Abstract: A window shutter unit (1) is for external mounting on a building. The window shutter unit comprises a plurality of hollow slats (2) mounted in a shutter frame. An air outlet opening (20) delivers ventilation air to a room in the building. A fresh air inlet system comprises air inlet openings (10) in the hollow slats (2), and a first air flow passage (8) from the individual slat to a common flow passage (9) leading to the air outlet opening (20).