Car dehumidifier bags: stopping condensation and damp inside your car

Condensation forms when warm, moist air touches a surface colder than the air's dew point. Inside a car in winter, that cold surface is the glass. The air in the cabin is warmed by the heater and by the people inside it, and warm air carries more water vapour than cold air. When it meets a cold windscreen, the vapour turns back into droplets.

Most of that moisture comes from the occupants and what they bring in:

• Breath and body heat: every passenger adds warm, humid air to a small, closed space.
• Wet coats, shoes and umbrellas: rain carried in on clothing evaporates into the cabin.
• Damp mats and upholstery: once soaked, they release moisture for days.
• Recirculated air: the recirculation setting keeps the same moist air inside rather than drawing in drier air from outside.

A car that mists up now and then on a wet morning is normal. A car that is damp every day, smells musty or has wet carpets usually has a fault letting water in.

Common causes of persistent damp

• Perished seals: worn or shrunken door, window, sunroof and windscreen seals let rain track inside.
• Blocked drains: sunroof drain tubes and other drain holes can clog with leaves and debris, so water that should run away ends up in the cabin.
• Heater matrix leak: a leaking matrix can leave damp carpets, a sweet smell like antifreeze and misting that will not clear. A coolant level that keeps dropping is a further clue.
• Clogged pollen filter: a blocked cabin filter restricts airflow, so the demister struggles to clear the glass.
• Wet carpets and underlay: water from any of the above pools in the carpet and soaks into the underlay, where it can take a long time to dry.

A car that mists heavily on damp days is often pointing to a water leak or a heater problem rather than the weather. The Triumph Stag Archive covers this in its winter driving guide.

The Highway Code says windows should be demisted thoroughly before setting off. Driving with a view obscured by mist is dangerous and can be illegal. The fastest results come from drying the air, not just warming it.

• Air conditioning on: air con removes moisture from the air, so use it with the heater, even in winter.
• Fresh air, not recirculation: switch recirculation off so drier outside air comes in.
• Demist setting: use the windscreen demist button where fitted; it sets the fan and vents for you.
• Build heat gradually: start the heater cooler and raise the temperature as the engine warms, with the airflow aimed at the glass.
• Open a window: in a car without air con, opening a window slightly lets humid air escape.
• Clean the inside of the glass: dirt and grease on the glass give moisture something to cling to, so clean it regularly with glass cleaner and a microfibre cloth.

A dehumidifier bag works alongside these steps. It does not replace them.

A car dehumidifier bag holds a desiccant: a material that draws water vapour out of the air and holds on to it. Left in a parked car, it slowly lowers the amount of moisture available to condense on the glass overnight. There are three common types.

Silica gel bags (reusable)

Most reusable car dehumidifier bags contain silica gel beads inside a fabric pouch. Many have a small indicator dot that is blue when dry and turns pink as the gel fills with moisture. When the indicator changes, the bag is dried out and used again.

Makers' instructions for drying typically involve a few minutes in a microwave on a modest power setting, commonly around 600 W. Some makers also allow natural drying, such as several hours in direct sunshine. Always follow the instructions printed on the bag, as times and methods differ between products.

Calcium chloride absorbers (single use)

Calcium chloride crystals absorb moisture and slowly dissolve into a salty liquid, or brine, that collects in a tub or in the lower part of a bag. When the crystals are used up, the unit is replaced or refilled. These absorbers take in a lot of moisture, but the liquid needs care:

• Spills: makers warn that the crystals and the salty solution should be kept away from metal, leather, fabrics and carpets. Calcium chloride may be corrosive to metals.
• Irritant: the crystals cause serious eye irritation and can irritate skin.
• Children: keep these products out of reach of children.

In a moving car, a tub of brine can tip over. That makes calcium chloride better suited to a parked or stored car, set level in a footwell or boot where a spill would do least harm.

Activated charcoal bags

Charcoal bags are sold mainly to absorb odours. They take up some water vapour, but they are not designed to dry a car cabin, and humid air shortens their life as odour absorbers. Treat them as a way to freshen a stale car, not as a dehumidifier.

• Dashboard: a bag at the base of the windscreen targets the glass that matters most for visibility.
• Footwells and under seats: these catch moisture from wet shoes and damp carpets.
• Boot: a bag here helps in a car that carries wet kit or has a damp boot floor.
• Door pockets: a useful spot for a smaller bag, and easy to check.

A single bag on the dashboard suits a small car with occasional misting. A larger or damper car benefits from two or more spread around the cabin and boot. Place each one where the indicator can be seen.

Secure any bag before driving. Makers warn that a loose unit can slide about, distract the driver or end up under the pedals, so keep bags clear of the pedals, gear lever and handbrake, and away from anywhere they could block the view.

