Yes, you can add insulation to a flexible air duct. Some flexible air ducts come pre-insulated, but if yours does not, you have the option to add external insulation wraps. These wraps improve energy efficiency and help reduce condensation within the duct. When adding insulation, it’s recommended to insulate the entire length of the ductwork to maintain consistent air temperature throughout its journey. This approach ensures that the heating or cooling system operates more effectively and efficiently.