St. Clair County is in IECC Climate Zone 4A (Mixed - Humid). Get accurate Manual J load calculations with official ASHRAE design temperatures for St. Clair County.
St. Clair County, Illinois is classified under IECC Climate Zone 4A (Mixed - Humid), a mixed-climate region with humid moisture conditions. With 31 ZIP codes and 1 communities, every HVAC installation in St. Clair County must use the ASHRAE design temperatures of 2°F (99% winter heating) and 94°F (1% summer cooling) for accurate Manual J load calculations.
With balanced heating and cooling loads, St. Clair County homes require well-rounded insulation — R-20 walls, R-49 ceilings, R-19 floors — and windows with both U-factor 0.32 and SHGC 0.4 to handle both winter lows and summer highs.
Minimum insulation R-values and window performance for Climate Zone 4A in St. Clair County, IL.
| Building Component | Minimum Requirement | Unit |
|---|---|---|
| Ceiling / Attic Insulation | R-49 | R-value |
| Wall Insulation | R-20 | R-value |
| Floor Insulation | R-19 | R-value |
| Window U-Factor | 0.32 | BTU/hr-ft²-°F |
| Window SHGC | 0.4 | Coefficient |
| Air Infiltration | 0.15 | ACH50 |
Climate Zone 4A requires precise load calculations. Upload your St. Clair County blueprint and get a professional Manual J report in 60 seconds— first report is FREE. Pre-loaded with St. Clair County's exact design temperatures (2°F winter / 94°F summer).
Enter your ZIP code to confirm it's in St. Clair County and get climate zone details.
Official ASHRAE design temperatures for St. Clair County, IL, used for accurate Manual J load calculations.
Minimum insulation and window requirements for St. Clair County per IECC 2021 energy code.
Climate Zone 4A: Mixed - Humid - True mixed climate with hot, humid summers and cold winters. Equal emphasis on heating and cooling.
1 cities and communities in St. Clair County share Climate Zone 4A requirements.
Compare climate zones with other counties in Illinois. Different zones require different HVAC equipment sizing.
Upload your St. Clair County blueprint and get a professional Manual J load calculation using official Zone 4A design temperatures. First report is FREE.
Join 10,000+ Illinois contractors who trust AutoHVAC