Buildings in cold climates have a high potential for carbon emission reduction due to the large difference between indoor and outdoor temperature. Existing research on carbon emission reduction of buildings in cold climates is case studies on a few locations. However, case studies cannot represent all cold climate regions because the available energy resources in different regions vary significantly. To fill this gap, we investigated the carbon emission reduction potential of building retrofits in cold climates of the United States (U.S.). The baseline building energy models are based on the Commercial Buildings Energy Consumption Survey, and the retrofitted building energy models are based on the Advanced Energy Design Guide 50% energy savings. Then, by simulating the baseline and retrofitted building energy models in cold climate regions, energy savings by retrofitting buildings can be predicted. Finally, the carbon emission reduction potential of building retrofits can be predicted by integrating energy savings with the emission factors of energy resources at corresponding times and states. This paper investigated the emission reduction potential for existing medium-sized office buildings in 18 states in cold climates of the U.S. from 2024 to 2050 under three different renewable energy cost scenarios. Based on the results, it is recommended that the retrofits of existing medium-sized office buildings in cold climates of the U.S. should focus on the buildings constructed before 1980 and the state of Utah, Michigan, Wisconsin, and Wyoming. Furthermore, the high renewable energy cost scenario has the most significant impact on the state of Iowa, while the low renewable energy cost scenario has the most significant impact on the state of Wisconsin.