Janus Gradient Porous Membranes with Switchable Radiative Cooling and Solar Heating for Building Thermal Management.

Passive building thermal management requires materials that adapt to seasonal changes by modulating their spectral properties, yet conventional materials exhibit fixed optical properties. Here, we report Janus gradient porous membranes that enable switchable thermal regulation by simple flipping: radiative cooling for summer and solar heating for winter. The membranes are fabricated via a solvent template-assisted evaporation-induced phase separation (ST-EIPS) process combined with strategic carbon black (CB) modification. The resulting asymmetric architecture is endowed with hierarchical porosity spanning from nano- to microscale, which facilitates a tailored solar-thermal response. When the original white surface faces upward, the membrane exhibits 96.06% solar reflectance and 94.69% emissivity in atmospheric transparency window, achieving subambient cooling of 11.7 °C under solar irradiance of 1101.3 W/m². Upon flipping to expose the carbon-modified black surface, solar absorptance reaches 97.07%, delivering a 22.5 °C temperature rise above ambient under winter conditions (923.2 W/m²). The Janus gradient porous membrane (JGPM) supports direct surface application with excellent mechanical robustness and strong component adhesion. Architectural prototype testing confirms the membrane's seasonal adaptability, demonstrating temperature reductions of 5.7 °C under high-temperature conditions and temperature increases of 10.3 °C in cool climates. The reliable performance across diverse environmental conditions underscores the potential of JGPM as a transformative solution for adaptive building thermal management.