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Insulation Materials - Polystyrene and rusting?


TitaniumSquirrel

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Hi All,

 

I am embarking on my narrowboat journey having just got a mooring license in Cambridge, but am now on the hunt for a boat itself! I've found quite a few I like, but am struggling to fully understand the issues surrounding insulation materials and the risks. It's not something that seems to be picked up on in any detail on the boat surveys I've seen?

 

A fair few of the ones I like happen to be insulated with polystyrene, which as I understand it is quite a risk for corrosion as well as wiring/fire concerns (for this post, I'm more interested on opinions over the corrosion issues). As polystyrene is vapour proof, if there are any air gaps left, then I understand the warm damp air from the boat gets in, condenses against the steel hull, and then corrodes it from the inside.

 

My question really comes down to how much of an issue is this (in the grand scheme of things, I assume it's nigh on impossible to avoid all corrosion no matter the material used)? I also assume that spray foam and things like Kingspan can have the exact same issue, as if any moisture gets between them and the hull then surely it will condense and cause corrosion and rusting there too?

 

Essentially, I have found quite a few boats that I like, and tick most of my other requirements, but all seem to be polystyrene.

 

Attached is one example from one of the boats, as you can see, it's already started rusting - but is this a real concern?!


Thanks!!!

 

Tom

WhatsApp Image 2021-11-03 at 17.59.14.jpeg

Edited by TitaniumSquirrel
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That was more or less standard in years gone by. I was perfectly happy to have polystyrene in mine. In practice the problem with PVC cable insulation seems to be far less that theory suggests. In any case it only applies if the insulation is touching the polystyrene. even if it is as long as the cables do not vibrate or move there should still be air insulation.

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7 minutes ago, Tony Brooks said:

That was more or less standard in years gone by. I was perfectly happy to have polystyrene in mine. In practice the problem with PVC cable insulation seems to be far less that theory suggests. In any case it only applies if the insulation is touching the polystyrene. even if it is as long as the cables do not vibrate or move there should still be air insulation.

 

Thanks Tony, I am more interested in the issue of water condensation and therefore rusting between the polystyrene and the steel hull. What was your experience with that?

Cheers

Tom

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I would expect that the general consensus would be that spray foam is best but as with everything it does depend on how well it is done.

We had a narrowboat for 14 years and it had polystyrene insulation and we had no obvious issues with rust but unless you are prepared to remove the woodwork to expose the insulation you will not be able to tell if there is an issue unless it is so bad that you see stains below the gunnels or on the floor. 

Your photograph seems to be of an air vent which will always show signs of rust because it is open to the atmosphere.

We now have a Dutch barge which has spray foam insulation and the two vent outlets in the rear toilet and shower both have rust around the edges, these areas are not painted and bare steel rusts very quickly, easily resolved with a coat of Vactan or similar.

I'm afraid that rust is just something one has to accept when you mix a steel boat and water, part of the joy of boating there is always something to repair, repaint or clean.

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There's nothing wrong with polystyrene per se, but there are better insulation materials these days. The main problem with polystyrene was the way it was installed on many boats, just loose sheets between the steel and wooden lining. 

 

Whatever slab insulation you use has to be bonded to the steel and joints filled with sprayfoam and/or taped with aluminum tape. Sprayfoam is a vapour barrier but rockwool should have a plastic sheet vapour barrier over it. I'm not sure about 3M Thinsulate? It may have it's own vapor barrier?

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Very little evidence on my then 20 year old ex hire boat when I modified and relined the back cabin. There was evidence low down around the back door where there had been persistent leaks. If it is tightly fitted to minimise and air gaps it would be difficult for water vapour to get to the steel, however without ripping out the linings you won't know how well the polystyrene has been fitted. I would far rather have polystyrene, preferably so called self extinguishing, that rock wool or glass fibre because that may absorb water from window frame to cabin side or vent leaks.

7 minutes ago, TitaniumSquirrel said:

 

Thanks Tony, I am more interested in the issue of water condensation and therefore rusting between the polystyrene and the steel hull. What was your experience with that?

Cheers

Tom

 

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I had occasion to repaint part of the inside of the hull in the engine bay. Whilst there I replaced the polystyrene with Kingspan just because it is a better insulator. I found no rust on the hull side when I removed the polystyrene. I also replaced the wood paneling whilst I was working in that area.

