Mould

Mould is a form of fungus, and while it can grow on the outside of a soft top car, it's more typically found inside.

Convertible cars are more prone to water ingress, and it is common for them to leak around the windows because of dirty or misshaped rubber seals. Once moisture has got into the car, it allows for mould to grow on any food source, which may be something as slight as the oils from your hands. 

Mould likes dark, shady areas, so you may first notice it on the headlining, but it is likely it's also growing on debris that has collected under the seats.

Mould spots are unsightly and the smell is unpleasant, but there is a greater reason to take the problem seriously. When mould spores are present in large quantities, they can have serious health effects.

What it means

Mould is a living growth that appears when a car's interior has been wet for some time. It often shows as grey, white, green or black patches on carpets, underlay, seat fabrics, seatbelts, headlinings and hidden foam. It usually follows a water leak, flooding incident, leaving the car open in heavy rain or long term condensation in a damp cabin. Even when you cannot see it yet, a strong musty, earthy smell is often a sign that mould is starting in the soft materials and underlays.

Why it matters

Mould is never the problem; it is the receipt for one. Growth inside a car means the interior has been wet for a sustained period, which points at a leak, a flood or serious condensation rather than at a valeting failure, and treating the growth without finding the water guarantees it returns.

Living with it is genuinely unpleasant, and for anyone with allergies or breathing difficulties it is worse than unpleasant. Materials suffer too, since prolonged growth stains fabrics, marks leather, attacks glues and backing materials and weakens foam and soundproofing. None of that responds to an air freshener or a wipe of the visible patches; the leak has to be found, the car dried properly and the affected areas decontaminated before anything smells right again.

Which cars turn up with mould?

Leak reports, insurance assessments and interior restoration quotes record it as mould present under carpets, mould on seat belts, mould on seat bases or mould and odour following flood. Cars carrying long-standing roof, sunroof, door or tailgate leaks are the usual candidates, along with anything parked up damp for months.

Context

Mould and mildew are the same family at different stages, and mould is the one that has had time to work down into foams and underlay rather than sitting on the surface. That depth changes the job entirely, because surface treatment cannot reach it and the repair becomes a strip-out rather than a clean. The full order of work is set out under water damage. What holds true here is narrower: shampooing a visible carpet while the leak is still running brings the smell and the growth back within weeks.

Common mistakes

• Trying to fix mould with air-fresheners or quick sprays rather than tackling the underlying water leak and damp.
• Cleaning only the visible surfaces and leaving soaked underlay, foam and soundproofing in place, so mould and smells quickly return.
• Leaving a wet car shut up on a warm day, effectively turning it into a greenhouse that encourages rapid mould growth.
• Assuming a cheap interior valet will solve serious mould and leak problems that really need proper leak diagnosis, strip-out, decontamination and controlled drying.

Authored by Danny Argent. Reviewed by Gary Wray. Last updated 16 May 2026.