How to get rid of condensation on a boat Part 1

Most production boats have notoriously poor ventilation and insulation. The result is a damp, mouldy and not very healthy, nor comfortable environment. They are hot when it’s warm, and freezing when it’s cold. Here’s how to get rid of the problems on a reasonable budget.

After waking up with a soaking wet duvet from condensation dripping from the skylights, and 4C in the salon, we decided enough was enough. And judging by the number of posts in various groups these days, we are not alone.

Drying duvets and mattresses in the sun on a crisp, cool Norwegian autumn day

With a background in something as exotic as building super coolers for computers a few decades ago, I already knew a thing or two on how to approach this problem, so I set to work.

This first part is meant to give you a basic understanding of what you’re up against, to empower you, so that you can make smarter choices when designing your own solution to the problem – cause very few boats are alike.

Part 2 contains a detailed description and tons of pics of exactly how we chose to combat our problems on our Beneteau 411. Rest assured though, despite being theoretical, this Part 1 is not an essay in thermal physics, and everybody should be able to get a firm grip of the basic concepts presented.

When done, you will not only have a nice, warm and dry boat with no mould and fresh air in the cooler months, you will also have a much cooler boat in the summer. And to top it off – it’ll be quieter inside too. The short version is as follows:

Get the heat up, the moisture out, and insulate your boat!

The condensation problem in a nutshell
Humans perspire and exhale in between 40 and 90 grams of water vapor per hour, depending on our basal metabolic rate, and our activity level at any given time. That means two people onboard could end up producing 2-3 liters of water vapor per day (that’s half to a whole a gallon for you US folks).

In addition, various activities during the day contributes to further worsen the situation:

  • Cooking with gas: 3 liters per day
  • Cooking with electricity: 2 liters per day
  • Dishwashing: 0.5 liters per day
  • Washing clothes: 0.5 liters per day
  • Drying clothes indoors: 1.5 liters per day

Unless we actively did something, the situation inside would quickly become shit.

The air’s capacity to hold onto that humidity depends on temperature. Relative Humidity (RH) tells us how much water vapor is in the air, compared to how much it could hold at that temperature, shown as a percentage.

For example, a relative humidity of 50 percent means the air is holding half of the water vapor it is able to hold. 100% RH means that the air is fully saturated, and that your life onboard is miserable.

This brings us over to the next technical term: The Dew Point (DP) is the temperature the air needs to drop to in order to reach a Relative Humidity (RH) of 100%. At this point the air cannot hold more water, is fully saturated, condensation forms, and your life becomes intolerable.

The graph below plots the DP across different Temperature and RH ranges. The colours indicate comfort levels. Legend is in the chart.

Example 1: A comfortable indoor temperature is 22C; At 50% RH, the dew point is 11C, and life onboard is good.

Should the temperature drop to 11C or below, as it easily would do at night with no heating, you have a serious condensation problem on your hands. This is why you must get the heat up.

Example 2: You and your partner have been out and about, and come back to a comfortable 22C and RH 50% aboard. But you met a few friends on the pier, and invited them over for a warming soup for dinner.

As the soup cooks, and you and your friends are enjoying a nice, warm cup of tea (or maybe some Norwegian Gløgg mulled hard cider?), the RH increases dramatically, and before long, it’s condensation time. This is why you must get the moisture out.

It’s easy enough when it’s warm outside and you can just open your hatches, but the cooler months demand other measures., especially in higher latitudes. Depending on whether you’re in a marina or at anchor, you (usually) have different tools and tactics at your disposal.

To reiterate “Get the heat up, the moisture out, and insulate your boat”

As you know by now, air’s capacity to hold humidity is dramatically increased as the air gets hotter. By warming up your boat, you also prepare the air for the next step in the process “get the moisture out”

Air circulation in itself will not help prevent condensation. All the fans in the world will not help prevent condensation. It simply won’t. It’ll just circulate air and moisture around inside your boat.

You need to make the hot air transport the humidity out of your boat. Then, and only then are you able to get a comfortable, condensation free boat. To do this, you either need to blow air in, or suck it out; over pressure, or under pressure.

This can be done actively with a fan sucking air out, or passively with a dorade vent pointing against the wind. For example.

The purpose of insulation is to prevent a layer of air to circulate, as only still air will insulate.

As warm, humid air meets a cool surface, like an uninsulated coach roof, condensation will form on those surfaces. Insulating these surfaces will create a barrier between the cold GRP (or whatever material your boat is made of), and prevent condensation forming on the surface.

This will keep the moisture in the air so that it can be transported out of your boat.

In addition, insulation means far less energy is needed to keep the heat up, thus saving your heating bill and energy consumption – crucial both for your budget, your battery bank, and the environment.

Come summer, insulation will prevent your boat becoming a greenhouse, making life far cooler and more comfortable aboard, so that you don’t need an AC – or need to run it far less than without.

As we all know, warm air is lighter than cold air, so it rises upwards, towards your coach roof. The roof therefore the most important place to treat with insulation.

Retrofitting insulation in the sides are often more complicated due to cabinets, galley, salon etc, but absolutely well worth it if you can. The more you insulate, the less energy you’ll use.

