Q: Does the FSK (Foil-Scrim-Kraft) facer used on Johns Manville insulations comply with ASTM C 921?
A: ASTM C 921 Standard Practice for Determining the Properties of Jacketing Material for Thermal Insulation is not a "Standard Specification", and it does not state required materials properties. It is not possible for a material to "comply" with ASTM C 921. It is a "Practice" which means it is intended to give some guidance in choosing appropriate materials.
The FSK facing used on Johns Manville Microlite® fiber glass duct wrap insulation and 800 Series Spin-Glas® fiber glass duct and equipment insulation complies with ASTM C 1136 Standard Specification for Flexible, Low Permeance Vapor Retarder for Thermal Insulation, Type II.