What Causes Windows to Crack by Themselves? (And What to Do About It)

Thermal Stress Cracks: The Most Common Cause

Thermal stress cracking is the most common cause of spontaneous window glass failure. Glass expands when heated and contracts when cooled — and when different areas of the same glass pane heat or cool at different rates, the resulting stress can exceed the glass's tensile strength and cause a crack. In residential windows, thermal stress most commonly originates at the glass edge, where the glass is shaded by the frame while the center of the pane is exposed to direct sun or significant temperature differential.

The classic thermal stress crack pattern starts at the glass edge, extends perpendicularly into the pane for a short distance, then curves to run more or less parallel to the edge. The crack typically looks smooth and follows a relatively gentle curve — very different from the branching, radiating pattern of an impact crack. In the DMV area, south and west-facing windows that experience intense afternoon sun in summer are the most common locations for thermal stress cracking.

Several conditions increase the risk of thermal stress cracking: low-emissivity (Low-E) coatings that absorb more solar energy than standard clear glass (the coating side of the glass can become significantly hotter than the shaded edge); interior window coverings — blinds, shades, or curtains — that trap heat against the glass and prevent air circulation, intensifying the temperature differential across the glass surface; adjacent objects that shade part of the glass while another part is in direct sun; and poorly fitting frames that prevent the glass from moving slightly as it expands and concentrates stress at the edge contact points. For more context on crack patterns and causes, see our article on stress cracks in windows.

Nickel Sulfide Inclusions in Tempered Glass

Tempered glass — the safety glass required in certain locations by building code — has a unique and rare failure mode with no equivalent in annealed glass: spontaneous fracture caused by nickel sulfide (NiS) inclusions. During the glass manufacturing process, tiny nickel sulfide particles can become trapped in the glass. These inclusions are chemically stable in their original state but undergo a phase transformation over time — a crystallographic change that slightly increases their volume. In annealed glass, this expansion is slow and the surrounding glass can accommodate it; in tempered glass, the high internal stresses created by the tempering process mean that a growing NiS inclusion can eventually trigger the entire pane to shatter catastrophically.

NiS-related spontaneous fracture is rare — it affects an estimated 1 in 10,000 to 1 in 50,000 tempered glass panes — but when it occurs, the failure is dramatic: the entire pane shatters at once into thousands of small fragments, typically with no prior warning or visible damage. The failure can occur years or even decades after installation. High-rise commercial buildings and canopy glazing applications manage this risk using heat-soak tested glass, which accelerates the NiS transformation during manufacturing to identify and discard vulnerable panes before installation.

Edge Damage Propagation

Glass is much more vulnerable to fracture originating from edge damage than from damage at its face surface. Small chips, scratches, or nicks at the glass edge — often introduced during fabrication, handling, or installation — create stress concentrations that can propagate as cracks over time, particularly when combined with thermal cycling or frame stress. A glass edge that appears minimally damaged at installation may show a slowly growing crack months or years later as the stress concentration at the chip site allows the crack to propagate across the pane.

This type of cracking typically starts at or very near the glass edge, often at a corner, and propagates in a relatively straight line before curving. It can be confused with a thermal stress crack but tends to originate from a specific visible edge damage point rather than from any edge-center temperature differential. Proper glass handling, adequate setting blocks, and correct bite (the amount of glass captured in the frame rabbet) during installation minimize this risk.

Frame Stress and Building Settlement

Building settlement — gradual movement of the structure over time — can cause window frames to rack (change shape from rectangular toward parallelogram) or compress, transmitting structural loads into the glass. In older DMV-area homes, foundation settlement, wood frame shrinkage, and decades of freeze-thaw cycling in the soil can produce measurable frame distortion. If the glass cannot move freely within the frame as the frame distorts, the glass absorbs the structural stress and may crack.

Cracks caused by frame stress typically follow stress lines across the pane rather than originating at the edge, and may recur if the frame stress is not addressed before installing replacement glass. A glass professional can identify frame stress as the cause and recommend shimming or frame adjustment to prevent recurrence.

What to Do About a Spontaneous Crack

A cracked window — regardless of cause — should be addressed promptly. A cracked pane in a double-pane IGU has a broken outer barrier that exposes the IGU seal to moisture and temperature, causing rapid seal failure in addition to the existing glass damage. A cracked pane in a single-pane window is a security, energy, and safety issue.

Virginia Glass Windows provides emergency glass replacement throughout Washington DC, Northern Virginia, and Maryland. Use our free photo estimate service to get a same-day assessment of a cracked window, or call (703) 470-7867 for emergency response.