Radiative thermoregulation can reduce the energy consumption for heating, ventilation and air-conditioning (HVAC) in buildings, and therefore contribute substantially to climate change mitigation. Electrochromism, a phenomenon in which a material exhibits reversible colour changes under an external electrical stimulus, can help control the heat balance of buildings in response to fluctuating weather conditions; however, its implementation has been largely limited to visible and near-infrared wavelength regimes. Here we develop an aqueous flexible electrochromic design for use as a building envelop based on graphene ultra-wideband transparent conductive electrode and reversible copper electrodeposition, in which the thermal emissivity can be tailored to vary between 0.07 and 0.92 with excellent long-term durability. Building energy simulations show that our design as building envelopes can save on year-round operational HVAC energy consumption across the United States by up to 43.1 MBtu on average in specific zones. Such dynamic emissivity tunability can further serve as a non-destructive technological solution to retrofit poorly insulated or historic buildings. Our work suggests a feasible pathway to radiative thermoregulation for more energy-efficient HVAC and solving some of the global climate change issues.