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 at least one, preferably two or three, series of cuts extending between major surfaces of the blanket with successive cuts of each series of cuts being separated by a series of separable connectors located intermediate the major surfaces of the blanket. Each series of separable connectors hold together adjacent sections of the blanket for handling, but are separable by hand 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 one or more of the series of separable connectors for insulating a cavity having a lesser width. Either major surface of the blanket may have a facing bonded thereto with one or more perforated lines or overlapping tabs in the facing permitting the facing to be separated by hand at each series of cuts and separable connectors. Preferably, the perforations of any perforated line(s) in the facing are closed by the bonding agent bonding the facing to the blanket so that the facing functions as a vapor barrier in spite of the perforations.