Blue–Green Infrastructure Strategies for Improvement of Outdoor Thermal Comfort in Post-Socialist High-Rise Residential Areas: A Case Study of Niš, Serbia

Urban densification in post-socialist cities has drastically reduced open and green spaces in high-rise housing areas (HRHAs), intensifying heat stress and degrading outdoor thermal comfort (OTC). These neighborhoods—shaped by socialist-era planning and, later, market-led infill—combine high built density, low greenery, and limited ventilation, making them critical testbeds for climate-adaptive regeneration. This study presents the first empirically validated ENVI-met assessment of blue–green infrastructure (BGI) performance in a post-socialist HRHA, using a representative courtyard in Niš, Serbia, during the 14 August 2024 heatwave. A 24 h field campaign (air temperature, humidity, wind speed, and mean radiant temperature) validated the model with high accuracy (R 2 = 0.92, RMSE = 1.1 °C for air temperature; R 2 = 0.88, RMSE = 3.5 K for Physiological Equivalent Temperature (PET). Four retrofit scenarios were simulated: S0 (existing), S1 (grass), S2 (grass + trees), and S3 (S2 + shallow pool). Across all scenarios, daytime PET indicated strong–extreme heat stress, peaking at 61.9 °C (16:00 h). The best configuration (S3) reduced PET by 2.68 °C (10:00 h) but <1 °C at peak hours, with acceptable comfort limited to 04:00–07:00 h. The results confirm that small-scale surface-level greening provides negligible thermal relief under a dense HRHA morphology. Urban morphological reform—optimizing height, spacing, ventilation, and integrated greening—is more effective for heat mitigation. Future work should include multi-seasonal field monitoring and human thermal-perception surveys to link microclimate improvement with exposure and health risk.