A foggy window that stays cloudy regardless of temperature is not a cleaning problem. It is a broken seal. The insulated glass unit (IGU) in a double pane window relies on a hermetic perimeter seal to keep the air gap dry and inert.
When that seal fails, moisture enters the cavity and the window loses its insulating performance. Advanced Window & Glass Repair handles foggy window repair and replacement across Northern Virginia, DC, and Maryland. This guide covers what causes seals to fail, how to confirm a seal has broken, and what the repair options actually are.
What Causes Window Seals to Fail
Thermal cycling is the primary cause of seal failure in residential windows. Every day the glass panes expand as they heat and contract as they cool. This constant movement stresses the seal at the perimeter of the IGU. Over years of cycling, the seal compound degrades and microscopic gaps form.
Moisture and UV exposure accelerate the process. Prolonged UV exposure breaks down the butyl and silicone compounds in the seal. The outer edge of the IGU seal is exposed to this directly. South-facing windows in direct sun typically fail sooner than north-facing windows.
Poor original installation contributes significantly to early seal failure. An IGU not seated correctly in the frame distributes stress unevenly across the seal during thermal movement. Improperly placed glazing tape or setting blocks cause the same problem. One section of the seal then fails before the rest.
Physical damage to the glass edge or the frame rebate can also compromise the seal. A knock to the corner of the glass during installation creates a point of stress that breaks down the seal faster than normal thermal cycling. The same applies when a frame has shifted enough to pinch the IGU edge.
How to Detect a Broken Window Seal
The most reliable indicator is persistent fogging between the panes. This condensation forms on the interior glass surfaces inside the cavity. It does not respond to changes in indoor humidity or outdoor temperature. If wiping the glass from either side has no effect on the haze, moisture is trapped inside the unit. The seal has failed.
A white or grey mineral film on the interior glass surfaces is a sign of long-term seal failure. As moisture enters and evaporates repeatedly, it leaves mineral deposits on the glass. This film is most visible in morning light at low angles.
Cold spots on the glass surface in winter indicate that the cavity gas has been replaced by ambient air. Argon or krypton fills the cavity in most modern IGUs. These gases insulate better than air. When the fill gas escapes, the insulating gap performs at reduced capacity. The glass surface becomes noticeably colder near the frame.
For professional foggy glass assessment and same-day repair, foggy glass repair and replacement covers double and triple pane units across Northern Virginia, DC, and Maryland.
Can a Broken Window Seal Be Repaired?
The seal between the two panes of an IGU cannot be resealed in place. The unit is a hermetically closed assembly. Once moisture has entered the cavity, the mineral deposits and internal contamination cannot be cleaned out through the existing seal. Attempts to inject dehumidifying chemicals through drilled holes provide temporary improvement at best. They do not restore the unit to original performance.
DIY repair kits that claim to defog double pane windows by drilling and injecting desiccant or chemical treatments do not restore thermal performance. They may temporarily reduce visible fogging by absorbing moisture present at the time of treatment. The seal remains compromised and new moisture continues to enter. These kits do not address the cause and are not a substitute for proper IGU replacement.
For double pane window repair and replacement, the correct repair is removing the failed IGU from the frame and installing a new sealed unit. This restores clarity and thermal performance without replacing the window frame.
When the Full IGU Needs Replacing
IGU replacement is the correct solution when the frame is structurally sound and the damage is limited to the glass unit and its seal. The glazier removes the failed unit, prepares the frame rebate, and installs a replacement IGU of the correct specification. Most residential IGU replacements take two to three hours on-site once the replacement unit has been supplied.
Full window replacement is necessary when the frame is rotted, warped, or structurally compromised. Fitting a new IGU into a damaged frame does not resolve the underlying problem. The new unit will fail sooner than it should. A professional inspection confirms whether the frame can accept a replacement IGU.
Specialty glass specifications including Low-E coatings, argon gas fills, laminated interlayers, or tempered requirements must be matched when ordering the replacement unit. Installing a standard clear IGU in a position that originally specified Low-E glass reduces the window’s thermal performance below its original specification. Advanced Window & Glass Repair matches the original glass specification on all IGU replacements across Northern Virginia, DC, and Maryland.
Cost of Broken Seal Repair vs IGU Replacement in Northern Virginia
The figures below reflect typical 2026 market rates for Northern Virginia, DC, and Maryland. Final pricing depends on glass specification, window size, frame access, and the total number of units being replaced in one visit.
| Service | Typical Cost Range (2026) |
| IGU replacement, standard size | $150 to $350 per unit |
| IGU replacement, large or custom | $300 to $600 per unit |
| IGU replacement with Low-E and argon | $200 to $450 per unit |
| Full window replacement (frame included) | $400 to $800 installed |
| Emergency or same-day service | Additional $75 to $150 |
Multi-window jobs attract lower per-unit pricing. The U.S. Department of Energy estimates that heat gain and loss through windows accounts for 25 to 30 percent of residential heating and cooling costs. For properties with multiple failed seals, the energy saving from restoring the IGU specification can offset a portion of the repair cost over time.
