Foggy windows, condensation between panes, and increased drafts are telltale signs of insulated glass unit (IGU) seal failure. This guide helps you identify the problem, understand the cause, and choose between repair, defogging, and full replacement.
Last Updated: March 2026
These six indicators help diagnose window seal failure, listed from most definitive to least. Multiple signs appearing together strongly indicate seal failure.
A milky, hazy, or foggy appearance between the two glass panes that cannot be wiped away from either surface.
Recommended Action: Seal has failed. IGU replacement or defogging recommended.
Water droplets visible between the panes, typically accumulating at the bottom of the window.
Recommended Action: Seal has failed. IGU replacement recommended (defogging may not resolve heavy moisture).
White, chalky, or crystalline deposits on the interior glass surfaces between panes.
Recommended Action: Seal has been failed for extended period. IGU replacement required -- deposits indicate permanent glass surface damage.
Reflections in the glass appear wavy or distorted, especially when viewed at an angle.
Recommended Action: Gas fill has leaked. Seal failure likely in progress. Monitor closely; plan for IGU replacement.
Noticeably louder traffic, wind, or other outside noise compared to other windows in the home. A failed seal means the insulating gas has escaped,
Recommended Action: Indicates gas fill loss. Inspect seal and consider IGU replacement for noise-sensitive rooms.
Cool air felt near the window surface even when the window is fully closed and locked. A failed IGU has lower insulating value,
Recommended Action: Check seal condition. May be seal failure or may indicate frame/weatherstripping issues -- professional inspection recommended.
Choosing between defogging, IGU replacement, and full window replacement depends on the age of the window, condition of the frame, and your budget.
| Option | Best When | Relative Cost | Thermal Performance | Lifespan |
|---|---|---|---|---|
| Defogging | Frame in excellent condition, budget-conscious, short-term fix needed | $ | Fair (no gas fill restored) | 3-5 years |
| IGU Replacement | Frame in good condition, window under 15-20 years old | $$ | Excellent (new seal + gas fill) | 15-25 years |
| Full Window Replacement | Frame damaged, window over 20 years old, upgrading performance | $$$ | Best (new frame + IGU + hardware) | 25-40 years |
Understanding the dual-seal system in modern insulated glass units helps explain why seals fail and what happens when they do.
The primary seal is made of polyisobutylene (PIB), a synthetic rubber with extremely low moisture vapor transmission rate. It is applied directly between the spacer bar and the glass surface, forming a near-impermeable barrier against moisture and gas migration. PIB remains flexible over its lifetime, accommodating thermal expansion and contraction. When the primary seal fails, moisture enters the IGU cavity and the insulating gas (argon or krypton) begins to escape.
The secondary seal is applied around the perimeter of the spacer bar, bonding the two glass panes together structurally. Silicone sealant provides excellent UV resistance and adhesion, making it the preferred choice for structural glazing applications. Polysulfide sealant offers superior gas retention (lower gas transmission rate than silicone) and is commonly used in residential IGUs where gas retention is important. Dual-seal systems with PIB primary and polysulfide secondary provide the longest seal life.
Window seal failure is caused by the breakdown of the primary and secondary seals in an insulated glass unit (IGU). The primary seal (polyisobutylene) keeps moisture out, while the secondary seal (silicone or polysulfide) provides structural bonding. Thermal cycling (daily heating and cooling) causes the IGU to expand and contract, stressing the seals over time. Excessive heat exposure (such as dark frames in direct sunlight), improper manufacturing, physical damage, and age all accelerate seal degradation. Most quality IGU seals last 15-25 years, though some fail sooner due to manufacturing defects.
In some cases, yes. Defogging (also called IGU restoration) involves drilling small holes in the glass, flushing out moisture, adding a desiccant, and resealing the holes. This procedure costs 50-70% less than full IGU replacement and can eliminate visible fog. However, defogging does not restore the insulating gas fill (argon or krypton) and does not repair the original seal -- the window will have reduced thermal performance compared to a new IGU. Defogging is a reasonable short-term solution for windows that are otherwise in good condition and where the frames are not damaged.
Wipe the glass surface with a dry cloth. If the condensation is on the exterior or interior surface of the glass, the cloth will remove it -- this is surface condensation and does not indicate seal failure. If the moisture or fog cannot be wiped away because it is between the two panes of a double-pane window, the seal has failed. Another test: the pattern of moisture between panes often appears as a hazy white or milky band that shifts position depending on temperature and humidity. Interior surface condensation typically appears at the bottom of windows first.
Yes, significantly. A properly sealed IGU with argon gas has a U-factor of approximately 0.25-0.30. When the seal fails, the insulating gas gradually escapes and is replaced by ambient air, increasing the U-factor to approximately 0.40-0.50. This represents a 30-50% reduction in insulating performance. Additionally, moisture inside the IGU can damage Low-E coatings over time, further reducing energy efficiency. A failed IGU also reduces sound insulation by approximately 3-5 STC points compared to an intact unit.
Quality IGU seals typically last 15-25 years under normal conditions. Premium manufacturers often offer 15-20 year warranties against seal failure. The lifespan depends on several factors: seal material quality (dual-seal systems last longer than single-seal), spacer type (warm-edge spacers reduce thermal stress on seals), climate (extreme temperature swings accelerate failure), sun exposure (south and west facing windows fail sooner), and frame color (dark frames absorb more heat). IGUs manufactured before the mid-1990s, when dual-seal technology became standard, tend to fail sooner.
If the window frame and hardware are in good condition and less than 15-20 years old, replacing just the insulated glass unit (IGU) is usually the most cost-effective option. IGU-only replacement costs 40-60% less than full window replacement and takes less time. However, if the frame is damaged, warped, or drafty even with the sash closed, or if the windows are more than 20 years old, full window replacement provides better long-term value. Full replacement also allows upgrading to improved frame materials, better hardware, and higher-performance glass.
Many window manufacturers offer warranties against seal failure, typically ranging from 10 to 20 years or even lifetime coverage on premium products. The warranty usually covers the IGU replacement but may or may not cover labor. To file a warranty claim, you need the original purchase documentation, installation date, and manufacturer information (usually found on the spacer bar between the glass panes or on a sticker on the frame). If you purchased the home and the windows were already installed, the warranty may still transfer to you depending on the manufacturer terms.
Our glass specialists inspect failed seals, assess frame condition, and recommend the most cost-effective solution. Free inspections and estimates throughout the DMV area, backed by our Comprehensive Warranty.
Take advantage of our special offers and save on your glass project.