How to determine that the seal has broken in a double-pane window.
The seal has broken long BEFORE condensation and fogging are visible in a double-pane window.
But how can you know that?
You can't. Unless you have a thermal camera!
One thing I do with Mighty Mo is to sweep the entire house. I'm looking for many things.
And one of those many things I'm looking at is to see how the windows are doing.
In this house none of the windows showed condensation. But one, only one, demonstrated the classic Bull's Eye pattern indicative of a broken seal.
The Bull's Eye happens as pressure differences begin due to the broken seal. As the gas inside the two panes dissipates air is drawn inside. As the pressure changes the two panes begin to get drawn together.
When it's cold outside, and at the time of this image the outdoor temperature was about 40F, a thermal camera can reveal that the two panes are closer, and cooler, in the middle than on the edges.
And the pattern that you see develops. Choosing a color palette which best demonstrates the phenomenon can make it very clear.
You can actually see that the top window is also beginning to draw closer together. Just beginning.
Also, notice (contrary to popular belief) that the thermal efficiency of the window is not decreased that much by the broken seal and the loss of the inert gas inside the panes. The temperature difference between the cooler center and the warmer edges is only about 10 degrees.
My recommendation: a thermal camera goes a long way toward evaluating a house for many different things. And windows are one of those things! Be sure to make a thermal image sweep an essential part of your home inspection. You'll be glad you did.