The building envelope can maintain a constant heat loss of the building under different ambient temperatures to avoid overheating and overcooling by tuning its emissivity. The usage of graphene as a transparent electrode also provides mechanical flexibility to the device, enabling it to retrofit the wall or roof of buildings with various shapes and be folded or rolled up for convenient transportation and storage. The building energy simulation across the U.S. further confirms such a smart building envelope can save more operational HVAC energy, especially in cold areas. It should also be noted that this electrochromic building envelope can reduce the usage of the embodied carbon of buildings' thermal insulation layer, contributing to a low-carbon sustainable future. We hope this research can pave the way for designing sustainable electrochromic devices in mid-IR modulation for smart building envelopes and deepen the fundamental understanding of the reversible metal electrodeposition.