Cool - Humid - Heating-dominated climate with cold winters and warm, humid summers. High heating loads, moderate cooling needs. Get IECC 2021 R-values, design temperatures, and free Manual J calculations.
IECC Climate Zone 5A (Cool - Humid) is a heating-dominated zone where furnace sizing and insulation R-values are the primary design concerns. Covering parts of 17 states including Connecticut, Illinois, Indiana and 14 more, Zone 5A has a winter 99% design temperature of 0°F and a summer 1% design temperature of 91°F — a 91°F temperature swing that HVAC systems must handle. The humid moisture regime means latent cooling loads are significant, and proper dehumidification capacity is critical for comfort. The IECC 2021 requires minimum wall insulation of R-20, ceiling insulation of R-49, and windows with U-factor 0.3 or lower.
Quick reference table for IECC 2021 minimum insulation and window specifications in Climate Zone 5A.
| Building Component | Zone 5A Requirement |
|---|---|
| Ceiling / Attic | R-49 |
| Exterior Walls | R-20 |
| Floor (over unconditioned space) | R-30 |
| Window U-Factor (max) | 0.30 |
| Window SHGC (max) | 0.40 |
| Typical Air Infiltration (ACH) | 0.15 |
Source: IECC 2021 Table R402.1.2. Values shown are minimums for residential construction in Zone 5A.
Official design temperatures for Climate Zone 5A per ASHRAE Fundamentals 2021. These values are critical for accurate Manual J load calculations.
99% design temperature for heating load calculations (coldest expected temperature)
1% design temperature for cooling load calculations (hottest expected temperature)
Summer wet-bulb for latent cooling load and dehumidification calculations
Minimum insulation and window specifications for Climate Zone 5A per the International Energy Conservation Code (IECC) 2021.
Minimum wall R-value (continuous insulation or cavity + continuous)
Minimum attic/ceiling R-value for energy compliance
Minimum floor R-value (over unconditioned space)
Maximum window U-factor (lower is better insulation)
Recommended Solar Heat Gain Coefficient
Typical ACH (air changes per hour) for load calculations
High humidity climate requiring significant moisture control
Typical equipment specifications for Climate Zone 5A installations.
Climate Zone 5A covers 17 states across the United States. Click a state to see city-specific climate data and Manual J load calculations.
Climate Zone 5A includes parts of Connecticut, Illinois, Indiana, Iowa, Kansas, Massachusetts, Michigan, Missouri, Nebraska, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, South Dakota, West Virginia, and Wisconsin. This zone represents a heating-dominated climate with humid moisture conditions.
The official ASHRAE design temperatures for Climate Zone 5A are:
For Climate Zone 5A (Cool - Humid), recommended HVAC equipment includes:
Typical AFUE: 95-98%
Typical SEER: 13-16 SEER
IECC 2021 requires the following minimum R-values for Climate Zone 5A:
Climate Zone 5A is heating-dominated. This means heating loads typically exceed cooling loads, requiring greater focus on furnace capacity and winter insulation values.
Zone 5A has a humid moisture regime. This means high humidity levels, significant dehumidification requirements in summer, and careful attention to latent cooling loads in Manual J calculations.
Our AI-powered Manual J calculator uses official Zone 5A design temperatures (0°F winter / 91°F summer) and IECC requirements for pinpoint accuracy. First report is FREE.
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