Correlation between indoor and outdoor temperatures during hot summers in Finnish residential buildings

As climate change intensifies, heatwaves’ impact on residential buildings becomes increasingly significant. This study aims to analyze the overheating risks in Finnish apartment building stock and explore the relationship between indoor and outdoor temperatures during the hot summer of 2021. Through a comprehensive field study in more than 6,000 apartments, the research examined the indoor temperature variations in Finnish apartment building stock during the hot summer of 2021. The findings revealed that almost 50% of the apartments experienced indoor temperatures exceeding 27°C for seven consecutive days. Moreover, the daily average temperature was higher than 27°C when the outdoor average temperature was higher than 21°C. There was a strong correlation between indoor daily maximum or average temperatures and the outdoor 5-day sliding average during short heatwaves. Notably, this correlation weakened in the face of longer heatwaves. This shows the effects of thermal mass in buildings giving occupants time to take proactive actions against heatwaves. The study highlights the increasing impact of heatwaves on Finnish residential buildings that should be considered in the design phase.