Flexible, water resistant, and available in a wide range of densities, expanding polyurethane foam has a variety of uses in both home and marine structures.
The lighter—less dense—kinds can be used for boat flotation, while the denser varieties make excellent insulation. Expanding polyurethane and urethane foams are especially convenient for filling awkward gaps that are otherwise difficult to reach.
These products can often be poured in multiple layers with excellent bonding between layers, and many can be finished with a laminate like a vinyl or polyester epoxy resin.
Like many synthetic materials, expanding polyurethane foam can pose dangers if not used correctly, so always carefully follow any manufacturer's instructions.