Insulation 2018-01-30T15:01:21+00:00


Narrow boat insulation is used not just for keeping warm in the winter but also for keeping cool in the summer and for reducing condensation from habitation. The main types of insulation found on narrowboats are expanded polystyrene or polyurethane (PU) foam.

Narrowboat spray foam insulation
Spray on polyurethane (PU) foam is a popular choice for insulation on a narrow boat
Narrowboat insulation applied during the fit out of a new narrow boat

Insulation… Expanded Polystyrene

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

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 styrene 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.

The above also applies to rigid polyurethane sheet and polyisocyanurate rigid sheet.

Insulation… Polyurethane (PU) Foam

By far and away the best option for insulation on a narrow boat is spray on polyurethane (PU) foam. It is closed cell, so is impervious to moisture penetration. There is no air gap between the insulation and the steel skin because it bonds to the steel.

No air gap (thermal break) means no condensation. Where there is the possibility of a thermal break, for example if a bearer has been stripped of its layer of insulation in order to provide an even surface, the break can be bridged with metallised foil insulation or bagged insulation under the timber sheet panel.

Because PU foam insulation is spray applied, it can be laid on as thick or thin as necessary, so bearers can be coated as well as flat panels. It can also be sprayed over timber batons, adding strength and moisture resistance to the structure. As well as a thermal insulator, it can also reduce vibration and noise carried by the hull.

This used to be a process that was restricted to professional application. However, it is now possible to DIY this process and a number of companies provide canal boat kits. There are a number of highly professional providers if you cannot DIY. A sailaway narrowboat / widebeam will usually come insulated, even a basic sailaway. It is only if you are buying a basic hull only that you would need to consider insulation application. However, if you are fully re-fitted an older narrowboat, you may find you need to replace the existing insulation.

Other Options

  • Thinsulate material – this is very thin and a superb insulator, but needs to be cut by hand and bonded to the steel shell with adhesive
  • Lambs wool – an environmentally friendly option but it must be fire retardant grade
  • Kingspan insulation – can be brought already veneered with timber

Hull Protection… Why is it important?


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