Method to produce a substrate with a foam layer and such a building panel

A method to produce a panel or a substrate element for flooring, comprising a substrate provided with a foam layer, the method comprising

• extruding, in an extrusion process, a substrate comprising a thermoplastic polymer and having a density of 800-2200 kg/m3,
• applying a foam layer on a first surface of the substrate after the substrate has been extruded, the foam layer having a foamed structure and comprising a thermoplastic polymer, wherein a density of the foam layer is less than the density of the substrate, and a thickness of the foam layer is less than a thickness of the substrate,
• wherein the foam layer is adhered to the substrate at least partly by heat from the extruded substrate, wherein the heat from the extruded substrate originates from the extrusion process,
• applying a surface layer on a second surface of the substrate, opposite the foam layer, wherein the surface layer comprises a decorative layer.