: Partially closed blinds or objects placed on a windowsill can create sharp "shadow lines" where one half of the glass is hot and the other is cold, leading to stress along that line.
This often happens if a radiator is blasting heat directly onto the window during a cold snap, or if a large piece of furniture is pushed right against the glass, trapping hot air in a localized pocket. The glass expands unevenly, creating tension that eventually snaps the pane. These cracks often start at the edge of the glass and travel inward. double glazing cracked on inside
You can often distinguish a thermal crack from an impact crack by its appearance. Impact cracks usually have a clear "starburst" point where the object hit the glass. In contrast, a thermal crack typically: : Partially closed blinds or objects placed on