Flexibilized, rigid plastic foam sheets with improved properties particularly desirable for low temperature and cryogenic insulation can be prepared by mechanical com­ pression of freshly expanded closed-cell thermoplastic foams. Thus an extruded foam sheet having a density of 20-100 kg/m³, a y-axial size size of 0.05 to 1.00 mm and a Y-axial compressive strength of at least 1.8 kg/cm² is flexibilized with 0.25 to 240 hours of expansion to give a flexibilized foam with improved elongation, workability, crack resistance and water vapor barrier properties.