Moisture in boats – condensation, mould and dehumidification

Do you have condensation on the windows, damp cushions or a musty smell on your boat? Start by checking whether rainwater is leaking in, whether water has collected in the bilge and whether air can circulate. Moisture problems often become more noticeable when a boat is left cold during winter storage. Once leaks and other moisture sources have been dealt with, a correctly installed desiccant dehumidifier can help keep humidity under control even at low temperatures.

Why do moisture and condensation build up on boats?

The hull, windows and metal fittings cool quickly in contact with water and outdoor air. When warmer, humid air inside the boat meets a sufficiently cold surface, water vapour may condense. This happens when the surface temperature reaches the air's dew point. You can see it as mist on the windows, but moisture can also collect on the inside of the hull, behind furnishings or beneath a berth.

Not every moisture problem is caused by condensation. Rain can enter around hatches, fittings and poorly sealed penetrations. Water may also collect in the bilge or be brought aboard on wet sails, clothing and equipment. Find out where the moisture is coming from before trying to remove it with a dehumidifier.

Temperature also explains why a boat may feel clammy after a change in the weather. As the air cools, its relative humidity (RH)rises even if no additional water has been introduced. The risk of microbial growth and material damage depends, among other things, on how long moisture remains, the temperature and the materials exposed. A single RH reading therefore does not mean you have mould.

What signs of moisture should you look for?

• Misting and water droplets on windows, hatches or cold hull surfaces.
• Musty smells when opening the cabin, lockers or storage compartments after a long period of storage.
• Discolouration or suspected microbial growth on cushions, wood, textiles and surfaces with poor air circulation.
• Corrosion and damp packaging around electronics, fittings and other items stored on board.
• Water in the bilge or moisture that returns after rain.

A musty smell is a reason to inspect the boat, but it does not in itself prove that mould is present. If you find suspected growth or damaged materials, investigate the cause and clean or repair the materials as appropriate. Read more about signs of moisture damage.

How to keep your boat drier during winter storage

Whether your boat stays in the water, stands ashore under a cover or is stored indoors, it makes sense to start with the things you can check yourself:

1. Stop water from getting in. Check hatch seals, covers, fittings and through-hull penetrations. Repair leaks and make sure covers shed rainwater and melting snow.
2. Empty and dry areas where water collects. Inspect the bilge and storage compartments. Remove wet rags, ropes, sails and other equipment that may release moisture during storage.
3. Allow air to circulate. Lift cushions away from cold surfaces or store them somewhere dry. Where appropriate, leave internal hatches and locker doors open so that air can reach confined spaces.
4. Assess the ventilation. Air exchange can remove moisture, but humid outdoor air does not always help inside a cold boat. Do not obstruct ventilation needed for safety or combustion appliances.
5. Monitor conditions during storage. A hygrometer that shows both temperature and RH can help you see whether humidity stays high for prolonged periods. Check the boat after changes in the weather too.

Moisture-absorbing sachets can help in small, enclosed lockers, but their capacity is limited in an entire cabin. A dehumidifier should not be used to compensate for an ongoing water leak.

Is a desiccant dehumidifier a good choice for a boat?

If humidity remains high for long periods after the sources of moisture have been addressed, a  desiccant dehumidifier may be a good option. Desiccant technology captures water vapour in a rotor. The moisture is then discharged through a moist-air hose while drier air is returned to the space. This means there is no water tank to empty.

A desiccant dehumidifier can operate even when the boat is cool. A conventional refrigerant dehumidifier can work well in warmer spaces, but its capacity often falls as the temperature drops. For further examples, read our guide to dehumidifiers for boats and winter storage.

EvoDry 6H 2.0 for smaller boats

Our EvoDry 6H 2.0 is a compact desiccant dehumidifier designed for applications including boats and other small spaces that are sometimes left unheated. The built-in humidistat controls dehumidification according to the humidity setting. You can also select continuous dehumidification when appropriate.

The model has a dehumidification capacity of 7.4 litres per day at 20 °C and 60% RH. This figure was measured under specified conditions; it is not the amount the unit will always remove from a cold boat. Capacity varies with temperature and humidity. Do not choose a dehumidifier based solely on the boat's length: also consider air volume, air leakage, moisture load and whether dry air can reach every compartment.

Plan the installation and electrical safety

The moist air must be discharged outside the boat in accordance with the installation instructions; otherwise moisture may return. Position the dehumidifier securely and protect it from the weather, keep airflow paths clear and make sure the filter can be accessed. Avoid places where water could splash onto the unit or collect near the electrical connection. Read the moist-air hose instructions for EvoDry 6H 2.0 before planning the outlet.