Williamson County is in IECC Climate Zone 5A (Cool - Humid). Get accurate Manual J load calculations with official ASHRAE design temperatures for Williamson County.
Williamson County, Illinois is classified under IECC Climate Zone 5A (Cool - Humid), a heating-dominated region with humid moisture conditions. With 32 ZIP codes and 1 communities, every HVAC installation in Williamson County must use the ASHRAE design temperatures of 7°F (99% winter heating) and 93°F (1% summer cooling) for accurate Manual J load calculations.
As a heating-dominated zone, Williamson County requires robust insulation — a minimum of R-20 in walls, R-49 in ceilings, and R-30 in floors. Windows must meet a U-factor of 0.3 or lower. These requirements ensure homes maintain comfort during the coldest 7°F design conditions.
Minimum insulation R-values and window performance for Climate Zone 5A in Williamson County, IL.
| Building Component | Minimum Requirement | Unit |
|---|---|---|
| Ceiling / Attic Insulation | R-49 | R-value |
| Wall Insulation | R-20 | R-value |
| Floor Insulation | R-30 | R-value |
| Window U-Factor | 0.3 | BTU/hr-ft²-°F |
| Window SHGC | 0.4 | Coefficient |
| Air Infiltration | 0.15 | ACH50 |
Climate Zone 5A requires precise load calculations. Upload your Williamson County blueprint and get a professional Manual J report in 60 seconds— first report is FREE. Pre-loaded with Williamson County's exact design temperatures (7°F winter / 93°F summer).
Enter your ZIP code to confirm it's in Williamson County and get climate zone details.
Official ASHRAE design temperatures for Williamson County, IL, used for accurate Manual J load calculations.
Minimum insulation and window requirements for Williamson County per IECC 2021 energy code.
Climate Zone 5A: Cool - Humid - Heating-dominated climate with cold winters and warm, humid summers. High heating loads, moderate cooling needs.
1 cities and communities in Williamson County share Climate Zone 5A requirements.
Compare climate zones with other counties in Illinois. Different zones require different HVAC equipment sizing.
Upload your Williamson County blueprint and get a professional Manual J load calculation using official Zone 5A design temperatures. First report is FREE.
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