A storage building can be perfectly dry while the inside of a car parked in it stays damp. For convertibles, and for any car with a leather and wood interior, that gap is where winter damage starts.
Most storage advice is about the outside of the car: salt, rust, tyres, batteries. The inside gets less attention, partly because it looks protected. It has a roof, doors and windows. That is also the problem. A closed car is a small, sealed space, and whatever water goes into it stays there.
The Canadian Conservation Institute, which advises museums on caring for their collections, publishes how quickly mould appears at different humidities. Leather is among the most vulnerable materials, especially when dirty or unfinished, because its surface can carry the proteins mould feeds on. For materials like that, the institute's figures for visible mould are roughly:
Clean fabric and wood need more, around 80 to 85%, before mould is likely. And the institute notes that any spell below 55% stops growth. Specialist storage aims for humidity around that line, as our self-storage article explains. Damp does other harm too: the institute lists veneer buckling, as the glue softens and the wood swells across its grain, among the effects of high humidity.
A winter lay-up runs from late October to early April, around 160 days. At 70%, that is long enough for leather to show mould. At 80%, it would take about a week and a half.
The building's humidity only reaches the inside of the car if the car's air can mix with the building's air. With the doors and windows shut, that happens slowly, and any water already inside the car decides the humidity in the cabin.
Air can only hold so much water. At 15 °C, a cubic metre of saturated air holds about 12.8 grams. Take a cabin with about three cubic metres of air: fully saturated, it holds under 40 grams. One damp carpet holding a cupful of water, about 250 grams, has more than six times what that air can carry. The cabin goes to 100% humidity and stays there until the water finds a way out.
A two-seater has less air than that, so the effect is stronger. And 100% sits at the top of the institute's table, the two-day row.
So dry the car out before it is stored. Lift the mats, check the carpets and the boot, look in the spare wheel well, and take out anything damp. Once it is dry, a car in a dry building can stay dry. Our winter storage guide also recommends moisture absorbers inside the car.
A folded hood is a damp cloth pressed into a tight space. The Triumph Stag Owners Club calls a folded damp hood "the primary cause of mildew and rotted stitching", and Robbins, a US convertible-top maker, warns that lowering or covering a hood when wet or damp "will lead to mildew growth". If the car arrives with its hood down after a wet drive, the hood needs to go up and dry before the car is put away.
For a winter lay-up, the advice is consistent: hood up. Folded for months, a hood sits in the same creases and keeps any damp that went down with it. Raised, the fabric is under gentle, even tension and air reaches both sides. Robbins also recommends keeping the hood latched up when the car isn't being driven, to help it hold its shape and resist shrinking.
Cars that spend the winter under a removable hardtop, with the fabric hood folded underneath, are the exception. For those, the rule is to fold the hood only when it is completely dry.
Fabric, seals and plastic stiffen in the cold. Robbins advises against raising or lowering a hood below 40 °F, which it gives as 5 °C, and a car's own manual may set its own limit. Plastic rear windows are the most fragile part: the Stag club notes that a hard fold, or folding in the cold, can crease one permanently. Get the hood up before the car goes into storage, while the weather is still mild.
Wood and leather respond to humidity in both directions. The same institute notes that very dry air cracks wood: heated rooms in a cold winter can fall to single-figure humidity, and wooden furniture cracks. A wood dash or a veneered door cap wants steady, moderate air.
That matters for owners storing at home, where a garage heated on and off through the winter can swing between damp and dry. A steady building held at or just under 55% stays below the mould line, and is far from the very dry air that cracks timber.
Our winter guide recommends a soft, breathable indoor cover on a clean car, and warns against plastic outdoor covers. A sealed cover traps a layer of air against the car. If the car's surface cools below the dew point of that trapped air, water forms on the paint and the hood, under the cover, out of sight. A breathable cover lets that moisture out.
Up and latched, once completely dry. If the car wears a hardtop for winter with the hood folded underneath, fold the hood only when it is dry.
Not necessarily. Heating on and off makes humidity swing, and very dry air cracks wood. Steady humidity around 55% suits leather and wood best.
They help keep a dry car dry. A wet carpet holds far more water than a cabin's air can, so dry the car out first.
Henry's Car Barn has been storing cars since 1984, at sites in Warwickshire, London, Hampshire and West Sussex, with a fifth at Thruxton from November 2026. Storage sits alongside collection management, transport across the UK and Europe, and on-site work including detailing and Liquid PPF installation.
If you're putting a convertible away for the winter, the most useful first conversation is about the car itself: what the hood and interior are made of, and how it has been kept until now.
