Quick answer

When to start using a dehumidifier in a campervan in the UK comes down to one morning: the one where water runs down the inside of the glass. In most of the country that lands between late September and mid October. A stored van wants one from the day you shut it up; an occupied van wants ventilation first.

Table of contents

What damp looks like before you can smell it

Damp in a van is usually condensation, and condensation follows temperature. Air holds roughly twice as much water vapour for every 10 degrees you warm it, so when a van cools overnight the surplus goes onto the coldest surface within reach: single glazed glass, the metal skin behind thin trim, any bolt running from outside to inside. That is why the windows show it first and why the damp patch turns up behind a locker. A caravan or a motorhome behaves the same way.

The ninety second check

Do this on a cold morning before you open anything: once fresh air gets in, half the evidence goes.

  • Water on the inside of the glass, with a wet line where it has run down.
  • Pooling low down in the sills, the seat runners and the door track, which is what rusts a door bottom from the inside.
  • A musty, cellar-like note in the first three seconds after opening, gone a minute later.
  • Black spotting on rubber seals, roof vent surrounds and around screw heads. Those dots are mould, not dirt.
  • Clammy bedding, and a hand under the mattress coming out wet: air is condensing on the cold board below the foam.
  • Misting between the panes of a double glazed window, which is a failed seal and not something a dehumidifier can touch.

The reading that settles it

Symptoms are late. Relative humidity is the share of the moisture the air could hold at that temperature, and mainstream damp guidance puts sustained readings above about 70 per cent in the zone where mould and mildew establish, aiming for 45 to 60 per cent indoors. Treat 60 per cent as the line for a stored van.

Leave a small battery hygrometer in permanently, and two if you can: head height, and low down in a locker. Cold corners read higher than the middle, and that is where trouble starts.

A stored van and a slept-in van are two different problems

In storage the van is shut, unoccupied and generating no moisture of its own. What it has instead is time, temperature swings, damp ground underneath and nobody to open a window, which makes the dehumidifier the main tool: it is the only thing working while you are elsewhere.

Occupied, the balance flips. Two adults asleep put a lot of water into a small volume overnight, the commonly cited figure being around half a litre each, and that is before the kettle, the wet coats and the dog. A small dehumidifier against that is bailing with a teacup.

Ventilation is the tool in a van you sleep in: a roof vent cracked at the downwind end, a window open on the sheltered side, all night, in the cold. It feels wrong and it works. One thing in an occupied van, open the vent; in a stored van, run a dehumidifier.

September to March, month by month

There is no national date, and anyone offering one is guessing. The trigger is the first run of nights where interior surfaces sit below the dew point of the air touching them, which turns on the overnight low and on how damp the van already is. Once lows settle into single figures and stay there, you are into it.

Geography moves that by a fortnight or more: inland Scotland and the north east before the south west coast, high ground before low, an exposed yard before a sheltered barn. Watch the mornings, not the diary.

September

Usually nothing yet, and still the month to start looking. Warm afternoons and clear cooling nights are what produce the first condensation, so a van can catch its owner out while the days still feel like summer.

October

For most of the country this is where the answer turns to yes. Overnight lows drop into single figures, the first mornings arrive with the windows running, and most UK vans go into storage now or in early November. The drying job below is an October one.

November

The peak. A van shut up in late October has four weeks of cold nights behind it, and if nothing is pulling moisture out, this is when the smell arrives and the black spots appear. Make the first check visit: empty the tank, read both hygrometers.

December and January

The coldest stretch, and the two months that expose the wrong type of unit. A stored van with a working desiccant unit and open vents needs little beyond a monthly look. A van lived in over Christmas is the opposite: more cooking, more bodies, more wet coats.

February and March

Recommissioning, and the classic mistake is stopping early because the evenings feel longer. February nights are as cold as January nights and March mornings still produce condensation. Keep going until the readings sit below 60 per cent without help, then check every outer-wall locker and the underside of the mattress.

Drying a van out and keeping it dry

The order matters more than the machine. A dehumidifier dropped into a van that was closed up wet spends the winter fighting water you left behind.

  1. Dry it before you shut it, not after. A dry breezy day, everything open, hours rather than minutes.
  2. Strip it out. Bedding, cushion covers, towels, fridge contents, anything that can go soft.
  3. Open every door you own. Lockers, wardrobe, shower, fridge and freezer. A closed locker on a cold outer wall goes first.
  4. Prop the cushions on edge or up on blocks. Foam flat on a cold base is the most reliable way to grow mould in a van.
  5. Leave the ventilation alone. A dehumidifier in a sealed box recycles the same air, and any leak keeps feeding it.
  6. Site the unit level, off the carpet, with clearance around the intake and in the middle of the living space.
  7. Use the humidistat. Around 55 per cent and let it cycle; running flat out all winter dries nothing extra.
  8. Run the drain hose on any layup longer than a fortnight. A tank that fills is a dehumidifier that stopped.
  9. Go back inside two weeks, then monthly.

