Types of Insulation for a Steel Boat

Insulation is important in a cruising boat for several reasons. First and foremost, it keeps the temperature inside the boat at a relatively consistent level ie. it keeps the boat warm in winter months and cool in summer months. Secondly, (and in a steel boat probably even more importantly than temperature) is it keeps condensation down to a manageable level. As someone who has lived on a steel boat through a (although relatively mild) Irish winter i can attest to the fact that steel does indeed sweat… a lot… Fibreglass boats suffer from this curse a lot less, mainly due to the typical construction of wooden core sandwiched between fibre glass which offers it’s own insulating properties. Wooden boats are the same in this regard because the wood itself is a fantastic insulator, therefore a lot less inside rain…

To keep myself warm through the winter months on Zora, i put “Kingspan” closed cell foam boards in place  between the steel stringers and frames in the forward cabin with the plan of removing them after the winter so we can spray foam the boat . While it did keep me warm, the problem i noticed was it almost impossible to eliminate the air gap between the steel and the insulation, which allowed condensation to form and drip out from any gaps In the boards. This had a very annoying tendency of dripping out of any gaps and finding its way into my ear when i was sleeping.

Another point that is worth bringing up is steel boats die from the inside out. Corrosion can progress unhindered, out of sight and out of mind behind your ceiling or overheads for years and years until one day while pressure washing the bottom, you make a hole. This is a very important point because it drives the ultimate decision on what type of insulation builders go for, as the primary reason for this corrosion is trapped water/condensation due to insulation. There are plenty of schools of thought on the different ways of stopping corrosion, one being that you can avoid it totally by encasing every bare metal surface in spray foam, and another being that you should be able to remove the insulation periodically for inspection and maintenance, with an air gap large enough to allow condensation to run into the bilge.

For now, lets go over a couple of different types of insulation we considered for Zora and the reasons we chose our preferred method.

 Board Foam Insulation

Plenty of steel boats are insulated using board foam, and plenty of people are happy with this set up. The argument here is that you can remove the boards to inspect the steel plating over time, however if you don’t completely stop air making it’s way behind the foam it can cause condensation. This condensation (which is a surprising amount of water especially when there are a few people on board) can get trapped behind the foam thanks to capillary action. Over time this water will find it’s way behind any poorly adhered coating and eat it’s way to freedom through your plating. The way to avoid this is to firstly ensure your coating is perfectly laid on to a well sand blasted hull and to then ensure the foam boards are sealed against the hull with either an adhesive or just fitted well to the steel. Another option is to leave a decent air gap (10mm+) behind the foam so that any condensation can run down the plating, through some limber holes, and collect in the bilge. While i spent the winter on board with the forward cabin insulated in this way, i definitely learned the importance of the above. I found a lot of water under the insulation after 6 months, and it made me shudder to think that if i had chosen this method permanently and not sealed my insulation correctly, the boat would have been swiss cheese after a decade or so. If you follow the principles above and keep in mind the importance of allowing condensation to flow easily into the bilge, you should have no issues long term.

Spray Foam Insulation

 An option that comes well recommended by a number of boat designers and builders is Closed Cell Polyurethane spray foam that is sprayed on the hull in two parts. While there are many kits available for the DIY builder, there are plenty of contractors that can do the job for not all that much more cost than a DIY kit. After looking at all the different options of DIY kits and professional contractors, we decided that this was a job that was best left to the professionals for several reasons. The main reason was that the 2 parts must be a very specific temperature in order to react properly. If the 2 parts fail to work together correctly, they can off gas for a long time which isn’t great for ones health. Another problem with the DIY kits is they rarely give the stated coverage, meaning you have to buy more to offset the inevitable mistakes and miscalculations. A contractor will usually be able to do the job in a few hours, you won’t have to dump the leftovers and they will be able to give you right coverage and depth of foam. Most importantly, they will have the equipment to keep the 2 components at the correct temperatures and if things go arse ways they are liable. All that said, i have seen some DIY jobs that have gone very well, so it all depends on how comfortable you are in your own abilities.

 There was an article shared around forums and discussions over the years where a steel boat was shown that had considerable and advanced corrosion. The article pointed out that the corrosion was due to the insulation foam that was sprayed into the boat. However, on closer inspection of the boat in it’s entirety you can see the deck was also very rotten, as was a lot of the cockpit and bilges, which leads me to believe the boat wasn’t protected properly from day one. I have seen plenty of steel trawlers who’s holds have been spray foamed originally, and the steel under it when removed after a long period for refoaming (think fish hold… they redo the foam periodically) has been perfect. Zora for example was originally spray foamed, and the yard that removed the foam after her accident told me that the steel under it was in perfect condition. Also an important point to remember is the foam MUST be a closed cell foam, as the open cell for will allow moisture to penetrate it and reach the steel. I have read reports of yards sandblasting their hulls and applying the foam directly to the prepared steel and skipping the paint step altogether. Zora was sandblasted and well coated in a 2 part epoxy paint, which after 3 years has shown no sign of degradation. I absolutely no concerns about using the spray foam.

In the image below, you can see how Zora is prepared for insulation. Pine battens are adhered to the hull using an adhesive called Stixall which proved to be incredibly strong in any tests we did. Also, these will be buried in the spray foam adding a lot more strength. We plan to add a frame of 9mm ply, then face it all with pine vertical tongue and groove before building our furniture. To keep things simple we plan to frame our bunks, galley and chart table with 2×2 pine and face it all with the same tongue and groove. Any joins will be covered by a hardwood varnished trim. While not exactly “yottie” to some, it will give a homely look to the boat while being extremely simple and easy to build, even with my considerable lack of woodworking experience.

There are other ways of insulating boats such as products like Armaflex, and Rock Wool insulation. Any research i did on Armaflex led me to think it was the same as using board foam, however you don’t need to have it quite as thick and it is easier to stick to the hull. For the price however, it didn’t appeal to me too highly. Rock wool on the other hand is a highly unsuitable way to insulate a boat. It can hold a lot of water, can easily wick moisture and it just not pleasant to use thanks to the glass fibre in it.

The next installment of this tech talk will go over how we are dealing with the insulation and more details on how we plan to cover it. 

Thanks for reading, any questions feel free to ask.