Quick answer

Lay the van up dry in October, before the first hard frost, and leave every vent open all winter. Strip out the bedding and cushions, drain the water system, then run a dehumidifier if you have mains or refillable moisture traps if you do not. Condensation still beading on the glass by mid morning means act now.

Table of contents

The signs of damp, and how to check them in ninety seconds

Damp announces itself long before it does structural damage, but only to someone who goes and looks. Check on a cold morning rather than a mild afternoon, when the panels are coldest and the moisture in the air has somewhere to land.

The first test is the smell, in the two seconds after you open the door. A cellar note that clears within a minute of a through draught is condensation in the soft furnishings; a musty smell still there after ten minutes means water is arriving from outside, which is a different problem.

Then look at the glass. Beads on the inside of the windows after a cold night mean the air holds more moisture than the cold surfaces can cope with. What matters is whether it has pooled on the sill, because standing water at a seal turns condensation into a rotten wall.

Black spot grows where air does not move: around window rubbers, along curtain hems, in the top corners of the gas locker, beside a roof light, and inside corner lockers and the wardrobe. Take a torch to those places rather than the middle of the van.

Press the flat of your hand under a cushion left against an outside wall. Damp foam feels cold and slightly heavy, and it does not warm under your palm the way dry foam does.

A hygrometer on the table turns all of this into a number. Between 40 and 60 percent relative humidity is fine in a stored van, and weeks above about 70 percent are mould conditions. Habitation checks add a damp meter, where under roughly 15 percent reads as dry and past 20 to 25 percent as wet, though meters vary enough that the trend matters more than one figure.

Where you look depends on what you own. On a touring caravan, check the awning rail, the panel joints, the gas locker corners and the roof to side seams. On a panel van conversion the cold surfaces are steel, so condensation forms behind furniture and under the mattress, and the first sign is often a rust bloom on a screw head. On a coachbuilt motorhome, start at the luton over the cab.

Compound or driveway: what your power supply decides

Settle this before you buy anything. A van on a storage compound with no electricity cannot have its air dried, so ventilation and absorbers only slow the problem down.

A van at home on a drive with a hook up is a different job: you can hold the humidity down all winter instead of watching it climb.

Check the storage contract first. Many compounds have no mains at the pitch, some ban unattended appliances, and some insurers dislike anything running in an empty van. Where mains is available, feed it through the van's own hook up inlet using a proper 25m caravan hook up cable rather than a domestic lead through a window, and keep the cable out of standing water.

Solar sits in between. A roof panel and a leisure battery will keep the battery healthy and can run a small ventilation fan, but neither will run a dehumidifier through a British December.

Ventilation, heating, dehumidifiers and traps, ranked

These four get presented as a menu of equal options, and they are not equal.

Ventilation is the base layer and it is not optional either way. A caravan is built with low and high level fixed vents so moist air can leave, and blocking them against cold or mice is the most common cause of a mouldy spring. Prop a roof light open and leave the internal doors, wardrobe and lockers open so air reaches the corners.

With mains, an active dehumidifier is where the money goes, and in an unheated van a desiccant machine beats a compressor one. Compressor units lose much of their capacity as the air drops towards single figures, which is where a stored van sits in January.

Heating on its own is the option we would drop. Warm air holds more moisture, so a tube heater in a closed van parks that moisture on the coldest panel every time it cycles off. A frost stat protects a water system you could not fully drain, but as a damp measure it wastes a hook up.

Refillable calcium chloride traps are the right answer with no mains. Put one at each end and one in the washroom, stand each on a tray because the brine stains fabric and corrodes metal, and expect to empty them. They pull a useful amount of water from a humid van, and nothing from a leak.

Put plainly: with a hook up we would run a desiccant dehumidifier on a humidistat, leave the vents open and skip the heater. With no power, open every vent, take the cushions home and put traps in the three coldest corners.

September to March, month by month

Treat this as a sequence rather than a diary. The first air frost reaches inland and northern areas well before the south west, and the gap between Aberdeen and Cornwall runs to two or three weeks, with another fortnight of swing between years. Work back from your own forecast.

September: the drying month

Nothing has gone wrong yet, and that is the point. The air still carries warmth, so a dry breezy day will dry a van out, and sealant cures properly now rather than sitting soft in the cold. Do the seal inspection and any resealing.

October: the lay-up window

For most of the UK this is when the van goes away properly. Drain the water system before the first hard frost, which in the north and the inland east can arrive in the first half of the month.

November: no more outdoor drying

Long nights and cold panels mean nothing dries outdoors from here until spring. Make the first check visit, empty the traps, and note the hygrometer reading so later visits have something to compare against.

