Why Has My Thermal Window Fogged?

Why Has My Thermal Window “Fogged?”

In homes everywhere, in climates hot or cold, wet or dry, homeowners have had the experience of their thermal windows fogging.” Why and how does this happen?

Essentially, it is because the seal, located at the edge of the glass and covered by the sash or frame, has failed for some reason, breaching what was once an air-tight chamber and allowing the window’s argon gas to leak out, making it possible for atmosphere and water vapor to seep in.

What causes a seal to fail? There are several possible culprits – one could simply be a poorly manufactured IGU (Insulated Glass Unit). Another might be settling of the house. As any glazier or carpenter will tell you, what used to a square or rectangular opening ten or fifteen years ago, isn’t necessarily so any more. Most likely, though, it’s what’s known as “solar pumping” – a fancy name for the never ending cycle a window is put through when heated during the day, then cooled at night. This constant cycle of expansion and contraction over time, can and does compromise the seal on even the most prestigious manufacturer’s thermal windows.

Can the seal on a thermal window (also commonly referred to as an insulated or double-pane window) be repaired? Short answer – no. Once the seal has been breached it is impossible to effectively re-seal a thermal unit. Nor can one of the panes simply be removed, the interior cleaned, and then replaced. What appears to be just condensation between the panes (referred to by manufacturers as “lites”) is actually a mixture of water vapor and the decaying remnants of the silica desiccant which is placed in the aluminum spacer between the panes during the manufacturing process to absorb moisture, but quickly becomes saturated when seal failure occurs. In time, this cloud permanently etches the glass. (Silica desiccant is what is in those small white packets that are used to keep products dry during shipping and storage) In addition, the sealant used in thermal-pane manufacturing is typically butyl, an extremely strong bonding agent which makes it virtually impossible to separate the glass panes intact. Sadly, the unit might leak, but wont come apart.

When seal failure does occur, it does so in small increments and the moisture build-up between the glass takes place slowly, is barely noticeable at first, and is often simply mistaken as a dirty window. Over time the process intensifies as more and more water vapor enters the thermal unit, spreading in an ever-widening haze until what was once just a small misty patch has become a classically “fogged window.” And like tooth decay, once it starts, it never gets better… only worse.

What can you do about fogged thermal windows? You have three options – One, do nothing; Two, contact a local window replacement company and get an entirely new window, frame and sash; And three, contact a glass repair company and replace just the fogged thermal unit. (Note: For the record, a window company typically installs entirely new windows in buildings, whereas a glass company typically repairs existing broken or fogged windows)

Obviously, doing nothing is the least expensive option. However, as time passes you may find that your view of the world outside (and out front) becomes less and less clear. On the other hand, fogged windows provide a certain degree of privacy for the occupants of the home, especially if one of them happens to be in the bathroom…

Having the entire window replaced is the most expensive option, but will certainly rectify the problem. However, window replacement companies are often set up to replace multiple windows and many have a minimum of four or five to accept a job. If you have a large number (6+) of fogged thermal-pane windows this might be your best option, as the economies of scale come into play – usually, the more windows you replace, the better price per window you get. Usually, that is.

Calling a local glass repair company is the fastest and least expensive way to correct the problem, as this is what they do on a regular basis. Most glass companies will gladly replace just a single window or two, or more, if necessary.

As the technology involved in the manufacture of insulated glass units continues to improve in the future, the problem of modern thermal windows fogging will almost certainly decline, but for the multitude of existing thermal windows out there from the past thirty or so years, the problem will not only continue, but worsen as time passes.