How old it is. Early failure is usually handling, and is normally covered by the unit's own guarantee.
Pressure cycling on a large or high-altitude installation
Less commonVisible bowing of the panes inward or outward, on large units, tall buildings or after transport over a mountain pass, sometimes with distorted reflections before any misting.
How to check: Whether the reflections in the glass are distorted. A cavity under sustained pressure difference works the seal continuously.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It separates a failed unit from room condensation in seconds, and those are a replacement and a ventilation question respectively — nothing else on this page matters until it is settled.
- 2
Inside on a cold morning is the room; outside on a cold morning is the unit performing well; neither face is a failed seal.
- 3
Blocked slots leave the unit's edge seal sitting in water, which is the single most common reason a whole elevation fails together rather than one unit ageing out.
- 4
One unit at fifteen years is ageing. Four in one season is a batch, an installation detail or a drainage problem, and replacing them without finding it repeats the failure.
- 5
Many units carry a manufacture date and a reference on the spacer, visible through the glass, which settles the age question and is what a guarantee claim needs.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where several units have failed on one elevation or within one season, since the cause is likely the frames, the drainage or the batch rather than the units themselves.
- Where the frame is timber and the glazing bead or the cill beneath a failed unit is soft to a probe, which is decay rather than a glazing fault.
- Where the glazing is at height, is large, or is in a door, because those units are heavy, are frequently toughened or laminated, and are not a two-person lift.
- Where the window is in a listed building or a conservation area, since the replacement unit's specification may be constrained and a like-for-like swap may not be permitted.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.