An insulating panel for a conditioned-air distribution duct, a distribution duct including at least one insulating panel, a method for manufacturing a distribution duct using at least one insulating panel, and to an instrument for cutting the panel. The insulating panel includes at least one insulating core based on mineral wool, preferably on rock wool, and possibly including an exterior layer, for example based on a thin skin of aluminum. On one exterior face a plurality of marks are provided that are straight and oblique with respect to a longitudinal direction of the panel, the marks forming two sets of opposing inclinations oriented at an angle with respect to the longitudinal direction.