# Condensation in a van: causes, ventilation and a routine that keeps it dry > Why vans get wet inside, how ventilation, insulation mats and heating help, what dehumidifiers can and cannot do, and how to prevent mould. Source: https://camping-spirit.com/magazine/van-condensation-and-ventilation/ *By CampingSpirit Editors, 2026-10-03* Anyone who sleeps in a van knows the morning picture: wet windows, drops on the roof vent, and a faint musty smell behind the cushions. Condensation in a camper is not a defect but physics in a small space. Whether it becomes a problem depends on how much moisture is produced and how quickly it gets out again. ## Why condensation forms Warm air holds more water vapour than cold air. When humid indoor air meets a cold surface such as a window, bare metal or a roof vent, it cools there and releases the water as droplets. A worked example from a German guide (camper.help): at 20 °C and 80 percent humidity, a 23 cubic metre interior holds more than 300 grams of water vapour. If the air cools to 0 °C, more than 200 millilitres of that condenses on surfaces. This comes from a single source, but the order of magnitude is plausible. The biggest source is you. According to Bürstner, the human body releases between 0.5 and 1 litre of water into its surroundings during sleep, through skin and breathing. Cooking, showering and wet clothes and towels add to it. Drying laundry inside the vehicle puts the moisture exactly where you do not want it. ## Ventilation: simple and effective Every source treats regular ventilation as the simplest measure. The Swiss touring club TCS mentions about five minutes of cross-ventilation, and Bürstner recommends five to ten minutes daily, plus during cooking and showering. Cross-ventilation means opening two opposite windows or doors at the same time so the air really changes. It is most useful in the morning and after cooking. A roof vent opened a crack or a tilted window helps during longer stays, especially overnight. Heat is lost this way, and that cannot be avoided. How wide a gap is enough in winter depends on the vehicle, the number of people and the weather; we found no reliable figure for that. A hygrometer shows whether your routine is working. As a target range, test.de and Bürstner give 40 to 60 percent relative humidity; camper.help recommends staying permanently below 70 percent. The TCS warns that if it regularly exceeds 70 percent, rust, mould and musty smells can follow. According to camper.help, some mould species appear from 70 percent and most grow from 80 percent. The US Environmental Protection Agency recommends keeping homes below 60 percent, ideally between 30 and 50 percent. That is advice for houses, but a usable reference point. ## Insulation, vapour barrier and thermal bridges The warmer the interior surfaces, the less water settles on them, so insulation also works against condensation. Promobil calls condensation the biggest problem in a motorhome and describes closed-cell elastomers such as Armaflex as insulation that is also vapour-tight. The idea is that humid room air should not reach the cold metal at all. If you are building a van yourself, check materials and layers carefully beforehand; which build-up suits your vehicle is not something we can say in general. Thermal bridges are places where heat flows out more easily, such as windows, doors and the cab. Condensation shows up there first. Insulating mats for windows and doors reduce it, according to Bürstner and test.de. For a rental van this is the most practical measure because it changes nothing permanently: put mats on the windscreen and side windows and take them off in the morning. ## Dehumidifiers and moisture absorbers Here the sources are sobering. Test.de writes that granules absorb far less moisture than enters the air on holiday and work only in closed spaces. With ventilation or a frequently opened door they help little. The TCS also sees only limited sense in mobile dehumidifiers because of the forced ventilation in the vehicle, and camper.help describes their effect as localised. They make more sense during longer standstill or for drying a wet tent and gear. In daily use they do not replace ventilation. ## Heating and drying in winter Warm air absorbs more moisture, and heated surfaces are less cold. Test.de therefore advises continuing to heat and ventilate after a trip until humidity is back in the 40 to 60 percent range; camper.help names at least one day at reduced heating. In frost, wipe the windows in the morning; camper.help notes that below zero water also leaves fabric by sublimation. Wet clothing belongs outside or in a room away from the van, such as a campsite drying room. ## Preventing and removing mould For storage from October to March the TCS recommends taking moisture-holding items such as upholstery, carpets, paper and textiles out of the vehicle, positioning fixed mattresses so air can circulate, and leaving cabinets and drawers open. If mould is visible: remove the moisture source first, clean with a suitable disinfectant, and wash affected textiles at 60 °C or more. The EPA advises scrubbing hard surfaces with detergent and water and drying them completely; porous materials often cannot be saved. ## A daily routine Wipe the windows in the morning and cross-ventilate for five to ten minutes. Ventilate while cooking and showering, dry laundry outside, put mats on the glass and keep an eye on a hygrometer. Now and then pull the mattress and cushions away from the outer wall. No routine keeps a van dry every night, and some figures here come from single sources, but removing moisture every day gets most of it right. ## Sources - [Schimmelpilz im Campingfahrzeug unerwünscht (TCS)](https://www.tcs.ch/de/camping-reisen/camping-insider/ratgeber/ausruestung/schimmelpilz-im-campingfahrzeug-unerwuenscht.php) - [Feuchtigkeit in Wohnmobil und Wohnwagen: 11 Tipps (Stiftung Warentest, test.de)](https://www.test.de/Feuchtigkeit-in-Wohnmobil-und-Wohnwagen-11-Tipps-fuer-weniger-Naesse-im-Camper-6190933-0/) - [Condensation in Motorhomes & Caravans: Causes, Solutions (Bürstner Help Center)](https://helpcenter.buerstner.com/hc/en-gb/articles/30914490326045-Condensate-Causes-Solutions) - [Ursachen und kleine Lösungen zur Schimmelvermeidung im Wohnmobil (camper.help)](https://camper.help/ursachen-und-kleine-loesungen-zur-schimmelvermeidung-im-wohnmobil/) - [Isolierung im Wohnmobil (promobil)](https://www.promobil.de/isolierung/) - [A Brief Guide to Mold, Moisture and Your Home (US EPA)](https://www.epa.gov/mold/brief-guide-mold-moisture-and-your-home)