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Condensation in Double-Paned Windows

What is Condensation?


Condensation is the accumulation of liquid water on relatively cold surfaces.

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Double-pane windows have a layer of gas (usually argon or air) trapped between two panes of glass that acts as insulation to reduce heat loss through the window. Other types of gas used in this space have various effects on heat gain or loss through the window. Some windows also have a thin film installed between panes that separates the space between the panes into two spaces, further reducing heat loss and heat gain through the window. If multiple-pane windows appear misty or foggy, it means that the seal protecting the window assembly has failed.

Why Double-Paned Windows Fail: Solar (Thermal) Pumping

Although double-paned windows appear to be stable, they actually experience a daily cycle of expansion and contraction caused by thermal pumping. Sunlight heats the airspace between the panes and causes the gas there to heat up and expand, pressurizing the space between the panes. At night, the window cools and the space between the panes contracts. This motion acts like the bellows of a forge and is called thermal pumping.

Over time, the constant pressure fluctuations caused by thermal pumping will stress the seal. Eventually, the seal will develop small fractures that will slowly grow in size, allowing increasing amounts of infiltration and exfiltration of air from the space between the panes.

Failure Factors

Windows on the sunny side of a home will experience larger temperature swings, resulting in greater amounts of thermal pumping, seal stress and failure rates.

Vinyl window frames have a higher coefficient of expansion resulting in greater long-term stress on the double-pane assembly, and a higher failure rate. Windows also experience batch failure, which describes production runs of windows, especially vinyl windows, that are defective, meaning that the pane assemblies have been manufactured with seals that have small defects that will cause the window to fail prematurely.

Recommendations for Failed Windows

According to industry experts, the glazing assembly can be replaced approximately 75% of the time. Occasionally, the sashes must be replaced, and only about 5% of those cases require that the entire window be replaced.

Inspectors should be aware that there are companies that claim to be able to repair misty windows through a process known as “defogging.”

This repair method proceeds in the following order:

  • A hole is drilled into the window, usually from the outside, and a cleaning solution is sprayed into the air chamber.
  • The solution and any other moisture are sucked out through a vacuum.
  • A defogger device is permanently inserted into the hole that will allow the release of moisture during thermal pumping.

In summary...

 Condensation appears between double-paned windows when the window is compromised, and removal of this water will not fix the seal itself. A window “repaired” in this manner, although absent of condensation, might not provide any additional insulation. This method is still fairly new and opinions about its effectiveness range widely. Regardless, “defogging” certainly allows for cosmetic improvement, which is of some value to homeowners. It may also reduce the potential for damage caused by condensation in the form of mold or rot. Some skepticism exists about the effectiveness and cost effectiveness of this method of repair.

In other words, condensation in double-paned windows indicates that the glazing assembly has failed and needs repair or replacement. Visible condensation can damage glazing and is the main indication of sealant failure





Source: https://www.nachi.org/condensation-double-paned-windows.htm