Troubleshooting Guide

Insulated glass unit failure: diagnosis, repair limits and replacement

A hazy or fogged window is one of the more common calls a glazier gets, and one of the easiest to misdiagnose from a distance. Some cases are a simple surface condensation issue that clears on its own; others mean the sealed unit has failed and needs to be replaced. This guide covers how to tell the two apart, what can and cannot be repaired, and what goes into ordering a correct replacement unit.

Written by the Gloria Glass installation team

What an insulated glass unit actually is

An insulated glass unit, or IGU, is two or more panes of glass separated by a spacer and sealed around the perimeter, with a sealed gap between the panes that is filled with air or an inert gas. The seal is what gives the unit its insulating performance, and it is also the single point most likely to fail over the unit's life.

Surface condensation versus seal failure

Condensation on the outer surfaces of a window is a humidity and temperature phenomenon, not a sign of a broken window. It typically appears on the room-side surface in cold weather when indoor humidity is high, or on the outside surface on humid mornings, and it clears as conditions change. It can be wiped away and it does not recur inside the sealed gap between panes.

Fogging or a persistent haze that appears between the two panes, and does not clear when the glass is wiped, indicates the seal has failed and moisture or air has entered the gas space. Once this happens it does not reverse itself; the fogging may come and go with temperature and humidity, but the underlying seal failure is permanent. Visible mineral deposits or a milky film that only shows under certain light are common signs of an older failure.

A quick way to tell them apart

• Condensation forms on an outer glass surface you can touch and wipe; seal failure fog is inside the sealed gap and cannot be reached.
• Condensation changes quickly with temperature and humidity; seal-failure haze may fluctuate but never fully clears.
• Seal failure is often worse at the edges of the unit, near the spacer, before it spreads.
• A unit with failed seals may also show reduced insulating performance, such as a noticeably colder pane in winter.

Where symptoms are ambiguous, a field inspection is the reliable way to confirm which is happening, since some early-stage failures are subtle and easy to mistake for condensation.

What can and cannot be repaired

A failed seal on a sealed insulated unit is not a repairable defect in the way a scratch or a chip might be. The seal and the gas fill are integral to how the unit was manufactured, and once moisture or outside air has entered the gap, cleaning or resealing does not restore the original condition or clear the existing haze permanently. The practical remedy is replacing the insulated glass unit itself.

If the surrounding frame is in good condition and only the glass unit has failed, the frame usually does not need to be replaced — the new unit is sized to fit the existing opening and glazing method. If the frame itself shows corrosion, rot or distortion, that is a separate problem that should be addressed alongside the glass replacement rather than after it.

Measuring and ordering a replacement unit

A replacement IGU needs to match the existing opening's daylight size and the frame's rabbet depth, and it needs a spacer profile compatible with the existing glazing method. Beyond the physical dimensions, a few makeup decisions matter:

• Spacer type — the material and profile separating the panes affects both the unit's edge performance and its appearance from close range.
• Pane makeup — whether each pane is monolithic, laminated, tinted or coated should match or intentionally upgrade the original specification.
• Gap fill — air or an inert gas fill affects insulating performance and is worth confirming against the original unit or project goals.
• Overall thickness — the combined thickness of panes, spacer and gap needs to fit the existing frame's glazing pocket.

Field measurement before ordering is important because frames settle and older openings are not always the size the original drawings suggest.

What to expect after replacement

A correctly sized and seated replacement unit should sit cleanly in the existing frame with no visible gap or distortion, and normal surface condensation behavior should return to whatever is typical for the building's climate and interior humidity — it is a byproduct of conditions, not something a new unit eliminates entirely. If haze or fogging reappears between the panes after replacement, that points to an installation or unit issue rather than an unavoidable outcome, and is worth raising with whoever supplied the unit.

A short diagnostic checklist

• Confirm whether haze is on an outer surface (condensation) or inside the sealed gap (seal failure).
• Check whether the condition clears with a wipe and changing conditions, or persists.
• Note whether the fogging is concentrated near the edges, which often signals an early failure.
• Inspect the surrounding frame for corrosion, rot or distortion before ordering a replacement.
• Field-measure the opening and rabbet depth rather than relying on original drawings.
• Decide the spacer, pane makeup and gap fill for the replacement unit.
• Confirm the replacement is seated and sealed cleanly once installed.

Where to go next

For repair and replacement scoping, see glass repair & replacement. If the fogging shows up on a storefront rather than a standard window, the broader diagnostic questions are covered in storefront condensation, seal failure or damage.