Get rid of condensation Part 2: How to insulate your boat!

In Part 1, we discussed the three pillars of condensation management; Heating, Insulation, and Ventilation. Now is the time for the practical implementation of these measures. Get ready for a shitload of pictures, a few surprises, facepalms and cursing…

The first part was written 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.

(So head on over to read Part 1 on how to get rid of condensation on a boat if you missed that one)

In contrast to Part 1, this second part is almost a pictorial, and though lengthy, I hope it should be very easy to follow. The work was done on our three cabin Beneteau Oceanis 411, 2001 model.

To reiterate from Part 1

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

And finally, before we dive into the fun part, it became clear that to provide you with enough detail, we needed to focus on simply one tactic in this post. And that’s why this post will only cover how we insulated our boat. The heating and ventilation will be published in a Part 3, possibly even a Part 4.

Choosing the right kind of insulation

You can’t just walk into Home Depot or your local hardware store, grab a pack of Rockwool, slap it onto your roof and expect good results from that.

Insulation for marine applications simply must not soak up water and humidity, so you absolutely need something with a closed cell structure.

There are many types to choose from, and among these are Armaflex and Aerogel. I curse myself for not thinking of natural cork when we made our final decision.

Despite cork being heavier than its synthetic counterparts, it is definitely something to consider, on it’s own merits alone, but also from a environmental perspective. So educate yourself, and don’t go for something just because I said so 😉

One thing you must be aware of though is that insulation is a game of building volume. The sole purpose of insulation is keeping air still, nothing more. That means the thicker insulation you can put up, the better.

Consequently, it is physically impossible for a 6mm thick material to insulate as well as 10 cm Armaflex, cork, Glava or Rockwool (provided it is installed correctly). It cannot be done; is nothing but marketing magic. (trust me on this – I am a marketer myself)

Long story short, we ended up with Armaflex Class 0, simply cause I’ve used it extensively a few decades ago in supercooling computers (so it was on top of my mind), it’s a fantastic product, super easy to work with, easily available, and relatively reasonably priced. Aerogel is a better product, but also pricier and harder to find.

We went for the self adhesive rolls with Microban. You need Microban to prevent mold on your boat. And I can tell you right away, you definitely want the self adhesive too – it is such a pleasure to work with!

We bought from these guys called Makitech in Norway. Top notch guys, great service, decent pricing. What’s not to like 🙂

The only drawback I can think of with Armaflex is that its surface easily snags or rips, so left unprotected it won’t last long. We were putting ceiling boards over it, so that was not an issue for us.

Materials and tools used

For starters, it is best to be two people on this job. The ceiling took us a total of 24 hours (2 people x 12h).

Note that 1) I am extremely fast and accurate with a knife, and 2) we have spent the past decade renovating our 4 story house together, so this was child’s play for us both.

In addition, we purchased a rotary desiccant dehumidifier (a Meaco DD8L Standard) and let it dry out the boat a full two weeks before we started the job, to make sure that the insulation would stick properly to the ceiling, and not come off because it was too wet.

  • 2 rolls Armaflex Class O, with Microban. 10mm X 1m X 10m.
  • A long ruler, ideally metal. We used a piece of plank that I verified straight. Minimum 1 meter. 1.2m is better.
  • A long, wide cutting board. Minimum 1 meter. 1.2m is better.
  • A carpet knife. More info below.
  • A tape measure.
  • Caulk gun + Tech7 or Sika, and a piece of cloth
  • Electric drill/screw driver
  • A paint scraper or a small crow bar
  • A blunt, flat tool, like a small drywall spatula/jointing knife
  • Angle grinder or multi tool + earmuffs, dust mask
  • Vacuum cleaner, hot water, soap, bucket, cloth etc

This piece of wood in the photo above worked as a ruler, got the job done, but it was less than ideal. It was verified straight, but because it was so thick, it pushed the knife out sideways, and I was always cutting at an angle instead of straight down. Annoying, but not detrimental to the job.

When it comes to the knife, you want something absolutely razor sharp. I used my trusty old Stanley carpet knife with brand new blades, but the best tool for the job is something with a longer blade. Something like this:

The reason for that is that you can extend the blade all the way out and then cut at a much shallower angle in the cut. This will give you a more “slicing motion” rather than the “pushing motion” a steeper angle will give.

