Builders enhance the energy efficiency of concrete structures by adding insulation--sometimes as a single layer on the face of the concrete and sometimes sandwiched in the center of a tilt-up panel or cast-in-place wall. In the case of insulating concrete forms (ICFs), concrete becomes the "meat" in the sandwich, encased between layers of insulation.
Probably the insulation material most widely used with concrete is rigid polystyrene foam, frequently referred to as EPS (expanded polystyrene foam). However, many contractors aren't aware that EPS is manufactured in two very different forms--extruded foam and molded bead foam. Both are rigid polystyrene plastic foams, but there the similarity ends. The different manufacturing processes result in significantly different cellular structures, each with its own thermal, mechanical, and moisture-resistance properties.