Why Do Car Windows Fog Up Inside in Winter? (and How to Stop It)

The short answer: your windows fog up when warm, moist air inside the car hits cold glass and the moisture condenses. Clear it fast with the defroster, the A/C on, recirculation off and the heat up. To stop it coming back, keep the inside of the glass clean and the carpets dry.

Here's why it happens, how to clear it, and what you can do to make it happen less often.

Why do car windows fog up on the inside?

Warm, moist air in the cabin meets cold glass, and the moisture condenses into tiny droplets. Breath, wet boots, snowy mats and wet coats all add moisture.

Consumer Reports describes it simply: in winter, moisture in the warm cabin air condenses on the cold glass. The colder it is outside and the more moisture there is inside, the faster it happens. That's why a car full of people in wet coats on a January morning fogs up almost instantly.

What's the fastest way to clear a foggy windshield?

Turn on the defroster, switch the A/C on, turn recirculation off, and set the heat high. The A/C dries the air while the heat keeps the cabin warm.

That's the method Consumer Reports recommends. Running the A/C in winter sounds backwards, but it works as a dehumidifier. CAA gives the same advice: your A/C is also a dehumidifier, so use it to clear foggy windows. Many cars switch the A/C on automatically when you select the windshield defrost setting; check your owner's manual for how yours works.

If it isn't snowing or raining, opening a window slightly also lets the humid air out.

Should recirculation be on or off to defog?

Off. Recirculation keeps the same humid air moving around the cabin. Fresh-air mode pulls in drier outside air, which clears the glass faster.

Both Consumer Reports and AAA say to turn recirculation off in winter. Cold winter air holds very little moisture, so bringing it in and warming it up dries the cabin out.

Does a dirty windshield fog up more?

Yes. Condensation clings to grime and film on the glass, so a dirty windshield fogs faster and looks smeary while it clears. A clean inside surface helps.

NAPA's guide to windshield fog calls a dirty windshield a magnet for condensation and recommends keeping it free of grime with a cleaner made for car glass. Dirty glass also adds glare from headlights and the low winter sun.

Wiping fog away with your hand makes it worse next time. NAPA points out that it leaves smudges that make it harder to see. And AAA notes that wiping inside fog is a quick fix that can leave streaks.

How do you clean the inside of a windshield properly?

Use an ammonia-free automotive glass cleaner and two clean microfibre cloths: one to clean, one to buff dry. Spray the cloth, not the glass, to keep cleaner off the dash.

Step by step:

1. Pick the right cleaner. Consumer Reports recommends an alcohol-based, ammonia-free automotive glass cleaner. AAA warns that ammonia-based cleaners can damage window tint. 2. Use clean microfibre. AAA suggests a waffle-weave microfibre cloth. Paper towels can leave streaks, and a dirty cloth just moves the film around. 3. Spray the cloth, not the windshield. Overspray runs down onto the dash and any screen nearby. Consumer Reports advises never spraying cleaner directly onto a touchscreen. 4. Wipe, then buff. Clean in overlapping passes, then buff dry with the second cloth. If greasy film keeps coming back, AAA suggests following up with an isopropyl alcohol cleaner. 5. Don't forget the corners. The bottom of the windshield, where it meets the dash, is the hardest part to reach and the easiest to miss.

Our post on what not to do when cleaning your car's interior covers the other surfaces nearby, like the dash and screen.

Do wet floor mats make car windows fog up?

They can. Snow and slush from boots melt into the mats and carpet, and that water slowly evaporates into the cabin air, adding moisture that ends up on the glass.

Knock snow off your boots before you get in, and use good rubber mats in winter. NAPA recommends rubber mats with raised sides and channels, since flat, cheap mats let water get everywhere, and warns that moisture that gets in can lead to mould. Empty and dry the mats regularly, and leave the car to air out when you can.

Salty meltwater leaves something else behind too: white salt rings in the carpet once it dries. Our guide to removing winter salt stains from car carpet explains how to deal with them.

If the carpet stays wet even when there's no snow in the car, or the fog keeps coming back no matter what you do, the water may be getting in from somewhere else. Cleaning can dry a carpet but can't fix a leak, so have a mechanic take a look.

Is it illegal to drive with foggy windows in Ontario?

Ontario's Highway Traffic Act requires your windshield and side windows to give you a clear view. Fog, frost or snow that blocks your view can get you an obstructed-view ticket.

Section 74 of the Highway Traffic Act says you can't drive unless the windshield and windows on either side of the driver give a clear view to the front and sides. As reported by AutoTrader.ca, an OPP sergeant put the obstructed-view ticket at $110 in Ontario. CAA's winter driving tips also remind drivers to clear all windows and mirrors before setting off, and not to forget the inside of the car.

The practical rule: wait until the glass is clear before you pull out. It's safer, and it takes less time than you'd think with the defroster set up properly.

Can a car detail help with foggy windows?

It helps: a detail cleans the inside glass and pulls salty moisture out of the carpet. It won't stop condensation, but fog gets easier to manage.

Interior glass and vents are part of our Light Interior Detail, and our Deep Interior Clean adds hot-water shampoo extraction and salt stain treatment for carpets that have soaked up a winter's worth of meltwater. Our interior car detailing page explains the options, and the services page compares them side by side. If you want to know how a detail fits into a winter routine, see our guide on how often to detail your car in Ontario.

Polish & Go is a mobile detailing service based in Milton, also serving Campbellville, Oakville, Georgetown, Halton Hills, Burlington, Mississauga and Brampton. Interior detailing runs year-round, and we work 9 a.m. to 9 p.m. every day. Book online, read our FAQ, or call 519-835-5580.