Ventilation air may be provided in buildings by means of natural or mechanical strategies. When a HVAC system is installed, thermal comfort and indoor air quality (IAQ) may be controlled with higher precision. However, especially between the 70s and the 90s, mechanical ventilation systems have been installed on formerly naturally ventilated buildings without providing any control for natural ventilation. The two ventilation systems are therefore overlapping, without any energy or comfort oriented control strategy, and the occupants are operating windows without any consistent understanding of IAQ and comfort conditions, neither of energy consumption. It may both lead to a substantial energy waste and to low indoor climate conditions. A typical university classroom with exhaust mechanical ventilation and operable windows without switching control has been assessed, in order to understand how occupant behaviour and different ventilation scenarios may influence indoor climate conditions.