Yes, you can add additional insulation to an R8 flex duct. While the R8 duct already comes with insulation, applying external insulation wraps can further enhance its energy efficiency and help reduce condensation if necessary. Insulating the flex duct can lead to comfort improvements and may pay for itself by reducing energy bills over time. However, it’s important to note that if your system already includes a furnace fan assisting airflow, the savings from adding insulation might be less significant.