Technical and financial analysis of energy efficiency measures taken for transforming of existing qualified residential buildings into nZEBs: The case of Istanbul

Building energy efficiency is considered one of the key components of sustainable development due to its significant impact on total energy consumption and greenhouse gas emissions. To improve the energy performance of buildings, energy efficiency upgrades are particularly necessary in the existing building stock. This study aims to evaluate the impact of various energy efficiency measures (EEMs) applicable to existing residential buildings in Istanbul, Turkey. The research examines individual measures such as building envelope improvements-including insulation on exterior walls (YLTM-D), roof (YLTM-Ç), and glazing systems (C1–C4); a radiant heating system (ISITMA-DS); a daylight sensor lighting control system (AYD-KONTROL); and a photovoltaic system. Subsequently, various combinations of all individual measures (S1–S22) are assessed, and all scenarios are compared in terms of energy efficiency, initial investment cost, annual energy cost, and global cost. When all EEMs were applied to the reference building, the maximum energy efficiency rate was calculated as 45%, while the highest cost-effectiveness rate was 1.34%. Compared to the reference building, the only cost-optimal scenario was the photovoltaic (PV) system, with a 14% energy efficiency rate and an additional cost of 390.49 TL/m². The study demonstrates that the energy performance of existing buildings can be improved through various EEMs; however, to increase cost-effectiveness, it is necessary to reduce real discount rates. In addition, the development of financial instruments to promote energy efficiency in residential buildings, along with mechanisms to distribute the financial burden among users, is considered likely to support the widespread adoption of such projects.