Condensation in double-paned windows

One of the common finds seen during home inspections is condensation or cloudiness inside double-paned windows.  Many people are curious as to why this happens and if anything can be done to prevent it, as it it unsightly to say the least.  

Double-paned windows have a layer of gas (usually either argon or air) that is trapped between the two panes of glass.  This gas acts as insulation that reduces energy loss through the window.  Other styles of double-paned windows have a thin film, typically known as Low-E Film, that is installed between the two panes, which further reduces the energy loss and gain through the window.  If you see condensation or cloudiness in between these panes, it means that the seal protecting the window assembly has failed.

Double-paned windows experience a daily cycle of expansion and contraction that is caused by thermal pumping.  Sunlight heats the airspace between the panes, and causes expansion and contraction of the gas in between the panes, which pressurizes the space between the two panes of glass.  Over time, this constant pressure that is put on the window panes stresses the seal enough to where small fractures develop, which cause the seal to fail, letting outside air in and out of the window panes.

Windows that are located on the sunny side of a home will experience larger temperature swings, which obviously results in greater amounts of thermal pumping, leading to higher failure rates of the windows.  Vinyl windows are the most likely windows to fail, because they have a higher coefficient of expansion, which results in greater long term stress on the double pane assembly.  Vinyl windows are also the most likely to have manufacturer defects, meaning there is small defects in the pane during manufacturing which causes the seal to fail prematurely. 

If windows that have a failed seal are left untended to, then irreversible damage can occur to the window itself.  One condition that can form is what is known as "riverbedding".  This is where condensed vapor between the glass panes forms water droplets that run down the length of the window, and actually carve narrow paths or grooves into the glass, very similar to how canyons are formed in nature.  Another condition is known as "silica haze".  Windows have silica pellets in between the panes that is designed to absorb moisture from any incoming air that enters the space between the window panes.  Once the silica gel has been saturated from constant outside air coming in, it will be eroded and accumulate as white "snowflakes" on the surface of the glass.  Once this happens, the only option is window replacement.

Whenever condensation is seen between window panes, the glazing assembly can usually be replaced.  Occasionally, the window sashes must be replaced.  It is on rare occasions that the entire window needs to be replaced.  There are companies out there that claim to be able to repair foggy windows through the process that they refer to as "defogging".  What they do is drill a small hole into the window, and spray a cleaning solution in between the window panes.  The solution and any moisture is then sucked out through a vacuum.  A defogger is then permanently inserted into the hole that was drilled that will allow the release of moisture during thermal pumping.  This process typically only removes the symptoms of a failed window seal, rather than addressing the issue altogether.

All in all, when you see condensation in between double-paned windows, the seal has failed and needs repair or replacement.



Source: InterNACHI