A system for packing and assembling a flexible duct that includes a sleeve. The sleeve includes a tubular vapor barrier and an insulating layer within the vapor barrier and includes a central cavity. Further, the sleeve is radially compressed and rolled along an axial direction. The system also includes an air core sized to fit within the central cavity of the insulating layer. The air core includes a sheet that blocks fluid flow therethrough and a structural support coiled within the sheet and the air core is flattenable and not within the rolled sleeve.