All of that assumes mains power. On a storage yard with hook-up you need a long lead, because the bollard is never where the van ends up: our review of a 25m caravan mains hook up cable covers what to check on the cheap ones. Plug into an RCD protected supply.

Batteries are not a way round this. A desiccant unit draws a few hundred watts for months on end, so even a large portable power station for a campervan is measured in hours at that load. No electricity on site means the passive option below.

Which type of unit works in a cold van

Types, not brands. The type is the whole decision, and getting it wrong is how people conclude that dehumidifiers do not work in vans.

Desiccant: the one to run in a stored van

Desiccant units draw moisture through an absorbent wheel rather than condensing it on a cold coil, so they keep working close to freezing and give off a little warmth as they run. That is what an unheated van needs in a British winter. On hook-up from October to March, buy this type.

Compressor: the wrong tool at the wrong time of year

A compressor unit chills a coil to condense water out of the air: fine in a warm room, poor in a cold one. Extraction falls away as the temperature drops, and below roughly 10 to 15 degrees many ice up and defrost instead of drying. These sell in the greatest numbers and disappoint in the months you wanted one for.

Passive moisture traps: the honest answer with no power

The refillable tubs and hanging bags hold calcium chloride, which pulls water out of the air and drips it into a reservoir. No power, nothing to fail, and for a van in a field with no hook-up they are the only thing that works. Capacity is finite, so a long van needs several and they want emptying every few weeks, and the collected liquid is corrosive enough to mark upholstery and aluminium. They hold a dry van steady rather than drying a wet one.

12V and mini thermoelectric units: not for a space this size

The small thermoelectric boxes are rated for a wardrobe or a locker and extract a fraction of what a mains unit manages. In a van sized volume with a cold shell they are close to decorative. Offered one of those or three moisture traps, take the traps.

Owners disagree about whether a stored van needs anything electrical at all, and the disagreement is fair: plenty get through a winter on traps alone. On hook-up we would still run a desiccant unit, because it holds a number rather than hoping for one.

Where a winter of this goes wrong

The unit that iced up in week two. A compressor dehumidifier in a cold van can spend December doing nothing while the tank stays reassuringly empty, and an empty tank gets read as a dry van. In November it means the opposite: check the coil for frost, and read the humidity, not the tank.

The tank that filled and stopped. Most units shut off when the reservoir is full, which on a van that went away damp can happen inside a week. After that it is an ornament with a light on.

The van sealed up tight. Taping the roof vent shut feels like the obvious move and is the exact opposite of one. Sealed, the same air cycles round while any leak tops it up, and moisture that used to show on the glass condenses behind a panel instead.

The trap left on a soft surface. Calcium chloride solution is corrosive, so a tub knocked over on a mattress leaves a stain and, on bare aluminium, worse. Tray, hard surface, out of the walkway.

Damp treated as condensation when it is a leak. If a single corner stays wet, or the reading will not come down whatever you run, water is getting in: roof vent surrounds, awning rail sealant, skylight seals, the seam above the sliding door. Get them checked, because a dehumidifier masks a leak for exactly as long as it takes the plywood behind the lining to go soft.

Running one instead of ventilating while you sleep. In an occupied van a dehumidifier supplements an open vent, it never replaces one. And separately from damp: any fuel burning heater needs its own air supply and a working carbon monoxide alarm.

Frequently asked questions

Does a campervan dehumidifier need to run all winter, or just in bursts?

In a stored van, continuously, but on a humidistat rather than flat out. Set it near 55 per cent and it cycles by itself, working hard for the first few days and barely at all afterwards. Burst running needs somebody there to switch it on, and over a layup nobody is.

Can you leave a dehumidifier running in a campervan unattended?

Yes, and a winter layup asks exactly that, provided the supply is RCD protected, the unit sits level with clearance around the intake, and the water drains away rather than filling a tank. Go back inside the first fortnight to confirm it is behaving.

What can you use if the storage site has no electricity?

Refillable calcium chloride moisture traps, several of them, in a tray on a hard surface. They need no power and hold a dry van steady through a winter, but they cannot rescue a van shut up wet. A power station will not run one for a season.

Will a heater do the same job as a dehumidifier?

Not on its own. Warming the air raises how much moisture it can hold without condensing, which stops the glass running but leaves the water in the van. A background heater plus open vents does work, because the warm wet air then leaves through the vent.

What humidity should a campervan sit at?

Between about 45 and 60 per cent relative humidity, and the ceiling matters far more than the floor. Sustained readings above roughly 70 per cent are where mould and mildew get going, so 60 is the line worth defending.

Will a dehumidifier stop the windows misting while you sleep in the van?

Partly, and less than you would like. Two people breathing put moisture into a small space faster than a domestic sized unit removes it, so the glass will still mist on a cold night. A cracked roof vent and a downwind window do more.