An elongated fibrous insulation blanket is pre-cut for custom fitting the insulation blanket into wall, floor, ceiling and roof cavities of different widths formed by the framework of a building. The blanket has one or more cuts extending for the length of the blanket which separate the blanket into two or more longitudinally extending sections. Adjacent sections of the blanket are joined together along the cut(s) by separable adhesive connectors which hold together the sections of the blanket for handling, but are separable by hand along the cut(s) so that the blanket can be handled as a unit for insulating a cavity having a predetermined width or easily separated by hand into two or more sections at a cut for insulating a cavity having a lesser width. The pre-cut insulation blanket may have a facing sheet, overlaying and bonded to a major surface of the blanket, that is separable by hand along the cut(s).