Caravan Damp and Dehumidifiers Over Winter: How to Stop Moisture in Storage

Winter Storage Guide

A practical guide from Bedam Leisure — caravan & motorhome specialists

To stop caravan damp over winter you need two things working together: enough through-ventilation to let moist air escape, and dehumidifiers to mop up what is left behind. Sealing the van up tight is the commonest mistake — a closed caravan in a British winter is a cold box full of damp air, and that is how mould gets into cushions and lockers.

Here is what happens to the air in a stored van, and what we would do about it.

1 The problem

Why a stored van gets damp even when it isn't leaking

People assume damp means a leak. Often it does — but plenty of watertight vans still come out of storage in March with black spotting behind the cushions. That is condensation, and three things cause it: upholstery, mattresses and timber hold water and release it slowly; the shell is cold, so outer walls and window reveals sit near the outside temperature all winter; and air that cannot move cannot carry moisture away.

So moisture lifts off the soft furnishings in the warmer part of the day, finds the coldest surface and condenses there overnight. Twenty weeks of that and the mould spores already in every fabric have all they need. As a working figure: under about 60% relative humidity mould struggles, in the 70s it thrives.

Condensation or a leak? Condensation forms on cold surfaces — window reveals, metal frames, the wall behind a cushion — and wipes away. A leak leaves a stain or soft patch in one place and gets worse. Our guide to damp checks and habitation servicing explains what meter readings mean.

2 Ventilation

Ventilation first — everything else is secondary

If you do one thing, do this: leave the van able to breathe. Caravans and motorhomes are built with permanent low-level and high-level ventilation so air can cross the interior. Those vents are not optional extras to tape over for the winter — blocking them is a safety problem as well as a damp one.

So make sure air gets in low and out high: floor vents not blocked by a wheel cover or a grass bank, roof vent clear of leaves. Open the internal doors, wardrobe and every locker so one body of air circulates through the van, and stand cushions and mattresses on edge rather than pressed against a cold outer wall.

Two products earn their place. A thermal vent cover cuts heat loss through a rooflight, raising the temperature of the coldest part of the ceiling. And if the van is at home with a hook-up, a powered roof vent does more for damp than any amount of crystals.

3 Dehumidifiers

Caravan dehumidifiers: what actually works in storage

Two different things are sold as dehumidifiers. A crystal moisture trap is a tub of hygroscopic salts that pulls water out of the air by chemistry alone — no power, nothing to go wrong, which matters because most stored vans have no electricity. An electric unit shifts far more water per day, but needs a hook-up and regular emptying, and a compressor model does little below about 15°C — most of a British winter in an unheated van. At home it is an excellent answer; in a storage compound, no answer at all.

So, for most owners: two or three crystal traps where the air is worst — washroom, living area, and the far end from the door. A single tub in the middle of a 7-metre caravan is the usual reason people conclude they do not work.

4 Covers

Covers: breathable only, and fitted properly

A cover is worth having: it keeps rain, sap and green algae off the roof and seals, and stops the frost and UV cycling that ages a sealant line. One rule — it must be breathable. A waterproof tarpaulin traps everything the van gives off against the bodywork and does more damage in a winter than five years of weather. Fit it on a dry day, pad the sharp corners and tension it, because a loose cover in a gale polishes the side of a caravan like wet-and-dry paper.

5 Before you lock up

The half hour that saves you a spring repair

Find any ingress first, dry the van out, then set it up to stay dry. Our complete winterising guide covers the water system and frost protection, and there is a motorhome storage checklist for vans.

On damp specifically: run a hand over every seal — windows, door, service hatches, rooflights, awning rail — because anything cracked or lifted is where water gets in, and resealing in October beats replacing a wall panel in April. Clear leaves and moss off the roof around the rooflight surrounds. Clean the fridge and leave its door on the winter catch; a shut fridge grows mould. Empty and rinse the cassette, blade open. Take home anything that holds moisture or attracts mice.

Then visit monthly. With a hook-up, an hour of heating is the most effective damp measure there is: warm air holds more moisture, so heating the van and then airing it carries that moisture out of the door. With no power it still pays to open up on a breezy day.