Van Condensation: Why a Roof Fan Is Usually the Answer

Someone posts this in r/vandwellers most weeks in the same shape. They started van life without knowing ventilation was a topic, cracked the windows, ran a fan, and still woke up to it. In one recent thread: “Woke up this morning and although condensation was less, the walls and roof were still wet.” (r/vandwellers)

The replies always say “get a MaxxFan”. That advice is correct and it is almost never explained, which is why people try the cheaper things first and lose a season to it.

Here is the explanation, and then the part nobody writes about: what to do when a standard fan will not physically fit your roof.

Condensation is a water balance, and you can do the arithmetic

Every night you add water to the air inside your van. Published figures put exhaled water vapour at roughly 100 to 200 ml per person per night, and skin perspiration adds more. Two people is comfortably half a litre before anyone boils a kettle, dries a jacket or cooks a meal inside.

Now the other half. Warm air holds more water than cold air. As air cools it reaches a temperature, the dew point, where it can no longer hold what it is carrying, and the surplus condenses onto whatever is coldest. In a van the coldest surface is bare metal, because sheet steel with outside air on the other side is an almost perfect heat sink. That is why the water appears on walls and roof rather than in the middle of the room.

So there are exactly three things you can do:

1. Get the humid air out before it reaches the metal. Ventilation. Largest lever by a wide margin.
2. Stop the metal being the coldest surface. Insulation, applied continuously with no air path behind it.
3. Pull water out of the air. Dehumidification and absorption. Smallest lever in a vehicle, for reasons below.

Almost every failed condensation fix is someone doing the third thing and hoping it substitutes for the first.

Why cracking a window and running a fan did not work

This is the specific step that disappoints people, and it deserves a straight answer rather than “you need more ventilation”.

A fan sitting on a shelf blowing across the bed moves air within the van. It makes you feel cooler and it does very little about total moisture, because the same wet air is still in the same box. What removes water is air exchange: humid inside air leaving, drier outside air replacing it.

A cracked window gives you passive exchange only. It depends on wind and on a temperature difference to drive it, and on a still night both can be close to zero. You have opened a hole; you have not created a flow.

A powered roof vent forces the exchange. It also does it in the right direction, because it sits at the highest point where the warmest, wettest air has already collected, and pushes it out of the van entirely.

The other half of the rule, and the one most often missed: you need two openings. Air will not leave through the roof unless something replaces it. A fan on the roof with the van otherwise sealed works against its own pressure. Crack a window at the opposite end and the same fan on a lower speed moves far more air. This is also the cheapest possible upgrade to a setup you already own.

What the powered options actually are

Roof vents in this market are standardised on a 14 by 14 inch opening. That is worth knowing before you shop, because it means the choice is not about size, it is about lid, control and whether it runs in rain.

Unit	Claimed capability	Rain-capable with lid up	Listed weight	Roughly
MaxxFan Deluxe 00-05100K	10 speeds, reversible, thermostat, manual lift	Yes, built-in rain shield	16.2 lb	$250 to $330
MaxxFan Deluxe 00-07500K	Same fan, powered lift and remote	Yes, built-in rain shield	Not published in listing data	$330 to $420
Fan-Tastic Vent 1200	3 speeds, manual crank lid	No, lid must close in rain	10.01 lb	$130 to $190

Weights and dimensions above are from the manufacturers’ Amazon listing data, retrieved 2026-08-18. Airflow claims are the makers’ own numbers. MaxxAir publishes figures in the region of 900 CFM for the Deluxe; nobody independent has published a measured value for it that we would stand behind, and that is a measurement job sitting on our bench list rather than a number we are going to repeat as fact.

The one distinction that matters for condensation is the rain shield. Condensation is worst on cold, wet, still nights, which is exactly when a fan without a built-in cover has to be closed. A vent you have to shut when it rains is a vent that is off precisely when you need it. That single fact is why the MaxxFan Deluxe is the default recommendation in every thread, and it is a better reason than brand loyalty.

Between the two Deluxe models, the difference is a powered lid and a remote. The manual-lift 5100K is the one to buy unless the vent will end up somewhere you genuinely cannot reach, in which case the powered lid stops being a luxury. Both have the same fan and the same rain shield.

The Fan-Tastic 1200 is the honest budget entry and it is a real option for a van that mostly gets used in dry weather. Buy it understanding what you gave up.

When a 14 by 14 fan will not fit: the low-clearance case

This is the question that has no good published answer, and it comes up constantly on older GM vans in particular. One r/vandwellers poster described a 1997 Savana with only around 8 inches of usable ceiling either side of a centre row of lights, and asked whether to give up on a roof fan and vent a portable air conditioner out of a window instead. (r/vandwellers)

The instinct is to look for a smaller fan. There essentially is not one. The 14 inch opening is an industry standard and the powered vent market is built around it, so shrinking the hole means leaving the category.