• Follow the stated time and power: do not exceed the microwave time or wattage on the instructions.
• Stay with it: do not leave a bag unattended in the microwave; makers advise monitoring the drying process.
• Let it cool: a freshly dried bag is very hot and releases hot vapour. Allow it to cool completely before handling it or putting it back in the car.
• Do not open the bag: cutting or tearing a pouch spills the contents.
• Children and pets: keep bags out of reach. Silica gel is generally considered non-toxic, but a packet can be a choking hazard for young children. If any contents are swallowed, seek medical advice.

Dehumidifier bags are a slow, passive aid. They will not:

• Fix a leak: if water is getting in, the source needs finding and repairing first.
• Dry a soaked car: a car with wet carpets needs drying before a bag can keep up.
• Work instantly: a bag reduces moisture over hours and days; it will not clear a misted screen on the morning of a journey.

To dry out a wet car, lift the mats, use a wet and dry vacuum on soaked carpet and leave the doors or windows open in a dry, covered space so air can circulate. Fans directed at the carpets speed things up. Acting quickly reduces the risk of mould.

For a classic car laid up over winter, damp is a bigger concern than a misted screen. Mould can spread across seats, carpets and leather trim, and moisture speeds corrosion. Insurance specialists, classic car magazines and owners' clubs suggest moisture absorbers inside a stored car, particularly when the windows are kept shut. For the wider preparation, see the winter lay-up guide and the lay-up checklist.

Inside the car and the garage

A closed car is a small space of its own. A dehumidifier in the garage dries the air around the car, but air inside a shut cabin and boot changes slowly. Bags in the cabin and boot deal with that trapped air. Calcium chloride tubs can sit level in the footwells of a car that is not being moved, and silica gel bags can go on the dashboard and parcel shelf.

For the garage itself, a desiccant dehumidifier is generally preferred in an unheated space, because compressor units lose effectiveness as temperatures fall. The guide to damp and condensation covers garage dehumidifiers in detail.

Under covers and inside enclosures

• Under a cover: a cover keeps dust off but does not dry the cabin, so bags inside the car still help. See the guide to car covers.
• In an airflow enclosure: the cabin is still a closed space inside the enclosure; see the Carcoon guide.
• In a portable garage: the car's cabin is still closed, so bags and a hygrometer inside it are worth adding. See the guide to portable garages.

Check, dry and measure

• Check on every visit: look at the indicators and empty or replace calcium chloride tubs before they fill.
• Dry silica bags promptly: a saturated bag absorbs nothing more.
• Use a hygrometer: a humidity monitor placed in the passenger compartment or boot shows what conditions inside the car are really like.

Conservation guidance from the Canadian Conservation Institute states that relative humidity of 65 per cent or more, with warmth and poor air circulation, promotes mould growth, and that growth is unlikely below 60 per cent with good air movement. Classic car storage guidance generally aims for around 50 per cent in the garage.

Not too dry: timber, leather and wood trim

Drier is not always better. Conservation and storage guidance warns that wood, leather and rubber can shrink and crack in very dry air, below about 40 per cent relative humidity. That matters most on cars with timber in their structure, such as the ash-framed MG T-series, and on any car with leather seats or a wooden dashboard. A garage dehumidifier with a humidistat set around 50 per cent keeps the air dry enough for the steel without overdrying the trim. The author's MG T-series archive covers this in detail in storage humidity and dehumidifiers.

The author of this site uses both electric dehumidifiers and dehumidifier bags for his classic cars: electric units to dry the air in the storage space, bags inside the cars themselves.

Some products sold as car dehumidifiers are small electric units rather than bags. Many compact models use a Peltier, or thermoelectric, element: one side gets cold, moisture condenses on it and drips into a small tank. These units are quiet and compact, but their moisture removal is modest, they suit very small spaces, and many stop when the tank is full until someone empties it.

A full-size desiccant or compressor dehumidifier is a different class of machine, intended for a room or garage rather than a car interior. If one is used in a garage, stand it on a stable, level surface and keep its air intakes clear of walls, covers and clutter.

• Using a bag to hide a leak: damp that keeps returning needs the cause found and fixed.
• Leaving recirculation on: it keeps moist air inside the car.
• Ignoring the indicator: a full bag does nothing more.
• Over-drying in the microwave: makers set a maximum time and power for a reason.
• Putting a hot bag straight back: let it cool completely first.
• Loose bags in the footwell: they can slide under the pedals.
• Calcium chloride tubs in a moving car: spilled brine can damage carpets and metal.
• Treating charcoal as a dehumidifier: it is an odour absorber first.

• Find and fix any leak first.
• Dry out wet carpets before relying on bags.
• Use air con with fresh air when driving.
• Keep the inside of the glass clean.
• Place a secured bag on the dashboard.
• Add bags in the footwells and boot for a damp or stored car.
• Check indicators and dry or replace when full.
• Fit a hygrometer inside a stored car.

For more on looking after a car off the road, return to the classic car storage hub.