Cavity wall insulation is installed in the space between exterior sheathing and interior finishes (typically drywall). Insulation materials commonly used include fiberglass (blown-in or batts), cellulose, mineral wool, and foam. Blown-in fiberglass is applied with specialized equipment that propels insulation into the cavity while a technician controls density. Proper density is critical; low density allows settling and reduces performance, while excessive density reduces effectiveness and cost. Cellulose is made from recycled paper and provides good thermal and sound performance. Cellulose is denser than fiberglass and settles less over time. Cellulose requires fire retardants meeting building code requirements. Mineral wool is noncombustible and provides good fire resistance. Foam insulation including polyurethane and polystyrene can be sprayed or injected. Foam expands to fill cavities and provides excellent sealing reducing air leakage. Foam requires proper humidity and temperature during application. Proper installation includes filling the entire cavity without voids. Voids reduce overall insulation value. Holes are drilled in the interior or exterior and insulation is blown in until cavities are full. Holes are then patched. Injection nozzles can provide more precise filling in irregular cavities. Installation quality is difficult to verify since the cavity is hidden after completion. Density requirements specified for the material ensure proper performance. Post-installation density testing may verify installation quality. Settling of some insulation materials over time can reduce performance; material selection should minimize settling. Energy code requirements specify minimum R-values for cavity insulation depending on climate zone.