Abstract: Valve (10) comprising a housing (20) with an air inlet (23) mounted at the end of a discharge air duct of a domestic appliance, and with an air outlet (24) communicating with the outside. The valve (10) includes a shoulder (41) sealed to the air inlet (23). A ball (42) is received in the housing (20) and is movable on a guide (43 between first and second positions. When there is no airflow from the duct (60), the ball sealingly engages the shoulder (41) under gravity so as to prevent air from being introduced into the duct from outside through gusts of wind. The shoulder (41) and the ball (42) can be made from an insulating material in order to reduce conductive heat loss in cold weather. When the domestic appliance is operating, the ball (42) moves towards the second position under the discharge airflow in order to evacuate said airflow. As soon as the appliance is stopped, the ball (42) resumes automatically its initial position.