Illinois is in IECC Climate Zone 5A (Cool - Humid). Get accurate Manual J load calculations with official design temperatures and IECC requirements.
Illinois falls entirely within IECC Climate Zone 5A (Cool - Humid), making it a heating-dominated state where furnace capacity and insulation R-values are the primary sizing concerns. With a winter 99% design temperature of 0°F and a summer 1% design temperature of 91°F, Illinois HVAC contractors must size systems for a 91°F temperature swing between extreme winter and summer conditions. Proper Manual J load calculations using these design temperatures are essential for code-compliant, right-sized installations that avoid callbacks and comfort complaints.
Minimum insulation R-values and window specifications required by the 2021 International Energy Conservation Code for Illinois.
| Component | Zone 5A |
|---|---|
| Ceiling / Attic | R-49 |
| Walls | R-20 |
| Floor | R-30 |
| Window U-Factor | 0.3 |
| Window SHGC | 0.4 |
Source: IECC 2021 Table R402.1.2. Requirements shown are for residential construction.
These ASHRAE design temperatures determine how Manual J load calculations size heating and cooling equipment for Illinois homes. The winter 99% value represents the coldest temperature expected, while the summer 1% value represents the hottest.
Cool - Humid
Based on Climate Zone 5A (Cool - Humid), here are typical HVAC equipment specifications for Illinois installations.
Annual Fuel Utilization Efficiency for Illinois furnaces
Seasonal Energy Efficiency Ratio for Illinois AC systems
Want a quick equipment estimate for your Illinois project? Use our free BTU calculator or AC tonnage calculator for a rough sizing, or upload blueprints for a professional Manual J report.
Illinois's climate zone data ensures proper equipment sizing. No more callbacks from oversized or undersized systems.
Illinois building departments require IECC-compliant Manual J calculations. Our tool uses official Illinois climate data.
Right-sized systems based on Illinois climate data reduce energy costs for homeowners and improve your reputation.
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