Damp, humidity and condensation

Rust needs water. On a car in a garage that water does not have to come from rain: it comes from the air. Every metal surface carries a thin layer of water molecules, and that layer grows as the air becomes more humid. The Canadian Conservation Institute (CCI), part of the Canadian government's heritage service, publishes the guidance many museums use. It says that:

• above about 65% relative humidity, all metal corrosion speeds up significantly;
• at moderate relative humidity, 40 to 55%, most metals show no immediate problems, and this range is also recommended for objects made of mixed materials;
• above 75%, the conditions it classes as "damp", corrosion and mould both accelerate rapidly.

A car is exactly that kind of mixed object: steel, aluminium, chrome, leather, wool, wood, rubber and paper all in one.

Salt changes everything

Salt attracts water. The CCI notes that salt-contaminated iron can corrode at a relative humidity as low as 20%, far below the level at which clean steel is at risk. Road salt in seams, sills and the underside will keep working in a garage that would otherwise be dry enough. Washing the car, underneath as well as on top, before it goes away is one of the most useful things you can do.

Mould

The CCI puts the point at which visible mould becomes possible on some surfaces at about 60% relative humidity. Sensitive materials such as leather can grow mould in about 100 days at 70%, ten days at 80% and two days at 90% or more; clean textiles, paper and wood typically need 80 to 85% before mould is likely at all. A stored car's interior, closed up through a damp winter, can reach those levels easily.

Two ideas explain almost everything about damp in a garage.

Relative humidity (RH) is how close the air is to being saturated with water vapour at its current temperature. Air at 100% RH can hold no more.

Dew point is the temperature at which air becomes saturated and its water vapour starts to condense into liquid.

Warm air can hold more water vapour than cold air. Warming a body of air without adding or removing water therefore lowers its relative humidity, and cooling it raises its relative humidity, until it reaches the dew point. Below that, water comes out as condensation. The Met Office explains dew forming the same way: when a surface cools below the dew point of the air touching it, water condenses on that surface.

Why garages "sweat"

A car, and the concrete floor beneath it, have a great deal of mass. They change temperature slowly. After a cold spell, the steel panels, chassis and floor may still be near freezing when milder, moister air arrives. The air touching them is cooled below its dew point and water condenses on the cold metal and concrete. You see it as a wet floor, misted glass and beads of water on the paint; you do not see it inside sills, box sections and door bottoms, which is where it does the most harm.

The spring thaw

Late winter and early spring bring the worst of this: a car that has been cold for months, followed by mild, damp days. It is the time of year to check the garage most often, and a good reason not to open the garage doors wide on a warm, humid day.

There is no single standard for storing cars, but the guidance from museums, insurers and specialists points the same way.

A sensible working target for a classic car garage is therefore below 60%, and ideally around 50%. The aim is steady conditions in that range, not the lowest possible figure.

Not too dry

Very dry air is rarely a problem in a British garage, but it is possible with a powerful dehumidifier in a small, well-sealed space. Hagerty UK warns that a dehumidifier set too high can draw moisture out of everything around it. Leather, wood and some trim materials can shrink and crack if kept very dry for long periods. If you have a humidistat, set it around 50% rather than at its lowest setting.

You cannot manage what you do not measure. A digital hygrometer, which shows temperature and relative humidity, is inexpensive and tells you more about your garage than any amount of guesswork.

• Put it near the car, at about the height of the sills, not on a cold outside wall or next to the dehumidifier's outlet.
• Use more than one if you can: one in the garage and one inside the car will show whether the interior is drier or damper than the building.
• Choose a recording type if possible. A hygrometer that logs readings, or shows the maximum and minimum since it was last reset, tells you what happened overnight and in a thaw, when you were not there.
• Read it at different times. A single reading on a dry afternoon says little. Look at the pattern over a few weeks.

If readings regularly sit above 60%, or rise above 70% in mild, wet weather, the garage needs work.

A dehumidifier is the most effective single tool for a damp garage. There are two main types, and they behave very differently in the cold.

Compressor (refrigerant) dehumidifiers

A compressor dehumidifier works like a small refrigerator. It cools a coil, and moisture from the air condenses on it and drips into a tank. It works well in a warm room. In a cold one, the coil has to be colder still, and close to freezing it ices up. The machine then stops dehumidifying while it defrosts. One manufacturer that sells both types says a compressor dehumidifier is generally suitable for rooms above 10°C, and that small compressor models will not perform well below 15°C. Which?, the consumer organisation, says refrigerant models typically struggle in colder spaces.

