What type of venting material should I use for the dryer exhaust?

You must use only rigid metal or flexible metal 4-inch diameter ductwork for exhausting the dryer to the outside. Do not use plastic or thin metal foil tubing, as these materials are known to collapse, become easily crushed, and trap lint, which can obstruct airflow and increase the risk of fire. The total length of flexible metal duct must not exceed 8 feet (2.4 m). (Pages 5, 16)