Camper van ventilation: preventing condensation

Two people produce about 1-2 litres of moisture per night through breathing and perspiration. Cooking, showering and wet clothes add another 1-3 litres per day. This moisture needs somewhere to go.

Without ventilation, this moisture settles on the coldest surfaces: windows, metal walls and corners behind cabinets. This leads to:

• Mould growth on walls and furniture
• Rust on the body panels (invisible behind insulation)
• A damp, musty atmosphere
• Health problems from mould spores

Good ventilation removes humid air and brings in dry outside air. Combined with insulation (which keeps surface temperatures above the dew point), this is the key to a dry, healthy camper van.

A roof fan is the most important ventilation tool in your camper van. The two most popular brands are:

MaxxFan Deluxe (40×40 cm opening)Most popular with self-builders. 10 speed settings, rain-resistant (can stay open in rain), built-in thermostat on the Electric model. Power consumption: 0.1A on speed 1 to 2.7A on speed 10 at 12V. Price: €250-€400.

Fiamma Turbo-Vent (40×40 cm opening)Large 10-blade fan, variable 12V motor, optional thermostat (Polar Control). Slightly cheaper than MaxxFan. Price: €150-€300.

Placement: mount the roof fan above the cooking area or in the centre of the camper. Heat and moisture rise, so above the kitchen is most effective.

One roof fan is the minimum. For a longer van (L3 or larger), consider a second ventilation point, such as a tilt-and-turn window on the opposite end.

In addition to active ventilation (roof fan), you need passive ventilation openings for constant air circulation, even when the fan is off:

• Place ventilation openings low in the camper (bottom of wall or floor) so cool, dry outside air can flow in while warm, humid air exits through the roof fan.
• Use insect screens on all openings.
• Ventilate cabinets and dead corners: drill small holes (15-20 mm) in the top and bottom of enclosed cabinets so air can circulate. Without this, mould forms behind and inside cabinets.
• Leave a gap of 10-15 mm between the back of furniture and the outer wall for air circulation.

For vehicle inspection, ventilation openings are mandatory in the living space. The exact requirements depend on your installation (gas requires additional ventilation).

With a gas installation, additional requirements apply: the gas compartment must have a ventilation opening of at least 100 cm² at the bottom, with a direct connection to the outside.

Even with good ventilation and insulation, you may still experience condensation. Here are proven solutions:

1. Run the roof fan while cooking: cooking steam is the biggest source of moisture. Set the fan to extract (blowing air out), not intake.
2. Dry wet clothes outside or in a sealed space with ventilation to the outside: not in the living area.
3. After sleeping, drive for 10-15 minutes with the heater on and a window open: this dries out the camper quickly.
4. Use a moisture absorber (e.g. CaCl₂ granules) in enclosed cabinets and under the bed as a backup, but don't rely on it as your only measure.
5. Insulate all metal surfaces: every uninsulated piece of metal becomes a condensation point. Pay extra attention to wheel arches, door posts and window frames.
6. Cook with a lid on the pan: this reduces steam release by 70-80%.
7. Consider a hygrometer (humidity meter): keep relative humidity below 60%. Above 70%, mould growth becomes inevitable.

Ventilating and insulating pull on the same rope. Every cubic metre of air you blow outside takes its heat with it, and you have to make that heat again.

For a living space of 8 m³ at 20 degrees inside and 5 below zero outside, worked out with the heating calculator:

• 0.3 air changes per hour: 20 W of ventilation loss
• 0.5 per hour: 34 W
• 1 per hour: 68 W
• 2 per hour: 136 W
• 3 per hour: 204 W

Put that next to a transmission loss of around 570 W through the walls of that same van. At normal ventilation, somewhere between 0.5 and 1 change per hour, ventilating therefore costs you 5 to 12 percent extra. That is manageable.

It only mounts up once you sleep with the roof hatch open. Three changes an hour costs 204 W, a third on top of your wall loss. At that point the heater is not too small, the window is too far open.

Moisture in the air only turns into water when it meets a surface colder than the dew point. Where that surface sits decides where you get mould.

In a camper the coldest spots are nearly always the same:

• The steel ribs and frames your insulation runs around instead of over
• The edges of windows and roof hatches
• The corner between wall and floor, where insulation often stops
• The back of cupboards pressed tight against an outer wall

More ventilation only helps so far there. What does help is making that spot warmer: laying insulation over the ribs rather than between them, and keeping furniture a centimetre off the wall so air can pass.

Two people breathing and sweating put roughly half a litre of water into the van per night. Cooking and showering come on top. That moisture has to go somewhere, and without ventilation that somewhere is your insulation.

A common roof fan draws around 25 W. Run it six hours a day and that is 150 Wh over 24 hours.

Set that against a daily use of around 450 Wh for fridge, lighting, water pump and sensors: the fan is a third of it. In summer you will not notice, because sun is plentiful. In autumn, exactly when you need it against condensation, it is a sizeable bite.

What you can do about it:

• Run it on a lower setting; most models more than halve their draw
• Run it during and after cooking and showering rather than all day
• Use a humidity switch that drives it on moisture instead of on a timer

Do count the fan in your energy balance. It sits in the list of common appliances in the electrical planner, so it does not fall outside your sum.

The aim is not to change as much air as possible, but to get the moisture out with as little heat loss as possible.

Short and vigorous beats permanently ajar. Five minutes with the roof hatch and a window open changes the air almost completely, while your furniture and panelling hold on to their heat. A window left ajar all night cools the whole interior instead, including everything you then have to warm up again.

Make sure there is an inlet and an outlet. A single roof hatch does little; air has to come from somewhere. A low vent in the sliding door and a high roof hatch give you a natural flow, because warm damp air rises by itself.

Ventilate at the moment you make the moisture. While cooking, during and after showering, and in the morning when you get up. Those are the three moments when most water goes into the air, and after them far less is needed.