Five Signs an Insulating Glass Unit Is Failing

2024 · HIHAUS

An insulating glass unit is two panes, a spacer and two seals. It is designed to keep the cavity dry for twenty-five years or more. When it does not, the cause is almost always the perimeter seal rather than the glass, and the symptoms appear in a predictable order.

1. Interior condensation that will not clear

Condensation on the room-side surface is normal in cold weather and wipes away. Condensation, fogging or a haze between the panes is a seal failure, and no amount of cleaning will remove it. This is the definitive symptom: once moisture is inside the cavity, the desiccant is saturated and the unit is finished.

2. A visible haze or white film

A milky film that develops gradually, often starting at one corner, is desiccant dust or a failed Low-E coating reacting to moisture. It reduces visible transmittance and, once the coating degrades, the U-factor the unit was certified for is no longer the U-factor the building has.

3. Water pooling in the cavity

Visible liquid water between panes means the secondary seal has failed completely. This is usually accompanied by a corroded spacer, which will continue to degrade and can stain the frame. Units in this condition should be scheduled for replacement rather than monitored.

4. Optical distortion or a wavy reflection

A unit that has lost pressure bows inward or outward, and the reflection in the glass becomes visibly wavy. This is also a structural warning: a bowed lite carries different stress and, in extreme cases, cracks from the edge.

5. Edge seal discoloration

Look at the perimeter of the unit where it meets the frame. A healthy primary seal is uniform. A failing one shows a gray or black line creeping inward, or a gap between the glass and the spacer. This is the earliest visible sign and the one most easily checked during a site walk.

Why it happens

Four causes cover most failures: an inadequate primary seal at manufacture, thermal movement in a frame that transfers stress to the unit edge, water standing in the glazing rebate because drainage is blocked, and mechanical damage during installation or cleaning. The first is a factory issue, the other three are site issues, and all four are avoidable with a specified seal system and a drained, ventilated rebate.

What to do about it

• Record the failure with photographs and the unit identification from the spacer label, which usually carries the production date.
• Check whether the failure is isolated or clustered. A cluster on one elevation points to a frame or installation problem, not a glass batch problem.
• Report inside the warranty period. Most manufacturers warrant seal integrity for a defined term, and claims made after it expires are a cost you absorb.
• When replacing, confirm the new unit matches the original make-up, because a mismatched thickness changes the rebate and the compression of the gaskets.

Every unit we ship carries a spacer label with the production date and the assembly specification, which is what makes a warranty claim verifiable rather than negotiable. If you have units in service that show any of the five signs, send us photographs and the label and we will tell you whether it is a seal failure or a rebate drainage problem.