Condensation in a campervan is not a minor inconvenience. It is the main cause of rot, mould, foul smells, and respiratory problems in UK van builds. And it is worse in the UK than almost anywhere else in Europe because of the specific combination of maritime climate, high humidity, and mild temperatures that keep the air saturated for months at a time.
Why Condensation Is Worse in UK Vans
The UK has a temperate maritime climate — meaning the air is humid year-round. Average relative humidity in the UK ranges from 70% in summer to 90%+ in winter. A van is a metal box that cools rapidly at night. When warm, moisture-laden air inside the van hits a cold surface (windows, bare metal panels, uninsulated roof), the water vapour condenses into liquid.
The problem compounds: the colder the surface, the more condensation. The more condensation, the more moisture soaks into upholstery, wood, and insulation. The more moisture trapped in materials, the longer it takes to dry out. In a UK winter with short daylight hours and limited ventilation, a van can stay damp for weeks.
A house has central heating, cavity wall insulation, and multiple occupants generating heat. A van has none of these advantages. Human breath alone releases about 400ml of water vapour per person per night. Add cooking steam, wet clothes, and damp dog, and you are generating 1–2 litres of water in a space the size of a Transit Custom. That water has to go somewhere. Without active management, it goes into your mattress, your clothes, and your walls.
Measuring Dew Point and Humidity
You cannot manage condensation effectively without data. Your body tells you the van feels "clammy", but by then mould spores are already germinating. You need a hygrometer.
A basic £5 hygrometer from Amazon tells you temperature and relative humidity. It is better than nothing, but the important metric is the dew point — the temperature at which the air becomes saturated and water condenses. If the dew point is 8°C and your van windows are 5°C, you get water on the glass.
More useful is a dew point alarm hygrometer. The ThermoPro TP60 (£15–18) shows the dew point and alerts you when conditions are approaching condensation threshold. The SensorPush wireless hygrometer (£45) logs data to your phone and graphs humidity trends over days — very useful for diagnosing whether your ventilation strategy is working.
What to look for:
- Relative humidity above 70% for more than a few hours is a warning sign
- Above 80% for sustained periods means condensation is forming somewhere, even if you cannot see it
- Above 90% means active water pooling is happening
The target for a healthy van interior overnight is 50–65% RH. Below 50% is difficult to maintain in a UK winter without aggressive heating and ventilation.
Passive Solutions — Ventilation
Ventilation is the cheapest and most effective condensation control. The physics is simple: replace moisture-laden interior air with drier exterior air. Even on a rainy UK day, outside air is typically less humid than the air inside a van occupied by two people.
Roof fans. The Maxxfan Deluxe is the gold standard. It moves 900 CFM on high, has a built-in rain sensor that closes the lid automatically, and can run on 12V at 0.5A on low (continuous running). At £320 for the Maxxfan Deluxe with remote and rain sensor, it is expensive for a fan. But it works.
The cheaper alternative is a Fiamma Turbo Plus or a Chinese equivalent (£60–100). These move less air (400–600 CFM), do not have rain sensors, and the manual closing is a nuisance. You forget, it rains, and your bedding gets wet. The Maxxfan's rain sensor alone is worth the upgrade for UK conditions.
Passive vents. Fixed roof vents (two small mushroom vents, one at each end of the van) rely on the stack effect — warm air rises and exits through the rear vent, drawing cool air in through the front. This works reasonably well in summer but poorly in winter when the temperature differential is lower. Add a vent booster fan (£30) to turn passive into active.
Window ventilation. Opening a roof vent and cracking a side window on opposite sides of the van creates cross-flow ventilation. Even a 10mm gap on the side window, combined with a roof vent on the rain sensor setting, keeps humidity below 70% on all but the most humid nights. In winter, this means sleeping in a colder van with a good sleeping bag rather than a warmer van with condensation.
Active Solutions — Heaters
The type of heater you run has a direct impact on condensation.
Diesel heaters produce dry heat. The combustion process draws air from outside the van and exhausts outside, so the heat generated is "dry" relative to propane. A 2kW diesel heater running at 40% duty cycle raises the internal temperature 5–8°C above outside, which lowers relative humidity significantly. Running a diesel heater overnight in winter keeps the dew point below the window temperature, preventing condensation on glass.