The best technique for cutting is: Hold your knife at a very shallow angle, and with a relatively light hand (let the razor edge of the knife do its job) do 2-3 shallower, longer, even passes, rather than one firm, deep cut all the way through. Do not use sawing motion.

The result is much less pulling on the Armaflex, and thus cleaner, faster cuts.

But I’m good with a knife, and didn’t feel like forking out the money for a new one just for this project. Go ahead. Call me a cheap bastard. Feel free to make things easier for yourself 🙂

Taking down the ceiling

In the Beneteau 411, and many other in the Oceanis series from Beneteau from around 1995-2005, it is incredibly easy remove the ceiling. As you can see in the pic below, they are simply plywood boards with tabs that lock into each other.

Each board is then fastened with 2 or 3 screws, and it took us roughly 5 minutes to remove all of the ceiling in the forward cabin; begin in one side, unscrew the two screws, pull the board to the side and out, and it should come loose.

Note that the very first in boards in each side are also fastened with Velcro. We used a small paint scraper to gently pull them loose after unscrewing the screws first. A small crowbar will work just as well.

After Lea had vacuumed and washed the ceiling boards, I started cutting Armaflex strips for her to glue into the groove in the boards.

Note that the board in the above photo is the middle board for the forward cabin. There are two downlights there. Do not cover these with insulation, but make sure to make room for them, as well as the power cord to prevent over heating. A very good idea is to change to LED bulbs to reduce the current going through.

This is what the stripped ceiling looks like in the forward cabin.

This clearly shows how much empty space is above the ceiling boards. And to our big, big surprise, we were able to fit double layers (a total of 20mm) insulation in the entire roof cavity, all over our boat. Roughly even 50% has a full 30mm.

Using my cell phone to see what’s inside the parts covered with inner liner, I discover a whole bunch of empty space. Space that I could insulate if only….

That’s 1/4 square meter extra insulation right there. Just pay damned close attention to where you cut, cause there are wires going in there….

Clearly on a roll, I decided to remove all the material and inner liner I could, while still maintaining structural integrity, avoiding power cords, and keeping the fastening for the ceiling boards intact. I used a combination of knocking (auditory), cell phone camera (visual) and probing to identify where to cut.

Measure twice – cut once!

I managed to remove all this from the forward cabin alone. But it was a lot of hard and messy work, took me at least 90 minutes with that Bosch saw, so I made a new plan for the salon/galley area….a much quicker and much cleaner plan…

What a fucking mess from the sawing! Fine dust all over the forward cabin!

For the biggest pieces of Armaflex, we simply folded them over, and marked the middle with a small piece of electrical tape. Likewise marked the center of the ceiling, then the two of us lifted the Armaflex up and fastened towards one side first, then the other side.

This is pretty much like putting up wallpaper in your ceiling, except from the fact that when this shit is stuck, it stays. You have absolutely no way of getting it off. At least not in one piece.

Finally I trimmed off the excess Armaflex in each side, using my fingers to trace the inner liner edge.

It is vital to make sure that the Armaflex is pressed firmly onto the roof all over, to avoid pockets of air where condensation can occur. Which it will.

Likewise, all seams must be filled with caulk, Tech7 or Sika, to prevent thermal bridges and condensation. In places where you can use double layers, you can overlap the layers instead, since it is much quicker than fiddling with caulk.

Slide your finger over the fresh caulk and close the seam pushing the Armaflex ends against each other. Use a cloth to wipe clean your finger and nozzle of the caulk gun.

With the entire ceiling now insulated with a double layer 10mm Armaflex, plus an additional 10mm inside the ceiling boards, we’ve managed to pack in more than twice the insulation we initially had planned so we’re very happy about that!

The wooden ring for the Dorade vent right above the chart table was fastened with some sort of caulk, so get a good grip on that and just twist and turn; It’ll come loose with relative ease.

The pipe for the Dorade vent

The salon would receive the same treatment as the forward cabin. Carefully knocking around, and inspecting with my cell phone, I decided that this area would be entirely safe to cut away. That is a HUGE piece of unnecessary inner liner!

The Bosch multi cutter was slow to work with, so I decided to bring out my angle grinder, equipped with diamond blade. It should cut through the fiberglass like a hot knife through butter.

