Roof Vent Cap Only is designed for use with 3" or 4" round ducts, providing reliable ventilation with a built-in back draft damper and bird screen to prevent unwanted debris and animals from entering. Crafted from durable galvanized steel, this vent cap features a sleek baked enamel black finish that ensures long-lasting protection and an attractive appearance. I for maintaining proper airflow while keeping your ventilation system secure and efficient. Model Number: 636