Ask The Beaverton Car Guy: Foggy Windows

By Larry “The Car Guy“ Ferguson

Hello Beaverton Readers!


This month’s article is in response to Barbara & Esther who both emailed in asking why their car windows keep fogging up more than usual?

What causes foggy Windows?

Windows fog up on the inside when warm, moist air in the car meets colder glass surfaces, causing condensation. In its simplest form, it’s too much water in the air, and it condenses on the cool glass. It condenses on the inside because the outside air is colder and when it’s colder outside that lowers the relative humidity point.

It’s like when you see your breath when it’s cold outside. Your breath generally has 100 percent humidity in it, but you’re only seeing it when it’s cold outside because that’s when it’s reaching the point where it’s causing fog.”

Humidity levels inside a car can increase from passengers breathing, clothes that are wet from rain, snow that people drag in on their boots, damp floor mats and — a classic example — an amorous couple. Even a hot pizza can elevate moisture levels in a car.

What to do?

When the inside windows fog up, I advise to turn on the defrosters (front and rear) while turning up the heat. The air conditioning comes on automatically when the windshield defroster is turned on in most current vehicles and this will dehumidify the air and clear windows faster.

Use warm air

Warm air contains more moisture, but it’s the relative humidity that matters, so warm, dry air coming through the defroster will clear the glass faster than cool air.

Use fresh air

Also keep in mind that it’s best to use fresh air instead of the recirculation mode because the latter will recycle the moist air that was the cause of the problem in the first place!

Newer cars

Keeping inside windows clear of condensation may be less of an issue than in the past. Several current vehicles, with automatic climate control systems have humidity sensors that can trigger anti-fogging modes. Of course having clean windows on the inside will help as well.

The system will take care of fogging by itself. It’s sensing the levels of fog and the temperatures on the glass, and it’s making corrections, either blocking recirculation or engaging the (air-conditioning) compressor or in the auto mode adjusting the blower, warming the air, going to defrost – all automatically. The Driver of the vehicle doesn’t have to do a thing!

Older cars

On Older vehicles without automatic climate control, it is best to manually set the heater to defrost, with the temperature on warm or hot, and make sure the air selector mode is set to fresh air. If the vehicle is equipped with Air Conditioning turn this on as well as it will automatically dehumidify in cabin of the vehicle.

Note that many of today’s vehicles have interior cabin filter located behind the glove box. These filters can not only become soiled and plugged with dust but they can also can get damp, which may cause windows to take longer to clear. Replacing this filter is only a 5 minute job.

Beaverton Readers: if you have a question or need automotive advice please contact me because chances are good that I just may have the answer and if not, then I’ll help you find it.

About the Car Guy: Larry’s specialty is locating cars, even hard to find cars, for everyday folk. He has been in the automotive industry for over 35 years and has several degrees in automotive technology. Email him at or call 503-930-1493