Why do windshield chips and cracks spread in the heat?
Because glass expands and contracts with temperature. When one part of the windshield is much hotter or colder than another — a sun-baked windshield hit with cold A/C, or a cold morning and the defroster — the stress concentrates at the weakest point: the chip. That’s when a small chip suddenly runs into a long crack.
101 Windshield Repair · Updated September 30, 2026
What’s happening inside the glass
A windshield is two layers of glass with a clear plastic layer between them. A rock chip breaks the outer layer and leaves tiny cracks, or “legs”, around the impact point. On its own, a chip can sit quietly for weeks.
Temperature is what wakes it up. Glass expands as it heats and shrinks as it cools. If the whole windshield changes temperature evenly, not much happens. But when part of it changes fast — cold air on the inside while the sun bakes the outside — the glass pulls against itself, and a chip is exactly where it gives way.
What makes a chip spread faster
- Blasting the A/C at a hot windshield after the car has sat in the sun.
- Running the defroster on high on a cold morning.
- A car wash with cold water on hot glass.
- Chips near the edge of the glass, where the windshield already carries more stress.
- Potholes, speed bumps and slamming doors, which flex the glass.
- Hot afternoons in the Valley and inland Ventura County, where 100° days are common.
What to do until it’s repaired
- Put a piece of clear tape over the chip to keep dirt and water out.
- Park in the shade or use a sunshade.
- Let the cabin cool down gradually: windows down first, then A/C on low, not aimed at the glass.
- Skip the car wash.
- Close doors gently.
Why fixing it early matters
A chip that’s filled and sealed can’t take in dirt or water and stops acting as a weak point. A small chip is also the quickest and best-looking repair. Once it has run into a crack, the repair is bigger and the line will be more visible.
Related questions
- Can a windshield crack spread while driving?
- Yes. Vibration, bumps and temperature changes from the A/C or defroster can all make a crack grow while you drive.
- How long does it take for a chip to turn into a crack?
- There’s no fixed time. Some chips stay the same for months; others crack the first hot afternoon. Heat, cold and bumps are what set them off.
- Will a repaired chip spread in the heat?
- A properly filled and cured chip is sealed and bonded, so it no longer acts as a weak point the way an open chip does.