Cold Weather Windshield Care That Prevents Cracks
Winter does not break windshields. Thermal shock does, and most of it is self-inflicted in the first two minutes of the drive.
Key Takeaways
- Never pour warm water on a frozen windshield
- Warm the defroster gradually rather than starting at maximum
- Lift wiper blades before scraping ice
- Any existing chip should be repaired before the first hard freeze
The thermal shock mechanism
Glass expands as it warms. If the inner surface warms rapidly while the outer surface stays frozen, the two layers try to be different sizes at the same time. The resulting stress is concentrated at any existing flaw, which is exactly what a chip is.
This is why so many cracks appear on the first genuinely cold morning of the year, in cars that were fine all autumn.
The right warm-up sequence
Start the defroster on low with the temperature moderate and let it come up over several minutes. Scrape mechanically while it warms rather than relying on heat. Never pour warm or hot water on frozen glass; the temperature differential is extreme and cracking is common.
Protect the wipers too
Blades frozen to the glass tear when the wipers are switched on, and a torn blade will score the windshield within a week. Lift the arms before an overnight freeze, or free the blades by hand before starting the car.
Fix chips before winter
A chip that survived summer is not stable, it is untested. Freeze-thaw cycling is far harder on damaged glass than heat is. In northern states the single best maintenance decision of the year is repairing every chip in October.