Underfloor Heating Underlay 2.5mm is an insulating floor underlay made from extruded XPS polystyrene that is designed for use under a variety of floating floors, particularly where underfloor heating or cooling is used. There are rectangular holes in the board that allow warm or cold air from the heating system to be distributed evenly across the floor. This floor insulation is available in the form of a rectangular panel, which makes it easy to install and provides effective thermal insulation for the floor. As a result, it helps maintain a comfortable room temperature and improves the efficiency of the heating or cooling system.