Improving building performance is key to tackling the challenges of climate change. Buildings are responsible for 30% of final energy consumption and 25% of greenhouse gas emissions in the European Union. The urgent need for energy efficiency improvement calls for the implementation of suitable policy instruments. Among them, energy performance certificates were introduced to raise awareness of building performance among owners and tenants and to motivate performance improvements. However, energy performance certificates certify the design more than the actual building, relying on standards and assumptions to compare buildings in different local contexts. This has resulted in observations of differences between the theoretical energy consumption on the one hand and the actual energy consumption on the other. This deviation is referred to as Energy Performance Gap (EPG), and typically concerns the difference between predicted performance of the design intent and the observed performance of the realized building over the time. Previous research has pointed to the importance of the EPG for the attainability of the energy and climate policy objectives (like the Swiss Energy Strategy 2050), given that only part of the predicted energy savings in buildings (based on standards) is typically realized.

This thesis reviewed the causes of the EPG and introduced the concept of optimal consumption to classify the causes and mitigation strategies for EPG. The quantitative analyses revealed the existence of an EPG for Swiss residential buildings, with a median of -11% per building (i.e., overall buildings typically consume less than would be supposed from their energy label). Moreover, after studying the retrofit of buildings, it was found that only 62.7% of the expected savings based on the theoretical values are actually achieved. Conversely, 96.4% of the expected savings defined on anticipated values, are actually achieved. Finally, the scenario analysis based on the energy and emission levels in 2021 highlighted that a much greater effort than made so far is required to achieve the 2050 objective, since it will be necessary to reduce GHG emissions by 96%.

Overall, this PhD thesis has advanced the understanding of the EPG in the residential sector by seeking to define the terminology used to identify different phenomena, and by quantifying them in the Swiss context. This thesis is indeed the most comprehensive analysis so far conducted in Switzerland on the topic of the EPG, and it adds important insights in the context of the Swiss energy transition.