Structural insulation within category A machinery spaces must have a barrier such as metal foil, sheet metal, cementitious coating, or other vapor barrier so that the surface of that insulation is impervious to oil and oil vapors. [CGD 74-127, 41 FR 3844, Jan. 26, 1976, as amended by CGD 74-127, 41 FR 4826, Feb. 2, 1976]