Car windows steaming up on the inside? How to clear them fast and stop it

Car windows steam up on the inside when damp air in the car touches glass that’s colder than the air’s dew point. To clear it fast, turn the air con on, recirculation off, the heat and fan up, and press the windscreen button. If it’s bad every morning, even when nobody’s been in the car, water is getting in or not getting out, and the cause finder below helps you work out where.

This guide follows the HanaHow episode, with UK and US words where they differ: demist and defog, windscreen and windshield, heater matrix and heater core.

Why car windows steam up

That mist is water, and much of it comes in with you. A study of how much water we lose through breathing worked out that at a heart rate of 140, someone breathes out about four times as much as at rest, about 60 to 70 ml an hour, which puts a resting person at roughly 15 to 17 ml an hour, and up to 20 ml when the air they breathe in is cold and dry. That’s about a tablespoon each, every hour. Wet coats, shoes and umbrellas add more, and the RAC says damp car mats and upholstery can mist the windows too.

Warm air can hold more water than cold air. When it touches something colder than its dew point, it can’t hold all of it any more, and the water comes out as tiny droplets on the glass.

In a car at 15°C and 80% humidity, the dew point is about 11.6°C, so any glass colder than that mists up. It doesn’t take much water to get there. A compact car has about 3 cubic metres of space inside, going by the US Environmental Protection Agency’s size classes, and at 15°C and 80% that air holds only about 31 g of water. It’s full at about 38 g, only 7 or 8 g more. Two people breathe out about 30 to 35 ml an hour, so without fresh air coming in they’d fill that gap in about a quarter of an hour.

Why the glass is so cold

The glass is usually much colder than the cabin air. It’s thin, and the cold and the wind outside pull the heat out of it. On a clear night it also loses heat to the sky: the Met Office says “surfaces such as car windscreens and grass tend to cool quicker under clear skies” than road surfaces, and that frost typically forms on nights with clear skies and light winds. That’s why a parked car can mist up overnight, or even frost on the inside.

How to clear it fast

1. Turn the air con on, even in winter. It chills the air past its dew point, the water drips out under the car, and the heater warms that air back up, dry. The AA says air con is “not just for summer”, and Mazda’s handbook says fogging “clears up more easily while the air conditioner is on”. Kia’s says water dripping under the passenger side when the air con runs is normal.
2. Turn recirculation off. Recirculation, the button with a car and a looping arrow, blows your own damp breath back round. Mazda’s handbook says “Do not use the recirculated air position in cold or rainy weather”, because the windows fog up. Cold outside air holds little water, so once the heater warms it, it’s dry.
3. Turn the heat and the fan up, and press the windscreen button. Hyundai’s handbook says to set the temperature and fan to their highest for maximum defrost. In many cars the windscreen button switches the air con and fresh air on for you: Hyundai’s and Mazda’s handbooks both say so. Don’t use the windscreen setting with the temperature on cold, though. Mazda warns it can make the outside of the windscreen fog up.
4. Turn the rear screen heater on for the back window. Kia’s handbook says it clears frost, fog and thin ice, and turns itself off after about 20 minutes. On some cars it heats the door mirrors too.

Don’t wipe it with your hand. The AA says you’ll leave greasy smears, and suggests a lint-free absorbent cloth if you need one. Your sleeve is also not a cloth.

Wait until every window is clear before you set off, and don’t leave the car running on the road while it clears. Highway Code rule 123 says you must not leave a parked vehicle unattended with the engine running.

If the air con won’t run

Mazda’s handbook says the air con “may not function when the outside temperature approaches 0°C”. Then use fresh air and full heat, and open a window a crack for a minute. The RAC says that without clever climate control, having the windows down can help clear the screen faster. On a cold morning the outside air is dry once it’s warmed, so you lose less than it sounds.

Find the cause

If it’s bad every morning, even when nobody’s been in it, water is getting in, or not getting out. Pick when it happens and what you’ve noticed, and the cause finder lists the likeliest causes first. The checks for each are below it.

Wet coats, mats and the dog

Start with what’s in the car. The RAC lists wet clothes, shoes, umbrellas and other things left to dry, and Epic Automotive Detailing adds damp towels from the dog and coffee cups. Take them out overnight, and dry wet mats indoors. The air in a small car holds only about 31 g of water, so anything wet can keep topping it up all night.

Then press a hand into the carpet in each footwell, and look in the boot. In winter, wet shoes can soak a mat on their own, but damp after a dry spell means a leak. The AA’s advice is to dry the area thoroughly first, then see where the water comes back. The AA says a heater or air con fault makes the carpet wet again in normal driving in dry weather, and of those two, a blocked air con drain is the cheaper one to rule out first.

