You get condensation in a car because you add water vapor to a sealed box all night and the glass is the coldest surface in that box. The fix is cross-flow: two openings, on opposite sides of the car and at opposite ends, so air has a path through rather than a single hole to sit in front of. One cracked window does almost nothing. Two do most of the work.
The mechanism, briefly
The National Weather Service defines the dew point as the temperature air needs to be cooled to in order to reach 100 percent relative humidity, and notes that if the air cools past that point, water vapor has to come out in liquid form. That is the entire condensation story in a car.
Two things happen while you sleep. You warm the air inside, which lets it hold more water. You also add water to it, through breathing and through your skin, continuously, for seven or eight hours. Meanwhile the glass sits at close to outside temperature because it is thin, uninsulated, and exposed on both sides. So you have humid air meeting a cold surface, and the water leaves the air at exactly that surface. Metal roof panels and the inside of the doors do the same thing more quietly, which is why the headliner feels damp even when you wiped the windows.
We have not found an official published figure for how much water a sleeping adult gives off per night, so we are not printing one. It is enough that people who sleep in cars wake up to running glass most nights, and enough that sealing the car makes it worse rather than better.
Cross-flow, not a crack
Air needs somewhere to come in and somewhere to go out. A single window open two inches gives you a small amount of exchange driven by whatever breeze happens to hit it. Two windows open half an inch each, on opposite sides, give you a route.
The practical setup:
- Open two windows on opposite sides of the car, ideally diagonal from each other (front left and rear right).
- Half an inch to an inch each is usually plenty. More is not better once air is moving.
- In wind, open the downwind side more than the upwind side, so the car is not pressurized and whistling all night.
- If you have a moonroof or a rear vent, use it as one of the two openings. High and low is a better pair than side and side, because warm wet air rises.
Crack the windows before you go to bed, not after the glass has already fogged. Once there is water sitting on the surfaces inside, it takes hours of airflow to get it back out.
Rain guards are what make this work in weather
The reason people sleep with the windows sealed is rain. Adhesive or in-channel rain guards (also sold as vent visors or window deflectors) sit above the glass and let you hold a window open an inch in steady rain without water running down the inside of the door. They are the single cheapest upgrade to sleeping in a car, and they are what turns “crack the windows” from advice into a habit you can keep in October.
Bug screens matter in the same way in summer. Fine mesh sleeves that pull over the whole door, or magnetic panels on a steel door frame, keep the window usable when the alternative is closing it. Test a magnet on your own door frame first: many modern cars have plastic trim exactly where you would want the magnet to stick.
Carbon monoxide: the one hard line
Do not run the engine to stay warm in a closed car, and do not run any combustion heater or stove inside one.
OSHA describes carbon monoxide as a poisonous, colorless, odorless and tasteless gas produced by the incomplete burning of carbon-containing fuels, and lists the internal combustion engine as one of the most common sources of exposure. It is harmful because it displaces oxygen in the blood and deprives the heart, brain, and other organs of oxygen, and OSHA states that large amounts can overcome you in minutes without warning, causing you to lose consciousness. The early symptoms it lists (headache, fatigue, dizziness, drowsiness, nausea) are also what being tired and cold in a car feels like, which is the problem.
CPSC’s guidance on engine-driven equipment makes the ventilation point directly: never use a generator inside a home or garage, even if doors and windows are open. Opening a window is not a mitigation for combustion exhaust in an enclosed space. That logic applies to a running vehicle in a garage, and it applies to a propane heater or stove in a car with the windows up.
There is one narrow exception, and it is an emergency one. The National Weather Service tells drivers stranded in winter weather to run the motor about ten minutes each hour for heat, to open a window for fresh air, and to make sure the exhaust pipe is not blocked. That is survival guidance for being stuck, with a clear tailpipe and an open window, not a way to heat a car you chose to sleep in. The intended answer to a cold night is a warmer sleeping bag, not an engine.
If you carry any fuel-burning appliance at all, a battery carbon monoxide alarm placed at head height is cheap and it does not care whether you were paying attention.
What does not work
Wiping the glass. It moves water from the window to your towel and back into the air. You will do it three times a night.
Running the defroster before bed. It clears the glass and warms the air, which then holds more moisture until the car cools and dumps all of it back onto the same windows.
Covering the windows tightly. A cover pressed flat against cold glass traps humid air against the coldest surface in the car. You will find the cover soaked and the glass behind it running. Covers still earn their place for privacy and heat, but they are not a moisture fix: see window covers for how to hang them without making this worse.
Sealing everything and hoping. The water has to go somewhere. If it cannot leave as vapor it will leave as liquid on your gear.
Cold nights, and the insulation trade
Insulation and ventilation get treated as opposites and they are not. Insulation raises the temperature of the interior surfaces, which means the glass and panels are less likely to be below the dew point. Ventilation removes the moisture that would land on them. Doing one without the other just moves the wet spot. The van insulation page covers the surface-temperature side of this in more depth, and van ventilation covers what a powered roof fan changes, which is mostly that you get real airflow without an open window in the rain.
In a car you cannot insulate much, so ventilation carries almost all of the load. Accept that, dress for it, and keep the openings.
The morning routine that keeps gear dry
Open everything for ten minutes before you drive off. Pull the bedding off the deck so the underside can dry, because the surface between a foam pad and a plywood deck stays wet longest and is where mildew starts (see building a sleeping platform for why that gap exists at all). Once a week, when there is sun, take the foam out and leave it out for an hour.
Where you park to do any of this is a separate question with a local answer, and the posted sign and the person who manages that lot beat anything on this site: start with sleeping in your car at rest areas.