This is the window industry’s hidden demon. Tiny, microscopic specks of nickel sulfide can be trapped in the float glass during manufacturing. Over years, these impurities slowly change crystalline structure, expanding by up to 4%. In a standard single pane, this causes a harmless chip. In a sealed, stressed IGU, it acts like a bullet from the inside. The crack often appears as a distinctive “figure eight” or butterfly shape, originating from a single point.
You don’t usually hear it happen. There’s no smash of baseball-bat glass, no howl of wind, no obvious intruder. You simply glance at the living room window one morning and see it: a thin, silvery line, or perhaps a web of fractures, snaking across the inner surface of the glass. double glazed window cracked inside
Consider it a gentle, visible reminder: every high-tech system, no matter how well sealed, has a lifespan. When the line appears, it’s time to give your window a second life—one clean, warm, crack-free unit at a time. If the crack is on the room-facing side (i.e., you can feel it) and the window is still fog-free, that is likely impact damage. You can temporarily cover with clear packing tape to prevent shards from falling. But if it’s between the panes, as described above, see Part 5—repair is not an option. This is the window industry’s hidden demon
Your window frame expands and contracts at a different rate than the glass. If the installer used too little glazing tape or overtightened the retaining beads, the frame pinches the IGU. Over a few seasons of expansion, the inner pane, which has less flexibility than the outer (being further from the external buffer), gives way. In a standard single pane, this causes a harmless chip
You touch the pane. It’s smooth. The crack is on the inside—but between the two sheets of glass.
On a sunny day, the gas between the panes heats up, expands, and presses outward on both glass sheets. At night, it cools, contracts, and pulls inward. This daily breathing is normal. But if the perimeter seal is old or poorly fitted, the pressure becomes uneven. Eventually, the inner glass—the side facing your conditioned home—shatters from fatigue.
This is the "hot coffee in a cold mug" effect, scaled up. Imagine a bright winter morning: The outer pane is freezing. The inner pane, warmed by your central heating, wants to expand. But the outer glass holds the entire unit rigid via the spacer. The result? Tension builds in the inner pane until it yields. You’ll often see these cracks starting perpendicularly from the edge, then arcing.