The ducting from the fan to the outside of the building has a strong effect on the airflow, noise, and energy use of the fan. Use the shortest, straightest duct routing possible for best performance, and avoid installing the fan with smaller ducts than recommended. Insulation around the ducts can reduce energy loss and inhibit mold growth. Fans installed with existing ducts may not achieve their rated airflow. An insulated flexible duct is recommended for the quietest possible installation. If a rigid duct is used, a short (1-3 feet) section of the insulated flexible duct will ensure quiet operation.