How We Are Controlling Mildew and Mold

If you own a boat controlling mildew and mold can be a daunting task.

Controlling mildew and mold on a boat is all we can hope to accomplish. For years we tried various treatments to eliminate mildew and mold only to find it popping up a few months later. We finally accepted the fact that mildew control was going to have to be a regular maintenance item.

When we lived in San Diego the mildew control required attention at least every quarter. But, when we arrived in the San Francisco Bay area in the fall and winter we realized that moisture trapped inside the boat in colder conditions was off the chart.

Controlling Mildew and Mold Becomes High Priority

As the temperature differential between inside the boat and outside the boat increased the amount of condensation in the boat increased. Condensation accumulates on all surfaces directly exposed to the outside even though you may not see it right away. It can drip from metal window frames, hatch frames, paneling and interior hull surfaces inside cabinets and below decks.

When the boat stays closed, as it usually does in the winter months, all this moisture has no place to go. So, the result is damaged wood trim, paneling, mechanical components below decks and of course mildew and even mold.

What Causes Mildew and Mold

What causes mildew?

  • Moisture. Moisture is one of the most important factors. …
  • Darkness. Mildew and Mold loves darkness. …
  • Stale air. Mildew and Mold spores exist in the environment naturally. So if you open windows from time to time they will come into your boat.
  • Cool temperatures. …
  • Evaporation. …
  • Sudden warmth. …
  • Dirt and grease.

The spores are always present. That means if you kill them today . . . they’re back tomorrow! The real goal is not to eliminate mildew or mold so much as it is to create an environment that limits its growth.

Going To Battle With Mildew and Mold

Considering the list above our goal is to limit the causes. Moisture inside the boat is at the top of the list. This can be done in a couple different ways. Chemically and mechanically.

Chemical Method

The most common method is the little silica gel pellets. Like the ones used in the little moisture absorbing packets in pill bottles. This stuff can be purchased in bulk at places like Walmart for just a few dollars. It’s a simple process where these silica gel pellets are suspended over a catch basin for the water drawn from the air. Or, you can get or make packets of the silica gel to place in drawers and small cabinets.

There are various companies and various designs of these contraptions that are pretty effective in small spaces, like cabinets, closets and closed storage lockers. They are not very effective for larger areas such as saloon (or solon?), galley, cabins and the like.

Mechanical Method

This group of machines comprises three types. All are electric powered but one type can use a 12v power source. The first is a Thermo-Electric Peltier type. The second is a Desiccant system that combines a heater with the silica gel pellets. The third is basically a heat pump, or compressor type. You can learn all about the different types Here.

If you’re away from the docks and only have 12 volt power available the thermostat-electric pettier type would be a good choice. They are helpful in small rooms but not as a stand alone solution. Incorporating other tactics such as improving ventilation, heat and light are also important.

The desiccant system, combining heat and silica gel, is probably a more efficient system than the peltier system. These are designed for small rooms. Depending on the size of your boat more than one may be required.

Removing moisture from the entire boat with one centrally located device can only be achieved by a compressor type device. These machines have a wide range of size and functions. From a small room to whole buildings. Prices also have a wide range. We’ve seen prices from about $150 to over $1,500. Most of these systems allow you to select your desired humidity level and can be set up to run continuously.

Passive Tactics

Once you have humidity under control consider the other elements that promote growth of mildew and mold, like air circulation, light, cleanliness and and temperature. Many boaters install solar powered vents to circulate air, open cabinet doors and drawers to allow air and light in. It’s difficult to address each of the possible mildew and mold enablers you encounter in the list above but the more of them you address the less time you will have to devote to cleaning it up.


For us the compressor type dehumidifier was the single most effective solution to our problem. In addition to sucking lots of moisture out of the air It is also a great air circulator and temperature controller. Since deploying this weapon against mildew and mold we have eliminated condensation inside our boat. Not just in the open spaces but also inside cabinets and drawers because we leave the doors and drawers open during the night for better ventilation in these areas.

If anyone has other suggestions for your fellow boaters let us know in the comments.

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