What builders actually do:

Move the hole, do not shrink it. Measure from inside to find where the cross ribs run, then site the opening in a bay between two of them. On GM vans the bay between the rearmost cross rib and the rear doors is a common choice, and it has a side benefit: putting the extract at the back and your intake at the cab windows sets up cross flow along the length of the van rather than a short circuit around the fan.

Frame or plate across the ribs where the roof is not flat. Where a flat 14 inch square is genuinely unavailable, the vent gets mounted on an adapter plate or a built-up frame that spans the ribbing and is sealed down to the roof. This is ordinary practice rather than a bodge, but it is fabrication, and it is the point at which a lot of people should get a quote instead of a jigsaw.

Check the headliner before you commit. You only need to pull the headliner over the square you intend to cut, and that is the moment to confirm there is no rib, wiring loom or brace hiding in the bay you picked.

And if the roof is truly not an option, the fallback is to maximise side ventilation rather than to accept none: rain guards let you sleep with the cab windows cracked in weather, which converts a fair-weather-only opening into a year-round one. That is a real improvement and it is not equivalent to a roof fan. Be clear with yourself about which you are buying.

On venting a portable air conditioner out of a window as an alternative: that solves a different problem. It is a cooling question, not a moisture question, and a single-hose portable unit in a van has its own issues that we cover in the air conditioning options map. It is not a condensation fix.

Insulation, and the trap in “just add more”

The second lever is keeping the metal above the dew point. The important word is continuous.

An insulation layer with air gaps behind it is worse than it looks on paper, because warm humid air finds the gap, reaches the cold steel behind your nice insulation, and condenses where you cannot see it or dry it. Hidden condensation is how vans rust from the inside.

This is why “should I pull the paneling and spray foam over my existing 1 inch rigid?” is a genuinely hard question and not a more-is-better one. Adding a second material over the first can trap moisture between the layers. If the existing rigid board is well adhered with no air path to the metal, sealing edges and addressing thermal bridges is usually higher value than adding thickness. If it was loosely fitted with gaps behind it, that is a real reason to pull it out.

We have not built and instrumented a comparison of insulation strategies, so treat the above as the reasoning rather than a result. Where it would take measurement to settle, it takes measurement.

The third lever, and why it is the smallest

Dehumidification gets recommended a lot and delivers least in a vehicle.

Compressor dehumidifiers lose capacity as temperature falls, and van condensation peaks on cold nights. The tool gets weakest exactly where the problem is worst.

Thermoelectric mini dehumidifiers, the small ones marketed for cars and closets, extract very little water for their power draw. In a van running off a house battery, that draw competes with things that matter more.

Calcium chloride absorbers genuinely pull water out of the air with no power at all, and they are cheap. What they cannot do is keep up with two people breathing. They work on a timescale of weeks.

Which gives absorbers a real and specific job: a van in storage. Nobody is adding moisture, nothing needs to happen fast, and slow passive extraction is exactly right for stopping a musty van over a wet winter. Use them for that and do not expect them to keep your walls dry while you sleep.

What to actually buy, in order

First, a hygrometer, and buy more than one. This is the cheapest thing on the page and it is the only way to stop guessing. Put one at ceiling height, one at floor level and one outside, and you can see what is actually happening: whether your relative humidity is climbing overnight, whether the fan is working, and how far your interior is from the dew point. A general comfort target of roughly 40 to 60 percent relative humidity is a reasonable thing to steer toward. A three pack of small digital hygrometers costs less than a tank of fuel and turns the whole problem from an argument into a reading.

Second, the roof fan, if you can fit one. The MaxxFan Deluxe 5100K is the default for the rain shield, not for the badge.

Third, the second opening. Free if you already have a window you are willing to crack. If weather is what stops you, in-channel rain guards let the cab windows sit open in rain, which is the difference between a vent you use in summer and one you use all year. The AVS set listed above is cut for Chevy Express and GMC Savana; check the fitment for your own van before ordering, because these are vehicle specific.

Fourth, and only for storage, a refillable calcium chloride absorber. Not a substitute for any of the above.

Things not on this list, deliberately: scented moisture products, which do nothing structural; and thermoelectric mini dehumidifiers, which draw power to accomplish very little in the conditions that matter.

Where this sits with the rest of the site

Condensation and heating are the same winter problem approached from two sides, and running a combustion heater changes the ventilation calculation rather than removing it. If you are heading into cold weather, read the diesel heater buyer’s guide and then what a home CO detector misses in a van, because a sealed van with a combustion appliance in it is the one configuration where ventilation stops being a comfort question.

If your reason for asking about airflow was heat rather than damp, the air conditioning options map is the right starting point.

Everything on this page that is a number is either a manufacturer claim, labelled, or a published figure with its source named. The only instrumented data VanTested has published comes from the unit in our own van, in the Vevor field log. Airflow measurement on roof vents is a real gap in this category and it is on our bench list.