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Insulation in a Boat- So Confusing

Steel like aluminum conduct sounds and heat extremely well. So it was clear to us in the beginning that at some point we need to insulate the boat.

Asking people who have built steel boats the simple and quick answers was: Spray on PUR (Polyurethane). I thought a few years ago that I’ve got the solution.

However, I started a research in internet and started talking myself to people who are more knowledgeable in insulation domain than I am.


Suddenly I realized this is not a simple subject and again…. full of compromises.


The problem with steel hull is that it is practically gas tight. So while we sailors create inside the boat humidity, it cannot escape through the hull. The humidity must get through the vents and companion way outside.

Also covering the insulation (whatever material it is) with an gas tight film by using plastics or aluminum foil is practically impossible. A boat has just way too many corners and we would have ended with a foil with a lot of holes.

Naturally any material that is anhygroscopic will not cause any problems in such a construction. PUR and PUR spray are anhygroscopic. However, they potentially release poisonous gases if there is a fire or a small fire in the boat. I would not like my family to be even remotely exposed to these poisonous gases. PUR can burn.

Armaflex or similar closed cell rubber material must be glued to the hull. Between the glued Armaflex and painted steel hull the potential water vapor has no chance to flow freely into the bilge. If the glued does not stick well or over time loses its adhesion it will potentially form pockets for condensed water.

Armaflex still forms a very nice-looking insulation surface. PUR spray will be a messy job and then in the end one must cut with a long knife to even the surface.

So after several sleepless nights I happened to be in Norway and on that large island there were several shipyard which did build large fishing and professional steel vessels like vessels to transport stuff to oil rigs far outside on the North Sea.

During a liunch break I walked in and happened to meet really experienced (older gentlemen) looking men sitting during their break and sipping coffee. I explained my dilemma and their short answer was: “Do not use PUR in any form. Use rock wool slabs and ensure that between rockwool and painted steel hull there is always air, i.e. a free path for the condensed water to run into bilge.” After that I slept well again.

This Norwegian expert statement, based obviously on huge cumulative experience, made a lot of sense.

First of all, rockwool does practically not absorb water. Try to put a piece of rockwool in a bucket of water. It will not get easily inside wet. Secondly, because it is impossible to make a non-permeable film between cabin air and the insulation the is to ensure at least a pathway for the water vapor that might reach in the end the colder painted steel hull.

So I went into Biltema and bought construction material, i.e. drainage sheet with diagonally placed spacers (studs) that create an air gap of 8 mm. Perfect (sorry boats are full of comprises). Then I took wide Gorilla tape and of course 70mm thick rock wool slabs.

All rockwool slabs were pressed slightly against the drainage sheet (hull) with narrow plywood sheet, which where mounted with bolts to the steel frame. Plywood sheet did have kind of forks to enhance fixing the position of rockwool slabs.


Finally that all was covered with FOAMALITE PVC 5mm thick sheets. Very light but strong material. Easy to cut though.


Now the yacht is still in the uninsulated steel sheet hangar and during summer the hangar air temperature can be really high. However, inside the boat it is nice cool to work. So the insulation seems to work at least in this situation. Excited to tested once the boat is launched.



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