A review of hybrid nature-based solutions integrating bio-PCM for thermal comfort and energy efficiency in buildings

Buildings consume a large portion of energy with substantial carbon emissions, where thermal comfort requirements dominate the operational use, intensifying urban warming. Building-integrated greenery (BIG) as a Nature-based Solution (NbS) helps to reduce heat gain and cooling loads through shading, evapotranspiration, soil thermal resistance, and carbon sequestration. Additionally, bio-PCMs can further stabilize indoor temperatures by storing and releasing heat to maintain thermal comfort. Although both techniques improve energy efficiency while reducing carbon emissions, correlational studies about their co-integration lack in-depth exploration. This review synthesizes and scopes the combined energy efficiency and decarbonization potential of these two strategies to unite their individual capabilities. Findings indicate bio-PCM application can achieve 20–30 % whole-building energy savings, while BIGs provide temperature moderation by 4 - 6 °C depending on climate and configuration. In addition, BIGs can achieve comparable cooling energy savings in many tropical microclimates to reduce urban heat island effects. Green walls can achieve around 17 % annual CO₂-emission reduction, whereas green roofs can achieve 375 gC/m² sequestration over two years. The bio-PCM performance depends on climate, comfort-aligned proper melting/solidification, and hygrothermal interaction with BIGs. Roof applications frequently outperform walls for bio-PCM, whereas BIG walls are advantageous for tall structures, leading to different integration techniques. The bio-PCM BIG co-integration feasibility is governed by moisture durability, leakage control for bio-PCM, rainwater management in BIGs, and fire code compliance. Although life-cycle assessments show their additive energy reduction and carbon savings, harmonized hygrothermal investigation and fire testing can ensure reliable performance and safety for building use.