Impact of urban form on energy performance, outdoor thermal comfort, and urban heat Island: A case study in Istanbul

Urban density and typology are critical design variables for optimizing building energy performance and outdoor thermal comfort conditions. However, these variables also significantly influence the microclimate in settlements. Therefore, this study adopted a holistic perspective on climate-responsive urban design, focusing on changes not only in buildings and urban blocks but also in atmospheric conditions such as urban heat island effect. Based on this, the study aims to determine the most optimal urban form by simultaneously evaluating energy consumption, outdoor thermal comfort conditions, and urban heat island effect across urban form alternatives generated with different design variables. The proposed approach was tested in Istanbul, a metropolitan city, considering factors such as increasing residential building demand due to rapid population growth, coexistence of high- and low-density residential areas, and rapidly changing microclimatic conditions influenced by city's topography. The complex relationships among energy consumption, outdoor thermal comfort condition, and urban heat island effect (performance criteria) in various urban form alternatives were evaluated based on design variables, including residential building typology (courtyard, scatter, high-rise, and slab), urban density (Floor area ratio / FAR: FAR1, FAR2, and FAR3), window-to-wall ratio (20 %, 40 %, 60 %, and 80 %), and road width (10 m, 15 m, and 20 m). Geometric models representing the temperate-humid climate of Istanbul were created for this research. Rhinoceros/Grasshopper and its plugins were used to develop parametric combinations (180 urban form alternatives) based on these design variables. The effects of the variables on performance criteria (EUI, Av.UTCI, and Av.UHI) were analyzed for each urban form. According to the results, energy efficiency for heating, cooling, and total loads varied by 73.66 %, 55.49 %, and 58.14 %, respectively, among alternatives. Changes in Average Universal Thermal Climate Index (Av.UTCI) and Average Urban Heat Island (Av.UHI) were smaller in alternatives with geometrically similar typologies. Regression analysis revealed that the FAR ratio was the most influential variable. The calculations using the weighted sum method revealed that urban form alternatives with the best conditions were achieved using Slab typologies (f = 0.0507), whereas the worst conditions were associated with Courtyard (f = 0.8726) and Scatter (f = 0.8316) typologies.