I also came up with this brilliant contraption of taping the vacuum cleaner to the angle grinder to more or less completely eliminate the fiberglass dust from the cutting.

While I felt certain I was ready for the Nobel Prize in Physics with that one, it turned out Physics had a very different opinion…

There was dust…

At least I got a chance to show how romantic I am, even after 11 years of marriage, so there’s that!

And there’s also the fact that I had managed to take out over 1 square meter inner lining in total. I’m glad we did this, but if you decide to do the same, your choice is basically the following:

  1. Saw away for hours with a slow, and somewhat messy Fein or Bosch.
  2. Get the job done in literal minutes, but have hours of cleanup.

Of the entire operation, taking out these panels has been the most time consuming part either way. You can easily knock off 4 hours of work omitting this.

Pro tip: Measure twice – cut once!

Sketch up where you want to cut with a pencil, then make the final lines with a sharpie. Then verify again before making the cut

There were a few blots of epoxy glue here and there. It was easily removed by giving it a bit of a tap with this paint scraper and a hammer. This is the same paint scraper that we used for taking down the velcroed ceiling boards:

It is important to get rid of these epoxy blots to get an even surface for the Armaflex, and to avoid the dreaded pockets of air where condensation can, and will, form, in between the coach roof and insulation.

A few days later, Lea and I was on our way to the boat to finish off the last bits of the insulation. Lea made some last minute changes in our plans due to the fact she decided to challenge the law of energy conservation, and thus went from biking to flying form.

She flew face first over the sidewalk, and into some thorn bushes that cushioned her fall, leaving her with only minor road rash, bruises all over, torn GoreTex pants, a concussion and hospitalization with EKG, MRI, the lot.

We’re just glad she missed the lamp post by a few centimeters 😬

Anyway…The magnificent Norwegian health care system delivered yet again, and this gave me the chance to finish the job singlehanded so to speak. And so I did. While it is certainly easier to be two on the job, I was surprised at how easy the installation went, even when the biggest piece of insulation in the entire project was going up.

I pre-cut the length and width of the sheet to 250cm X 89cm, then peeled back around 1 meter of the protecting plastic to expose the adhesive.

Starting in one corner, on port side, I lifted the sheet over my head, resting it there as I lined up the “aft” edge of the sheet, then started lining up the forward edge, then evened out in the middle, working my way from the side of the boat, towards the towards starboard.

And then it’s only a matter of putting the ceiling boards back up again. Easy enough for one guy, super easy for two.

Next up: insulating the hull

This section is quite specific to our Beneteau Oceanis 411, but I am sure people with other boats can glean loads of useful tips regardless.

If you do have a 411, I recommend doing things in this order (so you don’t have to fuck around and figure out how to get this and that unscrewed.)

All cabinets are fastened to the hull with brown plastic tabs and Philips screws. A shitload of brown plastic tabs and Philips screws. You’ll see them when you look into the cabinets.

Some are really fiddly to get to, but it really isn’t too fucking frustrating either. You’ll get the hang of things. At least we did. You’ll swear a lot though. At least we did.

Start with Starboard side and the salon, and remove the bookshelf. This gives you access to the forward cabinet, so remove that next. Then remove the cabinet towards the chart table, and the wall panel should come off reasonably easy; you have to bend it and slide it downwards until it stops on the chain plate rod, then slide it backwards, pivot around the chain plate rod, and over the section divider between the salon and chart table.

Do add heavy duty tape on top of the moldings to protect them from abrasion.

Insulated Beneteau 411

Insulating your boat is EVEN MORE FUN than it looks!!

You get reasonably easy access to the inside through these holes in the sofa backrest, where you can put up even more insulation.

To put things back, do everything in reverse. But don’t, not quite yet. Because with the forward cabinet out, you get access to the two screws on this side of the bulkhead wall holding the shelf on the other side in the forward cabin.

So unscrew those screws, then move to the forward head, remove the starboard cabinet with the mirror, and you’ll see two more screws on the bulkhead holding the other side of the shelf in the cabin.

Once they are removed, you can remove the shelf. Or maybe not. We couldn’t. Cause some fucker had decided to use four tubes of silicone to make sure it was REALLY FUCKING STUCK.

Swearing commenced, but we managed to gently (ohhhh ever so gently!!!) pry it loose with a crowbar, some wooden blocks for leverage (so you get better control of the force you apply), and a small drywall spatula to run along the edge of the shelf to try and loosen its grip a bit.

