Fogged windows: why it happens and fixes

Fogged car windows occur when warm, moist air inside your car comes into contact with the cooler surface of the glass, causing the moisture to condense into tiny water droplets. This common issue, especially during cooler or humid months, can be resolved with simple fixes and preventative measures. 

Why Car Windows Fog Up

Condensation forms on the inside of the windows when there is a significant temperature difference between the interior air and the cold glass. Factors that increase the moisture (humidity) inside the cabin include: 

• Breathing and Body Heat: Passengers’ breath naturally adds moisture to the air.
• Wet Items: Damp clothes, umbrellas, floor mats, or gym bags left in the car introduce excess moisture.
• Weather: Rain, snow, and general high humidity outside bring damp air into the vehicle.
• Dirty Windows: Dirt, oils, and grime on the glass provide more surface area for water vapour to cling to.
• Recirculation Mode: Using the air recirculation feature traps the existing moist air inside the cabin, compounding the problem.
• Leaks or Clogged Filters: Persistent fogging could indicate a deeper issue like a water leak through a door seal or a clogged cabin air filter reducing airflow. 

Fixes and Preventative Measures

Immediate action involves using your car’s built-in systems to dry the air and warm the glass. Long-term strategies focus on keeping the interior dry and clean. 

• Use the Air Conditioning (AC): The AC system is designed to remove moisture from the air (dehumidify it). Turn it on, even in winter, with the heater set to a comfortable temperature. The combination of warm, dry air is highly effective at clearing fog quickly.
• Activate Demister/Defroster Functions: Engage your car’s dedicated front demister and rear window defroster. These systems direct warm, dry air onto the glass and use electric heating elements, respectively, to evaporate moisture.
• Switch to Fresh Air Intake: Turn off the recirculation mode. Drawing in fresh outside air helps balance the humidity levels, whereas recirculation keeps moisture trapped inside.
• Crack a Window: If it’s safe to do so, opening a window slightly allows the moist inside air to escape and drier outside air to enter the cabin, speeding up the process. 

Preventative Measures

• Keep Windows Clean: Regularly clean the inside of your car windows with a dedicated glass cleaner. Clean glass provides less surface area for condensation to form.
• Remove Damp Items: Take wet clothes, umbrellas, and other sources of moisture out of the car when not in use.
• Use Moisture Absorbers: Place moisture-absorbing products like silica gel packets or specific car dehumidifier bags on your dashboard or under seats to passively reduce cabin humidity.
• Apply Anti-Fog Treatments: Commercial anti-fog sprays or treatments can be applied to the interior of clean windows to create a film that resists condensation.
• Check for Leaks: If fogging is a persistent issue, inspect floor mats and the boot for dampness, which may indicate a leak. Have the issue checked by a professional mechanic if necessary. 

What fogging actually is

Fogging is condensation, and it follows a simple rule: humid air touching a cold surface can drop water on that surface. In a car, the glass is often the coldest surface, so it becomes the collection point.

Inside fog and outside fog are different problems

Condensation can form on the inside or the outside, and the fix depends on which side is wet. A quick test works every time: lightly touch a corner of the glass. If it feels wet on the inside, you need to dry the cabin air. If the outside is wet, you need to manage outside humidity and surface temperature with airflow and wipers.

The physics is the dew point relationship. Fog forms when the glass surface temperature drops below the dew point temperature of the surrounding air. If the cabin air holds a lot of moisture, the dew point rises, so condensation happens at higher glass temperatures. 

Cars swing across those thresholds quickly. You can leave a warm house, step into a cold car, add body heat and breath moisture, then warm the cabin with the heater. Glass stays cold longer than air, so it becomes the first surface that hits the dew point condition.

The cabin produces moisture faster than most drivers realise

Every occupant adds water vapour with each breath. Wet shoes, rain jackets, umbrellas, and snow on floor mats add more. Even a small amount of meltwater hidden in carpet under mats can evaporate into the cabin air during a drive, raising humidity without any obvious puddle.

Short winter trips can amplify this. The heater warms air quickly, yet the glass warms slower. The cabin reaches a warm, humid state long before the windscreen reaches a temperature that resists condensation, so mist forms early and hangs around.

Fogging also spikes after a car wash or heavy rain. Water trapped in door seals, scuttle areas, and ventilation intakes can raise cabin humidity. If the cabin filter is damp or clogged, airflow drops and drying slows, which keeps condensation conditions in place longer.

Why winter makes it worse

Winter creates the perfect setup: cold glass, high moisture load, and repeated entry of wet gear. It is not one factor. It is a stack.

Cold glass and warm air create the perfect condensation gradient

Glass loses heat quickly overnight. In the morning, the windscreen is often close to outside ambient temperature. Once the engine warms, the heater pushes warm air into the cabin. Warm air holds more water vapour, so humidity can rise without the cabin feeling damp. When that warm, moisture rich air hits cold glass, condensation forms.

This is also why fog can get worse right after you turn the heater up. Heating alone increases the air’s capacity to carry moisture; it does not remove moisture. If the moisture source stays, the dew point can climb and the windscreen becomes a better target for condensation.

A second driver factor is ventilation discipline. Many drivers close vents, reduce fan speed, and use recirculation to “keep heat in.” That choice traps moisture, keeps the dew point high, and turns the cabin into a small greenhouse with cold glass walls.

Recirculation keeps moisture trapped, air conditioning removes it

Recirculation is useful for smoke, dust, and traffic fumes. It is bad for clearing mist. Recirculation keeps the same humid cabin air moving around the glass, so the humidity stays high. Guidance from multiple automotive sources recommends switching recirculation off during demisting so the system pulls in drier outside air instead of recycling humid air. 

