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Narrowboat Insulation.

Thought I would go with Spray Foam insulation for my new boat, but was told that it could trap moisture and Flame Retardant Polystyrene sheeting would be best, 50mm. on hull and 25mm. on cabin walls. Any thoughts on which would be more suitable? And do I need any on cabin roof? Many thanks.

Asked by: Anthony Burnell  | 7.56pm, Saturday 1 May

WW says:

Spray foam, if properly done, is probably the best insulation. If it is the genuine stuff, then it should be hydrophobic- and so shouldn't trap moisture. It is also very good at acting as sound insulation as well. It does need to be professionally applied, but is most likely to give long term value.
The person who gave you the advice might be confusing it with glass fibre (eg Rockwool type) insulation, which can trap moisture, but it is unlikely to.
Polystyrene insulation is tried and tested, however, you will need at least 25mm on the cabin roof as well (remember heat rises), and also bearers will need a thin covering, to avoid cold spots showing through the lining material.
One major disadvantage of polystyrene, is that you must not allow any electrical cables to come into contact with it. Polystyrene can (and will) over time, react with the PVC insulation on cables, causing it to break down, and localised electrical resistance in increase, with the possiblity of fire, and certainly voltage drop. If you do use polytyrene sheet, then all cable runs must be in conduit, right up to each individual fitting, remembering that 230VAC and 12V DC cannot share the same conduit.

Mark Langley  | 12.04AM, Sunday 2 May

Readers say:

Thanks for that, it was most informative. It sounds as though my builder is taking the easy option. How thick should the foam be to maximise heat retention and sound proofing? Again, many thanks.

Anthony Burnell  | 8.05AM, Sunday 2 May

Dear Mr. Langley, it seems that I have the same problem as Mr. Burnell and your answer was very interesting tome. May I ask you to name a few products - spray foams available at the market. I would be very interested to see if any of these products might be found in Serbia. Thanks in advance for your kind recommendation.

Lazar Stanojević  | 6.01PM, Wednesday 26 May

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