It’s winter, and the temperatures are dropping outside. One day you’re warming up your home and suddenly you notice something that you hadn’t seen during the summer. That perfectly clear window in the living room or the kitchen or somewhere else in the house suddenly looks foggy. You wipe it down from the inside — and from the outside — but the fog won’t go away. The next day, it warms up again outside, and to your surprise, the fog disappears again. So what’s going on?

That intermittent fogging during cold temperatures is an indication that you have what’s known as a "blown seal" in your insulated glass window. Here’s what happens:

Insulated glass windows, also known as double-pane windows, have two panes of glass that are held apart by a metal strip. The strip, usually somewhere between 1/4 and 3/4 of an inch wide, is adhered to the two glass panes with a flexible sealant material.

During the manufacturing process, moisture is evacuated from between the glass panes as they’re sealed together, forming a dead air space. It’s the combination of the two glass panes and the dead air that gives the window panes their additional insulating value, and helps keep the window warmer than one with a single pane of glass.

Depending on the type and design of the window, sometimes inert argon gas is used between the panes to increase the insulating value even further. Some windows also have decorative grids trapped between the panes as a design feature. The sealed, insulated glass units are then placed into the frame and held in place with molding strips, making up a complete window unit.

What happens when damage occurs

The sealed, insulated glass unit is designed to have quite a long life span; in theory, it should last as long as the window unit itself. However, sometimes there are flaws in the manufacturing process or, more likely, some type of impact damage occurs to the window. That can cause a small opening to appear in the seal between the glass and the spacer bar. It’s something you won’t see, but it’s enough to allow air to enter the space between the panes of glass.

You might be thinking that that’s no big deal, since that’s just an air space anyway, right? But the difference is that it’s designed to be a dead, dry air space. Now, with the broken seal, air that has moisture in it has been introduced.

During the summer, when the air temperatures outside are warm and the glass is also warm, that’s OK. But now, with the colder temperatures of winter, the outer pane of glass gets cold. The warm air inside your house is trying harder than ever to escape, and it carries moist air into the window cavity, where it hits that cold glass and condenses back into a liquid. The result is that fogging you see. And because it’s inside the window, you can’t do anything to get rid of it.

Replacement is the only option

Once you discover a window with a blown seal, your only option is to replace the insulated glass unit. You need to do that as soon as you discover the problem, as the window has lost its insulating value, and the trapped moisture can potentially lead to other problems. Not to mention the fact that you can’t see through the window!

The good news is that you must replace only the sealed glass unit, not the entire window. This is something that you need to leave to the pros. Contact a glass company in your area and have them make a site visit. They’ll examine the window, measure the insulated glass unit, including the size of the air space, and have a new one made up that matches. When the new one is ready, they’ll come back out, remove the moldings and the old unit, and install and seal the new unit in place.

If the window is relatively new and the glass unit fails, contact the company or the contractor where you purchased it. Home centers such as Lowe’s and Home Depot will typically replace insulated glass units that fail, as will many other retailers.

If you have windows that are damaged in an insurance-related claim, such as a fire, wind storm, or some type of impact such as a tree limb that falls, you may not be aware of the fact that a seal has been damaged until winter comes around and the fogging becomes obvious. For that reason, if you suspect any type of potential window damage, always make your insurance adjuster aware that you’ll be holding the claim open for possible future supplemental damage claims.

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