Blocked scuttle drains

The scuttle (US: cowl) is the tray along the base of the windscreen, under the plastic grille by the wipers. It has drains that leaves and dirt can block. The AA says a clogged scuttle can make water “overflow and enter the cabin through the ventilation system”.

It matters for misting because that’s where the car breathes in: Kia’s handbook says the air for the heating and cooling is drawn in through the grilles just ahead of the windscreen, and they shouldn’t be blocked by leaves, snow or ice. In Froy Whernside’s video, his estate car steamed up within minutes even on a dry day, he could hear water sloshing about when he went round corners, and the tray under the cover was “literally a pond”. Poking the first drain hole let the water gush straight out. He’d ignored the sloshing for months, and the standing water had rusted a suspension mount.

Lift the bonnet, take off the plastic cover if you need to, scoop the leaves out and clear the drain holes.

Blocked door and sunroof drains

Doors have small drain holes along the bottom edge, and sunroofs have drain tubes. The AA says that when they block, water “can seep into the car interior, often collecting on the floor or in door panels”. Clear them gently with a thin wire or a flexible tool, as the AA suggests, and check them now and then if you park under trees.

Leaking seals

The rubber seals around the doors, windows and boot can wear out and let water in. The AA says to check them if you find water inside after rain, and that a damaged windscreen seal can let water in during rain or a car wash. To find the leak, the AA suggests running a hose around the doors and the scuttle. Have someone watch inside. Water in the boot can point at the boot seal.

A blocked air con drain

The water the air con takes out of the air leaves through a drain tube, usually under the car near the passenger side. The AA says air con condensation is the most common cause of water leaking from a car, and it normally drips harmlessly underneath. If the drain blocks, it can pool inside instead. On a mild, damp day, after you’ve run the air con for a while, look for clear water dripping under the car. If there’s none and the passenger footwell is wet, the drain may be blocked.

A clogged pollen filter

The pollen filter (US: cabin air filter) cleans the air coming into the car. Kia’s handbook says that as dust builds up in it, airflow from the vents can drop, “resulting in moisture accumulation on the inside of the windshield even when the outside (fresh) air position is selected.” Another Kia handbook adds that an old filter can cause unpleasant smells. Kia’s handbook puts it behind the glovebox. Your handbook says where yours is and how often to change it.

A leaking heater matrix

The heater matrix (US: heater core) works like a small radiator: hot coolant from the engine runs through it, and the fan blows air through it into the car. When it leaks, the signs are:

• a sweet smell, or a sticky, sweet-smelling wet patch in a footwell. The AA says coolant is often green, yellow or pink.
• a greasy or oily film on the inside of the windscreen that smears when you wipe it and keeps coming back. A GM notice to owners, quoted in a US safety complaint, said a cracked heater core could give a strong coolant smell, vapour or mist from the heater vents, “and or an oily film on the windshield”. On its own, a film is more likely the cabin plastics (see clean the inside of the glass, below). It points at the heater matrix when it comes with a sweet smell, mist from the vents or a dropping coolant level.
• a coolant level that keeps dropping, with no leak under the bonnet. Check it only when the engine is cold: Kia’s handbook warns that hot coolant or steam could cause serious injury if you open the cap while it’s hot.

A carpet that gets wet in dry weather on its own can be the heater matrix or the air con drain, so check the drain first.

This one’s for a garage. The AA says a leaking heater core is a more complex job that may need expert attention, and that a full replacement can cost upwards of £1,000 for parts and labour, because of the labour involved. If the screen fogs up as you drive and won’t clear, stop somewhere safe. Coolant is poisonous. The UK Health Security Agency says car antifreeze can be 60 to 100% ethylene glycol, and that there have been many reported cases of children drinking it, often mistaking it for a soft drink. Keep it out of their reach.

Stop it coming back

Clean the inside of the glass

The plastics in a car give off vapour that settles on the glass as a hazy film. The car industry even has a standard test for it, DIN 75201, which defines fogging as “the condensation of evaporated volatile components from the interior of automobiles onto the automobile glass panes, especially on the windscreen”. Intertek, which runs these tests, describes it as a “light scattering film” on the glass, and the AA says this off-gassing can make the inside of the windscreen greasy. Fingerprints add to it. A dirty screen mists more easily, and the RAC says keeping it clean “will go a long way to stopping it misting up in the first place”.

Use glass cleaner and a clean, lint-free cloth, and buff it dry. Don’t use anything sharp or abrasive on the rear screen: Genesis’s handbook warns it can damage the rear screen heater.

Dry it out

Take wet coats and mats out overnight, and dry damp carpets thoroughly. A silica gel bag on the dashboard helps a car with no leak. In Hagerty’s test, most of the small pouches soaked up 22 to 24 g in half an hour in a steamer, about three quarters of what the air in a small car holds, and a bag of just over a kilo took 84 g in 20 minutes. A sock full of cat litter took 15 g, mostly in the sock, which Hagerty called “a very ineffective way to remove moisture from the air”. A bag won’t keep up with a wet carpet or a leak, so fix those first.

