A windscreen that mists up on a cold, damp morning is a familiar problem for drivers in Burgess Hill and across Mid Sussex once October arrives. Knowing how to demist a windscreen properly makes the difference between clearing the glass quickly and sitting on the drive wiping it with your sleeve. This guide explains why screens mist up, the fastest way to clear one, why the air conditioning does the job better than the heater alone, and when a screen that keeps misting points to a fault that needs looking at.
Why does a windscreen mist up?
A windscreen mists up when warm, moist air inside the car touches glass that is colder than it. The air cools at the glass, can no longer hold all of its water vapour, and the moisture settles as tiny droplets. That film of droplets is condensation, and it scatters light so you cannot see through it.
Autumn is when this starts in earnest. The glass is cold first thing, the air outside is damp, and everything you bring into the car adds water: your breath, wet coats and umbrellas, damp shoes and the rain that drips off them onto the carpets. Every passenger makes it worse, which is why a car full of people mists up far faster than one with a single driver.
Is the mist on the inside or the outside of the glass?
Most misting is on the inside, but on some cold, humid mornings it forms on the outside instead. The quick test is to wipe a small patch with your finger. If the mist comes away, it was on the inside. If it does not, it is on the outside, and the fix is different.
| Mist on the inside | Mist on the outside | |
|---|---|---|
| When it happens | Cold glass, moist air in the cabin | Glass colder than the damp air outside, often after the heater has warmed the cabin and the air outside is near freezing |
| Quick test | Wipes away with a finger from inside | Stays when wiped from inside |
| How to clear it | Air con on, fresh air, fan to the screen | Use the wipers, and turn the screen vents down or cooler so the glass is not warmed unevenly |
| How to prevent it | Keep the cabin dry and the inside of the glass clean | Usually clears once the screen and outside air temperatures even out |
What is the fastest way to demist a windscreen?
The fastest way to demist a windscreen is to switch the air conditioning on, select fresh air rather than recirculation, and direct the fan at the screen on its highest speed. Raise the temperature gradually rather than going straight to full heat, and switch on the heated rear screen.
Step by step:
- Start the engine and switch the air con on. It dries the air before it reaches the glass, which is what clears the mist.
- Turn recirculation off. Recirculating the cabin air keeps all the moisture inside the car. Cold outside air holds less water than the warm air you have been breathing in a closed cabin.
- Set the vents to the windscreen and the fan to full. The more dry air moves across the glass, the faster the droplets evaporate.
- Bring the temperature up gradually. Starting cool and warming up avoids blowing the first damp, lukewarm air from the vents onto cold glass.
- Switch on the heated rear screen and the heated windscreen if your car has one.
- On a car with climate control, press the front screen demist button. It usually turns on the air con, fresh air and full fan for you.
- If the car has no air con, open a window or two slightly to let damp air escape while the heater warms up.
Wiping the inside of the glass with a cloth clears a patch, but it leaves smears that catch the light from oncoming headlights, and the mist comes straight back because the air is still wet.
Why does air conditioning clear a misted windscreen fastest?
Air conditioning clears a misted windscreen fastest because it takes water out of the air. Air passing over the cold evaporator drops its moisture, which drains away under the car, so the air reaching the screen is dry and absorbs the mist from the glass instead of adding to it.
This is why the air con is worth running in autumn and winter, not just on hot days. The heater on its own warms the air but leaves the moisture in it. Dry air from the air con, then warmed by the heater, is the combination that clears glass quickest. The system is driven by the compressor, and our guide to how a car air conditioning compressor works explains how it moves refrigerant round to cool the evaporator.
Two things follow from this. First, if your air con has stopped working, the screen will take noticeably longer to clear, even with the heater on full. Second, running the system through the colder months keeps the seals and compressor lubricated, which helps it keep working when you need it in summer.
Why does my windscreen keep misting up?
A windscreen that keeps misting up, or will not clear with the air con on, usually means there is too much water in the car or not enough dry air reaching the glass. The common causes are water leaking in, a heater matrix leak, a blocked cabin filter, air con that is not working, or a stuck recirculation flap.
Water getting into the car
Damp carpets keep the cabin air wet no matter how long the fan runs. Check under the mats in the footwells and the boot. Water usually gets in through worn door or windscreen seals, blocked sunroof drain tubes, or blocked drains in the scuttle panel below the windscreen, where leaves collect every autumn. Finding and stopping the leak is the only lasting fix.
A leaking heater matrix
The heater matrix is a small radiator behind the dashboard that warms the air using hot engine coolant. When it leaks, coolant vapour comes out of the vents. The signs are a sweet smell in the car, a greasy film on the inside of the windscreen that smears when you wipe it, a damp passenger footwell, and a coolant level that keeps dropping. A heater matrix leak needs checking promptly, because losing coolant can lead to the engine overheating.
