The 2021 IECC increases insulation requirements, refines requirements for mechanical systems and adds new provisions related to power and lighting, compared to the 2018 model code. An Energy Dept.analysis found buildings meeting the 2021 version would see an average of 9.38% site energy savings. 

Illinois’ prior code was based on the 2018 IECC, but with amendments that reduced its stringency, says Robert Coslow, professional services administrator at the Illinois Capital Development Board (CDB). Its new code, which took effect this year, is just 0.5% less stringent than the 2021 model code, and only because the state opted to use insulation requirements from the 2024 IECC which were rolled back slightly from the 2021 model. 

For the average new homeowner, the updated code results in nearly 10% savings on utility bills, CDB Executive Director TJ Edwards said in a statement

“Illinois is committed to reducing energy consumption and greenhouse gas emissions,” Edwards said.

With this update, Illinois opted to have the International Code Council (ICC) publish its code for the first time. In the past, they used the IECC and then published state amendments.

“So if … you wanted to see what the Illinois code was, you had to take the model code and then you had to take our amendments and kind of overlay them together,” Coslow says. “It was a bit cumbersome.”

CDB made copies of the new Illinois energy code available to all municipalities in the state. Coslow says they wanted to make sure local officials were aware of the changes and that the newly published version is easier to use, enforce and get to users in the field. 

Illinois is also in the process of adopting a stretch energy code for communities looking to adopt more stringent requirements. While still subject to public review, the proposed code is based on the 2021 IECC for residential buildings and the 2024 model for commercial buildings. It includes electric readiness provisions, such as having the capacity to convert gas appliances to electric and connect rooftop solar systems, as well as promoting use of efficient heat pumps. 

The proposed stretch code has gone through CDB review and is about midway through the legislative process of being codified, Coslow says. He expects it will be available to the public in late October. 

Dominic Simms, CEO of ICC, praised Illinois for using building codes to improve energy efficiency and said in a statement that the state “has emerged as a leader on the national stage for its commitment to energy conservation.”

According to enr.com