Abstract: The technical field of the invention is that of building and public works, for example for lining the vertical walls of a building on the inside and/or on the outside, or for recovering water infiltrating through the ground in a plot. The present invention provides a closed insulating box in the form of a flat rectangular parallelepiped having two main faces in register which are spaced apart by four "lateral" faces. According to the invention, the six walls of the outer envelope of the box are rigid, thin, and made of a strong material, and at least one of the main walls is perforated by microholes, the main faces being interconnected by at least one internal stiffener, and the inside volume defined by the envelope is filled with rigid fragments of a granular material of cellular structure, and of relative density less than 1, and also having good thermal insulation properties.