January 20, 2026 · 6 min read
Why cold weather turns chips into cracks (and the defrost mistake everyone makes)
The morning it happens
It's minus three in the driveway. You start the car, crank the defrost to max, and go back inside for your coffee. When you come out, the little chip you'd been ignoring since November has a tail — ten centimetres of fresh crack, growing while you watch. Nothing hit the glass. The glass did it to itself, with your help.
This is the single most common origin story we hear in winter, and it's entirely preventable. Somewhere, a tiny gravel gremlin is very pleased with your defrost settings.
The physics, without the textbook
Glass expands when warm and contracts when cold — slightly, but measurably. When the whole windshield changes temperature together, nothing much happens. Trouble arrives when one region is hot while another is cold: the warm zone pushes outward while the cold zone holds rigid, and the boundary between them carries the stress. Intact glass shrugs this off. Glass with a chip cannot — a chip is a stress concentrator, a notch where all that boundary tension focuses on a few crack tips thinner than a hair.
Max defrost on a frozen windshield creates the steepest thermal gradient your car can produce: hot air flooding the inside surface at the base of the glass while the outside face sits at minus three. The stress peaks right where chips love to live — lower third, driver's side, courtesy of the truck in front of you last fall. When the concentrated tension exceeds what the fractured glass can hold, the crack runs. Cold nights double the trouble: overnight contraction squeezes the fracture, morning heat yanks it open. Every freeze-thaw cycle is another pull on the handle.
What to do instead, tomorrow morning
Start the car and set the defrost warm-ish and low, not hot and max. Let the cabin come up gradually over a few minutes — the gentler the gradient, the lower the stress. Scrape frost mechanically instead of melting it off with heat. Never, ever pour warm water on frozen glass (that's the express version of the same physics, and it sometimes finishes the job in one go).
If you're carrying a chip, two more moves: put a piece of clear packing tape over it to keep water out — water in the fracture freezes, expands, and wedges it wider — and book the repair this week rather than "when things calm down." Pothole season adds mechanical shock to the thermal story; the combination is why January and February are our busiest replacement months.
The economics of the cold snap
Here's what makes winter procrastination uniquely expensive. Rock chip repair is typically $0 out of pocket for drivers with ICBC comprehensive coverage — we confirm your coverage on-site before any work starts. The same damage, after one bad defrost morning, is a replacement — with a deductible attached in the typical case, plus camera recalibration if your car has one. Nothing about the stone changed; only the waiting did.
So when the forecast shows a cold snap, treat it as a deadline. The chip you fix on the mild Tuesday is a coffee-length errand. The one you leave for the frozen Thursday morning has a real chance of becoming a windshield.
Related questions
Does tape over the chip actually help, or is that a myth?
It helps for one specific reason: it keeps water and grit out of the fracture until repair. Water that freezes inside a chip expands and levers it wider. Tape isn't a fix — it's a bookmark that keeps the page from tearing.
How fast can a crack grow once it starts?
Anywhere from millimetres a week to across-the-windshield in one thermal shock. There's no reliable way to predict which — which is exactly why repairs are treated as time-sensitive.
Is parking outside making it worse?
Overnight cold soaks deepen the freeze-thaw cycling, so garage parking slows the clock. It doesn't stop it — the fix is still the fix.
Carrying a chip right now?
Small now. Expensive later. Today it’s likely covered.