Why do the windows on a car steam up on the inside, and should it worry you if you are about to buy one?

Quick answer: A bit of misting on a cold, damp morning is normal. It happens when warm, moist air inside the car meets cold glass. The quickest fix is warm air on the windscreen with the air-con switched on. The law says you must be able to see properly before you set off. But a car that mists up all the time, smells musty or has damp carpets may have a leak, blocked drains or a heater problem. On a used car, that is worth checking before you buy.

The clocks go back this Sunday, 25 October, so the evenings get dark an hour earlier and more of us will be driving home in the dark. Cold, damp evenings are when a misted screen catches people out.

At anycolourcar.com we sell quality used vehicles from The Old Garage on Genn Lane in Worsbrough, and our own technicians prepare every car and van at our Stairfoot service centre. This guide is what we would want any buyer around Barnsley, Sheffield, Wakefield and Rotherham to know, whoever they buy from.

Looking for your next car or van? Start on car search or call sales on 01226 574410.

Why do car windows steam up on the inside?

It is condensation. The AA explains that windscreen fog is caused by the difference in temperature between the inside and outside of the car, which makes moisture form on the inside of the glass. A warm, humid cabin and a cold screen are the classic combination.

The RAC says the moisture comes from your body heat and your breath, and that it builds up more when wet clothes, shoes, umbrellas, damp mats or upholstery are left in the car. It also lists leaks that let rainwater collect inside, and poor air conditioning and circulation.

TÜV NORD, the vehicle testing organisation, adds that cold air holds less moisture than warm air, which is why misting gets worse in autumn and overnight.

Is it illegal to drive with misted windows?

You must be able to see before you drive. Rule 229 of the Highway Code says that before you set off you MUST be able to see, and that you should make sure the mirrors are clear and the windows are demisted thoroughly.

The rule is backed by regulation 30 of the Road Vehicles (Construction and Use) Regulations 1986, which says all glass on a vehicle must be kept in a condition that does not obscure the driver's vision while it is being driven on a road.

So clearing a small patch and setting off is not enough. Wait until the glass is clear.

How do you demist a car windscreen quickly?

The AA's windscreen defrosting guide gives a simple routine:

Start the engine and turn the warm air blower onto the windscreen.
Switch on the heated rear window and heated mirrors, if the car has them.
Turn on the air-con. The AA points out it is not just for summer, because it removes moisture from the air.
If you can, let some outside air into the car as well.
Stay with the car while the engine is running, and do not drive off until all the glass is clear.

TÜV NORD also advises switching the fan from recirculation to fresh air, because recirculation only moves the moisture that is already in the cabin around the car.

The AA says not to wipe misted windows with your hand, as it leaves greasy smears and a ring could scratch the glass. Use a clean, lint-free cloth if you need to.

If the car has no air-con, the RAC says opening the windows can help, as the drier outside air reduces the water vapour inside. If the screen mists up while you are driving and you cannot see well, pull over where it is safe and wait for it to clear.

How do you stop a car steaming up in the first place?

A few habits make a big difference, based on advice from the RAC and TÜV NORD:

Take wet things out. Coats, umbrellas, wellies and wet dog towels left in the car overnight all add moisture.
Dry the mats. Damp footwell mats keep putting water back into the air.
Keep the inside of the glass clean. The AA says a hazy film builds up on the inside of the windscreen even if you do not smoke, and the RAC adds that dirt and grime can hold moisture.
Use the air-con all year. The RAC says regular use keeps the system in good working order and helps stop it smelling.
Look after the cabin filter. TÜV NORD says a dirty cabin air filter stops air circulating properly and leads to damp inside the car.
Clear the drains. TÜV NORD recommends checking the drains under the bonnet, and in the doors and sunroof, for leaves and dirt. If they block, water can get inside.

When is misting a sign of a problem?

If a car steams up badly every time you use it, even after the steps above, something is letting water in or keeping it there.

The RAC says that if you have removed wet items and are still having issues, there could be a leak, and in many cases you will not be able to fix it yourself. TÜV NORD names blocked drains and worn or porous door, window and sunroof seals as common causes, and warns that damp can lead to a musty smell and, in extreme cases, mould or rust.

