The solution is to insulate your boat with Foamseal. Not only does Foamseal effectively insulate the interior of a boat but it also protects the boat from corrosion, due to the foams ability to reduce the build-up of condensation on the steel. The whole application of a narrow generally takes less than a full day.
100s of narrow boat owners have chosen Foamseal to insulate and increase the longevity of their boats.