Panel (1) comprising, in succession, a first rigid board forming the back of the panel, a first thermally insulating layer (3) borne by the said backboard, an impervious covering (4) covering the said first thermally insulating layer, a second thermally insulating layer (5) which partially covers the said impervious covering and a second rigid board (6) covering the said second thermally insulating layer, characterized in that it comprises a film (8) which covers at least part of the said impervious covering which is not covered by the said second thermally insulating layer, the said film comprising at least one protective portion (9) and at least one spew portion (11) adjacent to the said protective portion, the said protective portion and the said spew portion being able to be detached from the said impervious covering independently of one another.