During the study two buildings have been analysed, both belong to 103 series buildings. The main objective of the work is to assess the efficiency of mechanical ventilation systems with heat recovery in renovated multi-family residential buildings. A comparison with natural ventilation systems is included. The paper focuses on data collection and analysis in two newly renovated buildings; both for energy efficiency and indoor air quality. It also shows one of the ways to solve space and low ceiling problems in existing multi-story apartment buildings by integrating supply air ducts in the insulation layer of the building facade.