Why Do My Car Windows Fog Up? How to Clear and Prevent It

Car windows fog up when warm, moist air inside the car touches cold glass and turns into tiny water drops. The fastest fix is to turn on defrost, switch the AC on, set the fan to high with warm air, and turn recirculation off. Cracking a window helps, too.

This guide explains why it happens, the best order of steps to clear the glass fast, and easy ways to keep it from coming back all fall and winter.

Why windows fog up

The air in your car holds moisture. When that air touches glass that is colder than the air, it cools down and can no longer hold all of the water. The extra moisture turns into a thin layer of tiny droplets. That is fog.

The moisture inside your car comes from:

• Your breath. Every person in the car adds moisture, and a full car fogs up faster.
• Wet coats, umbrellas, and shoes
• Damp floor mats and carpets
• Spilled drinks or a leaky seal
• Hot drinks and food
• Rainy or humid weather that gets in when doors open

In fall, nights are cold and days can be damp, so the glass is cool while the air in the car is warm and wet. That is why fog hits so often this time of year.

The fastest way to defog the inside (step by step)

Consumer Reports and AAA agree on the best order:

1. Turn on the defrost setting. Use the windshield defrost button, often a window symbol with wavy arrows. In most modern cars this also turns on the air conditioning and a higher fan speed.
2. Turn the AC on. Air conditioning pulls moisture out of the air. This is the key step, and you can run it with warm air.
3. Set the temperature to warm. Warm air heats the glass, which helps the fog clear.
4. Turn the fan to high and point the vents at the windshield.
5. Turn recirculation off. Choose fresh air mode instead. The recirculation button is the one with a car and a looping arrow. Recirculation keeps the damp air from your breath inside the car and makes fog worse.
6. Crack a window slightly. Letting a little humid air out can help, especially with a full car.

Most windshields clear in about a minute or two with these steps.

Quick fixes that make fog worse

Avoid these habits:

• Using recirculation. It traps moisture inside the car.
• Wiping with your hand or sleeve. This leaves oils and lint, and the glass fogs up again quickly.
• Using only heat with no AC. Warm air can hold more water, but it does not remove it.
• Turning the fan low. Air movement matters.
• Blowing air on your face only. Aim it at the glass.

What about the rear window?

Use the rear defroster button, which is the one with a window and wavy lines. It heats thin wires in the glass. Press it once and wait a few minutes. It may shut off by itself after about 10 to 15 minutes.

Side windows can be harder. Point the side vents toward them, or crack the windows open a little to let moist air out.

When the fog is on the outside

In warm, humid weather, the glass can get cold from your AC. Moist outdoor air then condenses on the outside of the windshield.

To clear it:

• Turn on the wipers. They wipe the fog off.
• Raise the temperature a little so the glass warms up.
• Point fewer vents at the glass, or turn the fan lower.
• Do not use defrost with cold air in summer.

How to keep car windows from fogging in the first place

1. Clean the glass inside

Dirt, dust, and oily film from your hands or from vinyl dashboards give water something to cling to. Clean glass fogs up less. Use a glass cleaner and a clean microfiber cloth, and wipe in one direction. Be sure to do the inside of the windshield and the rear window.

2. Keep the inside dry

• Shake off coats and umbrellas before getting in.
• Knock the water off shoes.
• Take out wet floor mats and dry them at home.
• Do not leave wet clothes or towels in the car.
• Park in a garage when you can, so the glass does not get cold and damp overnight.

3. Use your AC all year

Run the AC with the defrost setting, even in winter. In many cars the AC compressor turns on with defrost automatically. It dries the air and keeps the glass clear.

4. Use fresh air mode

Keep recirculation off unless you need it, for example in a smoky or dusty area for a short time.

5. Add a moisture absorber

Moisture-absorbing products can help. Silica gel packs, reusable dehumidifier bags, or a sock filled with crystal cat litter (silica based) can soak up water. Put them on the dashboard or under the seat. Do not use clumping litter. Replace or dry them out as the label says.

6. Replace the cabin air filter

A clogged cabin air filter cuts airflow and can hold moisture. Many carmakers say to change it about once a year or every 15,000 to 25,000 miles. Check your manual.

7. Use an anti-fog product

You can buy anti-fog sprays and wipes made for car glass. They leave a thin clear layer that keeps water from forming small droplets. Follow the label, and wipe off the extra so it does not streak.

Home tricks: do they work?

You may see tips online for shaving cream, dish soap, or potato rubbed on the glass. Here is the truth:

Skip these if you can and stick with the AC, clean glass, and a dry car.

Windshield fog and driving safety

Fog on the windshield cuts your vision, and that can lead to a crash. Do not drive until you can see clearly in all directions. If the glass is still hazy after a few minutes, pull over somewhere safe and clear it.

Do not wipe the glass while driving, and do not use your hand near the wheel. Use the defrost setting and wipers.

What if the car is always damp?

A car that fogs up every time may have a water leak or too much moisture inside. Check for:

• Wet carpets or wet spots under the floor mats
• A musty smell, like a damp basement. See our guide on why rooms smell musty. The same causes of damp air apply in a car.
• Clogged drain holes under the windshield cowl or in the doors
• Worn door seals or window seals
• A bad sunroof drain

If you find water, dry the carpet well and fix the leak. A damp car is also more likely to grow mold.

A greasy film and a sweet smell

If the inside of the windshield gets an oily, greasy film that smears when you wipe it, and you notice a sweet smell like syrup when the heat is on, you may have a leaking heater core. This can let engine coolant vapor into the cabin.

Coolant is toxic. Do not ignore this. Other signs include:

• A wet driver's side floor that feels slimy
• Low coolant with no sign of a leak outside
• Steam from the vents

Have a mechanic check it soon. Open a window for fresh air until it is fixed.

Winter tips

When it is below freezing, fog can turn to frost on the inside of the glass. Use the same defrost steps, then scrape gently from the inside with a soft plastic scraper only if needed. For the outside, use a scraper and de-icer, and do not pour hot water on the glass, because it can crack.

You can also make winter mornings easier by parking so that the windshield faces east, if you can. The morning sun will warm it up. Our guide on why windows at home get condensation shows how this same science works in your house.

The bottom line

Foggy car windows are caused by moisture in the air touching cold glass. To clear them fast, use defrost, AC on, warm air, fan high, and recirculation off. To keep them clear, keep the glass clean inside, keep the car dry, and run the AC with fresh air. If you see greasy film or smell something sweet, get your heater core checked.