Climate zones by state

Illinois Climate Zones by County

Illinois's 102 counties span climate zones 4A to 5A under the 2021 International Energy Conservation Code (IECC). Zone 5A is the most common, with 58 of the 102. For reference, Chicago and Springfield are in zone 5A and Carbondale is in 4A.

Insulation minimums for each zone in Illinois

ZoneCountiesAtticWood-frame wallsFloors over unheated spaceBasement walls
4A44R-60R-30, or 20+5ci, or 13+10ci, or R-20ciR-19R-10ci or R-13
5A58R-60R-30, or 20+5ci, or 13+10ci, or R-20ciR-30R-15ci, or R-19, or 13+5ci

Minimums from the 2021 IECC (Table R402.1.3). Many areas still enforce an older edition, so check with your local building department. See sources.

Which energy code Illinois uses

Illinois enforces the 2024 IECC with amendments for homes (in effect since Nov 2025). Source: US Department of Energy, September 2026. Local governments can adopt newer codes or amendments. See every state's energy code.

What changed from the 2018 map

Six counties moved one zone warmer in the 2021 map: Calhoun County (5 → 4), Clark County (5 → 4), Coles County (5 → 4), Cumberland County (5 → 4), Greene County (5 → 4), Jersey County (5 → 4). If your town still enforces the 2018 or 2015 code, the older zone applies to permits.

Every county in Illinois

NameZone (2021)Zone (2018)Details
Adams County5A5AInsulation →
Alexander County4A4AInsulation →
Bond County4A4AInsulation →
Boone County5A5AInsulation →
Brown County5A5AInsulation →
Bureau County5A5AInsulation →
Calhoun County4A5A changedInsulation →
Carroll County5A5AInsulation →
Cass County5A5AInsulation →
Champaign County5A5AInsulation →
Christian County4A4AInsulation →
Clark County4A5A changedInsulation →
Clay County4A4AInsulation →
Clinton County4A4AInsulation →
Coles County4A5A changedInsulation →
Cook County5A5AInsulation →
Crawford County4A4AInsulation →
Cumberland County4A5A changedInsulation →
De Witt County5A5AInsulation →
DeKalb County5A5AInsulation →
Douglas County5A5AInsulation →
DuPage County5A5AInsulation →
Edgar County5A5AInsulation →
Edwards County4A4AInsulation →
Effingham County4A4AInsulation →
Fayette County4A4AInsulation →
Ford County5A5AInsulation →
Franklin County4A4AInsulation →
Fulton County5A5AInsulation →
Gallatin County4A4AInsulation →
Greene County4A5A changedInsulation →
Grundy County5A5AInsulation →
Hamilton County4A4AInsulation →
Hancock County5A5AInsulation →
Hardin County4A4AInsulation →
Henderson County5A5AInsulation →
Henry County5A5AInsulation →
Iroquois County5A5AInsulation →
Jackson County4A4AInsulation →
Jasper County4A4AInsulation →
Jefferson County4A4AInsulation →
Jersey County4A5A changedInsulation →
Jo Daviess County5A5AInsulation →
Johnson County4A4AInsulation →
Kane County5A5AInsulation →
Kankakee County5A5AInsulation →
Kendall County5A5AInsulation →
Knox County5A5AInsulation →
La Salle County5A5AInsulation →
Lake County5A5AInsulation →
Lawrence County4A4AInsulation →
Lee County5A5AInsulation →
Livingston County5A5AInsulation →
Logan County5A5AInsulation →
Macon County5A5AInsulation →
Macoupin County4A4AInsulation →
Madison County4A4AInsulation →
Marion County4A4AInsulation →
Marshall County5A5AInsulation →
Mason County5A5AInsulation →
Massac County4A4AInsulation →
McDonough County5A5AInsulation →
McHenry County5A5AInsulation →
McLean County5A5AInsulation →
Menard County5A5AInsulation →
Mercer County5A5AInsulation →
Monroe County4A4AInsulation →
Montgomery County4A4AInsulation →
Morgan County5A5AInsulation →
Moultrie County5A5AInsulation →
Ogle County5A5AInsulation →
Peoria County5A5AInsulation →
Perry County4A4AInsulation →
Piatt County5A5AInsulation →
Pike County5A5AInsulation →
Pope County4A4AInsulation →
Pulaski County4A4AInsulation →
Putnam County5A5AInsulation →
Randolph County4A4AInsulation →
Richland County4A4AInsulation →
Rock Island County5A5AInsulation →
Saline County4A4AInsulation →
Sangamon County5A5AInsulation →
Schuyler County5A5AInsulation →
Scott County5A5AInsulation →
Shelby County4A4AInsulation →
St. Clair County4A4AInsulation →
Stark County5A5AInsulation →
Stephenson County5A5AInsulation →
Tazewell County5A5AInsulation →
Union County4A4AInsulation →
Vermilion County5A5AInsulation →
Wabash County4A4AInsulation →
Warren County5A5AInsulation →
Washington County4A4AInsulation →
Wayne County4A4AInsulation →
White County4A4AInsulation →
Whiteside County5A5AInsulation →
Will County5A5AInsulation →
Williamson County4A4AInsulation →
Winnebago County5A5AInsulation →
Woodford County5A5AInsulation →

Source: Pacific Northwest National Laboratory, Guide to Determining Climate Zone by County: Building America and IECC 2021 Updates (2022).

Frequently asked questions

What climate zone is Illinois?

Illinois spans IECC climate zones 4A to 5A; zone 5A is the most common. Find your county in the table above, or search by ZIP code.

How much attic insulation do I need in Illinois?

The 2021 IECC calls for R-60 in the attic everywhere in Illinois.

What energy code does Illinois use?

Illinois enforces the 2024 IECC with amendments for homes (in effect since Nov 2025). Source: US Department of Energy, September 2026. Local governments can adopt newer codes or amendments.

More for Illinois: Last frost dates in Illinois · Illinois planting zones · Frost line depth by state