Purpose: The purpose of this degree project is to study the energy performance of the apartment building located in Tensta.  This study will propose methods for energy efficiency by calculating the building's need for active heating in its existing condition and then studying and comparing different methods for energy efficiency. Method: To accomplish the purpose of this degree project, a literature study was performed to gather information as well as a case study of the apartment building. In addition to this, relevant calculations were performed to account for the most energy saving measure. Results: The results of this degree project shows that the apartment buildings yearly need for active heating is 193,3 MWh. By changing the current double-glazed windows to new triple-glazed windows the yearly need for active heating decreases by 15%. By installing an HRV-system the yearly need for active heating decreases by 31%. Conclusions: In summary, installing an HRV-system will account for more energy savings compared to changing the current double-glazed windows to new triple-glazed windows. However, this does not mean that installing an HRV-system is a better investment since this study only focuses on the energy savings generated by the different methods and not the cost of the different methods.