The studywas performed in an elementary Danish school where school hours are generally from 8:00 to 14:00 on schooldays. It is built of bricks and concrete in the early 1970s. The school is situated in a suburban area in an affluent local community close to Copenhagen with low-rise buildings (Fig.1). The classrooms are located in three buildings, Wing A, Wing B and the main building (Fig. 2). The classrooms are heated by water-filled radiators placed under the windows. The radiators are equipped with thermostatic valves. The classrooms in Wings A and B were recently refurbished and retrofitted with ventilation system as outlined below.