Evaluating the impact of district-level microclimate, urban microenvironment and occupant behaviour upon overheating of Nordic apartment buildings

• Validated simulations of Nordic buildings use high-resolution local weather data. • Microenvironment factors cause up to 1.5 °C variation; local weather adds 1.0 °C. • Occupant behavior impacts indoor temperatures up to 4.5 °C via balcony operation. • New buildings on average exceed 27 °C and old ones 33 °C, requiring active cooling. • Poor balcony door operation can increase peak cooling demand up to 2000 W per room. Climate change has amplified the frequency and intensity of heatwaves in Nordic countries, inducing a strong thermal discomfort in buildings that are optimized for cold climates. Most existing studies either isolate urban microenvironment factors or building-scale dynamics, ignoring their combined impact on indoor thermal environments, while currently used climate datasets typically ignore differences at the district level. Aiming to fill this gap, this work combines the impact of urban microenvironmental factors, building characteristics, and occupant behaviour on indoor overheating during heatwaves in the Helsinki region. Validated computer simulations mapping temperature variations at room, apartment, building, and district scales, showed that greenery and building density can substantially reduce indoor temperatures by up to 1.2 °C. At the building and apartment scales, limited wind exposure and intensified solar radiation induce instead overheating in middle-floor apartments and on south-facing façades by up to 0.8 °C. Passive cooling measures and mechanical balanced ventilation can partially moderate overheating, whilst occupant behaviour crucially reduces the indoor temperatures by up to 4.5 °C through optimal balcony doors and windows operation. Active cooling is therefore essential, with required capacities ranging from 200–850 W per room, spiking to ∼ 2000 W in scenarios with suboptimal occupant behaviour. These specific findings offer actionable insights for urban planners, architects, and policymakers towards optimized building design, strategic urban planning, and integration of active cooling systems. The methodology here introduced, grounded on high-resolution weather data and comprehensive computer simulations, is general and can be easily applied to other climates as well.