Overheating assessment for apartments during average and hot summers in the Nordic climate

Amidst rising ambient temperatures and frequent heatwaves, assessing summer indoor overheating is crucial. This study examined overheating in over 6000 apartments to show the temporal aspects of indoor overheating during the hot summer of 2021 compared to the average summer of 2020 in the Helsinki region of Finland, with additional consideration of the apartments' design year, size, and number of rooms. Data analysis was conducted employing ANOVA and post hoc tests to assess the impact of apartment characteristics on overheating. Finnish building codes and health-related regulations defined three temperature thresholds to determine overheating: 27°C, 30°C, and 32°C. The findings indicated that during the summer of 2020, 76%, 8%, and 1% of apartments exceeded 27°C, 30°C, and 32°C, respectively. In the hot summer of 2021, these proportions increased significantly to 96%, 32%, and 4%. The average number of degree hours above 27°C in 2021 was three times higher than in 2020. While nighttime temperatures were slightly lower, there was a considerable number of degree hours above 27°C. Smaller apartments exhibited a higher risk of overheating. Those constructed after 2012 demonstrated lower overheating risks during the hot summer.