Insulation is a one time job and fee – heating and AC is an ongoing cost. And it’s often noisy.

The type of insulation you’re after is something with a closed cell structure, not regular Rockwool or similar. A closed cell structure is the only type of insulation that is suitable for marine application, as it will prevent it soaking up water and moisture, bacterial growth etc.

There are a few types that fit the bill, and our choice is Armaflex. Here’s why:

It is what I used to prevent condensation and icing on the super coolers I built decades ago, and if it’s good enough to keep a processor from burning out, then it’s good enough to keep my ass warm in the winter too.

Armaflex is a dream to work with, super easy to shape and cut, insulates very well, is marine grade, fire retardant, has antimicrobial properties, and you can even get them as self adhesive rolls these days so you don’t have to mess around with sticky glue all over your boat either.

It is also used extensively in the HVAC industry, and easily accessible at a reasonable price from that kind of company. Sounds like a winner to me!

Other alternatives that others have used are Low-e, Aerogel and Dow. Aerogel is supposedly the superior product, even to Armaflex. I’m sure there must be others too. This is not at all an endorsement for these brands, nor the opposite – I simply don’t have any experience with them.

In the marina
When you have access to shore power, one of the most powerful tools in your arsenal is a dehumidifier. There are two main types of dehumidifiers: condensate dehumidifiers and rotary desiccant dehumidifiers.

The condensate dehumidifiers basically use a cold plate to induce condensation by lowering the temperature to below the dew point. As such, they are completely useless at lower ambient temperatures. But they are also cheap. The rotary desiccant dehumidifiers, however, are effective down to freezing, but not below.

We opted for a rotary type, specifically a Meaco dd8l standard, and at its lowest setting, it consumes only 30W so we can easily run it when at anchor, off the grid too.

At anchor
With no shore power, the dehumidifier is out for many. That is, unless you have massive solar power or a diesel generator running 24/7, or have a very low consumption dehumidifier, like we do.

Our 41’ boat was fitted with a Mikuni 3.5Kw diesel heater by the previous owner. What this does is to draw dry air from the outside, heating it up (making it super dry), creating an overpressure inside, thus forcing the hot, humid air out of our vents.

One issue that we have is that they only bothered to install hot air duct outlets in the stern cabins, and near the companionway in the salon. So we have had to install hot air ducts throughout the entire boat, all the way to the head in the forepeak.

But now we can take our morning dump in a gloriously warm head, without freezing our ass off – literally.

And that rounds up Part 1 in how to get a warm, dry boat on a reasonable budget. Next check out Part 2: How to insulate your boat .


    • Thanks Janet! At least on our boat it looks like a pretty quick and easy job to do the ceiling 🙂

  1. well written !
    Thats what we’re currently doing on our Moody 336.
    Getting the Armaflex into places where it has to be is a nightmare.
    It would be easier if we destroy the whole interior.. (joking)

    I found a calculater that calculates the U-value for air (which means: how big must your insulation under specific circumstances.) Its under ubakus dot de (and you do not need to log in, the demo is ok.
    I found that 4 cm is a minimum with our fibreglass hull and outside temperatures around -10°C.
    So we started firt to insulate the floor panels, next the ceiling, and then aftcabin and the rest as possible.

    • Thanks Mac! We are aware that 10mm is not a lot, but here on the west coast of Norway, in Stavanger, it rarely drops below 0C. And when it does, the air outside dries up a lot, not like the +10C and 95% humidity we have today 😮

      We’ll start with the ceiling, and hopefully some of the sides, and then take it from there 🙂

  2. This has been a particularly tough job for me as my boat has a plastic formed headliner. I am reluctant to take it off to prevent it becoming damaged so you know of any other alternative?

    • Hi Molly! We do have a GRP inner liner, but also ceiling boards that can be easily removed. Perhaps you have something like that too? What kind of boat do you have?

  3. Having sailed in northern waters and dealing with occasional fog plus dew every morning, I have launched my own quest to tackle humidity. I have a Webasto diesel truck heater installed, and it certainly provides dry heat to push the humid air out. But sometimes I don’t want the heat too. I’ve been toying with two ideas – a desiccating dehumidifier, and a seawater cooled radiator.
    A) Imagine a car radiator with seawater run through, and a fan. Might this both capture some humidity and drop the temp? Have you ever heard of anybody doing this? Of course only in colder waters.
    B) Take a desiccating dehumidifier, (which uses too much electricity, most of which is for heat) disassemble it and use the heat from the Webasto for the hot side of the dehumidifier. It would take some creative ducting, but use just a tiny bit of electricity.
    What do you think?

    • A) I’m not entirely sure what you’re drafting up, but my impression is that you want put a sea water cooled radiator inside of your warm boat.
      The sea water in the cooler months will typically be in between 0C and 10 C, well below the dew point inside your boat. Condensation will quickly form on your radiator, ant it will work on the same principle as the compressor type dehumidifiers. As such, it’s important that you keep your temp inside your boat over 15C, as they quickly loose efficiency below that temp. Sea water inside a car radiator, unless it’s stainless steel channels, is corrosion waiting to happen though 😉

      B) Hmmm…I understand the overall principle you’re getting at, but I’m not entirely sure about the details how you’d make it work…

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