What Is a Window Seal
Modern insulated glass units (IGUs) consist of two or three panes of glass separated by a spacer and sealed at the edges. The space between panes is filled with air or an insulating gas like argon or krypton. This gas-filled space creates an insulating barrier that reduces heat transfer through the window.
The seal around the edge keeps the gas in and moisture out. It's typically made of polyisobutylene (PIB) as a primary seal with a structural sealant like silicone or polyurethane as a secondary seal. Both seals must remain intact for the IGU to perform properly.
How Seals Fail
Seals degrade over time from UV exposure, temperature cycling, and chemical breakdown. Every time the window heats up in the sun, the gas inside expands and pushes against the seal. When it cools, the gas contracts and pulls on the seal. Thousands of these cycles eventually cause fatigue failure.
Manufacturing defects, improper installation, and physical damage can cause premature seal failure. Windows that receive intense sun exposure or experience extreme temperature swings fail faster than those in protected locations.
Signs of Seal Failure
The primary symptom is fogging or condensation between the glass panes that doesn't wipe away from either side. This moisture is trapped inside the sealed unit where you can't reach it.
Visible Fogging
Fog or mist visible between panes typically appears and disappears with temperature and humidity changes. On cool mornings, the condensation may be obvious. By midday, as the window warms, it may seem to clear up. This cycling pattern confirms seal failure.
Mineral Deposits
Over time, moisture cycling in and out leaves mineral deposits on the interior glass surfaces. These appear as white haze, streaks, or spots that become permanent. Even if you replace the IGU, these deposits remain visible until that point.
Distorted View
Failed seals can allow the panes to flex, creating subtle distortion when looking through the glass. This is more noticeable on larger windows where the panes have more room to move.
Impact on Home Performance
Failed seals reduce the window's insulating value but don't eliminate it entirely. According to energy studies, a failed seal reduces the IGU's insulating performance by approximately 10-20%. The window still provides better insulation than a single-pane window, but less than an intact IGU.
Energy Costs
One failed seal on a typical home has minimal impact on energy bills. Multiple failed seals, particularly on south-facing or west-facing windows, can noticeably increase heating and cooling costs. The National Fenestration Rating Council provides ratings that can help estimate the difference.
Comfort
Windows with failed seals may feel colder in winter when you stand near them. The reduced insulating value allows more heat transfer, creating cold spots. This is more noticeable in colder climates.
What Failed Seals Don't Affect
Failed seals don't compromise the window's ability to keep rain out. They don't affect structural integrity. They don't create safety hazards. The window still functions as a barrier between inside and outside. It just doesn't insulate as well and looks foggy.
Repair Options and Costs
Two main options exist for addressing failed window seals: replacing just the glass unit or replacing the entire window.
Glass-Only Replacement
If the window frame is in good condition, replacing just the IGU is usually possible. A glazier removes the old sealed unit and installs a new one in the existing frame. This costs $150-400 per window depending on size and glass specifications.
Glass-only replacement makes sense when the frames are solid and the windows are less than 20 years old. It's cost-effective and preserves the existing window style.
Full Window Replacement
Replacing the entire window, frame and all, costs $300-800 or more per window. This makes sense when frames are damaged, when the windows are very old, or when you want to upgrade to better-performing units.
Full replacement is typically unnecessary solely due to seal failure. But if the windows have other issues like rotted frames or broken hardware, seal failure may be the trigger that justifies complete replacement.
Defogging Services
Some companies offer defogging services that drill holes in the glass, clean out moisture and deposits, then reseal the holes. This costs $50-150 per window. Results are mixed. The underlying seal failure remains, so moisture may return. Most industry professionals consider this a temporary fix at best.
When to Address Failed Seals
Failed window seals rarely require immediate action. Unlike water intrusion through walls or roof leaks, seal failure doesn't cause progressive damage to other building components. You can live with failed seals indefinitely if the appearance doesn't bother you.
Priority Situations
Consider prioritizing repair when the window is in a high-visibility location where the fog is particularly bothersome, when multiple seals have failed on the same wall face creating noticeable efficiency loss, or when you're preparing to sell the home and want to address buyer concerns.
Low-Priority Situations
Failed seals on basement windows, closet windows, or other low-visibility locations can usually wait. Single failed seals in a house full of otherwise good windows are minor issues. If the house needs roof repairs, foundation work, or other significant maintenance, those take priority.
Failed Seals in Home Sales
During home inspections, failed window seals commonly appear in inspection reports. Buyers sometimes overreact to this finding, not understanding that it's a maintenance issue rather than a major defect.
Sellers can either repair failed seals before listing, offer a credit at closing, or simply disclose the condition and price accordingly. The repair cost is typically minor compared to other home sale considerations.
Negotiating Seal Repairs
When negotiating, buyers should request quotes before assuming costs. Failed seals on three or four windows might cost $600-1,600 to repair. That's a reasonable credit request but not a reason to renegotiate the entire deal.
Related Terms
IGU (Insulated Glass Unit)
The sealed assembly of two or three glass panes with gas-filled space between them. Also called a sealed unit or thermal pane. This is what fails when people refer to failed window seals.
Thermal Pane
Another term for double-pane or triple-pane insulated windows. The thermal refers to the insulating air or gas space between panes.
Low-E Coating
A microscopically thin metallic coating on glass that reflects infrared light while allowing visible light through. Low-E coatings improve window efficiency but are unrelated to seal integrity.
Argon Fill
Argon gas between window panes provides better insulation than air because it's denser and conducts less heat. When seals fail, the argon escapes and is replaced by regular air.