The most recent edition, the 2018 IECC, was published in August 2017, triggering the DOE review and determination process. In response, DOE and Pacific Northwest National Laboratory (PNNL) conducted a preliminary analysis to determine energy savings for the 2018 IECC residential provisions relative to the previous edition--the 2015 IECC. This report documents the methodology used to conduct the analysis and summarizes the results and findings.