Method for producing an inner trim panel for a motor vehicle, said method comprising the following steps: - producing a right-side stack and a wrong-side stack each comprising a reinforcement layer (2a, 2b) impervious to a rigid polyurethane foam, based on a thermoplastic material containing reinforcement fibres, said fibres being generally oriented in a given direction, and a porous fibrous layer (3a, 3b) based on fibres bonded together by a thermoplastic resin similar to said material, the external faces of said impervious and porous layers being clad with a respective porous protection layer (41a, 42a, 41b, 42b), - compressing each of said stacks between two plates heated to the melting point of said material, - disposing each of said stacks in a respective cooled mould provided with a punch and die, said protection layer of said reinforcement layer of said right-face stack being: -- either clad with a right-face cladding layer, -- or left unclad in order itself to form the right-face cladding layer, so as to achieve thermoforming thereof and the association of said layers with each other and to obtain a right-face shell and a wrong-face shell able to define, by superimposition of their respective edges, a closed volume delimited by the protection layers (42a, 42b) of the fibrous layers (3a, 3b), - disposing said shells facing each other in a reaction injection moulding mould so as to be able to define said volume, - injecting a rigid polyurethane foam precursor mixture into said volume so as to form an alveolar core, - after expansion and crosslinking of the foam, removing the assembly obtained from the mould. - producing a right-side stack and a wrong-side stack each comprising a reinforcement layer (2a, 2b) impervious to a rigid polyurethane foam, based on a thermoplastic material containing reinforcement fibres, said fibres being generally oriented in a given direction, and a porous fibrous layer (3a, 3b) based on fibres bonded together by a thermoplastic resin similar to said material, the external faces of said impervious and porous layers being clad with a respective porous protection layer (41a, 42a, 41b, 42b), - compressing each of said stacks between two plates heated to the melting point of said material, - disposing each of said stacks in a respective cooled mould provided with a punch and die, said protection layer of said reinforcement layer of said right-face stack being: -- either clad with a right-face cladding layer, -- or left unclad in order itself to form the right-face cladding layer, so as to achieve thermoforming thereof and the association of said layers with each other and to obtain a right-face shell and a wrong-face shell able to define, by superimposition of their respective edges, a closed volume delimited by the protection layers (42a, 42b) of the fibrous layers (3a, 3b), -- either clad with a right-face cladding layer, -- or left unclad in order itself to form the right-face cladding layer, so as to achieve thermoforming thereof and the association of said layers with each other and to obtain a right-face shell and a wrong-face shell able to define, by superimposition of their respective edges, a closed volume delimited by the protection layers (42a, 42b) of the fibrous layers (3a, 3b), - disposing said shells facing each other in a reaction injection moulding mould so as to be able to define said volume, - injecting a rigid polyurethane foam precursor mixture into said volume so as to form an alveolar core, - after expansion and crosslinking of the foam, removing the assembly obtained from the mould.