A rigid cellular foamed-plastic insulation material manufactured by extrusion of polystyrene in the presence of a blowing agent. The blowing agent dissipates out of the closed cell structure over time, creating a structure that resists liquid water penetration and vapor diffusion. The manufacturing process for XPS insulation results in a smooth surface skin. Typical density of 2 lb/ft³ and R-value of 5 per inch (0.029 W/mK).