Natural ventilation has the potential of achieving high ventilation rates for fresh air supply and summer cooling, although without the possibility of air to air heat recovery in winter. This study aims to compare the overall energetic efficiency and comfort provision of controlled natural ventilation in energy efficient offices and in the non-refurbished building stock with the related mechanically ventilated reference. The goal is to determine the energy savings for modern offices with low heating demand and higher summer cooling loads compared to existing office buildings, where heating loads dominate. To make sure that the simulation assumptions match the reality, the air-flow modelling of a single-sided ventilation strategy is validated by comparing it with results from tracer-gas measurements. Simulation results indicate that properly designed and controlled natural ventilation shows a good functionality in both state-of-the-art buildings and in the stock. In the moderate climate of Stuttgart/Germany no mechanical cooling is necessary. For state-of-the-art buildings ventilation energy losses in the heating season are easily compensated by lower cooling demand in the warm period of the year. In the building stock the conclusions are less clear, especially for densely occupied offices. Energy losses in the heating season can be compensated in the warm period, but primary energy savings are dependent on primary energy factors and hence how heat, cold and electricity are generated. (C) 2018 Elsevier B.V. All rights reserved.