In this study a new carbon equivalency metric was developed in order to quantify the compound carbon emissions that buildings are responsible in the built environment. This metric first analyses the rationale about the management of exergy balance among supply and demand involved in satisfying building power and energy loads. Then using the degree of the rationale found, direct carbon emissions from the building and avoidable secondary carbon emissions that the building is responsible due to exergy mismatches are calculated. Based on this metric a net-zero carbon building definition was introduced and its advantages for quantifying the actual impact of buildings on global sustainability were discussed in comparison to net-zero energy building and carbon neutral building concepts. A case study for an example net-zero energy building is presented, which reveals that the new carbon equivalency metric can indicate whether the building is actually environmentally neutral or not. Results show that the example building has negative impact on environment and global sustainability in terms of carbon emissions even though it is rated a net-zero building. This paper also discusses that although another new net-zero exergy building definition may reduce the shortcomings of the net-zero building definition, only the net-zero carbon building metric may accurately rate the environmental impact of buildings. Beyond carbon emissions from buildings, the same metric can be used for any variety of greenhouse emissions and sectors.