Abstract: Air quality in a interior space is monitored by a sensor and, to determine air renewal demand, the sensor signal is compared with a normal or reference value. For demand controlled ventilation, a switching signal is triggered for changing the amount of outside air supplied when air renewal demand changes. A reference ventilation is performed daily, at a time of day when outside air quality is satisfactory, typically in the early morning hours. Measured quality of the air from the reference ventilation is used in setting or resetting the reference value, e.g. adaptively.