Damp in a campervan — how to find it before it destroys the build

Of everything that can go wrong in a campervan, damp is the most insidious. An engine announces itself with noise, brakes with a squeal, a battery with no power. Water in a wall does nothing — it simply spends years quietly breaking down what holds the build together. By the time a visible stain or that characteristic musty smell appears, the repair costs many times more than the sealing that would have prevented it.

That’s also why damp is the most common and most expensive hidden fault when buying a used campervan. A vehicle can have a perfect chassis, a fresh inspection and low mileage while its walls are soaked, and repairing them means dismantling half the interior.

Where the water in the build comes from

A campervan build is a layered structure: outer skin, insulation and inner skin. As long as it stays sealed, it works beautifully. The trouble starts wherever something penetrates it — and in a campervan there are dozens of such places.

• Window and roof vent seals — the most common source of leaks
• Cable glands for solar panels, aerials and lights
• Mounting points for the awning, roof rack and rails
• Seams and trims joining skin panels
• Service hatches and fillers — water, fuel, gas
• Skin damage from branches, hail or minor knocks

On top of that comes a second, badly underrated source: moisture generated inside. Cooking, breathing, drying clothes and showering add up to several litres of water a day that has to go somewhere. On cold nights it condenses on the coldest surfaces — around windows, in corners and near the floor.

How to spot the problem early

The early symptoms are subtle and easy to dismiss. A characteristic smell when you open the campervan after a long stand, a wall that feels slightly soft under finger pressure, discoloured upholstery in a corner, a trim starting to lift. Each of those looks harmless on its own, but together they form a picture worth taking seriously.

• A musty smell when opening up after a few weeks parked
• Soft, spongy spots on the wall or floor
• Discolouration and staining on upholstery, especially in corners
• Trims lifting, skin panels bulging
• Condensation on the windows on cold nights
• Fixing screws that no longer grip in the material

Moisture readings — the only reliable test

Visual inspection only detects the problem once it’s already advanced. The actual condition is established with a moisture meter that measures the water content in the wall material. Readings are taken at a dozen or more points: under windows, in corners, along the floor, around hatches and near cable glands.

Interpreting the results is fairly simple. Values up to around 15 percent are treated as normal. The 15–20 percent range is a warning sign — something is letting water through and the source is worth finding. Above 20 percent we’re talking about genuine saturation, at which the material is already breaking down and the repair requires opening the structure.

What matters is measuring at many points and comparing them. A single raised reading under a window tells you more than an average across the whole vehicle, because it points to the specific place where water is getting in.

Why it’s so expensive

The water itself isn’t the problem — the problem is what it does to the material. Insulation soaks up moisture and loses its properties, wooden structural elements swell and then rot. In constructions using chipboard the process is especially fast: once the material delaminates it never returns to shape, even after drying.

Repairing a saturated wall therefore means not drying but opening the build, removing the ruined material, replacing insulation and rebuilding the structure. That’s work measured in days, and often requires dismantling furniture that was never designed to come apart. Hence the disproportion between the cost of prevention and the cost of repair.

Prevention that actually works

The single most important job is an annual inspection of the seals. Silicone and roofing sealants age under UV and temperature swings — after a few years they lose flexibility and start to crack. Replacing a seal is cheap and takes a few hours, and it prevents a problem that costs many times more to put right.

• Annual inspection of window, hatch and gland seals
• Replacing seals every 4–6 years, before they start cracking
• Regular airing, including in winter while parked
• Using the roof vents while cooking and after showering
• Moisture absorbers during long periods parked
• Checking after any knock, hailstorm or contact with a branch

Condensation — the problem you create yourself

Even a perfectly sealed campervan can get damp from the inside. Four people sleeping through the night in a closed vehicle exhale over a litre of water. Add cooking, drying jackets and heating, and the air inside ends up far more humid than outside and condenses on cold surfaces.

The answer isn’t to seal everything up but the opposite: controlled ventilation. A roof vent left ajar while the heating runs feels like wasting warmth, but it’s exactly what carries the moisture out. Campervans have permanent ventilation grilles for precisely this reason — taping them up for winter is one of the worst ideas going.

Winter storage and damp

A campervan closed up for four months without ventilation is ideal conditions for mould. Before storage it’s worth drying the interior thoroughly, emptying the water system, opening cupboard and fridge doors and leaving a roof vent ajar with its rain cover. Moisture absorbers should be replaced during the season, because once saturated they stop working.

If you can, look in on the campervan every few weeks and air it out. That’s ten minutes that saves you the spring discovery that the upholstery has bloomed with mould and a stain has appeared in the corner.

Damp when buying a used campervan

When viewing a campervan for sale, damp is the first thing worth checking — before mileage, before engine condition and before the equipment list. The seller doesn’t always know about it, and even an honest owner may have no idea that quiet structural decay is under way behind smart-looking trim.

• Ask for moisture readings or bring a meter with you
• Note the smell the moment it’s opened, before the vehicle airs out
• Press the walls by the windows and the floor in the corners
• Inspect the roof seals — fresh silicone can mask a repair
• Check whether trims and laminates are lifting
• Ask for the service history and records of any skin repairs

Freshly applied silicone on the roof isn’t a bad thing in itself, but it’s always worth asking what prompted it. An honest seller will tell you plainly; evasion is a signal to dig deeper.

What to do once you find damp

The first step is finding the source, not drying. Drying a wall without sealing the point where the water enters is wasted effort — the damp comes back within weeks. Only once it’s sealed does drying the structure and assessing how much material has to be replaced make sense.

With slight saturation caught early, sealing and drying is often enough. With advanced damp the build has to be opened up. The boundary runs roughly where the material starts losing its structure — which is exactly why regular measurement matters so much.

The places that leak most often

Workshop experience produces a fairly repeatable list. Windows come first — specifically their lower edge, where water running down the glass collects against the seal. Second place goes to roof vents, especially those fitted after the vehicle was bought, because retrofitted installations rarely match factory sealing.

The third typical point is the awning and roof rack mountings. These parts work while driving, vibrate and transfer loads into the skin, so the sealing around their bolts cracks faster than in static locations. Fourth are cable glands — particularly home-made ones from a solar panel installation, where a hole and some silicone were used instead of a proper gland.

It’s worth knowing this list because it lets you search methodically. If the meter shows raised moisture in a particular corner, the source is usually above that point — water runs downwards inside the structure and shows up lower than where it enters.

Can you fix it yourself?

Replacing the seal around a window or roof vent is within reach of anyone with basic practical skills, provided the right materials are used. What matters is removing all of the old sealant, degreasing the surface thoroughly and applying a sealant made for campervans, UV-resistant and staying flexible. Applying fresh silicone over old is the most common mistake and gives apparent sealing for one season.

Repairing the structure itself is another category entirely. It requires assessing how far the damage reaches, getting to the insulation layer and restoring the wall’s rigidity. Improvisation here usually ends with the problem returning within a year, and with having to dismantle whatever was patched together in the meantime.

Summary

Damp in a campervan is a problem that gives no symptoms for years and then produces the biggest bills of all campervan faults. Prevention comes down to three things: an annual seal inspection, regular moisture readings and sensible ventilation — including in winter.

If you want certainty about your campervan’s condition, or you’re checking a vehicle before buying, we take moisture readings as part of an inspection, along with checking the chassis and the 12 V system.