Insulated Beneteau 411

Next use a Dremel with fine tip to cut the corners around the windows, and a multi saw to cut the straight lines around the windows. When putting back together, a thin line of caulk is all you need to make things look 100% again 🙂

Again, you have to slide the wall panel downwards and pull towards you, WHILE ALSO bending it downwards in the middle to it becomes shorter lengthwise to make it come out, and do everything in reverse when putting things back together again.

Be REALLY fucking careful not to scrape the walls as you do this. You will swear so much doing this, but this work is character building and you will thank yourself when you’re done. At least that’s what you should try to tell yourself.

Insulated Beneteau 411

Next up is to layer as much insulation as you possibly can. We managed three layers to a total of 30 mm for most of the hull; only a few small sections prevented us using more than a single 10 mm layer. Also make sure to overlap the joints from the previous layers with the next as you work to prevent thermal bridges where you can.

When you have come this far, put everything back together again, then get ready to open the port side. This time start with just the port side of the forward cabin; take out the cabinets, and put them on the bed, then remove the wall panel and put that in the galley.

Insulated Beneteau 411


Insulated Beneteau 411

Again we managed 30 mm insulation on the port side. We put everything back together by doing the process in reverse, then moved on to the galley.

Insulated Beneteau 411

Remove the cabinets then use a Fein or Bosch saw to saw along the lower edge of the forward section of the wall panel cause it’s glued in. Use a sharp knife to remove the glue – you will have to redo this. Gray Tec7 is fine.

Insulated Beneteau 411

Remove the wall panel in the same way as the port side. Also make sure to protect the moldings with heavy duty tape here.

Insulated Beneteau 411

Again, layer with insulation as much as you can. Again we managed 30 mm nearly all over.

Insulating Beneteau 411

As an added bonus, insulating your boat gives you killer abs from the wide variety of poses you must endure – here in starboard aft cabin.

And with Part 2 concluded, was it worth it?

Was is worth it? Does insulating your boat actually work? Well – when I got to the boat the day after Lea fell on her face, it was a crisp, beautiful winter day, with temps just dipping below freezing.

The sole purpose of insulation is to prevent heat loss from inside your boat. When I got to the boat, there was quite a bit insulation up, but nearly half was missing; forward section is covered in frost, but the insulation stops right in front of the hatch and granny bar, and there is no ice what so ever cause the uninsulated warmer roof melts the ice away.

Update two years later: we can say absolutely yes! We have a nice, dry boat that takes very little to heat up, even mid winter in Norway, and it makes her sooo comfortable as a liveaboard boat. Even with sub-zero temperatures, we have 22C/71F inside and 45 % RH inside, and you actually don’t want it drier than that.

What we learned, and things to consider:

For starters, this has been a very rewarding project for both of us, and we’re extremely glad we did it. The temperature rose degree by degree as we worked our way through the boat, and the climate inside has improved dramatically, both because of the insulation, but clearly also because of the rotary desiccant dehumidifier we got.

Even with -5C outside, we keep an nice and comfy 18C inside with only 800W electrical oven in the salon. We keep the bedroom to 15C, but it’s so well insulated we sometimes wake up in the night because our bodies produce so much heat we heat up the room past 20C so it’s almost too warm, even with summer duvets!

This enables us to live comfortably onboard, even off the grid at sub zero temperatures, and we are definitely going to celebrate Christmas at anchor out in the middle of nowhere this year 🙂

Now that I know I can get 20mm insulation all over in the ceiling, one thing for you to consider is using 20mm instead of 10mm sheets. It will likely save you quite a bit of time (perhaps 3 hours?) on the cutting, but unless you buy a roll with 10mm in addition, you will not be able to fill all the small gaps (which is what took a looong time, but well worth it), indentations and the ceiling boards like we did.

Despite the unholy mess and the likely 2 hour clean up job taking out the unnecessary liner created, I am very glad we did it. It allowed us to insulate an additional ~1 m2 , and that does matter here up north. We also get a squeaky clean boat, which is nice too.

What’s next?

With the entire boat fully insulated, our priority right now is to do something with the heating situation, because nothing says pro live-aboarder like taking a mid winter morning dump on a warm shitter.

