What type of venting material is required for the dryer exhaust?

Only rigid metal or semi-rigid metal ductwork with a 4-inch (10.2 cm) diameter should be used for the dryer exhaust. Plastic or flexible foil venting must not be used, as these materials can collapse, trap lint, and create a fire hazard. If a flexible metal transition duct is needed between the dryer and the wall, it must be UL-listed and its total length must not exceed 8 feet (2.4 m). (Pages 5–6, 14–15)