Battic Door Premium In-Line Duct Back Draft Blocker 4 in. Diameter non-mechanical in-line one way air flow valve consists of a specially formulated fabric sleeve attached to an outer metal sleeve. fabric sleeve will allow air to pass through in direction of flow with no reduction in airflow silent, produces no metal clanging sounds, tested to UL723 standards stop backdrafts from your clothes dryer, bathroom exhaust fan, kitchen fan