Impacts of building energy retrofits on energy consumption, indoor environment, and hygrothermal performance in future climate scenarios

• Energy retrofits have shown a positive, lasting impact on IEQ in Nordic conditions. • Long-term surveys and measurements reveal sustained improvement in thermal comfort. • Continuous monitoring is vital for effective retrofitting and building performance. • Up to 15 % energy savings could be achieved via additional exterior wall insulation. • External wall retrofits ensure moisture safety under RCP8.5 2080 climate scenario. Climate change policies have driven new construction toward zero-energy buildings. However, majority of Europe’s existing, unrenovated building stock was built before 1990, remaining energy inefficient. With 90 % of these buildings expected to be still in use by 2050, energy retrofitting is the most practical strategy to reduce CO 2 emissions and energy use. This study combined repeated occupant surveys and long-term monitoring data of 13 multifamily buildings located in Southern Finland, and numerical simulations of a selected case building to assess the impact of energy retrofits on energy consumption, indoor environmental quality (IEQ), and hygrothermal performance under future climate scenarios. Occupant surveys and long-term monitoring showed increased thermal comfort, reduced draught and window condensation. In the case building, measured purchased energy use decreased by 52 %, showing strong agreement with validated energy simulation models. Among future retrofits studied, adding 150 mm of external wall insulation was superior, yielding additional annual energy savings of 15 %. External retrofitting emerged as the most moisture safe and energy efficient solution under RCP8.5 2080 climate scenario. In contrast, internal wall insulating was found to increase the risk of mould growth under future climate, where only 50 mm insulation could be safely added within 25-year lifespan. Based on the results, successful energy retrofits can lead to major reductions in energy use as well as enhanced IEQ and occupant comfort, while enhancing sustainable climate adaptability. The findings support strategic retrofitting alongside continuous monitoring of both IEQ, and energy use to better reflect real-world building performance.