Simply getting into the car in a wet jacket or bringing in a little snow on your shoes can turn the windscreen into an opaque wall within minutes. This is more than just an inconvenience: incorrectly adjusted ventilation can make the fogging worse and severely restrict visibility. The good news is that condensation caused by normal levels of moisture can usually be cleared fairly quickly.
If the windows fog up on the inside, switch on the windscreen defogger, increase the temperature and fan speed, turn on the air conditioning and switch off recirculation. Direct the airflow towards both the windscreen and the front side windows. If visibility does not improve quickly, do not drive blindly; wait in a safe place until the windows are clear.
Why car windows fog up
Fogging is actually a thin layer of tiny water droplets. When humid air inside the cabin comes into contact with cold glass, it cools to what is known as the dew point—the temperature at which water vapour begins to turn into liquid water. That is why the problem is most pronounced in autumn and winter, when the windows are cold and the car’s interior is damp.
You do not have to spill water inside the car for moisture to build up. It comes from the breath of the driver and passengers, wet jackets, umbrellas, snow on footwear and soaked textile floor mats. The more people there are in the car and the poorer the cabin ventilation, the faster the windows may fog up.
How to defog the windscreen properly
1. Switch on the defogging or defrosting mode
If the car has a button with a windscreen symbol, use it. Automatic climate control will usually direct the air towards the glass, increase the fan speed and, depending on the vehicle’s design, switch on the air conditioning. With manual controls, direct the airflow towards the windscreen and front side windows.
2. Increase the temperature and fan speed
Warm air heats the cold glass and helps the droplets evaporate. Set the fan to a higher speed so that the humid air next to the glass is replaced more quickly. There is no need to wait for the entire cabin to warm up—the key is to direct enough air straight onto the windows.
3. Use the air conditioning even in winter
Air conditioning is not only for cooling the car in summer. It removes moisture from the air at the evaporator, after which the dried air can be heated and directed onto the glass. Combining heat with air conditioning is therefore one of the most effective ways to clear fogged windows.
In very low outside temperatures, the air-conditioning compressor may not start in some cars. The threshold depends on the specific vehicle, so the owner’s manual takes precedence. However, you can still press the A/C button while defogging, and the control unit will determine whether the conditions allow the system to operate.
4. Switch off recirculation
When recirculation is on, the same air is repeatedly circulated inside the cabin. Moisture from breath, clothing and wet items therefore remains inside and may continue to accumulate. While defogging, switch to fresh air from outside.
Recirculation can be useful for briefly driving through a dusty area, for example, but it should not be used as a long-term setting in a damp car. After using it, restore the outside-air intake.
5. Open the windows only when it makes sense
Briefly opening the windows may help if the outside air is drier than the air inside the car. This allows the humid cabin air to be replaced by drier air more quickly. During rain or dense fog, however, opening the windows may not make much difference, so correctly setting the ventilation and air conditioning remains more important.
The most common defogging mistakes
One of the most common mistakes is leaving recirculation on. Although the fan is running, it continues to circulate humid air inside the car. Similarly ineffective is setting a weak airflow aimed only at the footwell while the windscreen receives too little warm, dry air.
There is no need to turn off the air conditioning for fear of getting cold—you can set the supplied air to a warm temperature. Wiping the glass with your hand or a dirty cloth is another mistake. This leaves greasy streaks that impair visibility and create a surface to which moisture can cling more easily.
If you need to wipe the glass in an emergency, use a clean microfibre cloth. However, this is only a temporary solution. The moisture remains inside the cabin, and the windows may quickly fog up again without ventilation.
When fogging is normal and when to investigate a problem
A one-off case of fogging after people in wet clothing get into the car is usually normal. However, it should gradually disappear once the ventilation is correctly set. If the windows fog up during almost every journey, take a long time to clear or the moisture returns even after the interior has been dried, it is worth checking the entire car.
A clogged cabin air filter
The cabin air filter traps dust and other contaminants from the air entering the interior. When it becomes clogged, it restricts airflow through the ventilation system, so less air reaches the glass and defogging takes longer. Another symptom may be noticeably weak airflow even at a higher fan setting.
The replacement interval is not the same for every vehicle. The car manufacturer’s service schedule, operating conditions and the condition of the filter are what matter. If you do not know when the filter was last replaced, checking it before autumn or winter is a sensible step.
A blocked air intake
The area below the windscreen, where the car draws in outside air, can become clogged with leaves, snow or ice. Debris restricts airflow and may also contribute to blocked drains. When clearing the car, it is therefore not enough to remove snow from the glass—the area around the air intakes should also be kept clear.
Water under the carpets or in the boot
Wet floor mats after rain do not necessarily indicate a fault. However, permanently damp carpeting, water beneath the upholstery, a wet area near a door or moisture in the spare-wheel well are warning signs. A musty smell also deserves attention, as it may accompany water that has been trapped for a long time.
The cause may be damaged or dirty seals, or a problem with the drains. Fogging alone, however, does not confirm a leak. Suspicion becomes stronger when it is accompanied by wet areas and moisture that repeatedly returns.
Air conditioning that does not dry the air
If the airflow is sufficient, the interior is not wet and the air conditioning does not help with defogging under suitable conditions, the system may need to be checked. However, a fault cannot be diagnosed from a single foggy morning. First rule out recirculation, a clogged filter, wet fabrics and low temperatures at which the system may not activate the compressor.
How to prevent windows from fogging up
Regularly remove wet umbrellas, clothing and other items that gradually release moisture into the cabin. Empty water from rubber mats and allow textile floor mats to dry thoroughly. After driving in snow, simply knocking the snow off your shoes before getting into the car can also help.
Clean the inside of the windows with a suitable automotive glass cleaner and a clean cloth. A greasy film caused by dust, fingerprints and unsuitable products reduces transparency and encourages tiny droplets to collect. Avoid improvised greasy coatings, which can produce distracting glare in the light of oncoming vehicles.
Before the cold season, check the cabin air filter, make sure the air intake is unobstructed and inspect the condition of the seals. If the car remains damp even after drying and the windows repeatedly fog up for no apparent reason, additional moisture absorbers will merely mask the problem. It is more important to find where water is entering or have the ventilation and air-conditioning system inspected.
Sources
Marcel Macko
I’m most interested in technology, the internet, bizarre news, and stories that make you stop and think, “There’s no way this can actually be true.” I follow what’s happening both at home and around the world and pick the topics I think are worth paying attention to. On Pitchoviny, I try to present them simply, factually, and without unnecessarily sensational headlines.