December: storms and blocked drainage

Leaves pile up in roof channels and around the awning rail, and standing water finds a tired seal within days. Clear the roof and check the cover.

January: the worst month for condensation

Surfaces are at their coldest, so ice on the inside of the glass is possible on a hard morning. Black spot starts here, out of sight, so put a torch into the corner lockers and behind the wardrobe.

February: mould shows on fabric

Anything soft left in the van declares itself now, on curtain hems, cushion piping and mattress corners. Catching it now means a wash and a dry rather than a replacement.

March: recommission and inspect

Open up on a dry day and do the smell test before you air the van out, because the first thirty seconds tell you what the winter did. Check the corners with a damp meter before the cushions go back.

Laying the van up so it stays dry

The order matters. Everything before the moisture control step is about getting water out of the van; only then is there any point trying to keep it out.

  1. Pick a dry day with a breeze. An afternoon of dry wind does more than a week of anything else, and a van laid up wet stays wet.
  2. Empty it completely. Food, bedding, towels, clothing and paper all come out. Take the cushions and mattress home if you can, or stand them on edge away from the walls.
  3. Clean and dry every surface. Wipe the walls and ceiling, clean the fridge and prop its door open on the catch, dry the shower and sinks, and take the cassette out to be cleaned and left open.
  4. Drain the water system fully. Taps to the mid position, drain down valve open, water heater plug out, pump and shower hose drained, waste traps emptied. Water left in a pipe over a hard frost splits fittings you will not find until spring.
  5. Inspect the outside. Roof lights, awning rail, panel joints, aerial and solar mounts, and any seam where the sealant has shrunk. A marginal seal in October leaks by February.
  6. Open everything inside. Internal doors, washroom, wardrobe, overhead lockers, seat bases and under bed boxes stay open, the fixed vents stay clear, one roof light stays propped.
  7. Set your moisture control. With mains, put the dehumidifier centrally, set the humidistat around 50 to 55 percent and run the drain hose into the shower tray. Without mains, traps at both ends and in the washroom, each on a tray.
  8. Cover, level and chock. Use a breathable cover over a clean dry van, never a plastic tarpaulin, which traps moisture against the panels. Check the van stands level so water runs off the roof.
  9. Book the visits. Monthly at least, plus a look after a big storm. A filled trap or a climbing hygrometer only helps if somebody reads it.

What goes wrong, and how to put it right

The sealed-up van is the classic. Somebody tapes over the vents against the cold or the mice, the moisture has nowhere to go, and the van opens in spring with mould across the ceiling. Recovery is unglamorous: clean the affected surfaces with a mould treatment suitable for the material, then leave the van open on dry days until the hygrometer settles under 60 percent.

Mould on upholstery is worse when it is dealt with wet. Brush the surface growth off outside on a dry day before you introduce any water, because scrubbing wet spores presses them into the foam. Wash removable covers if the label allows, then dry everything indoors rather than in the van. Foam that still smells after drying is usually past saving.

Frost damage hides until spring. An incompletely drained system splits a pipe, a pump body or the water heater, and the first sign is water under the van at the first fill in March.

The expensive failure is damp that only shows in spring, and it usually turns out not to be condensation at all. Climbing damp meter readings in one corner, a soft patch of floor, a stain spreading from a roof light or a ripple in the wallboard all point to water coming in through a failed seal, and no dehumidifier or trap will fix that. Get a damp report and reseal the entry point: delamination in a bonded wall grows more expensive every winter it is left.

Frequently asked questions

Should I leave a roof vent open in a caravan over winter?

Yes, prop at least one roof light on its ventilation catch and never block the fixed floor and high level vents. Those vents let moist air out and drier air in, and sealing a van up is the surest way to grow mould over a winter.

Do moisture traps actually work in a caravan?

Refillable calcium chloride traps do pull real water out of the air in a small closed space, and with no mains they are the best option you have. They work slowly and need emptying, so check them at every visit; one that fills unusually fast is a signal to go looking for a leak.

Can I leave a dehumidifier running in a caravan in storage?

Only if the site allows mains use and your insurer is happy with an appliance running unattended, and plenty of compounds say no to both. Where it is allowed, set the humidistat around 50 to 55 percent and run the drain hose into the shower tray so the tank cannot fill and stop.

Will a small heater stop damp in a stored caravan?

Not on its own. Warming the air raises how much moisture it can hold, but unless that air can leave through a vent it condenses on the coldest panel as soon as the heater cycles off. With mains, a dehumidifier does more for the same electricity.