Seal failure is the most common defect in double- and triple-pane windows. It occurs when the hermetic seal — made of PIB butyl rubber and silicone — that closes the insulated glass unit loses its integrity, allowing outside air (and its humidity) to enter the gas-filled cavity. Once humid air is inside, it condenses on the cold glass surfaces and the desiccant in the spacer bar quickly becomes saturated, resulting in permanent fogging that cannot be wiped away.
How Seals Fail
The primary mechanism is thermal cycling fatigue. Every day, the glass expands and contracts with temperature changes, stressing the seal bond at the glass edge. Over 15–20 years (thousands of cycles), the seal material fatigues and micro-cracks develop. Other contributing factors include:
- Poor original seal quality (low-cost windows)
- Direct water contact on the seal (missing or damaged spacer edge protection)
- Pressure washing the window edge
- Painting over the seal, blocking its flexibility
- Window installation problems that put the frame under stress
Diagnosing Seal Failure
The definitive sign is haze, fog, or visible moisture between the glass panes — not on the surface. To confirm it’s between the panes (not surface condensation), try wiping it: if it doesn’t wipe off, it’s inside the unit. Other signs include: rainbow-colored iridescence on the glass (caused by residue from the evaporated moisture), and a slightly hazy or dirty appearance even when both surfaces are clean.
Repair Options
- IGU replacement — remove the failed unit, install a new factory-sealed IGU in the existing sash and frame. Best option. Cost $150–$350 per unit. Restores full thermal performance.
- “Defogging” service — drill small holes, inject cleaning solution, re-plug. Removes haze temporarily but does not restore the gas fill or thermal performance. Not a lasting fix.
- Full window replacement — only warranted if the frame or sash is also damaged.
