The invention concerns a method for setting an insulating board consisting of glued glass or rock fibre mats (2), between two beams (P) of a structure. The method consists in separating a precut edge (B) of the board along a cut (3) thereof parallel to one of its side edges and located therefrom at a distance equal to its thickness (e). It is folded back at 90 DEG then the whole board is placed between the two beams (P). The width of the board is adapted by compressing the folded back edges at the distance D1 between the beams (P) by the elasticity of the edge (B) folded at 90 DEG whereof the fibres (2) extend perpendicular to the board top and bottom surfaces.
