An exhalation valve, an inhalation valve, a ventilator and a method for controlling ventilation are provided. An exhalation valve (10) or inhalation valve (10) for a medical ventilator (30) for controlling a flow of a fluid, especially a breathing gas, has an inlet (12) and an outlet (14) for the fluid. Between the inlet (12) and the outlet (14), a valve seat crater (16) and a diaphragm (18) are provided. The diaphragm (18) is movable relative to the valve seat crater (16) for influencing the flow through the valve seat crater (16). The valve seat crater (16) and the diaphragm (18) are arranged such that the valve seat crater (16) can partially be sealed by means of the diaphragm (18), with the diaphragm (18) and the valve seat crater (16) not closing completely on initial contact of the diaphragm (18) and the valve seat crater (16).