Desiccant dehumidifiers

A desiccant dehumidifier passes air over a wheel of moisture-absorbing material. A small heater then dries the wheel and the water is collected. Because it does not rely on a cold coil, its performance holds up at low temperatures. Which? says desiccant models are effective in both warm and cold spaces and advises choosing one for unheated spaces. The air they return is warmer than the room, by around 10 to 12°C according to the manufacturer quoted above, which slightly warms the garage.

Which type for a garage?

For an unheated British garage in winter, a desiccant dehumidifier is usually the better choice. A compressor model can make sense in a garage that stays above about 15°C, or in summer, and some owners use one of each across the year.

Sizing

Dehumidifiers are rated by how much water they can remove in a day under test conditions, which are usually warmer than a winter garage. As a general guide:

• Hagerty UK suggests a good household unit, extracting about 20 litres a day under test conditions, for a typical garage;
• a larger or draughty space needs more capacity, or better sealing first;
• no dehumidifier can keep up with a building that lets in rain or has large gaps to the outside.

Seal the building first, then size the machine.

Draining, controls and running costs

• Drain it continuously. A small tank fills in a day or two in a damp garage, and the machine then stops. Most units have a hose outlet; run it to a drain or through the wall so the unit can work unattended. Check the hose falls all the way and cannot freeze outside.
• Use the humidistat. A unit that switches itself on and off at a set humidity uses far less power than one running flat out.
• Check whether it restarts on its own after a power cut. If it does not, a short cut can leave it off for weeks.
• Keep it safe. Stand it where it cannot be knocked over, keep its cable clear of the car and the floor where water may lie, and do not cover its vents.

Running costs depend on the machine, the garage and your electricity tariff. Desiccant units draw more power than compressor units of similar size: one manufacturer quotes around 330 watts for a desiccant model against 150 to 260 watts for its compressor models, measured at 20°C and 60% RH. Which? reported in October 2026 that the dehumidifiers it rates as Best Buys cost around 7p an hour on average to run, and the least efficient around 17p. With a humidistat, a unit in a well-sealed garage runs for only part of each day.

A dehumidifier in a leaky building is drying the weather. Before buying one, deal with the obvious ways damp gets in:

• Doors. Up-and-over doors often have gaps at the sides, top and bottom. Fit brush or rubber seals, and a threshold seal where the door meets the floor.
• Gaps and holes. Seal gaps around windows, pipes and cables. The same work keeps out mice: a mouse can use a gap of 5mm.
• The roof. Check for leaks and slipped felt or tiles after heavy rain.
• Gutters and ground levels. Rain running off a roof or a sloping drive should be carried away from the walls and doorway.
• Wet things. A wet car, wet clothes, a damp lawnmower or bags of compost all add moisture.

Ventilation brings in outside air. That helps only when the outside air is drier than the air inside the garage. On a cold, dry, bright day it can; on a mild, wet day it brings moisture in, and that moist air then condenses on the cold car. Ventilation and dehumidifying also work against each other: a dehumidifier in a well-ventilated building is trying to dry the outside air.

The practical rule is:

• With a dehumidifier, keep the garage closed and as well sealed as you can.
• Without one, ventilation is better than nothing, because a sealed garage with no dehumidifier traps every drop of moisture that gets in. Some owners open the doors on dry, breezy days and keep them shut in damp, mild weather. A hygrometer inside and a look at the weather outside tell you which kind of day it is.

Concrete is porous, and a garage floor laid without a damp-proof membrane, or with one that has failed, lets moisture from the ground pass up through it. The Planning Portal advises that an existing garage floor may need upgrading for damp-proofing if the garage is converted to a room. A damp floor feeds moisture into the air all winter and keeps the underside of the car humid.

Testing the floor

A simple test shows whether moisture is coming up through the slab. Tape a square of clear plastic sheet, about 45cm across, tightly to the floor on all four sides and leave it for at least 24 hours. If the concrete under it has darkened or there is condensation on the underside of the sheet, moisture is coming through. The test, which is based on a recognised standard method, shows only whether moisture is present, not how much.

