Why Windshields Crack in Winter

The physics in plain English

Glass expands when it warms and contracts when it cools. When one part of a windshield is much warmer than another, the difference creates stress inside the glass. An undamaged windshield handles normal stress well. A chip, though, is a weak spot where stress concentrates, and a sudden temperature difference can make a crack grow out of it. That is why so many cracks appear when the defroster first hits cold glass, or when the sun hits a windshield that was frozen an hour earlier.

Water in the chip makes it worse

Moisture that gets into a chip can freeze and expand, pushing on the break from inside. Repeated freeze and thaw cycles, common in a Kansas winter, work the break a little more each time. Covering a chip with clear tape until it is repaired keeps water out.

Common winter mistakes

• Blasting the defroster on high heat right away. Start on low or with outside air and increase gradually, especially if there is a chip.
• Pouring hot water on a frozen windshield. It is one of the fastest ways to crack glass, chip or no chip. Use the defroster and a scraper instead.
• Scraping hard over a chip. Pressure on a damaged area can extend it. Scrape around it gently.
• Using wipers to break ice. Frozen blades can tear and drag grit across the glass; they can also strain the wiper motor.
• Slamming doors on a frozen car. The pressure pulse and jolt stress a cold, damaged windshield.
• Running summer washer fluid. It can freeze on the glass and in the lines.

A better winter morning routine

1. Start the vehicle and set the defroster to a low temperature with the fan on low or medium.
2. Clear loose snow with a brush, starting at the roof so it does not slide onto the windshield later.
3. Let the defroster soften the ice for a few minutes, then scrape gently, working around any chips.
4. Increase the heat gradually as the glass warms.
5. Use winter-rated washer fluid to clear salt and grime, and replace wipers that streak or chatter.
6. Clear the whole windshield, not just a peephole. Kansas law requires working wipers and prohibits driving with a view substantially obstructed by damage (K.S.A. 8-1741).

Remote start and garages

Remote start helps, as long as the climate control is not set to maximum defrost from the moment the engine starts. A garage keeps the windshield warmer overnight and reduces the temperature jump in the morning. Even parking facing east, so the morning sun warms the glass gradually, can help a little.

Summer has its own version

The same physics applies in a Wichita July. A windshield sitting in the sun can get very hot, and aiming full-blast air conditioning straight at it puts cold air against hot glass. Point the vents at the cabin first and let the glass cool more gradually, and use a sunshade when parked.

When a crack appears on a cold morning

• Note where it starts (a chip? the edge?) and how long it is.
• Keep the cabin temperature steady and avoid more sudden heat.
• Mark the end of the crack on the inside with a dry-erase marker to see whether it is growing.
• If it is in your line of sight, do not drive until it is dealt with.
• Get a quote. A short crack that has not reached the edge may still be repairable under the industry repair standard (ROLAGS repair standard (ANSI/AGSC/NWRD 002-2022)); a long or edge crack usually means replacement. See cracked windshield replacement.

Can windshields be replaced in cold weather?

Yes, but cold affects the job. Urethane adhesives have temperature limits and many cure more slowly in cold, dry air, which can lengthen the drive-away time. Shop installs in a heated bay avoid the problem. For mobile service in winter, a garage makes a big difference, and outdoor jobs may be rescheduled in snow, freezing rain or strong wind. See mobile vs in-shop replacement.

The cheapest winter fix: repair chips in the fall

The best way to avoid a winter crack is to repair chips before the first hard freeze. A chip repair costs a fraction of a windshield and fills the break with resin so water cannot get in and stress has less to work with. See rock chip repair.

Related services

• Rock Chip Repair: Fill rock chips from highways and gravel roads before they run into a crack.
• Cracked Windshield Replacement: When a crack is too long, at the edge or in your view, and what replacement involves.

Sources: ROLAGS repair standard (ANSI/AGSC/NWRD 002-2022); K.S.A. 8-1741.

Good to know: Requesting a quote is free and there is no obligation. Ask for a written quote that lists the glass, any calibration and the warranty terms.