I am upfitting a studio apartment over a detached garage in Charlotte, NC. Lower level of the garage is not conditioned. I have installed R-19 (kraft face up) batt insulation between the joists. The wood frame stairs will have sheetrock attached to the underside of the stringers which will create a cavity of varying thickness. What is the best way to insulate this cavity? Should batt insulation be installed to the backside of the treads and risers- leaving an airspace of varying dimension between the sheetrock or should the insulation be installed parallel to the stringers leaving a varying airspace between the insulation and backside of the treads and risers? Any advice/ rationale would be appreciated. Thanks!