Tips to Stop Condensation and Keep Your Boat Dry
As the cruising season winds down, it's time for all boat owners to start thinking about winterization. One of the biggest issues for Northwest boaters is how to keep moisture at bay during the cold winter months, especially if you are still using your boat. Mold and mildew not only can do serious damage to your upholstery, clothing, and carpets, but they can also create a health hazard for people with asthma or respiratory issues. Since mold and mildew thrive in a warm and wet environment, in this article, we take a look at two of the main things you need to do to combat their growth: reducing the relative humidity and ensuring proper ventilation onboard.
Reducing the Relative Humidity
It's the weekend and because you miss your boat, you've decided to spend the weekend at the dock. After a great meal and an action-packed movie, you're ready to hit the hay. But while lying in bed below the hatch, you suddenly feel something wet hit your forehead! Due to factors such as heating moist air, cooking with propane, or even two people breathing in an enclosed environment, you have increased the level of relative humidity (RH) within your boat. Relative humidity is the percentage of moisture in the air compared to how much moisture the air will hold before it reaches saturation point and turns into condensation – and when it gets too high it causes water to condense on colder surfaces….like your hatch, exacerbating the condensation problem.
There are multiple ways to reduce the relative humidity inside your boat, especially if you are in a marina with access to shore power, and minimize energy consumption when you are at anchor. So, let's review both powered options for when you are plugged in and non-powered options for when you're not.
Powered Options
Electric dehumidifiers are a great way to remove unwanted moisture from the air. There are many options, but one brand we recommend is Ironwood Pacific. It has no moving parts, and plugs directly into a standard 120 volt outlet. It can operate safely 24/7, and has a thermal cut-off switch which prevents overheating. It is small enough to be placed almost anywhere.
Products such as the DryWave Air Dryer help eliminate dampness, mildew, and condensation by raising the ambient dew point, which inhibits surface condensation. They are good for keeping smaller areas dry and are safe to operate.
If you live aboard you may want to consider a much larger dehumidifier, available at most home improvement stores. These can pull an amazing amount of water out of the air, potentially too much water if you're not careful in high humidity conditions. Thankfully, these bigger units allow you to set the level of humidity you are comfortable with and will shut down when that level is reached, so make sure you don't set it any lower than 50% or you may overdo it, causing wood to dry out and crack.
Non-Powered Options
There are a multitude of chemical-based desiccants that pull water out of the air, including products from DampRid, Dri-Z-Air, and NoDamp. While these come in different shapes and sizes, they all perform the same feat – capturing moisture and combining it with something like calcium chloride to create a non-toxic liquid solution in the bottom of the container.
With options like the hanging moisture absorber for closets, you just throw the unit away when it's full, while others collect the liquid that requires draining, which is then disposed of down your drain. All of these options need to be monitored over the winter for effectiveness, similar to how a cold surface would capture moisture, and refilled/replaced as needed.
An additional option is the non-powered Moso Natural products. Moso Natural products use highly porous, activated bamboo charcoal to eliminate odors naturally, absorb excess moisture, and reduce pollutants without relying on chemicals or masking them with fragrances. Their eco-friendly design works well for small, enclosed spaces, helping to prevent mold, mildew, and dampness and making them a long-lasting, versatile choice for natural air purification.
You may also consider Hypervent under-mattress pads. They are a simple yet highly effective solution for preventing condensation and mildew buildup beneath boat cushions and mattresses. Made from a breathable, rigid polymer mesh, Hypervent creates a ventilated space that promotes constant air circulation, allowing moisture to escape before it can accumulate. Lightweight, easy to cut, and mold-resistant, these pads are ideal for marine environments where trapped humidity is a common problem. By lifting bedding slightly off the surface, Hypervent helps keep sleeping areas dry, comfortable, and odor-free year-round.
Lastly, it's extremely helpful to cover windows and hatches with a protective layer – either inside, outside, or both. Using hatch covers can greatly reduce the temperature differential on the hatch surface, while adding a protective layer of plastic sheeting inside can create an excellent thermal barrier. Both of these options will greatly reduce the condensation problem and condensation around your portholes and hatches when you're spending time onboard.
Ensuring Proper Ventilation
Equally crucial as removing moisture from the air is keeping it moving. Many boats suffer from poor ventilation, but to keep your boat dry, it's essential to give warm damp air a way to escape — making proper ventilation a must. Stagnant air not only traps humidity but also creates ideal conditions for mold and mildew to flourish. Let's explore some effective ways to improve air circulation and provide the ventilation your boat needs.
