When a window fogs up on the inside where you cannot wipe it, the sealed insulating glass unit has failed at its edge — and the cause usually traces back to how the unit was made, glazed, and drained.
An insulating glass unit — two or more panes sealed around a spacer with an edge seal, often with an inert gas fill — depends entirely on that edge seal to keep moisture out and gas in. When the seal breaks down, moisture gets between the panes and condenses where no one can wipe it, the unit fogs permanently, and its thermal performance drops. The visible symptom is glass; the actual failure is at the sealed edge, and its causes are largely about manufacturing quality and how the unit is glazed and drained.
The failures are predictable. The sections below cover the four that ACE sees most — fogging between the panes, edge seal breakdown, gas loss and thermal performance, and condensation and replacement — and how detailing and specification reduce them.
These challenges rarely stay in one silo: the same discipline runs through lessons from through-wall flashing failures, our guidance on kickout flashing failures, and what we have learned from counterflashing and reglet failures, and it all ladders up to ACE’s broader envelope consulting for architects.
Fogging between the panes is the visible symptom of a failed insulating glass unit, because once the edge seal admits moist air, water condenses in the sealed cavity between the panes where it cannot be cleaned. The fog comes and goes with temperature at first, then becomes a permanent haze or mineral deposit.
It is important to recognize that this is not dirty glass and cannot be fixed by cleaning, since the moisture is inside the sealed unit, which is why the symptom reliably points to a seal failure.
Because the fog signals that the seal has already breached, it is a clear, visible marker that the unit’s performance is compromised and it is heading toward replacement.
Representative of the IGU failure patterns ACE finds in glazing investigation and peer-review work. Illustrative distribution, not a published statistic.
Edge seal breakdown is the actual failure behind the fog, because the primary and secondary seals around the spacer are what keep moisture out and any gas fill in, and they degrade when stressed by moisture, movement, UV exposure at the edge, or manufacturing defects. A well-made unit with a durable, properly protected edge seal resists that breakdown.
The desiccant in the spacer buys time by absorbing incidental moisture, but once it saturates, condensation begins, so a compromised seal is on a countdown from the moment it breaches.
Because the edge seal is doing the real work, unit quality and edge protection during glazing are the levers that most affect how long a unit lasts.
A sealed glass unit with a failing edge seal fogs between the panes, loses its gas fill and thermal value, and must be replaced.
Specify quality units and detail the glazing pocket to drain, keeping the unit edge out of standing water and protected from movement and UV.
The seals stay intact, the glass stays clear, the thermal performance holds, and premature replacement is avoided.
Gas loss and thermal performance decline together, because many units are filled with an inert gas to improve insulation, and a failing seal lets that gas escape and moist air in, so the unit loses the thermal performance it was specified for well before the fog is obvious. The building quietly loses energy efficiency.
This makes seal failure an energy issue as much as an appearance issue, since the assembly no longer performs to the values the design relied on.
Specifying quality units and protecting the seals is therefore about maintaining the thermal performance the envelope was designed around, not only about avoiding foggy glass.
Get the units and glazing-pocket drainage reviewed so the edge seals last as long as they should.
Schedule a consultationCall (866) 389-8883Condensation and replacement are the end state, because a failed unit cannot be repaired — the sealed assembly has to be replaced — and standing water in the glazing pocket is a leading accelerator of edge seal failure. Glazing pockets detailed to drain, so the unit’s edge is not sitting in water, materially extend seal life.
Weep paths and setting blocks that keep the unit edge above any incidental water in the pocket are small details with a large effect on how long the seals last.
The most valuable move overall is combining quality units with glazing-pocket drainage and edge protection, since that is what prevents the premature failures that lead to wholesale unit replacement.
In ACE’s field work, most insulating glass unit seal failures problems trace back to a few recurring locations — Edge seal breakdown, Standing water in glazing pocket, and Manufacturing / unit quality — rather than the open field of the wall or roof. The cost of resolving them climbs by roughly an order of magnitude at each stage: a detail corrected during design costs a fraction of the same fix during construction, which is itself far cheaper than a repair once the facility is operating. That economics is why we push envelope decisions as early in the project as possible.
Specify durable insulating glass units and glazing pockets detailed to drain, with setting blocks that keep the unit edge out of standing water.
Confirm the units are set correctly and the glazing-pocket weeps and drainage are open and functioning.
Distinguish seal-failed units (internal fog) from surface condensation, and address glazing-pocket drainage when replacing.
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