A pipe insulation assembly includes a length of tubular insulation with a longitudinally extending slit so that the tubular insulation can be passed over and mounted on a length of pipe and a jacket overlaying the outer surface of the tubular insulation. To facilitate the installation of the pipe insulation assembly, a ruler, which is associated with the jacket and, preferably, no longer visible when the pipe insulation is mounted on a pipe, extends for the length of the jacket and is located adjacent the slit in the tubular insulation. The ruler has units of linear measurement marked thereon for measuring the pipe insulation assembly in the direction of the length of the pipe insulation assembly.