Batt insulation (mostly made of fiberglass) is sectionals of insuation that are used to fill voids in the walls between studs and also between ceiling joists for multible story configurations. It is also used for dapening vibrations to reduce sound coming from rooms such as upstairs, bathrooms, and laundry rooms. These batts come premade at different R-values depending on the makeup and thickness. These factors need to be considered when looking to fill between different size "cubbys" (4", 6",10", 12", etc).