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In my experience the steel under polystyrene might 'rust' a bit but will not corrode badly enough to worry about. More concerning is damp that might run down and collect under the floor and start to produce evil corrosion and scale on the lower sides and bottom. When people build boats they want to get the floor down and start fitting it out asap with no thought about ever having a look at what is rotting or corroding away under the floor.

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36 minutes ago, Bee said:

In my experience the steel under polystyrene might 'rust' a bit but will not corrode badly enough to worry about. More concerning is damp that might run down and collect under the floor and start to produce evil corrosion and scale on the lower sides and bottom. When people build boats they want to get the floor down and start fitting it out asap with no thought about ever having a look at what is rotting or corroding away under the floor.

 

So true. I can't understand why the floors aren't fitted after the bulkheads and linings so they can be easily lifted.   Even our GRP cruisers that had an inner hull moulding the majority of the floor could be lifted as required.

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Since you are the same chap who wants to live off grid without fossil fuel, as I said on the other thread, insulation is key. And polystyrene isn’t very good. If I were you I would only countenance a spray foam insulated boat. Although I suppose maybe you plan is to buy a polystyrene insulated boat, gut it and apply spray foam in thick layers?

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I used commercial foam insulation panels (the kind with aluminium skins) and sealed gaps as best as I could with aluminium tape. After 3 years it's all looking fine behind the ones I checked.

As others have said, keeping moisture laden air out is key.

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Looking at the OP's photograph I suspect the holes for the mushrooms were cut and no protection was given to the bare metal edges of the holes. 

 

Our boat has Polystyrene and Rockwool. It's definitely not as thermally efficient as sprayfoam but it's still possible to get the cabin too warm even at this time of year. 

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If the assumption is that polystyrene chemically causes corrosion of steel, in my experience I have never found it to be so.

 

The effect on pvc wiring insulation is also minimal.

I have found, the poly erodes and sticks to the cable. It is claimed that this causes the insulation to become either softer or brittle, again I have not found this to be so.

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2 minutes ago, Tracy D'arth said:

If the assumption is that polystyrene chemically causes corrosion of steel, in my experience I have never found it to be so.

 

The effect on pvc wiring insulation is also minimal.

I have found, the poly erodes and sticks to the cable. It is claimed that this causes the insulation to become either softer or brittle, again I have not found this to be so.

 

I totally agree with your thoughts re wirirng insulation. All my observations seem to point towards the wiring destroying the polystyrene rather than the other way round. However when I have said this explicitly in the past other have sought to prove me wrong.

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5 minutes ago, Tracy D'arth said:

The effect on pvc wiring insulation is also minimal.

 

I see the same. Polystyrene has no effect on PVC cable insulation, it is the other way around. PVC cables touching polystyrene will eat into the polystyrene but the cables themselves remain unaffected. I see this happen on the electric thermal stores I fix for a living, all the time.

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I understand that in theory the migration of plasticiser in the pvc into the polystyrene causes the pvc to become brittle. But in practice after the initial migration the polystyrene structure collapses and there is no longer any in contact with the pvc, and thus the migration stops before any real harm has been done.

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In your case polystyrene would be a waste of time.Thick spray foam is what you need.

Having had boats with both the difference is huge plus the fact that with polystyrene you get the added benefit of condensation 😉

don't forget double glazing either as windows account for a large percentage of thermal transfer and condensation.

 

Loddon has both spray foam and double glazing and is warm and dry in the winter and coolish in summer.

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On 03/11/2021 at 19:07, blackrose said:

There's nothing wrong with polystyrene per se, but there are better insulation materials these days. The main problem with polystyrene was the way it was installed on many boats, just loose sheets between the steel and wooden lining. 

 

Whatever slab insulation you use has to be bonded to the steel and joints filled with sprayfoam and/or taped with aluminum tape. Sprayfoam is a vapour barrier but rockwool should have a plastic sheet vapour barrier over it. I'm not sure about 3M Thinsulate? It may have it's own vapor barrier?

 

My boat is insulated with 3M Thinsulate and yes one side is a vapour barrier. On the hull sides it is glued directly to the hull and where water tanks have been insulated with it, it has been loose laid. No signs of corrosion where I have bothered to access it.

 

It's heat insulation properties are marginally less than sprayfoam, but being a fabric it has a consistent thickness, unlike sprayfoam where the thickness depends on the skull of the person spraying it. Subjectively, sound insulation seems better than sprayfoam.

Edited by cuthound
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