Flattened Glass in Thermopane Windows

Thermopane or Dual paned windows include two panels of glass, separated by a spacer bar and covered together. Some windows were produced with Argon gas injected in to the cavity between the glass panels to improve energy efficiency. Argon was utilized 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 vacuum.

Other Causes of Collapsed Cup

There are other possible causes of a collapsed glass window. During the manufacturing of large thermopane units, one panel of glass is laid over top the other panel, separated from the spacer bar, and then are sealed 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 is definitely less pressure or gas on the inside of the window cavity, which can lead to a Collapsed glass condition after it is installed and cooled. Preferably, after the large unit has been manufactured, it would be placed upright and briefly vented with a breather tube and resealed to allow the cavity to equalize. The large thermopane that is not equalized will have a lower pressure within the windows cavity after it is stood upward.

Some smaller thermopane glass devices are manufactured with single strength glass (1/16″). This glass can, due to its weakness, flex inward in severe cold conditions, reducing the insulating qualities of the window.

Lower pressure in a very glass cavity is a significant problem in colder climates, because the gasoline (air) within a collapsed window cavity has a partial vacuum (low pressure) which contracts, and causes the 2 panels of glass to bend inward.

Factors that Contribute to Flattened Glass

If the Argon placed inside 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 because it was constructed, and leaves hardly any room for the glass to bend 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 evoking the air left inside the window cavity to contract further, pulling both 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, making less air in the window hole.
How to know if you have Collapsed Glass

The telltale sign of Flattened glass is a faint rainbow colored 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 homeowner may think that because the oval condensation spot disappears during warmer temperatures, 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 Flattened Glass

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

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


Collapsed glass has become more of a problem as windows age and the original Argon gas disappears from the window cavities. As energy prices rise, it is more important than ever before to restore the windows insulating characteristics and save the glass from future replacement.