Vinyl windows facing a balcony beneath an overhang.

Stress cracks happen most frequently on windows beneath an overhang.

Window glass can crack because of mechanical force or thermal force. Mechanical forces include wind or flexing of the window frame due to nearby construction or other environmental factors. Thermal stress may result in window cracks when various parts of the glass expand at different rates. When the stress of expansion overcomes the strength of the glass, a crack forms. Just as when you pour cold water into a hot baking dish and it cracks, extreme cold on a window’s surface may lead to stress cracks, particularly if the window is large and a single pane (i.e., not insulated).

Stress cracks happen most frequently on large windows that are beneath an overhang or recessed in an outward protruding room. Shadow lines create stress factors because sunny, hot areas of glass may expand while shaded, cool areas of the same glass may contract. Dramatic changes in temperature (such as overnight) can also cause stress cracks in windows. Pay attention to where there’s shade on glass, especially on south-facing windows, and consider awnings or insulating glazing options to reduce the potential for cracking.

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