Failed Window Seals:
What Fogged Windows Actually Mean
That persistent haze between the panes of glass is not something you can clean. It means the sealed unit has failed, the insulating gas is gone, moisture has moved in, and the window is no longer performing the way it was designed to. Here is what buyers need to understand about this finding.
What a Failed Window Seal Actually Is
Modern residential windows are not single panes of glass. They are insulated glazing units (IGUs) made of two or three panes of glass separated by a spacer and sealed around the perimeter to create an airtight chamber. That chamber is often filled with an inert gas, typically argon (krypton in some higher-end units), which conducts heat less efficiently than air. The gas fill, combined with the air gap between panes, is what gives a double-pane window its insulating performance.
The perimeter seal is what holds all of this together. It is a flexible sealant, typically silicone, polyisobutylene, or a similar material, bonded to the spacer and the glass around the full perimeter of the unit. When that seal is intact, the gas stays in, moisture-laden outdoor air stays out, and the unit performs as designed.
A failed seal is a breach anywhere in that perimeter. Once the seal is compromised, the inert gas begins to escape and is replaced by whatever air is on the other side: outdoor air, which in the Portland metro and SW Washington for much of the year means humid air carrying significant moisture. That moisture enters the space between the panes and condenses on the interior glass surfaces where temperatures are cooler. The result is the persistent fogging, hazing, or mineral deposit streaking that buyers see on inspection day and cannot wipe off, because it is between the glass, not on the surface.
Why It Matters Beyond Appearance
The most visible consequence of a failed seal is cosmetic: the fogging or hazing that obscures the view. But buyers who think of it as purely cosmetic are missing the more significant issue, which is the loss of thermal performance.
Skylights fail the same way, and the post on skylight leaks, condensation and fogged glass covers how that looks overhead.
When the inert gas escapes from a failed IGU and is replaced by air, the window’s insulating efficiency drops. The argon or krypton fill in a double-pane window is responsible for a meaningful portion of the unit’s overall R-value. A window that started with an R-value of around 3 to 4 when new loses some of that resistance to heat transfer once the gas fill is replaced by air. In practical terms, that means more heat loss through the window in winter, more heat gain in summer, and higher energy costs, spread across every window that has failed.
In the Portland metro and SW Washington, where the heating season runs six or more months, the performance difference between intact and failed window seals in a home with multiple failed units becomes a real operating cost issue, not just a visual one.
There is also a moisture management dimension. Windows that have failed seals often show frame deterioration as well. The breach in the seal that allowed moisture into the IGU is often accompanied by moisture reaching the frame material itself. In wood-framed windows, this can lead to paint failure, swelling, and eventually wood decay at the frame and sill. In vinyl or aluminum-framed windows, moisture infiltration at the frame seal can compromise the overall weathertightness of the window installation even if the frame material itself does not decay.
The other relevant factor locally is the duration of the wet season. Once a seal fails, fogging develops quickly in our climate because outdoor air during the October-through-April period consistently carries high moisture content. A seal that fails in November can show visible fogging within weeks. This means that windows inspected in winter or early spring in the Portland metro tend to reveal failed seals more readily than those inspected in the dry summer months, though the underlying condition exists regardless of season.
Homes built or fully re-windowed in the 1990s and early 2000s have windows that are now roughly 20 to 35 years old, at or past the expected service life of most residential IGU products from that era. Multiple simultaneous seal failures in a home of this vintage are common and expected, not a sign of unusual product failure.
Real Inspection Photos
What Failed Seals Look Like in Russ's Reports
Photos from recent inspections showing fogging and haze trapped between the panes.
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Advanced seal failure: haze and spotting inside the glass unit that cannot be wiped off. This home had failed seals at many windows. -
Condensation collecting along the bottom of the glass unit. Early failures often show up near the edges before spreading across the pane. -
Comparing panes side by side helps. The right arched pane is hazy from a failed seal while the left one is still clear.
How to Identify a Failed Seal at a Showing
You do not need an inspector to spot a failed window seal. The signs are visible to any buyer who knows what to look for during a showing. Check every window in the home, including basement windows and windows in less-used rooms where fogging may be more advanced.
What to look for: Any persistent haze, cloudiness, or streaking that appears to be between the panes of glass rather than on the surface. The classic appearance is a milky or grayish cloudiness in the center of the glass or toward the edges. More advanced failures show white or gray mineral deposit streaks, the dried residue of water that has repeatedly condensed and evaporated inside the unit. These streaks do not follow any consistent pattern and cannot be cleaned because they are on the interior surfaces of the sealed unit.
How to confirm it is not surface condensation: Run your finger across the glass surface. Surface condensation is on the glass and will smear or wipe. A failed seal produces fogging inside the unit that does not respond to wiping at all. If the haze is present in the morning and gone by afternoon when the sun warms the glass, it may be surface condensation related to high indoor humidity rather than a seal failure. True IGU failure produces fogging that is either persistent regardless of conditions or that leaves permanent mineral staining.
Check the edges first: Seal failures often start at the corners and edges of the unit where the perimeter sealant is most stressed. Early-stage failures may show only slight cloudiness at one corner of the window before spreading toward the center as the condition progresses.
IGU Replacement vs. Full Window Replacement: How to Decide
When a seal has failed, there are two repair paths. Understanding the difference helps buyers see what correction actually involves.
| Factor | IGU Replacement | Full Window Replacement |
|---|---|---|
| What is replaced | Glass unit only, existing frame stays | Entire window including frame and glass |
| Best when | Frame is in good condition, window is standard size | Frame is damaged, window is obsolete, or multiple issues exist |
| Typical cost (Portland metro) | $150 – $400 per window | $400 – $1,200+ per window |
| Disruption | Lower; glazier removes and replaces glass unit | Higher; involves interior and possibly exterior trim work |
| Limitation | Frame must be compatible with replacement IGU | Higher upfront cost, more lead time |
The practical decision point is the condition of the frame. If the frame (the vinyl, aluminum, wood, or fiberglass surrounding the glass) is in good structural condition, a glazing specialist can remove the failed glass unit and install a new IGU in the existing frame. This is less expensive and less disruptive than full replacement, and it restores the window’s thermal performance.
Full window replacement makes more sense when the frame has moisture damage or wood decay, when the window is a non-standard size that makes IGU sourcing difficult, when the hardware is failing alongside the glass, or when the window is old enough that replacing the IGU alone would leave an aging frame that will need replacement soon anyway. In those cases, the additional cost of full replacement eliminates the likelihood of returning to the same window in a few years with additional problems.
When multiple windows have failed seals, as is common on homes where the windows are all from the same installation era, a conversation with a window contractor about whether to repair some and replace others, or to do a full-home window replacement on a phased basis, is worth having before settling on a repair plan.
What the Count of Failed Windows Means
A single failed window seal is a minor finding, typically a few hundred dollars to correct. Five or ten failed windows on a home where all the windows are from the same 1990s installation is a different picture. That is a window system at end of life, not an isolated defect, and it warrants a real estimate.
When the inspection documents multiple failed seals, a written estimate from a qualified glazing specialist or window contractor for either IGU replacement or full replacement across all affected units shows what correction will actually cost. How repairs are handled is worked out between the buyer and seller, with the buyer’s agent.
Window seal findings are sometimes described as cosmetic. Technically, failed seals reduce thermal performance, can accompany frame deterioration, and represent a repair that every owner will eventually need to make. The fogging is the visible symptom. The thermal and moisture performance loss is the real issue. Both are documented in the inspection report.
Failed Window Seals FAQ
