Here, to tackle the bottleneck, we design a cost-effective Cu-based aqueous electrochromic mid-IR building envelope. The adoption of aqueous cupric solution makes the electrochromic device nonflammable and much more cost-effective than its counterparts, and using the ultra-wideband transparent conductive electrode allows considerable dynamic range of thermal emissivity tunability from 0.07 to 0.92. The switching is stable, non-volatile, efficient, and mechanically flexible. After 1,800 deposition-stripping cycles, the emissivity contrast only reduces by 21%, thanks to the ultrathin Pt catalyst layer. The density functional theory (DFT) calculation also confirms the uniform and durable deposition of Cu on the Pt-modified electrode. Finally, the rigorous building energy calculation shows that our aqueous dynamic building envelope can save 8.4% of the U.S. buildings' HVAC energy consumption on average among climate zones 5-8 and reduce the embodied carbon of the insulation layer.