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

• Unpleasant and potentially unhealthy: Mould spores and the musty smell make the car an unpleasant place to sit, and prolonged exposure is generally considered undesirable, especially for people with allergies or breathing issues.
• Shows the car has been wet: Mould is a symptom of a damp problem – usually a water leak, flood or serious condensation – not just a simple valeting issue.
• Damages materials over time: Left long enough, mould can stain fabrics, mark leather, attack glues and backing materials and weaken foam and soundproofing.
• Not cured by a quick freshen-up: A simple spray of air-freshener or wiping only what you can see will not solve it. The leak must be found, the car dried properly and affected areas decontaminated.

Where you’ll see it

You will see mould mentioned on leak reports, insurance assessments and interior restoration quotes. Common notes include mould present under carpets, mould on seat belts, mould on seat bases or mould and odour following flood. It is especially common on cars with long-standing roof, sunroof, door or tailgate leaks, or vehicles that have been parked up damp for months.

Context

Mould sits in the same family as mildew, fungus and other biological contamination, but tends to be more serious than a bit of light surface damp. In a proper repair process the sequence is usually: find and fix the water leak, strip out affected trims and carpets, remove and, if necessary, replace soaked soundproofing and foam, then dry the shell and components thoroughly before cleaning and treating surfaces to neutralise mould and odour. Only then should the car be rebuilt. Simply shampooing the visible carpet with the leak still active usually means the smell and growth will return.

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.

Written by Danny Argent. Last updated 03/12/2025 17:55