As a complete newby to this group I feel somewhat humbled by the expertise demonstrated in the replies.
We are currently in the process of buying our first narrowboat & one of my concerns is that she is insulated with either polyurethane or polystyrene sheets. This is a 2005 Aqualine vessel which seems fairly well thought of.
The type of insulation is one of my greatest worries, as I would imagine that the air gap between the polystyrene sheets & the cold steel wall of the hull, would be a likely cause of condensation on the cold surface. Clearly those vessels with a spray foam insulation, will not be susceptible to the same concern, as the air gap should (in theory at least) be non existent.