Anti-fog sprays and shaving foam

Anti-fog sprays and shaving foam leave a thin layer that makes condensation spread into a clear film instead of droplets, so you can see through it. Boston’s Museum of Science explains that shaving cream’s surfactants stop water forming large droplets. They hide the mist rather than remove the water, and they need putting on again. In ChrisFix’s test on a steamed-up window, shaving cream worked as well as Rain-X Anti-Fog, while a cut potato and baby shampoo left a residue and hand sanitiser didn’t work. That’s an informal test: we couldn’t find an independent one.

What the law says

UK

Highway Code rule 229 says that before you set off, “you MUST be able to see”, and to “make sure the mirrors are clear and the windows are demisted thoroughly”. The capital MUST marks the legal part; demisting thoroughly is the advice that goes with it. The law behind it is regulation 30 of the Road Vehicles (Construction and Use) Regulations 1986, which says the glass must be kept so it “does not obscure the vision of the driver”, and the Highway Code’s maintenance annex adds that “windscreens and windows MUST be kept clean and free from obstructions to vision”.

There’s no single offence for misted windows, and the penalty depends on which one the police use. In England, Scotland and Wales:

• Glass that obscures your view (regulation 30): a £50 fixed penalty and no points.
• Driving where you can’t have a full view of the road ahead (regulation 104): £100 and three points.
• Using a vehicle in a dangerous condition: £100 and three points.

So driving when you can’t see out properly can mean a £100 fine and three points, though not every stop does. If it goes to court, the maximum fine in England and Wales is £1,000 for the first two, or £2,500 for the dangerous condition offence in a car, and for the two offences with points the court can also ban you from driving. Northern Ireland has its own Highway Code, which we haven’t covered here.

US

Every new car, truck and bus sold in the US must have a windshield defroster and defogger, under Federal Motor Vehicle Safety Standard 103: “Each vehicle shall have a windshield defrosting and defogging system.” Rules for drivers are set by each state and vary. Minnesota’s, for example, says: “No person shall drive any motor vehicle with the windshield or front side windows covered with steam or frost to such an extent as to prevent proper vision.” Check your own state’s vehicle code.

Sources

• Dew point: the Magnus formula with Alduchov and Eskridge’s coefficients, Improved Magnus form approximation of saturation vapor pressure, 1996. Car size: US EPA size classes on fueleconomy.gov (compact: 100 to 109 cubic feet).
• Breath: Zieliński and Przybylski, How much water is lost during breathing?, Pneumonologia i Alergologia Polska, 2012, and Comprehensive analysis of heat and water exchanges in the human lungs, Frontiers in Physiology, 2021.
• Met Office: How does frost form?
• The AA: How to defrost a car windscreen, Why is my car leaking water?, Why is my car heater not working?, How to clean a windscreen and Ask Edmund: water in the driver’s foot well. RAC: How to demist your windscreen in double-quick time.
• Owner’s handbooks: Mazda CX-3 operating tips (recirculation, air con near 0°C), MX-5 defrosting and defogging and Mazda3 climate control; Hyundai windshield defrosting and defogging and manual heating and air conditioning; Kia heating and air conditioning, rear window defroster, engine coolant and climate control air filter (2024 handbook, 2026 handbook); Genesis rear window defroster (mirror heaters).
• Heater core: NHTSA complaints for the 2008 Saturn VUE (complaint 10426571 quotes GM’s programme 09206A). UK Health Security Agency: Ethylene glycol: general information.
• Inside film and anti-fog: DIN 75201, Intertek on fogging tests, durable antifog coatings, Molecules, 2024, and Museum of Science, Boston, de-fog your bathroom mirror. Hagerty UK: cheap dehumidifiers reviewed, 18 November 2022.
• UK law: the Highway Code, rules 226 to 237 (rule 229) and rules 103 to 158 (rule 123) and annex 6; Road Vehicles (Construction and Use) Regulations 1986 regulation 30 and regulation 104; Road Traffic Act 1988 section 40A and section 41D and section 42; Sentencing Act 2020 section 122 (fine levels); the Fixed Penalty Order 2000, Road Traffic Offenders Act 1988 schedule 2 and GOV.UK endorsement codes.
• US law: FMVSS 103, 49 CFR 571.103 and Minnesota Statutes 169.71.
• Videos: ChrisFix, How to Stop Car Windows from Steaming Up. Scotty Kilmer, Never Get Foggy Car Windows Again. Epic Automotive Detailing, How to Stop Car Windows Fogging up FOREVER! Froy Whernside, Car Windows Steaming Up? Windscreen Drain Holes Blocked.