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Window Seal Failure: Signs, Causes & When to Replace
Cloudy glass between panes? That's a failed window seal. Virginia Glass Windows explains exactly how to identify seal failure and what your options are in the DMV area.
6
Warning Signs
6
Known Causes
6
Solution Steps
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Professional Assessment
Common Symptoms
Cloudy, hazy, or milky appearance between the glass panes that cannot be wiped from either surface
Condensation or moisture droplets visible between the panes, typically most prominent in the morning
White mineral deposits or streaks between the panes from evaporated moisture
Oil or oily streaks between the panes on older windows with failed desiccant
Reduced visibility through the affected window compared to others in the same room
Haze that moves or changes over time as temperature or humidity conditions change
What Causes This
Thermal cycling is the primary mechanism that eventually destroys all IGU seals. Each day, as the temperature rises, the gas between the panes (air, argon, or krypton) expands and puts outward pressure on the sealant. Each night, as temperatures drop, the gas contracts and the sealant must resist inward pressure. Over thousands of these daily cycles across years of operation, the primary and secondary sealants at the glass edge fatigue and develop micro-cracks that eventually allow moisture vapor to penetrate the unit. In the DMV, the temperature differential between a hot summer day and a cold winter night can exceed 100°F, making the thermal cycling stress particularly aggressive.
UV degradation of the primary sealant (typically polyisobutylene, or PIB) causes the material to become brittle over time. PIB is highly resistant to UV when protected by the secondary sealant and the spacer frame, but when those outer layers are compromised by scratches, impacts, or improper installation, UV reaches the PIB and degrades its flexibility. Once the primary sealant becomes brittle, the thermal cycling that it normally accommodates elastically instead creates micro-fractures.
Installation errors that leave insufficient sightline (the distance from the glass edge to the visible portion of the frame) expose the edge seal to direct UV. Proper installation standards specify minimum sightlines to protect the seal from UV and mechanical damage. Windows that were installed with the glass edge too close to the exterior edge of the frame accelerate seal failure and warrant early inspection.
Physical edge damage from improper cleaning, frame contact, or installation trauma compromises the secondary seal at the point of damage. Prying tools used to adjust window frames, cleaning brushes with metal components, or impacts at the glass edge during installation can breach the secondary sealant. The damage may not be visible from the interior but creates a pathway for moisture vapor to enter the unit.
Low-quality or aged desiccant losing its capacity to absorb moisture causes the seal failure to become visible. New IGUs contain a desiccant column (typically molecular sieve) in the spacer that absorbs residual moisture in the gas space during initial pressurization and continues to absorb minor infiltration over the life of the unit. When the desiccant is exhausted, further moisture infiltration cannot be captured, and visible fogging begins. Desiccant exhaustion is often the final step in a seal failure process that began years earlier.
Pressure washing or aggressive chemical cleaning of the window exterior can drive water into the edge seal area under high pressure. The sealant joint between the glass and the spacer frame is not designed to resist the several hundred PSI generated by a pressure washer at close range. Direct pressure washing of window glass is a documented cause of premature seal failure and is particularly prevalent in the DMV market where many homeowners use pressure washing as part of annual exterior maintenance.
How to Fix It
Solution Steps
1Confirm the diagnosis by distinguishing inter-pane moisture from interior condensation. Run your finger across the affected area. If the haze or condensation is between the panes and cannot be reached from either side, it is a failed IGU seal. If it wipes away from the room-side surface, it is interior condensation from humidity, not seal failure.
2Check the window age and warranty status. Most residential windows carry a 10–20 year warranty on the IGU seal, and many manufacturers (Andersen, Pella, Marvin, and others) will provide a replacement IGU at reduced or no cost within the warranty period. Locate the window documentation or manufacturer label (typically on the frame or edge of the glass) and contact the manufacturer before spending money on out-of-warranty repair.
3For out-of-warranty windows with seal failure, the repair option is IGU replacement — removing and replacing the glass unit within the existing sash frame. This preserves the frame and hardware and is typically 40–70% less expensive than full window replacement. The replacement IGU is custom fabricated to the exact dimensions of the failed unit, including glass thickness, spacer dimensions, and any specialty glass (Low-E, tempered, laminated) that matches the original specification.
4Obtain measurements of the failed IGU to order a replacement. The relevant dimensions are the overall glass size (height and width), the glass thickness, and the spacer width (visible as the silver or black bar between the panes at the edge). A glass professional can take these measurements during a service visit. Submit photos at /photo-estimate and Virginia Glass Windows can provide a preliminary assessment before the measurement visit.
5Consider whether full window replacement is warranted alongside IGU replacement. If the window frame, weatherstripping, and hardware are all in good condition, IGU replacement is the correct repair. If the frame is rotted, the hardware is failing, or the window does not meet current energy code, full replacement may be more economical over the window's remaining useful life. Virginia Glass Windows provides a professional assessment that considers both options.
6After IGU replacement, address any contributing factors that accelerated the seal failure. If the failed unit was on a window with heating vents directed at it, redirect the vents. If the windows were regularly pressure washed, update cleaning protocols. Replacement IGUs typically last 15–25 years when the contributing factors are managed.
When to Call a Professional
When the warranty status is unclear and professional documentation of the failure is needed for a warranty claim
When the failed IGU is in a unit with specialty glass (tempered, laminated, patterned, or wired) that requires matching
When multiple windows show seal failure simultaneously, suggesting a systematic installation or product quality issue
When the failed window is in a location requiring ladder access or where glass handling is complex (large units, high locations)
When full window replacement vs. IGU replacement is uncertain — a professional assessment provides cost-benefit clarity
Stay Ahead
Prevention Tips
Never pressure wash window glass directly. Clean window glass with a squeegee and mild soap, or a dilute vinegar solution. If using a pressure washer on exterior surfaces, maintain a minimum distance of 4 feet from window glass and use a wide-angle tip.
Avoid applying aftermarket window films with aggressive adhesives to the interior of IGUs. Some films trap heat at the glass surface and accelerate seal stress.
Inspect IGU edge seals annually by examining the edge of the glass visible at the frame. Dark or oily staining at the glass edge, or visible cracking in the secondary sealant, indicates early-stage failure before interior fogging begins.
Keep heating vents and baseboard heaters directed away from window glass. Consistent high heat at the glass edge accelerates thermal cycling stress on the seal.
When replacing IGUs, choose argon-filled units with warm-edge spacer systems. Warm-edge spacers maintain a higher glass edge temperature, reducing the thermal differential that fatigues the seal.
Common Questions
FAQs About This Problem
Several products claim to drill a small hole in the IGU, dehumidify the air space, insert new desiccant, and reseal the hole. These methods have inconsistent long-term results and are not universally recommended by glass professionals. Proper IGU replacement is the reliable solution. The cost difference between a questionable repair and a proper replacement IGU is often small.
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