A blocked pollen or cabin filter
The cabin filter cleans the air coming into the car through the vents. When it is clogged with dust, pollen and leaves, airflow to the windscreen drops and the screen takes much longer to clear. Weak airflow from the vents on full fan is the giveaway. Our article on the cabin air filter covers what it does and when it needs changing.
Air conditioning that is not working
If the air con is switched on but not cooling, it is not drying the air either, so demisting falls back on the heater alone. Low refrigerant, a compressor fault or an electrical fault can all stop it working. If the screen clears slowly and the air con does not blow cold on a mild day, that is a strong hint.
A recirculation flap stuck closed
A flap behind the dashboard switches between fresh air and recirculated air. If it sticks on recirculation, the car keeps reusing its own moist air and the windows mist up even with the setting on fresh air. You may notice the button makes no difference to the sound of the airflow.
Dirty glass on the inside
A film of grime on the inside of the windscreen gives condensation something to cling to, so a dirty screen mists faster and clears slower. Cleaning the inside of the glass with a proper glass cleaner and a dry microfibre cloth makes a visible difference.
How do you keep your windscreen clear through autumn and winter?
You keep a windscreen clear through autumn and winter by keeping the cabin dry, the inside of the glass clean, the screenwash topped up with a winter-strength mix, and the wiper blades in good condition. Running the air con regularly and changing the cabin filter at service time help too.
- Keep water out of the cabin. Shake off umbrellas and coats before getting in, and dry out wet floor mats rather than leaving them in the car.
- Clean the inside of the glass regularly through the colder months.
- Use screenwash, not plain water. A winter-strength screenwash mix resists freezing and cuts through the road film that builds up on wet roads.
- Check the wiper blades. Blades that smear, judder or leave streaks will not clear rain or mist from the outside of the glass.
- Never pour hot water on a frozen windscreen. The sudden temperature change can crack the glass. Use the heater, an ice scraper and de-icer instead.
Our winter car care checklist for Burgess Hill drivers covers wiper blades, screenwash, tyres, the battery and coolant in more detail.
Is it illegal to drive with a misted windscreen?
Yes, driving when you cannot see properly out of the car is against the law. Rule 229 of the Highway Code says you must be able to see, so you must clear snow and ice from all your windows, and it advises making sure the windows are demisted thoroughly before you set off.
Visibility is also part of the MOT. The DVSA’s MOT inspection manual has a whole section on it, which covers the driver’s view through the windscreen, the condition of the glass, the wipers and the washers. Wipers that do not clear the screen and washers that do not supply enough fluid are both checked. Keeping them in good order helps your car pass, and more importantly keeps the screen clear in bad weather.
Frequently asked questions
Why does my car mist up when it rains?
A car mists up more in the rain because the air outside is already close to saturated, and you bring extra water in on coats, umbrellas and shoes. With more moisture in the cabin and cold, wet glass, condensation forms quickly. Switching the air con on with fresh air and the fan on the windscreen clears it fastest. If the car mists up every time it rains even when you are dry, check the carpets for water getting in.
Should I use hot or cold air to demist my windscreen?
Dry air matters more than hot air. The quickest method is the air con on to dry the air, with the temperature raised gradually so the glass warms as the moisture is removed. Full heat straight away with the air con off can make the mist worse at first, because the first air from the vents is lukewarm and damp. Once the screen has cleared, keep some warm, dry air on the glass to stop it misting again.
Why does my windscreen mist up when the air con is on?
If the inside of the screen mists with the air con on, the system is probably not working properly, the cabin filter may be blocked, or the car may be set to recirculation. If the mist is on the outside, cold air from the air con has chilled the glass below the temperature of the damp air outside. Use the wipers and turn the temperature up slightly, and the outside mist will clear.
Why does my car smell sweet and the windows mist up?
A sweet smell inside the car together with a greasy mist on the windscreen is a classic sign of a leaking heater matrix. Coolant escaping from the matrix behind the dashboard turns to vapour, comes out of the vents and settles on the glass. You may also find a damp passenger footwell and a coolant level that keeps dropping. Have it checked soon, because a coolant leak can lead to the engine overheating.
Windscreen that won’t clear? Get it checked in Burgess Hill
If your windscreen keeps misting up, the air con is not blowing cold, or there is a sweet smell and a greasy film on the glass, we can check the air conditioning and track down the cause. We look after drivers in Burgess Hill and across Mid Sussex. Get in touch to book your car in.