The IECC addresses energy efficiency on several fronts including cost savings, reduced energy usage, conservation of natural resources and the impact of energy usage on the environment. For the most current adoptions details go to International Code Adoptions

Key changes include:

Log homes designed in accordance with the standard ICC 400, Standards on the Design and Construction of Log Structures, are exempt from the building thermal envelope requirements of the IECC.
The maximum allowable fenestration U-factors in Table R402.1.2 (for the prescriptive compliance path) for climates zones 3 through 8 have been reduced from the values in the 2015 edition.
The ICC/RESNET 380 standard has been included as one of standards that can be used for determining the air leakage rate of a building or dwelling unit.
The Energy Rating Index compliance alternative index values have been increased slightly however, the method for determining an index is now required to be in accordance with standard ICC/RESNET 301.
Revisions to interior and exterior lighting power budgets and better clarity for lighting controls.
Clarity that regardless of design methodology, system commissioning is required.
New limits on heated or cooled vestibules.
Mechanical provisions reorganized based on equipment type rather than design methodology.