UL Design Number FIRM-FILL® Underlayment Thickness Number of Hours Subfloor Joists Resilient or Furring Channels Gypsum Board Batts & Blankets J917 1/2-1" 2 Precast Concrete J919 3/4" 2-4 Precast Concrete J920 3/4" 2-3 Precast Concrete J924 1/2" - 3/4" 2-4 Precast Concrete J927 3/4" 2 Precast Concrete J931 1/2" 2-3 Precast Concrete J957 1/2" 2-4 Precast Concrete J991 1/2-1" 1-4 Precast Concrete J994 1/2-1" 2 Precast Concrete