A pipe insulation system includes a tubular core of fibrous insulation and a flexible closed cell foam layer overlaying the core and forming an outer surface of the system. The tubular core has a first longitudinal slit extending completely through the wall of the core and a second longitudinal slit in the inner surface of the tubular core, opposite the first slit, that forms a hinge permitting the tubular core to be opened and closed to place the system about a pipe. Preferably, the foam layer has a water vapor permeance rating of 0.02 perms or less and the pipe insulation system includes a seal for sealing the first slit so that an outer surface layer of the sealed system formed by the foam layer and seal has a water vapor permeance rating of 0.02 perms or less.