Types of Boat Dehumidifiers and Their Effectiveness
| Type | Description | Key Features |
|---|---|---|
| Desiccant Dehumidifier | Uses moisture-absorbing materials like silica gel to extract humidity from enclosed boat spaces. | Silent, no power needed (for passive units), ideal for lockers and cabins. |
| Compressor Dehumidifier | Draws in moist air and condenses it using refrigerant coils, collecting water in a tank. | High capacity, efficient in warm/humid climates, great for larger boats. |
| Air Dryer | Circulates dry air to prevent condensation and mildew in enclosed spaces. | Low energy usage, compact design, suitable for year-round use in marinas. |
| Ventilation System | Promotes air exchange with the outside environment to reduce trapped moisture. | Passive or solar-powered options, prevents stale air, maintenance-free. |
| Heater Dehumidifier | Warms the air to raise dew point, reducing relative humidity and surface condensation. | Great in cold climates, protects electronics, dual-purpose heating and drying. |
Air Dryers
Air Dryers are an excellent solution for combating damp, musty conditions aboard your boat. Designed specifically for marine environments, these compact and efficient portable air dryers gently warm and circulate air in enclosed spaces like cabins, lockers, and engine rooms. By maintaining consistent airflow and reducing moisture buildup, they help prevent mildew, mold, and corrosion, common issues in boats stored in humid or cold conditions. Their low-profile design and energy-efficient operation make them a reliable, set-it-and-forget-it tool for year-round protection against marine dampness. We stock several from trusted brands.
However, keep in mind that they are electrically powered, and most boatyards will not let you leave dehumidifiers plugged in while unattended. So, you may want to look for other solutions to keep your boat dry during winter storage.
Vents
There are many types of vents you can install, but below are two types that, if properly placed, will keep air circulating throughout your boat year-round.
Cowl Vents—These vents are installed through the deck in conjunction with dorade boxes to keep water out. They are normally placed fore and aft, allowing air to move in and out of the boat. The tops of the vents can be turned in any direction so that you can position them to catch the most air, no matter where the boat is located. We carry a wide selection of these vents from Nicro, Sea-Dog, and Vetus Denounden.
Solar Vents—These vents are installed either through the deck or in a hatch lens. They are powered by solar technology with a
built-in rechargeable battery, allowing them to operate continuously—day and night—without drawing from your boat’s electrical system. This makes them an ideal low-maintenance solution for maintaining airflow in cabins, lockers, and other
enclosed spaces. For the most effective air exchange, installing two vents is recommended: one configured for air intake and the other for exhaust. This setup promotes a steady circulation of fresh air, helping to reduce condensation,
mildew, and stale odors onboard.
The vents include an on/off switch so you can easily deactivate them when ventilation isn’t needed. They are also designed with a watertight closure feature that allows you to completely seal them off during rough conditions or
green water exposure when underway. We offer a selection of reliable and durable solar vents from trusted marine brands like Nicro and Marinco.
Heaters
While you can't run them 24/7 when you are not onboard, a built-in cabin heater is a great option for warming the air within your boat when you are. Cabin heaters come in diesel, propane, or electric models, depending on how much heat you want to provide, how much power you have available (on or off the dock), and how large your boat is. We stock a multitude of popular cabin heaters from Dickinson, Heatercraft, King, Red Dot, and Sig Marine..
Portable electric heaters are also an option for providing heat while plugged in at the dock. The TrueNorth has a nice low profile (so it can't tip over) with an automatic thermostat to maintain a preselected temperature and an “Anti-Freeze” setting (the heater activates if the temperature drops below 38°F (3°C), if you just want to protect your boat from freezing temperatures. The Pali is an engine compartment heater - which protects your engine and/or bilge against freezing damage. The Pali also uses ceramic elements that self-limit to a predetermined temperature and cannot overheat.
While any one of these heaters is great for keeping your boat warm, heating the dry air isn't the whole story. You need to combine that warmth with proper ventilation (like a slightly cracked hatch or porthole) to really dry out a damp boat. Without ventilation, moisture is stuck inside, and while the heat will draw the moisture into the air, without anywhere to go, it will soon be condensing on cooler surfaces like your hatch, putting us back to square one. With even a small amount of ventilation, the warm, moisture-laden air rises and vents out the opening, taking the water with it. While this will allow a little cool air into the boat, the circulation of that fresh air is what ultimately helps dry out your boat, since warm air can hold more moisture than cool air. It's important to remember that cabin heaters should only be run while you are onboard and can monitor them safely.
Best Practices for Using Heaters on Your Boat
Using heaters effectively is crucial when it comes to keeping your boat warm and reducing condensation. A well-functioning heater can maintain a comfortable temperature inside your boat and help combat the effects of moisture. Diesel heaters are a popular choice among boaters, as they provide consistent heat and can be connected to the boat's fuel system. Additionally, propane or butane heaters offer an alternative that can be more economical, especially when shore power is not available.
When utilizing heaters, ensure that they are equipped with thermostats for optimal temperature control. This feature enables the heater to maintain a set temperature, preventing overheating and excessive energy consumption. Furthermore, remember that heating alone is not sufficient; combining warmth with proper ventilation will help remove moisture from the air, ensuring an optimal environment onboard.