If windows in your home or business are fogging up and the glass is not clearing, a failed seal is the most likely cause. Advanced Window & Glass Repair carries out IGU assessments and replacements across Northern Virginia, DC, and Maryland. Call (571) 351-3692 or book through the contact page.
How to Extend Window Seal Life
Seal longevity is affected by maintenance practices as much as installation quality. A few consistent habits significantly extend the interval between seal failures.
Keep the glass edges and frame channels clear of water pooling. Standing water against the base of the IGU accelerates seal degradation at the lower edge, which is where most seal failures start. Check that the weep holes in the frame are clear and draining correctly after rain.
Clean glass with mild, pH-neutral cleaners only. Solvent-based or alkaline cleaners degrade the butyl compound used in most IGU perimeter seals when applied repeatedly near the glass edge. Apply cleaning solution to a cloth rather than spraying directly at the glass to avoid saturation of the seal area.
Inspect external caulking and glazing tape annually. The external seal between the frame and the IGU is the first line of defence against moisture. Where this external seal has cracked or pulled away from the glass, repair it promptly with appropriate glazing sealant before water reaches the IGU perimeter.
Avoid pressure washing glass directly. The high-pressure water jet forces water under the glazing tape and into the frame rebate, which can compromise both the external seal and the IGU perimeter in a single session. For guidance on how window performance and seal integrity affect residential energy costs over time, the U.S. Department of Energy guidance on window performance and replacement covers the factors relevant to homes in Northern Virginia, DC, and Maryland.
Conclusion
A broken window seal is not a surface problem and cannot be fixed from the outside. Once moisture enters the IGU cavity, the unit needs to be replaced. The good news is that IGU replacement is significantly cheaper than full window replacement. In most residential cases, the frame is sound enough to accept a new unit without further work.
For homeowners across Northern Virginia, DC, and Maryland dealing with persistent fogging or cold spots near double pane windows, a professional assessment is the first step. Advanced Window & Glass Repair inspects both the glass and the frame to confirm the right course of action before any work is carried out. For guidance on the related issue of condensation forming on the room-side surface rather than between the panes, how to fix condensation inside windows covers the causes and solutions for that separate problem.
Advanced Window & Glass Repair provides IGU replacement and foggy window repair across Northern Virginia, DC, and Maryland. Call to book an assessment, or reach the team through the contact page.
Frequently Asked Questions
Why does my double pane window fog up on the inside of the glass?
Fogging between the panes of a double pane window means the IGU seal has failed. Humid outside air enters the cavity, and as temperatures change, that moisture condenses on the interior glass surfaces. The fogging does not clear because the moisture is trapped inside a sealed space. Once the seal fails, the only effective solution is replacing the IGU. Fogging on the room-side surface of the glass is a different issue caused by high indoor humidity and is addressed differently.
How long does an IGU typically last before the seal fails?
Well-installed IGUs in a residential setting typically last 15 to 25 years before seal failure becomes likely. South-facing windows in direct sunlight, windows in high-humidity environments, or units that were not correctly installed may fail sooner. The first indication is usually minor fogging at the lower edge of the pane, which worsens over weeks and months as more moisture enters the cavity.
Will my energy bills go up if my window seal fails?
A failed seal allows the inert gas fill (usually argon) to escape and be replaced by ambient air. Argon slows heat transfer better than air, so losing the fill reduces the window’s thermal resistance. The difference in heating and cooling costs depends on the number of affected windows, the climate, and the original glass specification. For a single window in a well-insulated property, the impact on bills is small. For multiple failed windows across the same property, the cumulative effect is measurable.
Can I fix a broken window seal myself?
A broken IGU seal cannot be repaired from the outside, and the IGU itself cannot be resealed without specialist equipment. DIY kits that involve drilling holes and injecting desiccant provide temporary reduction in visible fogging but do not restore the IGU’s thermal performance or stop further moisture ingress. Professional IGU replacement is the only method that restores the window to its original specification and performance.
How do I know if my window needs a new IGU or a full replacement?
If the frame is structurally sound, the sash operates correctly, and the only problem is fogging or a failed seal, an IGU replacement is sufficient. If the frame is rotted, warped, or cracked, or if the window has dropped out of square and no longer sits correctly in the opening, the frame needs to be addressed as part of the work. A site inspection by a qualified glazier confirms which applies before any material is ordered.
Does window orientation affect how quickly seals fail?
South-facing and west-facing windows are exposed to more direct solar radiation, which increases the temperature differential the IGU experiences during daily thermal cycling. This accelerates seal fatigue compared to north-facing windows that receive less direct sun. In the Northern Virginia, DC, and Maryland climate, south-facing double pane windows typically fail a few years earlier than equivalent north-facing units. This is a useful factor to consider when planning a multi-window replacement project.
