A Switchable Dual‐Mode Janus Composite for All‐Weather Passive Radiative Cooling and Solar‐Joule Heating in Energy‐Efficient Buildings

ABSTRACT Adaptive thermal regulation across diurnal and seasonal cycles remains a critical challenge for energy‐efficient buildings. A switchable dual‐mode Janus‐structured composite is developed by integrating a polyoxymethylene fiber/acrylate resin/BaSO 4 aerogel cooling layer with a graphene aerogel/paraffin wax (PW) phase‐change composite heating layer to achieve synergistic passive radiative cooling, solar‐Joule heating, and latent‐heat storage. The heating layer exhibits high thermal conductivity, high solar‐to‐latent heat conversion efficiency, high latent heat capacity, and high electrical conductivity. This permits a maximum sub‐ambient temperature rise of 17.4°C across the day. The cooling layer delivers a solar reflectivity of 96.2%, a mid‐infrared emissivity of 94.0%, and a low thermal conductivity of 0.053 W m −1 K −1 , achieving a maximum sub‐ambient temperature drop of 21.3°C and a genuine radiative net cooling power of 110.4 W m −2 , with an additional transient thermal buffering provided by the latent heat of PW (239.7 J g −1 ). Outdoor tests verify diurnal regulation and paraffin‐induced temperature hysteresis. Building energy simulations across 15 Chinese cities demonstrate annual energy savings exceeding 10.8% in most regions, with reductions of 9.5 and 15.1 MJ m −2 in heating and cooling demands, respectively. This work provides an effective strategy for all‐weather thermal management and sustainable energy savings under diverse climatic conditions.