Learn how to distinguish indoor condensation, exterior condensation, and fog trapped between window panes—and when to replace the glass, sash, or complete window.

Gentek Window Glass Guide for Iowa Contractors

Fog Between Window Panes: Seal Failure, Glass Repair, or Full Replacement?

Foggy glass can mean three very different things. Moisture may be sitting on the room-side surface, forming temporarily on the outdoor surface, or trapped inside an insulated glass unit. Only the third condition normally indicates an insulated-glass seal failure. Correctly identifying the location of the moisture prevents unnecessary window replacements and avoids promising that a humidity problem can be solved with new glass.

First, determine exactly where the moisture is located

Run a clean, dry cloth over the inside surface of the glass. If the moisture wipes away, it is room-side condensation. If the inside is dry, safely check whether the outdoor surface is damp. When neither accessible surface changes and the haze appears suspended between panes, the insulated glass unit may have lost its seal.

Moisture location	Likely explanation	Typical response
Room-side glass surface	Indoor humidity meeting cold glass	Manage humidity, airflow, and surface temperature
Outdoor glass surface	Exterior air reaches its dew point on cool glass	Usually temporary; monitor rather than repair
Between sealed panes	Possible insulated-glass seal failure	Document, check warranty, and evaluate glass or sash replacement

The key diagnostic rule

If the haze wipes off, it is not trapped between the panes. If it cannot be reached from either side and changes with temperature or sunlight, an internal glass condition is more likely.

What an insulated glass unit actually is

Most modern replacement windows use an insulated glass unit, often called an IGU. Two or three panes are separated by spacer systems and sealed around the perimeter. Depending on the selected Gentek glass package, the spaces may include argon or krypton gas and one or more Low-E coatings.

The edge seal is designed to limit moisture entry and retain the intended insulating environment. It must tolerate daily and seasonal expansion, contraction, temperature differences, solar exposure, wind pressure, and movement of the window sash. No sealed glass unit should be described as incapable of failure.

What happens when the glass seal fails

When the perimeter seal no longer controls moisture and gas movement as intended, humid air can enter the space between panes. As temperatures change, moisture may condense internally and later evaporate, leaving mineral deposits, streaks, or a cloudy film. Early failure can appear only during certain mornings or seasons; advanced failure may remain visible most of the time.

Intermittent fog that cannot be wiped from either surface
Water droplets or a haze between panes
White, gray, or rainbow-like mineral residue
Internal streaks that change as the unit warms
Reduced visual clarity or a permanently cloudy appearance

A failed seal can reduce the glass unit’s intended thermal performance, but appearance is often the first and most obvious complaint. The homeowner sees a dull view, uneven reflections, and glass that never looks clean.

Indoor condensation is not the same as seal failure

During an Iowa winter, warm indoor air carries moisture toward colder surfaces. When glass temperature falls below the air’s dew point, water forms on the interior surface. Cooking, showers, humidifiers, drying firewood, houseplants, newly poured concrete, and tightly sealed construction can all raise indoor humidity.

Interior condensation can occur on a perfectly intact window. Heavy or recurring moisture still matters because it can damage wood casing, drywall, paint, and nearby finishes. The solution may involve reducing indoor humidity, using kitchen and bath exhaust fans, improving airflow, opening blinds, and checking the heating and ventilation system—not replacing an otherwise sound insulated glass unit.

Exterior condensation can be a sign of cool outer glass

Outdoor condensation sometimes appears on high-performance glass during calm, humid mornings. If the insulated unit reduces heat transfer from inside, the exterior pane may remain cool enough for outdoor moisture to condense on it. The fog usually clears as sunlight, wind, or temperature changes warm the surface.

Exterior condensation does not automatically mean the seal has failed. Confirm that it is truly on the outdoor surface and inspect for patterns that might involve screens, roof overhangs, landscaping, or shading.

Why insulated glass seals fail

A contractor should avoid declaring one cause without evidence. Glass seals may be affected by age, repeated thermal cycling, manufacturing defects, edge damage, frame distortion, improper setting, blocked drainage, excessive heat, chemical exposure, or water that remains against the glass edge.

Normal aging: Sealants and spacers experience years of temperature and pressure changes.
Installation stress: An out-of-square or bowed frame can place uneven stress on the sash and glass.
Drainage problems: Blocked weeps or trapped water may expose the glass edge to prolonged moisture.
Impact or handling damage: A damaged glass edge can compromise the sealed unit.
Excessive localized heat: Aftermarket films, dark coverings, or unusual shading patterns can create temperature differences.
Product defect: A covered manufacturing issue may qualify for warranty consideration.

