The applicable building energy code that determines the minimum insulation requirements for commercial roofs with insulation entirely above the deck in Illinois is the 2018 Illinois Energy Conservation Code (based on the 2018 International Energy Conservation Code with state-specific amendments). The minimum insulation requirements apply both to new construction and roof replacements on existing buildings.
The minimum requirement for insulation installed entirely above the roof deck is R-30 for all climate zones. This code is effective July 1, 2019.
Sample roof assembly diagrams and additional information can be found in PIMA’s Illinois code fact sheet.