We have a 3.5Kw Mikuni diesel heater that the previous owner installed, but the previous owner was a lazy ass and only bothered to install heating ducts in the stern cabins and the companionway. We have fixed this too, and have installed some electrical ovens for when on shore power.

In short, there will definitely be a Part 3 about heating and ventilation, quite possibly even a Part 4, as we tackle the remaining challenges, so make sure to follow us on Facebook to get the updates, and check out our DIY and refit section for other projects, and the full list of DYI and refit projects we’ve done, and haven’t had the chance to write about….yet….

Fire off all your questions below, and I’ll make sure to answer the best I can!


    • The 411 gets its stiffness from the grid liner in the hull, and the balsa core in the roof sandwich. The liner in the ceiling is simply there to hold the ceiling plates in place and make things look nice 🙂

      You do get a feel for things when you’re nearly born into a boat, have built two boats before you’re 15 years old, and fully renovated an old house from the core and up 😉

      The only thing I was nervous about was the very real possibility of cutting a few wires in the forward cabin

  1. I would also throw my self off a bicycle rather than be inside that cabin covered in fibreglass dust 😉

    Don’t think I could bring myself to cut that all out but it’s an impressive project

  2. I am allso planning insulation with armaflex, i am aboard now. The most humidity is coming from my windowframes and hatches.
    Allso the Hull is condensating and gives water in the bilges .
    But anyhow, i was planning the ceiling but allso the decks, and later on the hullsides to the waterline . I have a Autoterm (Planar) 4kw heater from Russia, with only 1 airduct which blows from companionway to the forecabin. Doing a Great job.
    I live aboard and when the whole boat is heated, the hot air goes automaticly to the aftcabin too. Becouse of natural aircirculation. At this moment it is -8°C outside, my heater is at 1/3 blowing and inside it is 23°C and the humidity is 40%..outside 88%.
    There is a lot of snow and ice on deck.
    The most of moist is coming in the evening when outertemp, drasticly comes down. Is again on windowframes and hatches. We have 5 vents on the boat.
    So, mainquestion is the Hull and under the decks, all solid glass, no coring, insulate the same thicknes as rooftop?

    • Hi Arthur! It makes sense that the window frames and hatches is where most condensation forms, as they are (likely) in aluminum, and therefore acts as a thermal bridge to the outside.

      One trick would be to build a well insulated box to put on top of your hatches. Apparently works really well, but keep in mind not to compromise escape routes in case of fire or other emergencies. As such, these needs to be easily removed from the inside 🙂

  3. Hei Serre,
    Much enjoying reading through your blog. Hope to sail over from Scotland sometime as my daughter is in Oslo…lockdown…and lack of a boat rather holding things back.
    Your description and detail is great, but where I am slightly confused is was there an inner grip liner behind the ply panels? ( so ingenious how they are fitted).
    I see the deck already has a core so providing a good bit of sealed off insulation before you start. Much more difficult to single skin grp as sealing the insulation from vapour is in reality not 100% possible. I know from working as an Architect that some flat roofs are calculated by limiting the amount of moisture build up over winter against it breathing out in summer.
    One last comment, did you consider fitting a heat recovery vent?

    • Hi Mark, glad you liked it! I’m actually not quite sure what you mean by inner grip liner?

      Heat recovery vent has certainly been considered, but as we are planning to switch to a slightly bigger boat, hopefully next year( to live aboard permanently) then it doesn’t make sense to spend TOO much time rigging that up.

      We have the 3.5kw diesel heater, and also 2.6kw electric heating, and with the insulation, we feel this should be sufficient to get us through the next winter season 🙂

      • For the heating: hat about a heatpump such as marinaire, using less energy and being better for the environment than electrical and also, i believe, able to dehumidify?

  4. Hi Sverre
    Nice article. Veldig gøy 🙂 Thanks for taking the time and effort to write it on top of the actual hard, dusty and messy work. I am curious about the cork you mention because I am considering insulating my boat with cork. What thickness cork were you considering? I live and sail mostly in Denmark, with my family in an old “Kaskelot”.

    All the best! Og godt nytår!

    • Hey thanks! The thickness was pretty much determined by how much I could fit in there in the various spaces. I just added layer by layer until I maxed out. I know there exist spray on cork products these days…you may wanna check that out. Don’t remember any name though 🙂

      Godt nyttår!

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