Performance of modern mechanical and natural ventilation systems in apartment buildings under a changing Nordic climate

Ventilation systems in new buildings and retrofitted ventilation systems in old buildings in Finland are designed to consistently provide a high level of indoor environment along with high energy efficiency. New buildings are implemented with balanced mechanical ventilation with kitchen boosting, which requires precise control of supply and exhaust airflows to preserve balance. Retrofitted buildings are equipped with self-regulating inlet devices to mitigate the natural ventilation dependency on outdoor conditions. The main objective of the study is to investigate the performance of these modern ventilation systems of apartment buildings in various operational conditions, such as occupant behaviour, possible maintenance shortages, microbial pollution resistance and urban microenvironment under changing climate. The performance analysis of the mechanical ventilation in the new buildings showed a significant imbalance between the supply and exhaust airflows during boosting conditions so buildings with high airtightness might encounter significant pressure difference over the envelope. Additionally, analysis of intermittent and continuous nighttime mechanical ventilation long-term operation under humid and cold with microbial pollution showed the extinction of colonies by the end of the experiment regardless of the chosen strategy. The performance analysis of a retrofitted apartment building with natural ventilation showed a significant effect of occupant behaviour, poor maintenance and the possibility of opening windows on the CO2 concentration besides the effect of outdoor conditions. The apartments on higher floors showed high levels of CO2 and risk for overheating, and small apartments tend to overheat more despite a higher air change rate. Improving the envelope airtightness does not benefit the indoor air quality. The effect of urban microenvironment parameters on the performance of large building stock was assessed to understand its ability to mitigate overheating during heatwaves. The most substantial influence of the urban microenvironment was observed during short heatwaves. The leading factors of indoor temperature decrement were low sea distance and high levels of greenery around buildings. The influence of urban microenvironment factors during the long midsummer heatwave was lower, but the green view index showed the most consistent temperature-reducing influence. This research can present the performance of modern ventilation systems in the climate of the Nordic region, emphasizing the need for a precise understanding of each new step in building energy-efficiency advisement to prevent shortcomings and ensure high indoor conditions.