When an insulating glass unit (IGU) fails, the most visible symptom may be fogging between the panes. The actual cause, however, can begin years earlier through moisture exposure, edge seal damage, manufacturing variation, incompatible glazing materials, or improper installation.
Preventing failure requires more than selecting the right glass. The spacer, sealants, desiccant, fabrication process, frame design, handling practices, and drainage system must continue working together throughout the life of the unit.
What Does IGU Failure Look Like?
A failed IGU does not always announce itself immediately. Some problems develop slowly as moisture enters the sealed cavity or gas escapes through a damaged edge seal.
Common warning signs include:
- Condensation or fog that cannot be wiped from either exposed glass surface
- Mineral deposits, staining, or haze within the sealed cavity
- Corrosion or deterioration of a coating inside the unit
- Visible separation between the spacer and glass
- Sealant that appears cracked, displaced, or detached
- Glass movement, excessive deflection, or changes in the appearance of reflected images
- Reduced thermal performance caused by gas loss or moisture intrusion
It is important to distinguish seal failure from normal surface condensation. Moisture on the room side glass may be related to indoor humidity, outdoor temperature, air circulation, or the thermal performance of the complete window. Exterior condensation can form when the outside glass surface becomes colder than the outdoor dew point.
Condensation between the panes is different. Because the moisture is inside the sealed cavity, it usually indicates that the edge system is no longer controlling moisture as intended.
The Edge Seal Is the Main Line of Defense
The perimeter of an IGU carries much of the responsibility for its durability. It maintains the space between the glass panes while limiting moisture entry and gas loss.
A dual seal system divides those responsibilities between two materials. The primary seal, commonly polyisobutylene, acts as the main moisture vapor and gas barrier. The secondary seal provides adhesion and structural support while helping the unit withstand temperature changes, wind pressure, frame movement, and handling.
The spacer also affects performance. Its design determines how the glass is separated, how sealants are applied, and how much thermal transfer occurs around the perimeter. Open joints, poorly formed corners, inconsistent sealant coverage, and damaged spacer material can create weak points.
Desiccant absorbs small amounts of moisture left inside the unit during production and moisture that may gradually pass through the edge system. It cannot compensate indefinitely for a damaged or poorly constructed seal. Once its moisture holding capacity has been exhausted, condensation can begin forming inside the cavity.
Our Endur IG construction uses a continuous stainless steel warm edge spacer with formed corners, a polyisobutylene primary seal, molecular sieve desiccant, and a silicone secondary seal. Our published data reports a seal failure rate of 0.10% after twenty years.
Moisture Exposure Can Accelerate Failure
Even a well constructed IGU should not be left sitting in water.
Window and curtain wall systems are designed to manage the water that passes exterior gaskets and seals. Drainage paths and weep openings allow that water to leave the glazing pocket. When those paths are blocked, the edge of the IGU may remain exposed to moisture for extended periods.
Standing water can place continued pressure on the edge seal, especially when combined with heat and ultraviolet exposure. Freeze and thaw cycles may add further stress in cold climates.
Preventive measures include properly designed glazing pockets, clear drainage openings, correctly positioned setting blocks, and sealants that direct water away from the IGU edge. Drainage should be checked during manufacturing and installation rather than assumed based on frame design alone.
Material Compatibility Matters
Sealants, setting blocks, gaskets, tapes, cleaning products, lubricants, and other glazing materials can come into contact with the IGU edge. Not every material is chemically compatible with every edge seal system.
Incompatible products may soften sealants, reduce adhesion, cause discoloration, or allow chemicals to migrate into the sealed cavity. This can lead to edge seal deterioration or chemical fogging that affects the appearance of the unit.
Compatibility should be confirmed before a glazing material is approved for production. Suppliers should provide written guidance, and testing may be necessary when a new sealant, gasket, tape, or cleaner is introduced.
Substituting a material without reviewing compatibility can create a failure risk even when the replacement appears physically similar to the original product.
Production Variation Can Create Hidden Weaknesses
Many IGU defects begin with small production inconsistencies that are difficult to identify after the unit has been sealed.
Glass must be clean so the sealants can bond to the intended surface. Debris, moisture, fingerprints, coating residue, or cleaning solution left near the edge can affect adhesion. Incorrect spacer placement, incomplete primary seal coverage, inadequate secondary seal depth, and improper unit thickness may also reduce durability.
Gas filled units require additional controls. The initial gas concentration should be measured, and the cavity must be sealed properly to retain that gas over time. A unit can appear acceptable while leaving production even if its gas fill or seal coverage is outside specification.
Our insulating glass quality systems monitor washer water quality, glass defects, unit thickness, argon fill, coating color, and center of glass thickness. These checks help identify variation before a unit is sealed and shipped.
Handling Damage Is Not Always Obvious
An IGU can leave the production line in acceptable condition and sustain damage during storage, transportation, window assembly, or installation.
Impacts against the edge can damage the glass, spacer, or seal without creating an immediate crack. Dragging units across racks, placing excessive pressure on one corner, using improper setting blocks, or allowing units to rub against metal surfaces can also create localized damage.
Units should be stored and transported according to the supplier’s instructions. In most applications, this means keeping them properly supported, protected from standing water, and secured against movement. Glass edges and corners deserve particular attention because damage in these areas may affect both glass strength and seal integrity.
Protective films, labels, and packaging materials should also be removed according to product instructions. Leaving temporary materials in place beyond their intended exposure period can make removal difficult or affect the appearance of the glass.
Design Conditions Can Stress the Unit
Temperature and atmospheric pressure constantly affect sealed insulating glass.
As the gas inside the cavity warms, it expands. As it cools, it contracts. Changes in elevation between the manufacturing facility and installation site can also create a pressure difference between the cavity and the surrounding air. These forces cause the glass panes to deflect inward or outward.
Some deflection is expected. Excessive movement, however, can strain the edge seal, distort reflected images, or increase glass stress. Large units, narrow cavities, unusual shapes, high elevations, and certain glass thickness combinations require additional review.
The frame must also provide adequate support without restricting normal glass movement. Improper edge clearance, uneven setting blocks, excessive glazing pressure, or frame deflection can transfer stress into the IGU.
Glass breakage does not always mean the seal failed first. Impact, edge damage, thermal stress, structural loading, and installation pressure can break a pane even when the edge seal remains intact. Selecting the correct glass type and thickness is therefore part of preventing complete unit failure.
Prevention Is Shared Across the Product Life
Long-term IGU performance depends on quality control throughout fabrication, transportation, window assembly, and installation. Durable edge seal construction, compatible glazing materials, careful handling, proper setting blocks, clear drainage paths, and adequate edge clearance all help protect the sealed cavity.
When a failure occurs, the glass, frame, seal, drainage openings, and installation conditions should be reviewed before the unit is replaced. Finding the actual cause can prevent the same problem from affecting the replacement glass.
We pay close attention to the production details that affect seal durability and IGU performance. This helps window and door manufacturers limit early failures, service calls, and warranty expenses.
