Hot Summers in Nordic Apartments: Exploring the Correlation between Outdoor Weather Conditions and Indoor Temperature

As the incidence of extended hot summers in the Nordic climate increases due to climate change, non-mechanically cooled apartments face high risks of overheating. Hence, this study aimed to investigate the temporal effects of heatwaves on indoor temperatures and examine the correlation between outdoor weather conditions and indoor temperature levels. A comprehensive field study was conducted across over 6000 apartments in the Helsinki region during the hot summer of 2021 and its heatwaves. Results indicated that nearly half of the apartments experienced indoor temperatures above 27 °C for over 7 consecutive days. It was found that an outdoor daily average temperature of 19 °C could cause indoor daily average temperatures higher than 27 °C. Further, the study revealed a strong correlation between indoor temperatures and outdoor 5-day moving average temperature, allowing occupants time to take preventative measures. Additionally, a linear relationship was found between the indoor average temperature, the outdoor 5-day moving average temperature, and the 7-day moving average solar radiation. The strength of the correlation and the magnitude of the effects of outdoor temperature and solar radiation varied depending on the duration of heatwaves. This highlights the importance of considering heatwaves in the design and renovation of residential buildings in the Nordic climate.