What is claimed is:1. A method of making a faced, pre-cut fibrous insulation blanket, comprising:providing a fibrous insulation blanket; the fibrous insulation blanket having a length, a width and a thickness; the length of the fibrous insulation blanket being at least three times the width of the fibrous insulation blanket; the width of the fibrous insulation blanket being substantially equal to the width of a standard cavity to be insulated; the fibrous insulation blanket having a first major surface and a second major surface;forming a first cut in the first major surface of the fibrous insulation blanket extending for the length of the fibrous insulation blanket; the first cut having a depth greater than one half of the thickness of the fibrous insulation blanket and less than the thickness of the fibrous insulation blanket whereby the fibrous insulation blanket can be handled as a unit for insulating a cavity having a standard cavity width or easily torn apart at the first cut by hand for insulating a cavity having less than a standard cavity width;feeding the pre-cut fibrous insulation blanket into a facing station;providing a facing with a longitudinally extending perforated line extending for the length of the facing;applying a bonding agent to a major surface of the facing for bonding the facing to the second major surface of the fibrous insulation blanket and for closing the perforations of the perforated line so that the facing sheet functions as a vapor barrier; andbringing the facing sheet into contact with the second major surface of the fibrous insulation blanket with the perforated line aligned with the first cut in the fibrous insulation blanket and bonding the facing sheet to the fibrous insulation blanket whereby the faced fibrous insulation blanket formed can be handled as a unit for insulating a cavity having a standard cavity width or easily torn apart at the first cut and the aligned perforated line by hand for insulating a cavity having less than a standard cavity width.2. The method of making a faced, pre-cut fibrous insulation blanket according to claim 1, wherein: the bonding agent is a bituminous material.3. The method of making a faced, pre-cut fibrous insulation blanket according to claim 2, wherein: the first cut has a depth extending to within one quarter of an inch of the second major surface of the fibrous insulation blanket.4. The method of making a faced, pre-cut fibrous insulation blanket according to claim 1, including: forming a second cut in the first major surface of the fibrous insulation blanket which extends parallel to the first cut and is substantially identical to the first cut; and wherein the facing sheet has a second perforated line which is aligned with the second cut when the facing is bonded to the fibrous insulation blanket.