An assembly having a structure provided with an interior volume in which is present for example at least one fluid capable of circulating in said volume and under the action of circulation means. Thermally insulating elements of VIP construction are arranged around a layer containing a PCM and extending around the peripheral wall that surrounds the volume. Protrusions fixed to the peripheral wall delimit spaces in which the thermally insulating elements are positioned. A sleeve extends around the protrusions and the insulating elements.