Diesel heaters
Diesel heaters are a reliable choice for maintaining warmth on your boat during cold months. Diesel heaters operate by using a diesel fuel source, generating heat that is circulated throughout the cabin. One of the advantages of diesel heaters is their energy efficiency; they consume less fuel compared to other heating methods, making them cost-effective in the long run. Additionally, diesel heaters are often designed to work in low temperatures, ensuring consistent performance in chilly conditions.
However, when choosing a diesel heater, it's essential to consider installation requirements and space constraints. Some models may need to be professionally installed, which could add to the overall cost. Moreover, regular maintenance is crucial to ensure safe and efficient operation. By following the manufacturer's guidelines, you can enjoy the benefits of a diesel heater while keeping your boat cozy and dry.
Propane/Butane heaters
Propane and butane heaters are commonly used on boats for their portability and ease of use. These heaters can be connected to existing gas lines or used with portable tanks, offering flexibility for boaters. One significant advantage is their rapid heating capability; they can quickly raise the temperature inside your boat, making them ideal for short-term use during outings.
However, safety should always be a priority when using propane or butane heaters. Proper ventilation is essential to prevent the buildup of harmful gases, and it's crucial to monitor the heater while in operation. Additionally, consider the energy consumption of these heaters; while they can be economical for short uses, prolonged operation can lead to increased fuel costs. By ensuring safe and efficient usage, propane and butane heaters can effectively contribute to a warm, condensation-free environment onboard.
Tube heaters
Tube heaters on boats are essentially long, cylindrical heating elements, often enclosed in a protective tube, that are used to provide warmth in various areas of the vessel. They are a relatively simple and efficient way to heat specific spaces without requiring a complex or bulky heating system. Tube heaters can be mounted in various locations, such as cabins, engine rooms, storage lockers, or bilges.
Compressor Dehumidifiers
Compressor dehumidifiers operate by drawing in moist air and cooling it using a cold surface, which causes the water vapor to condense into liquid water. This liquid water is then collected in a tank. One of the advantages of compressor units is their high drying capacity, making them particularly effective in warm and humid conditions. They can quickly lower the relative humidity (RH) inside your boat, which is essential for preventing mold and mildew growth.
However, it's important to consider the power consumption of these units. While they can efficiently remove moisture, they may consume more energy, especially if used continuously. This is why it's crucial to monitor the humidity levels and adjust the settings accordingly, ideally keeping the RH between 50% to 60%. Additionally, noise levels may be a concern; many compressor dehumidifiers can be quite loud, which might be disruptive in the confined space of a boat.
Keeping Your Boat Clean
While all of the options above will help keep mold at bay, the last important step is to remove as much mold and mildew as possible before it gets a chance to really take hold. Wiping surfaces down with a 10% bleach solution is a great way to kill mold – but it's a lot of work to reach every nook and cranny on your boat. Thankfully, there is a great all-natural product that helps you do this with no harsh chemicals – Kanberra Gel.
Insulating Your Boat to Prevent Condensation
Insulation plays a pivotal role in minimizing condensation within your fiberglass boat. By effectively reducing the temperature difference between the interior and the cold surfaces of the hull, insulation helps to maintain a warmer environment inside, which in turn decreases the likelihood of moisture accumulation. When properly installed, closed-cell foam insulation acts as a barrier to cold air, preventing it from reaching the warmer, humid air inside the cabin. This technique not only keeps the interior more comfortable but also protects against potential damage caused by insulation-related condensation on cold surfaces, thus contributing to lower energy consumption.
To maximize the benefits of insulation, ensure that you cover all vulnerable areas, including compartments and below-deck surfaces. This thorough approach helps keep the surfaces above the dew point temperature, thus trapping less moisture. Furthermore, the use of breathable membranes can allow any trapped moisture to escape while still providing insulation, ensuring that your boat remains dry and free from the detrimental effects of condensation, which is especially important for maintaining dry bedding.
Are there specific materials or coatings that help stop condensation in fiberglass boats?
Yes, specific materials like closed-cell foam insulation and breathable membranes can effectively reduce condensation in fiberglass boats. Additionally, applying specialized coatings such as anti-condensation paint can help minimize moisture buildup. Using these solutions ensures a drier environment, enhancing comfort and longevity for your boat's interior.
As you can imagine, many factors will determine how much moisture you need to remove from your boat—including how much time you spend onboard, especially if you are at anchor, how damp an area you live in, and how cold the ambient temperature is. It may take a little experimentation to get it right, but if you try combining the ideas above, the first step will ensure you return to a fresh, clean boat with a pleasant fragrance next spring.
We hope you've enjoyed this article on keeping your boat dry – but if you have any additional questions, please don't hesitate to contact our product experts at (800) 426-6930.