Window seals keep the insulated glass unit intact and prevent air and moisture from passing through the window assembly. When a seal fails, the window loses its insulating ability, develops condensation between the panes, and allows energy to escape. This is one of the most common window problems in residential and commercial buildings.
Here is how window seal failure happens, how to identify it, and what to do about it.
How Window Seals Work
Modern windows use insulated glass units, also known as IGUs. An IGU consists of two or three panes of glass separated by a spacer bar and sealed around the perimeter. The space between the panes is filled with air or an inert gas, typically argon or krypton, which reduces heat transfer.
The perimeter seal holds the assembly together and keeps the gas fill in place. Most seals use a combination of a primary sealant, usually polyisobutylene, and a secondary sealant, such as silicone or polysulfide. The spacer bar also contains a desiccant material that absorbs any residual moisture inside the unit.
When the perimeter seal breaks, the gas fill escapes and outside air enters the space between the panes. The desiccant becomes saturated, and moisture begins to accumulate inside the unit.
Signs of Window Seal Failure
The most visible sign of seal failure is condensation or fog between the glass panes. This moisture is trapped inside the IGU and cannot be wiped away from either the interior or exterior surface. It may appear and disappear with temperature changes, but it worsens over time as more moisture enters the unit.
Distorted or wavy glass is another indicator. When the gas fill escapes, atmospheric pressure pushes the panes inward slightly. This distortion can be visible at certain angles and causes reflections to appear wavy.
Increased energy costs without a clear cause may point to seal failure across multiple windows. Each failed unit reduces the thermal performance of the wall it sits in.
Drafts near the window frame may indicate that the frame sealant has also failed, though this is a separate issue from the IGU seal. Both should be inspected together.
What Causes Seal Failure
Age is the primary factor. Most window seals have a functional lifespan of 15 to 25 years depending on the quality of the seal, the materials used, and the conditions the window faces.
Direct sun exposure accelerates seal degradation. UV radiation breaks down the sealant over time, and thermal cycling from sun exposure expands and contracts the seal daily. South- and west-facing windows tend to fail sooner than north-facing ones.
Poor manufacturing quality shortens seal life. Windows with inadequate sealant application, contaminated spacer bars, or low-quality desiccant fail sooner than properly manufactured units.
Excessive moisture exposure from rain, humidity, and condensation at the frame contributes to seal breakdown from the outside. Improper installation that allows water to pool at the sill or penetrate the frame accelerates this process.
Repairing vs Replacing a Failed Sealed Unit
Once the seal on an IGU has failed, the unit cannot be resealed in place. The gas fill is gone, the desiccant is saturated, and the internal surfaces may have mineral deposits from evaporated moisture.
The most effective solution is to replace the insulated glass unit. This involves removing the glass from the sash or frame, ordering a new IGU to the same dimensions and specifications, and installing the replacement with new glazing tape or sealant.
In many cases, the sash and frame are still in good condition and only the glass unit needs replacement. This is significantly less expensive than replacing the full window.
Full window replacement is recommended when the frame is also damaged, when the window is outdated and underperforming, or when multiple windows in the home have failed seals. Replacing all affected windows at once is more cost-effective per unit and provides a consistent upgrade across the property.
Preventing Seal Failure
Proper installation is the first line of defense. Windows should be installed with correct flashing, drainage, and sealant to prevent water from reaching the glass unit.
Choosing windows with quality seals and spacer bars extends the lifespan of the IGU. Warm-edge spacer technology reduces thermal stress at the seal edge and improves long-term performance.
Maintaining the window frame, sill, and exterior caulking prevents moisture from reaching the seal. Repainting and recaulking wood frames on a regular schedule helps protect the glass unit.
Avoiding pressure washing directly on window seals reduces the risk of forcing water past the sealant.