The RSIC installation guide calls for a 1/4″ gap between the ceiling gypsum board and the perimeter of the ceiling. This gap should then be filled with acoustical caulking. The gap is then taped and finished like a regular ceiling / wall intersection. The caulking provides 2 functions. It fills the void and bonds the ceiling to the wall with a flexible material ensuring an air tight ceiling. This air tight ceiling stops noise from flanking around the gypsum board and through what may be a weak area of the ceiling. The second function ensures that in the event of a movement the corner will remain sealed. Typically mud is pushed into the corner and tape is placed over the mud. This drywall mud creates an initial hard bond between the wall and ceiling. If there is any movement that bond can be broken, resulting in a gap that lets noise leak through.