Air conditioning matters even in winter. The air conditioning system dehumidifies air by cooling it across the evaporator, which condenses moisture out of the airflow, then the heater can warm that now drier air before it hits the glass. That combination clears mist faster than heat alone, which is why motoring organisations and manufacturers commonly advise running air conditioning during demisting. 

This is also why a car with a weak air conditioning system often struggles to demist. If the system cannot pull moisture out of the air efficiently, the cabin stays humid and the windscreen keeps fogging, even with the fan on high.

Fix it fast in the moment

Clearing fog quickly means changing two variables at once: lower cabin humidity and raise glass temperature. The fastest settings do both.

The quickest setting sequence for inside fog

Start by directing airflow to the windscreen and front side windows. Those are the surfaces you need clear first, and the side glass is often the first to fog again. Use high fan speed. Clearing fog is an airflow job, not a gentle comfort setting job.

Turn air conditioning on. Even if the air feels cold at first, the system is removing moisture. Then set temperature high. Warm, dry air evaporates condensation faster than warm, humid air. Manufacturer guidance and motoring organisations recommend using the defog or defrost mode, turning off recirculation, using high fan speed, and running air conditioning for best results. 

Finish by turning recirculation off. This brings in outside air that is often drier than the cabin air created by wet clothing and breath. If outside air is very humid, the air conditioning step becomes even more valuable, since it still removes moisture from the incoming air stream.

A reliable quick sequence

• Windscreen defog or defrost mode on 

• Recirculation off 

• Air conditioning on 

• Fan speed high 

• Temperature high until glass clears 

If the outside is fogging, treat it like rain, not demisting

Outside fog is a different mechanism. Warm, moist outside air meets a colder windscreen, so moisture condenses on the exterior surface. The cabin settings still help a little by warming the glass, yet the primary tools are wipers and airflow across the exterior surface.

Use wipers and washer fluid to remove the film and keep it from forming a continuous layer. A clean windscreen makes a real difference here. Dirt, oily residue, and old washer fluid leave a thin film that water clings to, which turns light mist into a smeared sheet.

If outside fogging persists during humid conditions, keep some warm air on the screen. The goal is to keep the windscreen slightly warmer than outside air at the surface level. That reduces the temperature difference that drives exterior condensation.

Prevent it from coming back

Fast demisting is useful. Prevention is where you win time every morning.

Reduce moisture sources and keep glass clean

Start with the obvious moisture load. Remove wet items from the cabin. Shake out floor mats. If carpets stay damp, dry them. A small amount of hidden water can keep cabin humidity high for days, especially in winter when the car is parked cold overnight.

Clean the inside of the windscreen with a proper glass cleaner and a lint free cloth. Interior glass builds a film from plastics off gassing, skin oils, and vaping or smoking residue. That film gives condensation a surface to cling to, which makes fogging happen faster and clear slower. A clean surface fogs less and clears faster under the same HVAC settings.

Use air conditioning regularly in winter, even for short periods. This keeps seals lubricated and keeps the system functional, and it also dries the cabin during the drive. Many demisting guides recommend air conditioning during winter demisting, which aligns with how moisture removal works in vehicle HVAC systems. 

If fogging is constant, suspect a fault, not the weather

Some cars fog constantly even with correct settings. That pattern often points to a moisture source or HVAC issue. Water ingress from a leaking door seal, blocked scuttle drain, or sunroof drain can keep carpets damp. That turns the cabin into a permanent humidity source.

A sweet smell, greasy film on the inside of the windscreen, or unexplained coolant loss can point to a heater core leak. That leak introduces moisture into the cabin airflow. It also leaves residue that makes fogging worse. If you see that pattern, demisting becomes a temporary fix, not a solution.

Air conditioning faults also show up as poor demisting. If the system does not run, does not cool, or cycles oddly, it cannot pull moisture out of the airflow efficiently. Several demisting guides flag air conditioning problems as a common reason a car will not demist properly. 

Safety, rules, and the fixes to avoid

Fogged glass is a visibility hazard. That makes it a road safety issue, and there is clear guidance on the driver responsibility side.

Clear visibility is a driving requirement, not a preference

UK guidance for adverse weather explicitly calls out the use of windscreen wipers and demisters as part of safe driving in poor visibility conditions.  The practical takeaway is simple. If you cannot see clearly, you should not be moving at speed, and you should not be relying on a small cleared patch in front of the steering wheel.

Rear visibility matters too. Rear window defoggers became common for a reason. NHTSA notes that rear defoggers can help with backing and lane changes in rain or winter conditions.  Use them early, not once the rear glass becomes opaque.

Build a habit. As soon as you start the car in winter, turn on front and rear demist functions. The system works better when it gets ahead of condensation rather than chasing it.

Avoid water on hot glass and other risky shortcuts

Pouring water on a cold or frosted windscreen is a bad idea. Rapid temperature change risks cracking glass, and the water can freeze and create a worse problem. Motoring guidance advises using heaters and air conditioning for defrosting and avoiding water methods that can freeze or cause damage. 

Wiping the inside of the windscreen with a sleeve or paper towel often smears oils and leaves lint, which creates a film that fogging clings to next time. If you must wipe, use a clean microfibre cloth reserved for glass.

Leaving recirculation on during demisting is another common mistake. It traps humidity in the cabin and slows clearing. Manufacturer and motoring guidance repeatedly points drivers toward recirculation off for faster clearing. 

The reliable fix is not clever. It is airflow to the glass, air conditioning on, temperature high, recirculation off, fan speed high. Once you understand the moisture and temperature relationship, the settings stop feeling like guesswork and start feeling like control.