Collapsed Glass in Thermopane Windows

Thermopane or Dual paned windows contain two panels of glass, separated by a spacer bar and sealed together. Some windows were manufactured with Argon gas injected into the cavity between the glass panels to improve energy efficiency. Argon was used because of its higher insulating qualities more than ambient air. In some situations the Argon gas will dissipate from the window cavity causing the window tooth cavity to have a partial negative pressure or even vacuum.

Other Causes of Collapsed Cup

There are other possible causes of a collapsed glass window. During the manufacturing of large thermopane units, one board of glass is laid over top the other panel, separated with the spacer bar, and then are covered together. The top panel of bigger thermopane units will naturally flex downward in the center due to its weight, as it is sealed into the thermopane. The result can be less pressure or gas on the inside of the window cavity, which can result in a Collapsed glass condition after it is installed and cooled. Ideally, after the large unit has been produced, it would be placed upright and briefly vented with a breather tube and resealed to allow the cavity in order to equalize. The large thermopane that is not equalized will have a lower pressure within the windows cavity after it is stood up.

Some smaller thermopane glass models are manufactured with single strength glass (1/16″). This glass can, because of its weakness, flex inward in extreme cold conditions, reducing the insulating characteristics of the window.

Lower pressure inside a glass cavity is a significant concern in colder climates, because the gasoline (air) within a collapsed window tooth cavity has a partial vacuum (low pressure) which contracts, and causes both panels of glass to flex inward.

Factors that Contribute to Collapsed Glass

If the Argon placed within the thermopane unit has dissipated with the seal.
If the spacer bar (the visible, in most cases silver, bar that will runs around the perimeter of the thermopane unit) is a narrow type as it was constructed, and leaves very little room for the glass to flex before the glass touches.
If the cup in the unit is single strength (1/16″) which is weaker and easier to flex.
If the temperature decreases causing the air left inside the window cavity to contract further, pulling the 2 panels of glass inward.
When the thermopane consist of large panels of glass where the top panel of glass flexes downward during construction and is sealed in that position, leaving behind less air in the window cavity.
How to know if you have Collapsed Cup

The telltale sign of Collapsed glass is a faint rainbow coloured spot in the center of the window (this is where the glass panels are touching), and in some cases an oval or round condensation spot in the center of the window on the inside of the home. The home owner may think that because the oval moisture build-up or condensation spot disappears during warmer temps, the problem may have resolved itself, but that is not the case. The condition will probably recur and the heat loss through the cup will resume.

Problems associated with Collapsed Glass

A windows R-value is mostly determined by the amount of space inside the window’s cavity. When the space between the glass is reduced, the insulating high quality (R-value) of the thermopane unit will be reduced. Collapsed Glass causes the 2 panels of glass to flex in, reducing the space inside the window’s cavity, which reduces the insulation qualities of the window. This intense flexing of the glass panels may also lead to premature seal failure, which will then require thermopane replacement. In some cases the glass can be flexed so forcibly together that one or both panels will shatter.

Repair Flattened Glass

Collapsed Glass can be fixed. Using specialized tools a specialist can penetrate the glass, which will relieve the negative pressure and equalize the window cavity with all the outside environment.
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A clear seal is then placed over the hole in order to re-seal the window. This will regain the insulating qualities of the window minus the original argon. If the collapsed glass occurs in a tempered cup window (patio door or additional large units where tempering is required) the glass can’t be drilled as it can in an annealed unit (regular thermopane). These tempered glass models can be removed from the window framework allowing the procedure to be accomplished simply by penetrating the seal and spacer, allowing ambient air in to equalize the window cavity, and then re-sealing the unit.


Collapsed glass is becoming more of a problem as windows age group and the original Argon gas dissipates from the window cavities. As power prices rise, it is more important than ever to restore the windows insulating features and save the glass through future replacement.

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