Van Ventilation: Roof Fans, Air Exchange, and Getting Moisture Out

A powered roof fan is the fix for a wet van because it is the only thing in the vehicle that removes water. Insulation redistributes heat. A dehumidifier costs you power you probably do not have. Air exchange is the mechanism, and a fan is how you get air exchange on a still night with the doors shut.

EPA’s framing for indoor air applies cleanly here. Source control first, then increase the amount of outdoor air coming indoors. In a van the sources are you and your cooking, and the outdoor air arrives through a hole in the roof.

What the fan is actually doing

Your cabin air carries moisture. Every hour you sleep in the van, more goes into it. Every hour the fan runs, some of that air leaves and drier outside air replaces it.

The target is a humidity level, not an airflow number. EPA says indoor relative humidity should be kept below 60 percent, ideally between 30 to 50 percent. That is a house guideline, not a vehicle standard, and no ventilation code covers a van. But it is a useful line: if a cheap hygrometer on your wall reads above 60 on a cold night, you have more moisture than the van is clearing, and the wall cavities are getting the surplus. What happens to it in there is covered in van insulation and condensation.

Here is the number we could not verify, and it is the one everyone wants: how much water two people put into a van overnight. The USDA Forest Products Laboratory publishes a paper on water vapor generation in residential buildings, and it blocks automated access, so we could not read it and will not quote figures from secondhand summaries of it. We have no verified pints-per-night number for a van and are not going to make one up. Use the hygrometer instead. It measures your actual van, with your actual habits, which is better than any table.

Sizing, honestly

The van ventilation market runs on CFM claims, and the manufacturers are not always the source of them.

Maxxair’s own product pages for the MaxxFan Deluxe, checked 2026-07-18, specify a 10-speed reversible fan with intake and exhaust, a fit for standard 14 by 14 inch roof openings, 12 volt operation, a thermostat, and a built-in rain shield described as letting the fan run in the rain or while the vehicle is moving. Those pages do not publish a CFM rating. Retail listings and review sites quote figures like “over 900 CFM” at specific amp draws. Those numbers may well be right, but they are not on the manufacturer’s spec page we read, so we are not repeating them as fact.

What that leaves you with is a decision made on features rather than a headline airflow number, which is probably the more useful basis anyway:

• Reversible or not. Exhaust is what you want for moisture. Intake is what you want for cooling on a hot evening. Reversible covers both.
• Rain shield. A fan you have to close when it rains is not available on the nights you most need it. This is the single feature that separates a van fan from a cheap RV vent.
• Speed control and thermostat. A fan you can run slowly all night at low current draw beats a fan that is either off or loud.
• Current draw. This is the spec that interacts with the rest of your build. Work it into your power budget before you size the battery, because a fan running eight hours a night is a real load, not a rounding error.

One fan, in a 14 by 14 opening, exhausting, with a window cracked at the far end of the van, is the standard configuration for a reason. It is enough for most builds most of the time. Two fans is a hot-climate decision, not a moisture decision.

Give the air a path

A fan with nothing open is a fan fighting itself. It needs an inlet.

Crack a window at the opposite end from the fan. That gives you a defined path along the length of the van, so the air being replaced is the air you slept in, not the air in the six inches under the fan. In cold weather people close everything, run the fan, and wonder why it did not help.

DOE’s building science material on natural ventilation is a good description of the free version: cross-ventilation using prevailing breezes, and the stack effect, where air enters low, absorbs heat, rises, and exits high. Both are real and both work when conditions cooperate. Neither is reliable on a windless night, and a windless cold night is exactly the condensation scenario. The powered fan exists because you cannot schedule a breeze.

The carbon monoxide part

Any fuel-burning appliance in a van produces carbon monoxide, and so does the engine.

OSHA’s fact sheet describes CO as poisonous, colorless, odorless, and tasteless, produced by the incomplete burning of carbon-containing material such as natural gas, gasoline, kerosene, oil, propane, coal, or wood, and identifies the internal combustion engine as one of the most common sources of exposure. It explains the mechanism as CO displacing oxygen in the blood and depriving the heart, brain, and other vital organs, and states that large amounts can overcome a person in minutes without warning, causing loss of consciousness and suffocation. Initial symptoms it lists are headache, fatigue, dizziness, drowsiness, and nausea, which are also what a normal bad night in a van feels like. That is why the gas is dangerous rather than merely unpleasant.

EPA’s guidance is short and specific. Do not use gasoline-powered engines or generators in enclosed spaces. Do not sleep in any room with an unvented gas or kerosene space heater. EPA also notes that CO detectors have varied in laboratory testing, with some failing to alarm even at very high levels and others alarming at levels that pose no immediate risk.

The line for a van build: a sealed-combustion heater that takes its air from outside and exhausts outside is a different device from an open flame in the cabin, and a CO alarm is a backstop, not permission. Do not treat the roof fan as your safety margin. It is there to move moisture.

If you are still deciding when to cut the roof, note that the fan opening belongs in the second step of the build order, while the van is empty and you can still reach both sides of the metal.