Collapsed Glass in Thermopane Windows

Thermopane or Dual paned windows consist of 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 increase energy efficiency. Argon was used because of its higher insulating qualities over ambient air. In some situations the Argon gas will dissipate out of the window cavity causing the window cavity to have a partial negative pressure or vacuum.

Other Causes of Collapsed Glass

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 by the spacer bar, and then are sealed together. The top panel of larger thermopane units will naturally flex downward in the center due to its weight, as it is sealed into the thermopane. The result is 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. Ideally, 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 window cavity after it is stood up.

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

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

Factors that Contribute to Collapsed Glass

  • If the Argon placed inside the thermopane unit has dissipated through 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 as it was constructed, and leaves very little room for the glass to flex before the glass touches.
  • If the glass 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 two panels of glass inward.
  • If 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 less air in the window cavity.

How to know if you have Collapsed Glass

The telltale sign of Collapsed 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 glass 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 quality (R-value) of the thermopane unit is reduced. Collapsed Glass causes the two panels of glass to flex in, reducing the space inside the window’s cavity, which reduces the insulating qualities of the window. This extreme flexing of the glass panels can 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 Collapsed Glass

Collapsed Glass can be repaired. Using specialized tools a technician can penetrate the glass, which will relieve the negative pressure and equalize the window cavity with the outside environment. A clear seal is then placed over the hole to re-seal the window. This will restore the insulating qualities of the window minus the original argon. If the collapsed glass occurs in a tempered glass window (patio door or other large units where tempering is 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 frame allowing the procedure to be accomplished 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 and the original Argon gas dissipates from the window cavities. As energy prices rise, it is more important than ever to restore the windows insulating characteristics and save the glass from future replacement.