Passive radiative cooling for vertical building facades: From fundamental constraints to practical strategies

Rising space-cooling demand driven by urbanisation and climate change has increased interest in passive alternatives to air conditioning. Passive radiative cooling (PRC) dissipates heat through the atmospheric window (8–13 μm) without energy input. However, research has largely focused on rooftops, while vertical facades remain underexplored. Their cooling performance is constrained by a limited sky view factor (SVF ≤ 0.5) and strong longwave radiation from surrounding warm surfaces, reducing the theoretical maximum net cooling power to roughly half that of horizontal systems. To address these constraints, this review critically examines four strategy classes: (i) spectral control, which confines emission to the atmospheric window while suppressing out-of-band parasitic heat gain, (ii) directional control, which redirects thermal radiation toward the sky while limiting absorption from surrounding surfaces, (iii) coupled spectral–directional approaches, which synergistically combine both mechanisms, and (iv) hybrid passive cooling systems, which extend cooling performance beyond radiative limits. Key research gaps are identified, including overestimation of cooling performance in simplified single-facade models, the difficulty of achieving sub-ambient cooling under realistic urban conditions, and the lack of scalable, durable systems suitable for facade deployment. By connecting…