The role of private outdoor greenery in enhancing indoor air quality, thermal comfort and energy efficiency: a critical review

Urbanisation and climate change are degrading indoor conditions by intensifying heat stress, air pollution and reliance on mechanical cooling, yet the role of private residential greenery in mitigating these effects remains insufficiently understood. This review is aimed at critically evaluating how private outdoor greenery influences indoor air quality (IAQ), thermal comfort and energy efficiency in residential buildings. A systematic screening of 3750 Scopus records identified 54 relevant studies, which were synthesised to assess design determinants, climatic sensitivity and methodological approaches. Quantitative findings indicate that private greenery can reduce indoor air temperature by 3.4°C, lower mean radiant temperature by up to 22.9°C and achieve up to 25% cooling energy savings, depending on tree placement and density. Reported magnitudes vary widely across climates, vegetation configurations and methodological approaches. However, direct empirical evidence linking private outdoor greenery to measured indoor IAQ improvements remains limited, with many IAQ-related findings inferred from pollutant deposition, ventilation interactions or outdoor air quality changes rather than direct indoor measurements. Integrated assessments of IAQ, thermal comfort and energy performance are rare in residential contexts, and design thresholds remain undefined. This review contributes a consolidated evidence base and identifies critical research directions to support climate-responsive, evidence-based residential greening strategies.