Collapsed Glass in Thermopane Windows

Thermopane or Dual paned windows consist of two panels of glass, divided by a spacer bar and covered together. Some windows were manufactured with Argon gas injected into the cavity between the glass panels to boost energy efficiency. Argon was utilized because of its higher insulating qualities over ambient air. In some situations the particular 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 more than top the other panel, separated by spacer bar, and then are sealed together. The top panel of bigger thermopane units will naturally flex downwards in the center due to its weight, since it is sealed into the thermopane. The result is usually less pressure or gas within the window cavity, which can lead to a Collapsed glass condition right after it is installed and cooled. Ideally, after the large unit has been manufactured, it would be placed upright and quickly vented with a breather tube plus 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 units are manufactured with single strength glass (1/16″). This glass can, due to its weakness, flex inward in intense cold conditions, reducing the insulating characteristics of the window.

Lower pressure in an exceedingly glass cavity is a significant issue in colder climates, because the gas (air) within a collapsed window hole has a partial vacuum (low pressure) which contracts, and causes both panels of glass to bend 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 runs around the perimeter of the thermopane unit) is a narrow type since it was constructed, and leaves hardly any room for the glass to flex before the glass touches.
If the cup in the unit is single power (1/16″) which is weaker and simpler to flex.
If the temperature decreases causing the air left inside the window hole 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 structure and is sealed in that position, leaving behind less air in the window tooth cavity.
How to know if you have Collapsed Cup

The telltale sign of Flattened glass is a faint rainbow coloured spot in the center of the window (this is where the glass panels are usually touching), and in some cases an oval or even round condensation spot in the center of the particular window on the inside of the home.
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The homeowner may think that because the oval condensation spot disappears during warmer temperatures, the problem may have resolved itself, yet that is not the case. The condition will probably recur and the heat loss through the glass will resume.

Problems associated with Collapsed 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 2 panels of glass to flex in, reducing the space inside the window’s cavity, which reduces the insulating qualities of the window. This intense flexing of the glass panels may also lead to premature seal failure, that 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 repaired. 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 usually then placed over the hole to re-seal the window. This will bring back the insulating qualities of the windows minus the original argon. If the collapsed glass occurs in a tempered glass window (patio door or other large units where tempering can be required) the glass can’t be drilled as it can in an annealed unit (regular thermopane). These tempered glass units can be removed from the window body allowing the procedure to be accomplished 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 group and the original Argon gas goes away from the window cavities. As power prices rise, it is more important than ever before to restore the windows insulating characteristics and save the glass from future replacement.

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