How to Tell If a Window Seal Has Failed

Tree in rainy weather through window

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Single-pane windows were normal for hundreds of years. Now, it is rare to find a window that is not sealed. Double- and triple-pane windows are composed of two or three panes of glass within an air-tight frame; in other words, they are sealed. When this seal fails, they are less effective as weather barriers.

But how do you know that the seal has failed? Determining this is vital because your chilly house interior may be due to other factors that are easier and cheaper to solve.

When Failure Is an Option

Windows are subjected to the same brutal elements as are roofing and siding. Roofing and siding are hardy materials; windows are delicately constructed devices with moving parts. The element that forms the core of windows is the Insulated Glass Unit or IGU.

While failure is not intentionally built into IGUs, it is not unexpected. This is the secret that window manufacturers do not include in their sales literature.

It's not uncommon for a house to have at least one window with a failed seal. The cause can either be an immediate, outright failure of the seal or a slow, expected leakage over time.

Even in climates that are not considered extreme, it's common to find double-pane, sealed IGU's. The reason they are sealed so tightly is they have an air vacuum between the panes. Sometimes this air vacuum is replaced with a low conductivity inert gas like argon or krypton.

It is this very combination of two, and even three, sheets of glass and the gas within that keeps the cold away.

Makers Admit to Leaking IGU's

Major glass manufacturer Vitro Architectural Glass (formerly PPG) outlines how window seals can fail:

The partial pressure differentials between the air outside and the gas inside cause both argon and krypton to naturally escape an IGU. Even when an IGU is perfectly constructed, the gas will escape at a rate of about one percent per year, and that rate is much faster when the IGU is poorly made.

Trucking new window units over high altitudes such as the Rocky Mountains is one way to break IGU seals even before the window is installed. Some manufacturers operate regional factories for this reason.

Detection Tips

  • Clean Them First: Since your test is all visual, you need to provide yourself with a blank slate. Thoroughly clean both the inside and the outside of the window, so you are certain that you are not looking at external moisture.
  • Moisture Within the IGU: Fogging, hazing, or moisture between the two panes of glass means that a window seal has failed. While a window can certainly fail and not have any moisture between the two panes, it is more common to see moisture between the two panes.
  • Glass Distortion: This test is easier to see with larger sheets of glass. Vitro Architectural Glass notes that as the krypton or argon gas leaks out, "air doesn’t backfill into the IGU [and] the two lites of glass begin to collapse into the center of the unit, which can cause the glass to look distorted or even break." By standing at a distance outside and looking at the reflection, you can see if the glass is more distorted than those of windows not affected.
  • Save Those Warranties: If you are in the habit of tossing out warranties, this is one product where you want to keep this information. Premature window seal failure is clearly a product failure, not a function of poor installation.

    One certain way not to detect a failed window seal is by putting your hand on the glass. Unless it is viciously cold -- far below freezing -- it is doubtful that the "feel test" will work. 

    What to Do With a Failed Window Seal?

    Often, it is not necessary to undertake an expensive window replacement.

    Instead, look at lower-cost solutions, such as calling in the warranty and having only the affected IGU replaced. Window sashes can be removed and replaced by glaziers or by the company you purchased from.

    A second option is to call a defogging company. After drilling a tiny hole in the glass, the company expels the condensation and then installs a valve and seal. Reviews of such fixes are mixed, as this does not address the cause of the initial leak.

    Finally, you can simply ignore the failed seal. Yes, this may sound like home renovation heresy. But if you live in a moderate climate, your need for sealed IGUs will be less substantial than if you live in a place of brutal, extreme temperatures. Weigh the cost of new windows with the increased energy cost, and you may be surprised to realize that it is cheaper to keep the window -- failed seal and all.