All modern windows have insulated glass units, which means you have a glass sandwich with gas in the middle. This gas is sealed in between two panes of glass, and no air or moisture shalt find its way betwixt, as long as the seal is intact. When this glass seal fails, you’ll eventually see condensation between the two panes of glass, and this can lead to a permanent dirty, fogged look. More on that topic here: Failed Window Seals vs. Fogged Glass.
There is, however, a subset of a failed window seal that doesn’t cause condensation or fogged glass, and it’s called collapsed glass. This happens when the gas between two panes of glass leaks out, but no air leaks back in, creating a vacuum. This can be so severe in some cases that the two pieces of glass actually look like they’re touching. Thank you for demonstrating, pops.
As I mentioned in last week’s blog post about melted vinyl siding, you can get some crazy-looking reflections from windows with collapsed glass. This creates a magnifying-glass effect, concentrating enough sunlight to actually deform vinyl siding.
The photo above, taken by Boston home inspector Donald Lovering, shows a home with melted vinyl siding. This is a good example because the siding is quite badly melted. It’s a great example because those four-pointed stars of light are all concentrated reflections from the neighbor’s house. And this is a fantastic example because the windows on this house also have collapsed glass. To know which ones have collapsed glass, just look at the reflections on those bottom windows closely:
You’ll notice the window on the far right has a normal reflection, but the other three windows have distorted reflections, which are probably caused by collapsed glass. While fun-house glass reflections are a telltale sign of collapsed glass, what’s the real consequence of this? A slightly lower insulating ability. On cold days, we can clearly pick this up with our infrared cameras, because windows with collapsed glass show a cold oval in the middle.
For a home inspector, it’s fair to simply call all of these conditions a failed seal. This slightly lowered insulating ability won’t have any major effect on the heating and cooling bills for a home, but it’s a defect nevertheless, and will typically lead to fogged glass.