The high-performance mid-IR electrochromism, nonflammability, inexpensiveness, and flexibility (Figures and) of the Cu-aqueous mid-IR modulation device allow the application of smart building envelopes for thermal management by electrical bias (Figure ). To experimentally demonstrate such applicability, we attached the electrochromic device onto a heater and measured the heat loss through the device under different ambient temperatures (Figure ). Detailed experimental design is shown in the Method. (Figure ) Without the ability to change the emissivity, the high-e/low-e building envelope tends to overcool/overheat the building in a cold/hot environment, confirmed by the high/low heat loss (over 100 W/m 2 or below 50 W/m 2 ). In contrast, the electrochromic device can vary its emissivity in response to different ambient temperatures to maintain the constant heat loss (near 75 W/m 2 ) for the thermal comfort of the buildings.