Types of insulation found on narrowboats
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Sat Apr 21, 2018 at 9:20am

Types of insulation found in narrowboats


Spray Foam

Spray on polyurethane foam is an ideal way to insulate narrow boats. The foam is sprayed directly onto the inside of the hull and adheres well to steel hulls and most surfaces. As the polyurethane foam is sprayed it expands which makes it perfect for getting into voids and under battens. 


Spray applied polyurethane foam insulation differs from traditional insulation (sheet insulation and rock wool) in several ways. Firstly the speed of application of sprayed foam insulation is a major advantage over sheet insulation and rock wool. Spray foam is sprayed onto the inside of the boat whereas sheet insulation and rock wool have to be accurately cut to size and fitted. Polyurethane foam also has a far better insulation value than rock wool, and this translates to thinner insulation.


Rock wool

Rock wool Insulation is made from stone, with a non-directional fibre orientation and a higher density, which means not only good levels of insulation, it traps sound waves and dampens vibration.


Easy to cut to fit around cables, pipes, sockets and services,  Insulation is quick and easy to friction fit without leaving gaps or cracks, which can significantly reduce performance.

Good quality rockwool insulation repels water and is vapour permeable, and resists rot and mould.

Being made from stone also means exceptional fire performance again, good quality rock wool is capable of withstanding temperatures of up to 1,177C and achieving the highest Euroclass A1 fire resistance classification. rock wool insulation is ideal for use not only in boats, but in residential applications such as home offices, studies, bedrooms and bathrooms, and TV media and gaming rooms.


Early narrow boats for leisure were insulated by big sheets of expanded polystyrene. This material is still used today by some converters and it has its place in terms of cost and ease of use. There are potential issues though.

PVC wiring reacts with polystyrene insulation. Polystyrene causes plasticisers used in the manufacture of older cabling to migrate thus causing the insulation to go brittle. The potential issue is where there is direct contact of the styrene and PVC sheath close to metal parts that may cause a short circuit if the cable breaks or the insulation cracks. Boat builders are aware of this now and so will run services via conduit if they are using polystyrene sheet.

If you are the owner of an older narrowboat it may be worth checking the integrity of the wiring where it is in contact with polystyrene sheeting. It is good to know that there is polystyrene resistant cabling available for repairs and upgrades. Polystyrene sheets must also be fire retardant.

The other issue is efficiency. It is impossible to completely cover the steel shell with sheets of polystyrene. The sheets have to be meticulously sealed to form a vapour barrier and this barrier can be breached over time. When this happens warm air from the interior gets around the polystyrene sheeting and condenses on the cold steel of the outer shell. This condensation can be considerable. It is not beyond the realms of possibility that over 10L of water could be produced in a very cold month. The only place this condensation can go is into the bilges. Many owners just don’t know it’s there. Come spring and summer, the bilges dry out and there is no issue. However, over time this wetting and drying will cause corrosion. The last place an owner needs corrosion is in a cabin bilge where it is often impossible to see. We would highly recommend making an access hole into the cabin bilge at the rear of the narrowboat so any build up of condensation can be removed as soon as possible.

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Rachel | Tue May 4, 2021 at 5:48pm
Hi there, when purchasing a narrowboat, do you have any advice for the kind of year/ era after which insulation became predominantly spray foam rather than polystyrene? For example, if an owner doesn't know what kind of insulation is installed, would it be reasonable to guess that a boat built post 1990 is spray foam insulated, or would you need to find out for certain with a survey/ BSS ? Many thanks!
Venetian Marina | Wed May 5, 2021 at 11:32am
Hi Rachel, Spray foam started to be used in narrowboats from the early 2000's. But during this time builders also used rockwool. Spray foam has now become the standard for insulation on narrowboats.The new the boat the more chance the insulation is spray foam. If the sale advert doesn't tell you what insulation has been used, a surveyor should normally be able to check for you.
michael robinson | Sun Nov 21, 2021 at 3:52pm
i have a job to fit out a narrow boat for a friend and would like to know if i can use rockwool insulation ,i have read that this is ok but would like to check with you before going ahead .it is and old boat and has some polystyrene in it and have been told that this is dangerous many thanks mick

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