The built environment is one of the biggest energy consumers. Therefore, various methods have been created to estimate and calculate the energy consumption during the lifetime of buildings. However, the actual energy use differs substantially from the design energy use. For this reason, this thesis aims to study the gap between actual and design energy use. Due to the great share of residential buildings in the building sector this research is focused only on residential buildings. The research revealed five categories of reasons behind the gap. These are a) occupants’ characteristics, b) occupants’ behavior, c) technical aspects of building, d) knowledge of the industry, e) calculation processes. Each of these categories contains a number of reasons with the most outstanding being i) the age of the occupants ii) the heating habits of the occupants iii) the correct installations iv) the understanding of technologies and building methods v) the assumptions used in the models. A closer look to the residential sector of Greece, the United Kingdom and the Netherlands showed that there is a significant lack of research and thus data on this topic. Nevertheless, the available data showed that the residential sector, in the United Kingdom and the Netherlands, exhibits the same reasons that lead to the gap between actual and design energy use. The reasons that have been confirmed for the two countries are falling into the categories c) technical aspects of the building and d) knowledge of the industry. Further research in the areas of a) occupants’ characteristics, b) occupants’ behavior, c) technical aspects of building, d) knowledge of the industry, e) calculation processes is recommended. In order to bring the design energy use closer to the actual energy use statistical data are required which will reveal the main reasons behind the gap in each country. Therefore, data bases which will contain the design calculations as well as the tests and monitoring results are required. The focus of the policy makers should be on three groups, the designers, the constructors, and the occupants. While the possible improvements include less complex designs, better education of the constructors on the heat losses and detailed guidelines to the occupants for efficient use of the building and conscious use of energy
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