Broken seals on windows can be hard to detect or easy to see if they are badly fogged up.
Sometimes they show up when the sun shines on them and other times they do not show themselves at all. Sometimes the seals fail and moisture, either from inside the home or outside the home, finds its way in between the glass.
Since ambient air has some amount of moisture in it (and sometimes quite a lot), when the seal of the window fails, air will find its way in between the panes. There are desiccant materials inside these sealed units to absorb small amounts of moisture. Depending on how saturated the desiccant material is, will be consistent with the amount of fogging that shows up between the panes.
Humidity levels on both sides of the glass, as well as indoor and outdoor temperature differentials, will also affect how much fogging there is.
Time—or how long the seals have been broken--is also a factor. If the window has been leaking for long enough, considerable staining of the glass between the panes can occur even though there might not be any obvious moisture at any particular moment in time. When the staining gets bad enough, it can be very difficult to see through the glass.
While failed seals are for the most part a cosmetic concern, and do not seriously affect thermal efficiency of the window, over time they can become a more serious problem. Clearly if the water starts to build up to the point that it looks like a terrarium, in the winter the water could freeze when the power goes out and the glass could break.
Whether and/or when the glass should be replaced has to be made on a case by case basis.
Failed seals on windows are one of the hidden costs we are not told about when we bought the windows originally.
There is one trick you can try at home to see if you your windows have broken seals. It will not tell you that you do not have a broken seal, but it will often tell you when you do.
The ice cube trick.
Any frozen item will work, like a frozen bag of peas, a sports ice-pack, (you guys in the frozen NE can go grab an icicle) but a regular old ice cube held in a tissue works great. You simply hold the ice-cube on the surface of the glass for 15 to 30 seconds. If there is moisture between the panes of glass it will condense on the cold spot leaving a foggy shape the same shape as your ice-cube but on the other side of the glass. When you dry the surface of the glass to keep the melting ice-cube from running all over the place—the foggy area will not disappear, as can be seen in the center of this next picture.
Remember though, this is no guarantee that you do not have a broken seal. It merely tells you that there currently is moisture between the panes of glass, and the moisture is consistent with a broken seal. I find this trick works about 50% of the time, your mileage may vary.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board