Foggy Windows? How to Diagnose IGU Seal Failure

You have probably noticed it gradually — a slight haze on one or two windows that you cannot seem to clean no matter how much glass cleaner you spray. You wipe the inside, you wipe the outside, and the cloudiness remains. That persistent fog between your window panes is not dirt. It is a failed insulated glass unit seal, and it is one of the most common window problems in Utah homes built between 1990 and 2010.

Understanding what has happened inside your window — and what your real options are — can save you from either overspending on unnecessary full replacements or underspending on a band-aid that does not last.

What Is an Insulated Glass Unit (IGU)?

An insulated glass unit is the sealed assembly of two or three glass panes separated by spacer bars and filled with an inert gas, typically argon or krypton. The entire perimeter is sealed with a dual-seal system: an inner seal (usually polyisobutylene) that blocks moisture, and an outer seal (silicone or polysulfide) that provides structural integrity and UV resistance.

This sealed assembly is the core of what makes modern windows energy-efficient. The gas fill between the panes conducts heat about 34% less efficiently than regular air. Combined with low-E coatings on the glass surfaces, a properly sealed IGU can achieve U-factors as low as 0.25 — meaning it only allows a quarter of the heat transfer that a single-pane window permits.

When the seal fails, the insulating gas leaks out and is replaced by regular air containing moisture. That moisture condenses inside the sealed space when the glass surfaces reach dew point temperature, creating the fog you see.

How IGU Seals Fail

Seal failure is a gradual process, not a sudden event. The dual-seal system degrades over years of exposure to thermal stress, UV radiation, and moisture. Here is what happens:

Phase 1 — Outer seal degradation: UV radiation and thermal cycling break down the outer structural sealant. Micro-cracks develop, especially at corners and along the bottom edge where moisture can wick upward. This phase can last years with no visible symptoms.

Phase 2 — Gas migration: As the outer seal weakens, the insulating gas (argon or krypton) slowly leaks out. Because these gases are lighter than nitrogen (the primary component of air), they escape more readily through micro-cracks. The window's thermal performance begins declining, though you may not notice fog yet.

Phase 3 — Moisture infiltration: Once the outer seal is compromised, the inner moisture seal is exposed to higher stress. Humidity from the air enters the interstitial space. Initially, the desiccant material in the spacer bar absorbs this moisture. When the desiccant is saturated, moisture begins condensing on the interior glass surfaces.

Phase 4 — Visible fog and mineral deposits: Now you see the problem. The fog may appear and disappear with temperature and humidity changes at first, but it becomes increasingly persistent. Over months and years, dissolved minerals in the condensing water deposit on the glass, creating permanent white or iridescent staining that remains even if the moisture temporarily clears.

Diagnosing Seal Failure vs. Other Condensation

Before assuming your IGU has failed, make sure you are looking at the right type of condensation. This distinction is critical because the solutions are completely different.

Between-pane fog (IGU failure): You cannot touch it or wipe it away from either side. It appears inside the sealed glass assembly. Often worse at the bottom of the window. May clear temporarily on warm days and return when temperatures drop. This requires glass or window replacement.

Interior condensation (humidity issue): Forms on the room-facing surface of the glass. You can touch it, wipe it with your finger, and see the streak. Most common on cold mornings, in bathrooms, and in kitchens. This is typically a humidity management issue, not a window failure.

Exterior condensation (good sign): Forms on the outdoor surface of the glass, often as dew or frost on cool mornings. This actually indicates excellent window insulation — the exterior glass stays cool because heat is not transferring through the window. No action needed.

A simple finger test settles it: if you can touch the fog, it is surface condensation. If you cannot, it is between the panes and the IGU has failed.

Why IGU Failure Is Common in Utah

Utah's environment is particularly hard on IGU seals, which is why many homeowners along the Wasatch Front see failures sooner than the expected 20-year seal life.

Intense UV radiation: At 4,200 feet elevation in Salt Lake City and 6,000+ feet in mountain communities, UV intensity is 15-25% higher than at sea level. UV is the primary degradation mechanism for both the outer sealant and the low-E coatings. South- and west-facing windows bear the brunt, often failing 5-7 years before their north-facing counterparts.

Extreme thermal cycling: Utah windows experience dramatic daily temperature swings. A February day might range from 5 degrees F overnight to 45 degrees F in afternoon sun. Each cycle causes the glass panes, spacer bars, and sealants to expand and contract at different rates. Over 15-20 years, this adds up to thousands of stress cycles that fatigue the seals.

Low winter humidity with high indoor humidification: Because Utah's outdoor air is extremely dry in winter, homeowners run humidifiers aggressively. This creates a strong moisture pressure gradient pushing humid indoor air toward the cold window assembly, accelerating the assault on the inner moisture seal.

