When Does Facade Glass Need to Be Replaced?

Glass is not a fit-and-forget material

Modern commercial glazing is engineered to last decades, but it operates in a harsh environment. UV radiation, thermal cycling, wind loading, building movement, and moisture exposure all take a toll on the glass itself and on the sealing systems that keep it in place.

The glass industry distinguishes between failures that require immediate replacement and conditions that can be monitored. Understanding the difference saves building owners from unnecessary replacement costs while ensuring genuine failures are addressed promptly.

Sealant degradation

Structural silicone sealant bonds the glass panel to the frame in structural glazing systems. Over time, UV exposure and thermal cycling cause the sealant to harden, crack, and lose adhesion. When the structural seal fails, the glass panel is no longer reliably held in the frame.

Sealant life varies depending on the product, orientation, and climate. North-facing sealant in Australian conditions degrades faster than south-facing due to higher UV exposure. A general expectation is 15 to 25 years for structural silicone, but environmental factors can shorten this significantly.

Sealant condition assessments should be included in every facade inspection programme. Adhesion testing (peel tests and cut tests on representative samples) provides objective data on remaining sealant life.

Thermal stress fractures

Thermal stress fractures occur when one part of a glass panel is significantly hotter than another. The hot area expands while the cooler area restrains it, creating tensile stress at the boundary. When that stress exceeds the glass edge strength, a crack initiates from the edge and propagates across the panel.

Common causes include:

• Partial shading from adjacent buildings, columns, or spandrel panels
• Dark-coloured blinds or furnishings directly behind the glass
• Reflections from adjacent buildings concentrating solar energy
• Air conditioning vents blowing cold air onto the glass interior surface

Thermal fractures typically start from the glass edge and run perpendicular to the edge before curving. The fracture pattern is diagnostic. If thermal stress fractures are recurring, the root cause must be addressed, not just the broken panel replaced.

Nickel sulfide inclusions

Nickel sulfide (NiS) inclusions are microscopic contaminants that can be present in toughened (tempered) glass. They undergo a slow phase change over time that creates internal stress. When the stress exceeds the glass strength, the panel shatters spontaneously without any external load.

NiS failures are random and unpredictable. They can occur months or decades after installation. The characteristic sign is a butterfly-shaped fracture origin visible on the broken edge, with two small fragments at the origin point.

Heat soak testing (AS/NZS 2208) subjects toughened glass to sustained elevated temperature to accelerate the NiS phase change and force failures before installation. Heat soaked glass significantly reduces but does not eliminate the risk of in-service NiS failure.

For buildings with a history of spontaneous breakage, a glass audit should assess whether heat soaked glass was specified and whether remaining panels are at risk.

Insulated glass unit (IGU) seal failure

Double-glazed units rely on an edge seal to maintain the gas fill (typically argon) between the two glass panes. When the seal fails, moisture enters the cavity and the unit fogs or shows condensation between the panes.

Seal failure is progressive. Early-stage failure shows intermittent fogging that clears in warm weather. Advanced failure shows permanent fogging, mineral deposits on the inner glass surfaces, and reduced thermal performance.

AS/NZS 4666 covers the requirements for insulated glass units. Failed IGUs cannot be repaired in situ. The unit must be removed and a new IGU manufactured and installed.

Laminated glass delamination

Laminated glass consists of two or more glass plies bonded by a plastic interlayer (typically PVB or SGP). Delamination occurs when the interlayer separates from the glass, usually starting at the edges and progressing inward.

Delamination is caused by moisture ingress at the glass edge, UV degradation of the interlayer, and inadequate edge protection. It appears as milky or cloudy areas at the panel edges that grow over time.

Delaminated laminated glass loses its post-breakage performance. The interlayer is supposed to hold broken glass fragments in place, but delamination compromises this critical safety function. Replacement is required.

When to repair vs replace

Minor sealant cracking on non-structural weather seals can often be repaired by removing and replacing the sealant. Surface scratches and minor chips can sometimes be polished out.

Replacement is required when:

• Structural sealant has failed adhesion tests
• The glass is cracked or fractured
• IGU seals have failed (fogging)
• Laminated glass is delaminating
• The glass specification no longer meets current NCC requirements for safety or energy performance

References

• AS 1288:2021: Glass in buildings - Selection and installation
• AS/NZS 4666:2012: Insulating glass units
• AS/NZS 4667:2000: Quality requirements for cut-to-size and processed glass
• AS/NZS 2208:1996: Safety glazing materials in buildings (heat soak testing)