Treating the floor

• Liquid damp-proof membranes are coatings, often two-part epoxy, designed to be painted onto an existing concrete floor to hold back moisture from below. They need a clean, sound surface and careful preparation.
• Floor paints and sealers reduce dust and make spills easier to clean up, but not all of them stop moisture coming through. Check the product is intended as a damp-proof membrane if that is what you need.
• Drainage outside often matters more than any coating. Water standing against the walls or running under the door is the first thing to fix.

Warming the air lowers its relative humidity, so a heated garage feels and measures drier. Heating does not remove any water, though. When the heating goes off, the relative humidity rises again, and the cycle of warming and cooling can make condensation worse on surfaces that stay cold.

The evidence from heritage buildings favours dehumidifying. A 2020 study by David Thickett of English Heritage, published in Studies in Conservation, compared the two approaches in pairs of storage rooms at Dover Castle and Fort Brockhurst, all set to 50% relative humidity. The dehumidified rooms stayed within a 40 to 65% band about 98 to 99% of the time, against about 80 to 90% for the rooms with humidity-controlled heating, and used roughly a third of the energy. Heating did less well mainly because humidity rose in warm summer months, when the heaters' upper temperature limit switched them off. It is one study of masonry buildings, and the author cautions against applying the results too widely, but it points the same way as the physics.

Two kinds of heater make matters worse. Paraffin and bottled gas heaters without a flue release a lot of water vapour as they burn, as council housing advice points out. They also bring fire and carbon monoxide risks into a building that holds a car full of petrol. Do not use them in a garage to dry a car.

A cover protects a car from dust, knocks and drips. It does not dry the air, and the wrong cover makes damp worse.

Indoor covers

An indoor cover is light and breathable, essentially a shaped dust sheet. Because air can pass through it, it does not trap condensation in a dry garage. Make sure the car is clean and dry before you cover it, because grit under a cover can mark the paint and moisture under it has nowhere to go. Footman James recommends breathable, fitted covers indoors, and advises avoiding fabrics that trap heat and moisture.

Outdoor covers

An outdoor cover has a harder job. It must keep rain off while letting moisture out. Good ones have several layers: a soft lining against the paint, a breathable middle layer and a water-repellent outer layer. Hagerty UK warns that a fully waterproof, non-breathable cover can trap moisture, advance corrosion and lift lacquer, and that a cover allowed to move in the wind can scratch the paint. Choose one that fits and can be strapped down.

Airflow enclosures

An airflow enclosure, often called a car bubble, is a plastic envelope around the car with fans that move air through it continuously. Moving air helps stop condensation settling on the car, and the plastic keeps off dust and drips. Points to weigh:

• It needs power all the time. If the fans stop, the enclosure becomes a plastic bag around the car, so plan for power cuts.
• It does not dry the air. It circulates the air from the garage. In a damp building, a dehumidifier in the garage, or one designed to work with the enclosure, is still needed.
• Outdoor versions exist, but they share the limits of any outdoor storage and need a secure power supply.

• Do not wrap the car in plastic sheeting, polythene or a tarpaulin. They trap moisture against the paint and chrome. Footman James and Hagerty UK both warn against covers that do not let the car breathe.
• Do not cover a wet or salty car.
• Do not dry a garage with a paraffin or bottled gas heater.
• Do not rely on heating alone. It moves the problem around rather than removing water.
• Do not leave a dehumidifier with a full tank. It will have stopped working.
• Do not open the doors wide on a mild, wet day to "air" a cold car. That is when condensation is worst.

On each visit, and at least once a month through the winter:

• Read the hygrometer and note the highest and lowest figures.
• Look under the car for wet patches on the floor and drips from the sills.
• Open the doors and boot. A musty smell is the first sign of mould. Look at the carpets, the undersides of seats, the headlining and the hood lining for white or grey bloom, and feel under mats for damp.
• Check the glass for misting inside, which means the cabin is damper than it should be.
• Look at bright metal and chrome for dulling or spots.
• Check moisture absorbers in the cabin and boot, and empty or replace them when full.

If you find mould, dry the car first and only then clean the affected areas, following the trim maker's or a specialist's advice for leather and fabrics. Mould on a stored car is a sign that the storage, not just the car, needs attention.

For the full set of tasks when putting a car away, see winter lay-up step by step and the printable checklist. Mice like a damp, quiet garage as much as mould does: see mice and rats.