Concrete Underpad. TVM’s Concrete Underpad consists of a double layer of polyethylene bubbles, sandwiched between a metalized aluminum sheet, and a white polyethylene surface. It is ideal for many under-slab insulation applications, as the aluminum surface is treated with a coating designed to protect it from corrosion. Using TVM Concrete Underpad in your construction project improves the efficiency of radiant heating systems, and significantly reduces heat-loss to the ground below. Concrete Underpad is also an approved vapor barrier. Size: 500 sq.ft Rolls