Gas heaters (Propex, Truma) burn propane from a bottle. Combustion of propane produces water vapour at a rate of about 1.6 litres of water per kg of propane burned. A Propex HS2000 running for 8 hours on low produces roughly 0.5–0.7 litres of water vapour. This water goes into the air inside the van (the Propex is a warm-air heater that vents combustion externally, but the exhaust gas does not enter the cabin — the water vapour from the burning process is not the issue; rather, the moisture is already present in the air). Actually, propane combustion in a balanced-flue heater does NOT introduce water vapour into the cabin. The issue is more nuanced: gas cookers (unflued) release all combustion products, including water, into the van. Heated air holds more moisture than cold air. So a gas heater that recirculates cabin air will heat that air, increasing its capacity to hold moisture, but the moisture content in the air does not change. The real benefit is that warmer air holds more moisture before condensing, so simply raising the temperature reduces visible condensation even if total water content is unchanged.
Electric fan heaters (when on EHU) produce completely dry heat. Zero combustion, zero moisture added. Ideal for condensation management but requires shore power — not practical for off-grid UK van life.
The practical takeaway: if you have a diesel heater, use it. It is the best tool for winter condensation control. If you rely on a gas heater, run it at a consistent temperature (avoid large temperature swings) and pair it with active ventilation.
Insulation Choices
Your insulation choice is a decision you make during the build — and it is hard to undo. The wrong insulation will trap moisture inside your walls for years, causing hidden rot.
Closed-cell insulation (Armaflex, Kingspan, Celotex). Closed-cell foam does not absorb water. This is the critical property. If moisture gets behind the vapour barrier (and it will), closed-cell insulation will not wick it along the panel. Armaflex is the preferred material for UK van builds — it is flexible, self-adhesive, 10–19mm thick, and R-values of 0.35–0.55 per layer. Cost is about £10–15 per sheet.
Open-cell insulation (sheep's wool, recycled polyester). These materials absorb and release moisture — they are "breathable". In a van with perfect ventilation and no leaks, sheep's wool works well and provides excellent acoustic damping. In a British winter with daily condensation cycles, open-cell insulation absorbs moisture, takes days to dry, and becomes a breeding ground for mould. There are successful builds using sheep's wool in the UK, but they require exceptional ventilation discipline.
Vapour barriers. The most contentious topic in UK van building. In theory, a vapour barrier (VCL) on the warm side of the insulation stops warm, moisture-laden air from reaching the cold outer skin. In practice, every screw, rivet, and cable penetration punctures it. Some builders argue that a vapour barrier in a van is impossible to seal completely and therefore useless. Others run a sealed VCL with taped joints and accept that minor punctures will not significantly reduce performance.
The pragmatic UK approach: use closed-cell insulation (Armaflex or Celotex), run a vapour barrier on the warm side with taped joints and sealed penetrations, and accept that some moisture will get behind it. The closed-cell insulation will not absorb or transport that moisture, so it remains manageable. Ventilate aggressively.
Thermal bridging. Every metal rib, cross-member, and window frame is a thermal bridge — a path for heat to escape and cold to penetrate. In a UK winter, condensation forms on thermal bridges first. Covering exposed metal with Armaflex strips or neoprene tape reduces this. Magnetic window covers trap a layer of air against the glass, significantly reducing condensation on windows (the most visible form).
Winter-Specific Strategies
Window covers. The single biggest improvement for UK winter condensation. A set of Thermal Blind Company covers or DIY Reflectix panels cut to fit each window creates a warm barrier. The air gap between the window and the cover prevents the interior air from contacting the cold glass. On a van with eight windows, this prevents litres of condensation per night.
DIY approach: buy Reflectix double-reflective insulation (£15 for a 30cm x 5m roll), trace each window onto cardstock, transfer to Reflectix, and cut. Attach using suction cups or magnets. Cost: about £20 for a full set of window covers.
Silver screens (external covers) are even more effective — they prevent the glass from getting cold in the first place. But they must be fitted from outside, which is impractical for frequent stops.
Cooking without ventilation. The worst condensation event in any van is cooking. Boiling pasta releases 500–800ml of steam into the van in 10 minutes. In winter, van lifers often close all vents and hatches while cooking to keep the heat in — which traps all that steam.