Can foggy windows be defogged?

Some services drill small holes, vent the cavity, and clean or dry portions of the unit. This may improve appearance in certain cases, but it does not recreate the original factory-sealed insulating glass system or restore lost insulating gas. It should not be presented as equivalent to a new IGU.

If appearance is the only concern and the homeowner understands the limitations, a defogging service may be considered. For a lasting restoration of the sealed glass assembly, replacement glass or a manufacturer-approved sash is normally the more defensible approach.

Replace the glass, the sash, or the complete window?

Replace the insulated glass unit

Glass-only replacement may make sense when the frame, sash, balances, locks, weatherstripping, and installation remain sound. The new IGU must match the required thickness, dimensions, Low-E surface, safety glazing, spacer, grid pattern, and glazing method.

Replace the complete sash

A manufacturer-supplied sash can preserve factory glazing and may simplify service on an operable unit. It can also address sash distortion or damaged sash components. Exact model, size, color, handing, glass, and grid information are essential.

Replace the whole window

Complete replacement is more reasonable when the frame is deteriorated or distorted, hardware and weatherstripping are failing, parts are unavailable, several units have problems, the existing installation leaks, or the homeowner wants a meaningful performance and design upgrade.

Do not sell a complete window solely because it creates a larger ticket. If a warranted sash or glass unit solves the verified problem, that is the honest recommendation. Conversely, repeatedly replacing glass in a damaged frame is false economy.

Aesthetics and matching older windows

A replacement glass unit must do more than fit. Low-E coatings can have a subtle reflected color, and glass from different production periods may not match perfectly. New grids can differ in profile, width, color, or alignment from surrounding windows. Tempered glass stamps may also be visible where safety glazing is required.

Compare the repaired window from inside and outside. On a prominent front elevation, replacing one unit with noticeably different glass can look worse than the original fogging under certain light. When several windows are aged or failed, grouping replacements by elevation may create a more consistent appearance.

Check the Gentek warranty before ordering a repair

Gentek publishes written window and patio-door warranties and provides a process for submitting claims. Coverage depends on the exact product, purchase date, owner, installation, reported condition, exclusions, and documentation. Optional glass-breakage coverage is separate from seal-failure coverage.

Locate the window label and serial number before removing the sash or ordering glass. Photograph the complete window, the internal fogging, the label, and the surrounding installation. Keep the original invoice when available. Do not promise approval until the manufacturer or authorized warranty administrator confirms coverage.

Contractor inspection checklist

Verify whether moisture is inside, outside, or between the panes.
Record the manufacturer, series, serial number, glass markings, size, and approximate age.
Photograph the condition in lighting that clearly shows the haze or deposits.
Inspect frame and sash geometry, drainage openings, glazing beads, locks, and weatherstrip.
Check for impact, edge damage, aftermarket film, unusual heat exposure, or trapped water.
Confirm safety-glazing, Low-E, grid, tint, spacer, and thickness requirements.
Review the current written warranty before quoting replacement material.

Frequently asked questions

How can I tell if a double-pane window seal has failed?

Moisture, haze, or deposits that cannot be wiped from either accessible glass surface are common signs. Confirm the location before assuming the sealed unit has failed.

Can fog between window panes be repaired?

The insulated glass unit or complete sash can often be replaced while retaining a sound frame. Defogging may improve appearance but does not recreate the original factory-sealed system.

Does condensation mean I need new windows?

Not necessarily. Interior condensation usually involves indoor humidity and cold glass. Exterior condensation is often temporary. Moisture trapped between panes is the condition most associated with seal failure.

Is a failed window seal covered by warranty?

Coverage depends on the exact manufacturer warranty, product, purchase date, ownership, installation, exclusions, and documentation. Locate the serial number and submit the condition for review before promising coverage.

Should I replace the glass or the whole window?

Replace only the glass or sash when the frame and installation remain sound. Consider complete replacement when there is frame damage, leakage, failed hardware, unavailable parts, multiple problems, or a planned performance upgrade.

Evaluate fogged Gentek windows in the Quad Cities

Suburban Wholesale and Supply helps Davenport, Bettendorf, and Quad Cities contractors identify Gentek replacement windows, insulated glass packages, sash information, current warranty resources, and ordering requirements. Bring clear photos, label information, dimensions, glass markings, and a description of when the fog appears so the condition can be evaluated accurately.

Fog Between Window Panes: Repair or Replace?