Washington spans 2 IECC climate zones. Find your exact zone, design temperatures, and get free Manual J load calculations for any Washington location.
Washington spans 2 IECC climate zones (Zone 4C, Zone 5B), which means insulation requirements, design temperatures, and equipment specs vary significantly depending on location. The primary zone, Zone 4C (Mixed - Marine), is a mixed climate, with winter design temperatures as low as 29°F and summer highs reaching 85°F. HVAC contractors working across Washington need to verify the exact climate zone for each job site — using the wrong zone data leads to oversized or undersized equipment, failed inspections, and unhappy homeowners.
Minimum insulation R-values and window specifications required by the 2021 International Energy Conservation Code for Washington.
| Component | Zone 4C | Zone 5B |
|---|---|---|
| Ceiling / Attic | R-49 | R-49 |
| Walls | R-20 | R-20 |
| Floor | R-19 | R-30 |
| Window U-Factor | 0.32 | 0.3 |
| Window SHGC | 0.4 | 0.4 |
Source: IECC 2021 Table R402.1.2. Requirements shown are for residential construction. Washington contractors must use the zone that corresponds to the job site location.
These ASHRAE design temperatures determine how Manual J load calculations size heating and cooling equipment for Washington homes. The winter 99% value represents the coldest temperature expected, while the summer 1% value represents the hottest.
Mixed - Marine
Cool - Dry
Based on Climate Zone 4C (Mixed - Marine), here are typical HVAC equipment specifications for Washington installations.
Annual Fuel Utilization Efficiency for Washington furnaces
Seasonal Energy Efficiency Ratio for Washington AC systems
Washington's climate zone data ensures proper equipment sizing. No more callbacks from oversized or undersized systems.
Washington building departments require IECC-compliant Manual J calculations. Our tool uses official Washington climate data.
Right-sized systems based on Washington climate data reduce energy costs for homeowners and improve your reputation.
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