1990s-era window quality: Many of Utah's suburban developments in the Salt Lake Valley, Utah County, and Davis County were built during the construction boom of the mid-1990s to early 2000s. The windows installed in these homes were often builder-grade units with single-seal (rather than dual-seal) IGU construction and aluminum spacers — both factors that reduce seal longevity.

Glass-Only Replacement vs. Full Window Replacement

When you confirm IGU seal failure, you have two paths forward. Each has clear advantages and limitations.

Glass-Only (IGU) Replacement

A technician removes the sash, extracts the failed glass unit from the frame, and installs a new IGU of the same dimensions. The frame, sash, hardware, and weatherstripping remain in place.

Cost: $150-$350 per window, depending on glass size, coatings, and gas fill options.

Best when:

• The window is under 10 years old
• The frame, sash, and hardware are in excellent condition
• Only one or two windows are affected
• The window brand and model still have available IGU replacements
• The frame material is vinyl or fiberglass (not degrading)

Limitations:

• Does not address frame-level insulation, worn weatherstripping, or degraded hardware
• The new IGU is only as good as the aging frame holding it
• If the frame design contributed to the seal failure (poor drainage, inadequate clearance), the new IGU may fail on the same timeline
• Not all window models support field-replaceable glass units

Full Window Replacement

The entire window assembly — frame, sash, glass, hardware, and weatherstripping — is removed and replaced with a new unit. This can be done as an insert (new window into the existing rough opening) or full-frame (new window with new framing).

Cost: $500-$1,200 per window installed for insert replacement, depending on size, material, and glass options.

Best when:

• Windows are over 15 years old
• Multiple windows have failed seals (signal of house-wide aging)
• Frames show signs of wear: cracking, warping, discoloration, difficulty operating
• You want to upgrade to better glass technology (triple-pane, improved low-E)
• The original windows were builder-grade with known short lifespans
• You want to address condensation, drafts, and energy efficiency in one project

Advantages:

• New seals, new gas fill, new low-E coatings, new weatherstripping, new hardware
• Improved energy ratings (modern windows significantly outperform those from 15-20 years ago)
• New manufacturer warranty (typically 20 years on IGU, lifetime on vinyl frames)
• Addresses the complete building envelope at the window opening

How to Choose the Right Fix

Run through this decision framework:

Choose glass-only replacement if all of the following are true:

1. Only 1-3 windows are affected
2. Those windows are less than 10 years old
3. Frames are in excellent condition (no cracks, warps, or rot)
4. Hardware operates smoothly
5. Weatherstripping is intact and sealing well
6. You are not planning to sell the home in the next 5 years

Choose full window replacement if any of the following are true:

1. More than 3 windows have failed seals
2. Windows are over 15 years old
3. Frames show any deterioration
4. You feel drafts around the window even when closed
5. Other replacement signs are present (difficulty opening, visible damage, high energy bills)
6. You are planning to sell within 5 years and want to maximize home value

For most Utah homes experiencing IGU failure, especially those built in the 1990s and early 2000s, full window replacement is the more practical choice. When one seal fails after 15-20 years, the remaining windows are on borrowed time. Replacing them systematically — starting with the most affected or the most important rooms — avoids the frustration of chasing individual failures over the next several years.

Finding a Qualified Provider in Utah

Whether you choose glass-only or full replacement, the quality of the installation matters as much as the quality of the product. When evaluating providers:

For glass-only replacement:

• Ask if they can match the original IGU specifications (thickness, low-E coating position, gas fill)
• Verify they use dual-seal IGU construction (not single-seal)
• Ask about their warranty on the replacement glass unit
• Confirm the technician will inspect the frame drainage system and weatherstripping during the service

For full window replacement:

• Request NFRC-certified performance data for the specific window they propose
• Ask about their installation warranty (separate from the manufacturer's product warranty)
• Verify they spray-foam or backer-rod and caulk the gap between the window frame and rough opening — this air sealing detail is critical for preventing future condensation
• Ask for references from other Utah homeowners, ideally in your area

The difference between a window that lasts 25 years and one that fails in 10 often comes down to installation quality and material selection, not just price.

What to Do Right Now

If you suspect IGU seal failure, start with the finger test on each window. Document which windows are affected, which direction they face, and approximately when you first noticed the fog. This information helps any provider you consult give you an accurate assessment.

While failed IGU seals are not an emergency — your home is not going to flood or collapse — they do get worse over time. The mineral deposits that etch your glass during fog-and-clear cycles become permanent if left for years. And every day with a failed seal means reduced insulation and higher energy costs.

For a complete understanding of all types of window condensation — not just seal failure — and when each type warrants action, read the comprehensive condensation and mold guide. If your foggy windows are part of a broader pattern of window aging, the warning signs guide can help you assess whether a whole-home window project makes sense.