Abstract: Cryogenic fluid tank for a launch rocket, comprising at least one wall delimiting a storage space for cryogenic fluid, said wall being provided, on at least part of the internal surface thereof, with thermal insulation comprising a so-called top layer intended to be in contact with the fluid stored in the storage space, characterized in that the top layer comprises an insulating material covered, on the face thereof intended to be in contact with the cryogenic fluid stored in the tank, with an impervious enclosure forming an impervious barrier between the cryogenic fluid stored and the insulating material, and in that the insulating material of the top layer is completely encapsulated in an impervious enclosure, that is to say the insulating material is covered by the impervious enclosure over its entire surface.