Fogging of the inside of the windows is caused by moisture - duh!! Well it may sound simple but the question is "Where is the moisture coming from and why isn't it leaving the inside of the car??"
One source of moisture is a leak in the heater core. Check for dampness in the rugs where your feet rest - mainly on the passenger side front.
Second, moisture is brought in with snow from your feet (in cold climates naturally) and remains in the carpets.
Third, moisture is brought in with your body. Every breath you take pumps moisture into the inside of the car. Perspiration, even in the winter, will bring moisture in the car with you. Four people in a standard car will fog up the windows in a few minutes - if and only if you don't remove the moisture from the car!!
That is the key - removing any moisture from the inside of the car no matter how it got in there in the first place. Normally, the ventilation system of your car is capable of removing the moisture IF you have it turned on, and IF you have it set for FRESH AIR, not set in the recirculating air mode. 99% of all car ventilation systems have the capability of turning the system off and/or recirculating the inside air as well as bringing in fresh air from the outside.
If you have it set for recirculating then the moisture stays inside the car and condenses on the inside of the windows once the relative humidity get high enough. Sometimes this is not the operators fault, but rather a faulty control cable or vacuum control for the baffle that switches between recirculating and fresh.
Make certain that the system is on fresh. To test this, close all the windows, turn the fan onto the highest setting and then open one window about a half an inch. Standing outside the car you will feel a strong wind blowing out the window - if you don't feel it then it is not bringing in fresh air - it is recirculating the inside air!! If you have it set on the fresh position then something is broken.
All modern cars have vents in the doors which act to allow air to exit the car even if the windows are closed. These vents can get clogged up with dust and debris so make sure they are clean to ensure that air can leave the car with all that moisture it contains.
Turning the fan to the highest position and cracking open a window will allow a lot more flow of the moist air from the interior of the car. On vehicles with "wing windows", open them an inch or so and the moisture will leave even faster. If you don't like the rushing noise of a window open next to you then crack open a rear window - an added benefit is that your mother-in-law won't be able to hear you and your spouse talking about her!!
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