The refurbishment, conversion or upgrading of existing buildings always involves energy-efficiency issues. Upgrading a building to improve its energy efficiency requires careful consideration if works are to be effective, economical and avoid damaging the historic character of the building. The selection in applying measures result from the level of building conservation and the building condition. The basic subject of this research is the existing housing stock presented through residential buildings in the central part of Zagreb from the period prior to 1940. The year of construction of buildings (1870-1940) provide a general idea about the method of construction and the materials that were used, and these are the factors influencing the building’s energy performance. They are two and three-story buildings of massive brick walls without thermal insulation and single-glazed double windows, with unheated basements and attics, with vaulted and wooden ceilings and with gable roofs. The street facades are rendered with ornamentation. The research will analyze energy performance of buildings of this period and demonstrate energy-efficiency upgrade measures in terms of their constructional details and energy performance benefits. This paper will show the importance for integrated approach balancing between energy efficiency and protection requirements. The concepts range from a simple energy-efficiency improvement for a building envelope to more complex and expensive methods such as extensions and even demolition work. Possible constructional measures include improving the thermal insulation, renewing the windows, eliminating thermal bridges or decreasing the solar heat gains by providing sunshades. For buildings protected by conservation orders, it is clear that non-intrusive upgrading can ensure that a traditional building has the potential to the long-term viability.