There is one more cause worth knowing about. Haynes explains that the heater matrix, which uses engine coolant to warm the cabin, sits deep behind the dashboard and occasionally leaks. If a car is losing coolant and also has condensation on the inside of the windows, Haynes says the matrix could be at fault. Check the carpet under the dashboard for moisture and use your nose: a leaking matrix will usually make the interior smell of coolant, and the heater may not work as well.

A musty smell from the vents is a different thing. The RAC says it is usually caused by bacteria and mould in the ventilation system, often when the air-con has not been used regularly, and that a cleaning treatment and a new cabin filter may help.

What to check for damp on a used car viewing

A damp day is a good day to view, because a car that has been standing in the rain is more likely to show a leak. Work through these on the viewing or the test drive:

Get in and close the doors. Watch whether the glass mists up quickly with the engine off.
Run the demist routine. With the engine on, the blower on the screen and the air-con on, the screen should start to clear. If the air-con light does not come on, or the screen stays fogged, ask why.
Feel the carpets. Press down in all four footwells, under the mats, and in the passenger footwell under the dashboard. Wet or spongy carpet needs explaining.
Use your nose. A musty smell can mean water has been getting in for a while. A smell of coolant points towards the heater.
Look in the boot. Lift the boot floor and check for water around the spare wheel well. Our guide to spare wheels, repair kits and run-flats covers what else to look for under there.
Check the seals and headlining. Look at the rubber seals around the doors and sunroof for splits, and at the roof lining for water marks.
Check the heated rear screen. Switch it on and make sure it starts to clear the glass.

If you find damp, ask the seller what has caused it and what has been done. Get any promise to fix it in writing before you pay. Our guide to buying a used car in winter covers the heater, lights and battery checks that go with this.

What we do before a vehicle leaves us

Every car and van at anycolourcar.com is prepared by our own technicians at our Stairfoot service centre in Barnsley.

If you are viewing a car with us on a damp day and want to see the screen clear, just ask. We are happy to start it up and run the demister with you.

Every vehicle we sell comes with a brand-new 12-month MOT, booked and paid for by us at a local DVSA-approved testing station, plus 6 months' warranty and 12 months' AA breakdown cover on every retail sale.

We do not pay sales commission, so you get straight answers and no pressure. Part exchange is welcome. If you cannot get here in daylight, we can send a walk-around video.

A little misting on a cold morning is normal. A car that never clears, smells musty or has wet carpets is telling you something. Check before you buy.

FAQ

Why does my car steam up when it rains?

Rain adds moisture to the air inside the car from wet clothes, umbrellas and shoes, and the glass is cold. When warm, humid air meets the cold screen, it condenses.

Should I use hot or cold air to demist my windscreen?

The AA recommends the warm air blower on the windscreen together with the air-con, plus some outside air if possible. The air-con removes moisture while the warm air clears the glass.

Does the air-con help in winter?

Yes. Both the AA and the RAC say air-con removes moisture from the air in the car, which helps demist the windows. The RAC also says regular use keeps the system in good working order.

Is it illegal to drive with a misted windscreen?

The Highway Code says you must be able to see before you set off, and the law says the glass must not obscure your view while you drive. Wait until all the windows are clear.

Why does my car steam up even when it is parked?

Moisture already in the car condenses on the glass as it cools overnight. If it happens a lot, look for wet mats, a blocked drain or a worn seal letting water in.

In short

Misting is just condensation, and on a cold Barnsley morning most cars do it. Warm air on the screen, the air-con on and a bit of fresh air will usually clear it. You must wait until you can see properly before you drive. A car that steams up all the time, smells musty or has wet carpets is a different matter, so check for damp before you buy.

For the rest of the viewing, use our used car buying checklist for Barnsley and our used vehicle test drive checklist.

Ready to look? Browse car search or call sales on 01226 574410.

Hero image: photo of a misted car windscreen at night by AhmadElq, CC BY-SA 4.0, cropped and placed beside our own graphic.

anycolourcar.com · The Old Garage, Genn Lane, Worsbrough, Barnsley S70 6TF · Sales 01226 574410

Why does my car steam up inside? Condensation checks on a used car in Barnsley