Abstract: A method and a system for controlling ventilation of an indoor area of a building, comprising the steps of: ventilating the indoor area by means of mechanical ventilation and natural ventilation according to a ventilation mode selected among a plurality of ventilation modes, a set of adjustable control parameters (31; 32; 33; 34; 35) and at least one measurement value from a sensor, wherein each ventilation mode is associated with the set of adjustable control parameters (31; 32; 33; 34; 35), each having an adjustable value selected among a group of mode dependent adjustable values, and/or a set of fixed control parameters (34) each having a mode dependent fixed value, and controlling the mechanical ventilation and the natural ventilation by comparing the measurement value from the sensor with a corresponding value of the control parameters of the ventilation mode such that a desired indoor climate defined by the ventilation mode is obtained.