The invention relates to a method of manufacturing a self-supporting case (3, 7) intended to provide thermal insulation for a tank, with a fluid tight membrane, for storing a fluid, said method involving: a step of supplying a bottom panel (10) and a cover panel (11); a step of manufacturing a plurality of bearing walls (14), said step of manufacturing involving, for each bearing wall (14): o arranging in a mould a composite material comprising a fibre reinforced thermoplastic matrix; o inserting fixings inside the mould; o forming the composite material (14), during which forming the fixings are set in the mass of said bearing wall (14), interposing the bearing walls (14) between the bottom panel (10) and the cover panel (11) in such a way that the bottom panel (10) and the cover panel (11) are spaced apart in a thickness direction of the case (3, 7) and that the bearing walls (14) extend in the thickness direction, and fixing the bearing walls (14) to the bottom panel (10) and/or to the cover panel (14) using said fixings, lining the plurality of compartments (15) formed between the bearing walls (14) with an insulating lining. The invention also relates to a case thus manufactured and to a sealed and thermally insulating tank comprising a thermal insulation barrier comprising a plurality of cases as mentioned hereinabove.