I claim the ornamental design for insulating material, as shown and described.
FIG. 1 is a perspective view of the insulating material in a slightly folded state;
FIG. 2 is a perspective view of the insulating material in a flat state;
FIG. 3 is an edge detail perspective view of the insulating material in which a pattern applied to at least one surface of the insulating material is depicted; and,
FIG. 4 is a top view of a portion of the insulating material in which the pattern applied to the surface of the insulating material is depicted.