A "pressure crack" typically looks like a long, wavy line that curves across the pane. This is more common in units that were manufactured at sea level and then installed in high-altitude locations, or in units where the primary seal has failed, allowing the internal pressure to fluctuate wildly. Hidden Manufacturing and Installation Flaws
Heavy Curtains or Blinds: Thick window coverings can trap a layer of hot air between the fabric and the glass. On a sunny day, this localized heat builds up rapidly, causing the center of the inner pane to expand while the edges tucked into the frame stay cold.
Double glazing works by trapping a layer of inert gas (usually Argon) between two panes, creating an insulating barrier. Once the seal is breached by a crack, that gas escapes, and moisture-laden air rushes in.
A crack on the inside of the unit might look like a mere cosmetic flaw, especially if the glass hasn't fallen out. However, the "double" in double glazing is about to become meaningless.
Unfortunately, once a pane is cracked, the "unit" is compromised. The vacuum or gas (argon) between the panes will leak out, leading to condensation and a loss of insulation. The only permanent fix is to have a professional glazier replace the individual double-glazed sealed unit.
Similarly, if the window was installed too tightly into the frame without enough room for natural movement, any shift in the building’s foundation or extreme weather can force the frame to squeeze the glass until it breaks. Can You Repair a Cracked Inner Pane?