In November 2019, the Department of Energy released a determination of the impact on energy efficiency resulting from the 2018 International Energy Conservation Code.
DOE found that compared to the 2015 IECC, residential buildings constructed to comply with the 2018 IECC can expect approximately 1.97% savings in energy costs, 1.91% savings in source energy (the total raw energy required to operate the building) and 1.68% savings in site energy (the amount of heat and electricity consumed by a building).
The determination included a breakdown of relative energy savings by climate zone. Buildings in Colorado, which includes areas in Climate Zones 4 through 7, saw energy cost savings that ranged from 1.93% in Zone 7 to 2.22% in Zone 4. Most of the state, including the Denver metro area, is in Zone 5, which saw energy cost savings of 2.01%.
DOE examined a total of 47 changes between the 2015 and 2018 codes. It found that 11 of those changes resulted in decreased energy use, while three changes increased energy used in a building. The vast majority (64%) of changes between the two codes were administrative.