Solutions:
- Cook outside when possible (jet boil, camping stove under the tailgate)
- Use a lid on every pan (reduces steam by 70%)
- Run the roof fan on extraction during cooking, even in winter. The 30 minutes of fan operation costs negligible battery power.
- Open a window 50mm and run the fan on exhaust — creates negative pressure that pulls steam out
- Use a pressure cooker (Aldi sell one for £25). Steam release is minimal with a pressure cooker.
Drying wet gear. UK van life means wet clothes, wet boots, wet tent gear. Drying inside the van pumps litres of water into the air. Designate one area of the van for drying (near the diesel heater outlet is best) and run the heater on low with a roof vent open. The warm dry air picks up moisture from the clothes and the vent expels it.
DIY Condensation Prevention Checklist
This is the routine that keeps a van dry through a UK winter:
- Hygrometer visible at eye level. Check before sleeping and on waking.
- Roof vent on rain sensor overnight, fan on low extraction
- One side window cracked 10mm (if dry) or no window crack with diesel heater running
- Diesel heater set to 16°C minimum overnight (prevents temperature dropping below dew point)
- All windows covered with internal covers before dark
- Gas cooking done with lid on pan and fan on max extraction
- Wet clothes hung in shower area (if separated) or near heater outlet
- No unvented gas heaters running indoors
- Soft surfaces (mattress, cushions) lifted periodically to check for hidden condensation
- External moisture wiped from roof and side panels every morning with a microfibre cloth
What to Do If Mould Has Already Taken Hold
If you smell mustiness or see black spots, act immediately. Mould spreads fast in vans.
Step 1 — Identify the source. Is it a leak (water ingress from roof vent, window seal, or body seam) or condensation? A leak will produce concentrated water tracks. Condensation mould is more diffuse — along window frames, around thermal bridges, on the underside of cushions.
Step 2 — Remove affected materials. Porous materials (carpet, upholstery, unsealed wood) that are visibly mouldy need to be removed or treated. Non-porous surfaces (metal, plastic, sealed wood) can be cleaned.
Step 3 — Clean with white vinegar or hydrogen peroxide. Do not use bleach on porous surfaces — the chlorine stays in the material and continues to degrade it. For sealed surfaces, a 50/50 white vinegar and water solution kills most mould spores. For stubborn cases, 3% hydrogen peroxide (from any chemist) is more effective.
Step 4 — Dry thoroughly. After cleaning, run the diesel heater at 25°C for 2–3 hours with maximum ventilation. Use a dehumidifier if you have EHU access. The van must be bone dry before you replace any materials.
Step 5 — Address the cause. If the mould was from condensation, your ventilation or heating strategy was insufficient. Increase ventilation, run the heater more consistently, or improve window covers.
Step 6 — Prevent recurrence. Microfibre cloth in the cab every morning to wipe down windows. Leave cushion gaps for airflow. Keep a moisture absorber (Sainsbury's do a 500ml unit for £3) in the wardrobe area. Replace monthly.
Products That Actually Work
- Maxxfan Deluxe — £320 — Overpriced but the only roof fan with a reliable rain sensor. If you spend winters in your van, this pays for itself in peace of mind.
- Kampa Dometic 12V Dry Air — £50 — A portable dehumidifier that works on a Peltier element. These are useless for whole-van dehumidification (they remove about 200ml/day, which is nothing). Do not waste your money. Only a compressor dehumidifier (12V or mains) can meaningfully reduce humidity.
- Unibond Aero 360 — £12 — A passive moisture absorber that works in wardrobes and small enclosed spaces. Effective for its size but cannot handle a whole van.
- ThermoPro TP60 hygrometer — £15 — Essential. Buy two, one at waist level and one in the roof (the warmest, most humid air collects at the ceiling).
- Reflectix double-reflective insulation roll — £15 — DIY window covers for a full-sized van.
- Vent booster fan — £30 — Attach inside a roof vent mushroom to convert passive to active extraction.
The Hard Truth
No product eliminates condensation in a UK winter van. The combination of active ventilation (a fan running continuously), dry heat (diesel heater), and physical separation (window covers) manages the problem. If you have all three, you will wake to dry windows and a van that does not smell. If you are missing any one, you will have water on the glass and a damp mattress base within a week.
The van lifers who do not struggle with condensation are the ones who run their diesel heater all night, leave a vent cracked, and have good window covers. That is the formula. Everything else is optimisation.
