Damp in the garage – causes and how to reduce it

Does your garage become damp when you park a wet car, or have you noticed condensation on the garage door and rust on your tools? Start by finding out where the water comes from and how it can dry out. Fix leaks and check the ventilation first. If humidity stays high for long periods, a dehumidifier can help – a desiccant dehumidifier is often a good choice for an unheated garage.

Why does a garage become damp?

Imagine driving into the garage on a winter's day, with snow packed into the wheel arches and slush on the tyres. As the snow melts, water collects on the floor and starts to evaporate. If the garage is cool and air exchange is limited, it can take a long time for the moisture to escape.

There are several common causes:

• Wet vehicles and equipment: Cars, bicycles, workwear and tools bring rainwater and snow indoors.
• Condensation: Moist air meeting a cold garage door, metal roof or another cold surface can release water as condensation.
• Water ingress: Rain or meltwater can enter around the garage door, through the roof or from the ground if drainage is inadequate.
• Slow drying: If air movement is limited or the outdoor air is humid, water can linger. More ventilation does not always lower the humidity.

Moisture that remains in building materials can eventually cause damage. Read more about signs of moisture damage and about moisture risks in buildings, as explained by Sweden's National Board of Housing, Building and Planning (Boverket).

How the seasons affect moisture in different ways

Autumn: Rain and high humidity mean that vehicles and equipment often come into the garage wet. As temperatures change, moisture can condense on cold surfaces. Pay particular attention to whether the garage door seals properly and whether the roof and gutters direct water away.

Winter: Snow and ice melting off the car can leave large puddles on the floor. Salt and moisture also put unprotected metal parts at risk. Remove standing water rather than letting all of it evaporate inside the garage.

Spring and summer: Melting snow and heavy rain can expose problems with ground levels and drainage. Warm, humid outdoor air can also cool against a cold garage floor or walls. In some conditions, increasing ventilation can raise relative humidity rather than lower it.

How can you tell if your garage is too damp?

A wet floor after parking a snow-covered car is not unusual. But if the garage stays damp or the problem keeps coming back, you should investigate why.

• Condensation on the inside of the garage door, windows or other cold surfaces.
• A musty or earthy smell and discolouration on timber or other materials.
• Surface rust on tools, bicycles or other metal parts.
• Cardboard boxes, fabrics or wooden items that feel damp.
• Water on the floor that does not dry out between uses.

A hygrometer can help you see how humidity changes when you park the car and when the weather changes. Measure the temperature too. A single relative humidity reading does not tell you whether the garage has suffered moisture damage. Record the readings over a few weeks to see whether humidity rises briefly or remains high.

How to reduce moisture in your garage

1. Stop water getting in. Check the roof, garage door seals, downpipes and slope of the surrounding ground. Do not allow rainwater or meltwater to flow towards and into the garage.
2. Provide a way for water to drain away. Keep the floor and any existing drains clear. Remove standing water, and avoid placing moisture-sensitive items directly on a wet floor.
3. Check the ventilation. Ventilation is needed to remove exhaust fumes, odours and other pollutants. Air exchange can also help the garage dry when outdoor conditions are suitable, but warm, humid outdoor air can cause more condensation in a cold garage. Never run a combustion engine inside the garage.
4. Monitor the moisture levels. Check whether condensation and odours disappear once the cause has been addressed. If humidity remains high, controlled dehumidification may be the next step.

Want to compare the options in more detail? Read our guide to ventilation versus a dehumidifier in a garage.

Which type of dehumidifier is suitable for a garage?

In an unheated garage, the dehumidifier needs to work even when the temperature drops. Our EvoDry desiccant dehumidifiers absorb moisture from the air using a rotor containing a moisture-absorbing material. A heated airflow regenerates the rotor, and the moisture is carried outside as humid exhaust air through a duct. The treated, drier air returns to the garage. This is why desiccant technology is often a good choice for cold or intermittently heated garages.

If you keep your garage warmer and the temperature stays fairly steady, a refrigerant dehumidifier may also be suitable. It cools the air so that water condenses into a tank or drains away. No single temperature threshold applies to every model. See our comparison of desiccant and refrigerant dehumidifiers.

Advantages and limitations of desiccant dehumidification

A key advantage in an unheated garage is that the technology does not need to condense water on a cold surface inside the unit. A duct-connected EvoDry also has no water tank to empty. With humidity control, the dehumidifier can adjust its operation as conditions change.

However, installation needs planning. The humid-air duct must discharge outside, dry air needs to reach the whole garage, and the unit uses electricity for its fan and regeneration heater. Filters and other service items also need maintenance as specified in the manual. Dehumidification is therefore not maintenance-free, nor does it replace ventilation for exhaust fumes and other pollutants.

Size the dehumidifier for more than floor area

A garage where you park a wet car every evening has a different moisture load from one used to store a dry motorbike for months. Consider the garage's air volume, temperature, door openings and moisture load – not just its floor area.

Also think about where the dehumidifier will be installed and how the air will reach the entire garage. In garages divided into sections, air distribution may need extra planning. On a duct-connected desiccant dehumidifier, the humid air must be exhausted outside in accordance with the model's installation instructions. Follow our guide to installing a dehumidifier before planning duct openings and positioning.