A concept specially suited for concrete, ferro-cement, or other one-piece hulls. If hull is built with double walls, a thick wall in the outer side, exposed to elements, and a thinner in the inner side, separated by a few centimetres, if the space between outer and inner hull is divided into cells, and cells filled with a solid foam, e.g. Expanded Polystyrene, it may act both as blocking water leaks from outside, and as bidirectional sound and thermal insulation adding little weight and complexity to design and construction.