Mixed - Marine - Marine climate with mild, wet winters and cool summers. Moderate heating needs, minimal cooling. Get IECC 2021 R-values, design temperatures, and free Manual J calculations.
IECC Climate Zone 4C (Mixed - Marine) is a mixed-climate zone requiring careful balance between heating and cooling equipment sizing. Covering parts of 3 states including California, Oregon, Washington, Zone 4C has a winter 99% design temperature of 29°F and a summer 1% design temperature of 85°F — a 56°F temperature swing that HVAC systems must handle. The marine moisture regime brings moderate year-round moisture from ocean influence, with condensation prevention as a key concern. The IECC 2021 requires minimum wall insulation of R-20, ceiling insulation of R-49, and windows with U-factor 0.32 or lower.
Quick reference table for IECC 2021 minimum insulation and window specifications in Climate Zone 4C.
| Building Component | Zone 4C Requirement |
|---|---|
| Ceiling / Attic | R-49 |
| Exterior Walls | R-20 |
| Floor (over unconditioned space) | R-19 |
| Window U-Factor (max) | 0.32 |
| 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 4C.
Official design temperatures for Climate Zone 4C 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 4C 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
Moderate temperatures with high moisture from ocean influence
Typical equipment specifications for Climate Zone 4C installations.
Climate Zone 4C covers 3 states across the United States. Click a state to see city-specific climate data and Manual J load calculations.
Climate Zone 4C includes parts of California, Oregon, and Washington. This zone represents a balanced climate with marine moisture conditions.
The official ASHRAE design temperatures for Climate Zone 4C are:
For Climate Zone 4C (Mixed - Marine), recommended HVAC equipment includes:
Typical AFUE: 92-95%
Typical SEER: 14 SEER
IECC 2021 requires the following minimum R-values for Climate Zone 4C:
Climate Zone 4C is a balanced climate. This means heating and cooling loads are relatively balanced, requiring careful sizing of both heating and cooling equipment.
Zone 4C has a marine moisture regime. This means coastal climate with marine influences, moderate humidity, and potential for condensation issues that require proper vapor barriers.
Our AI-powered Manual J calculator uses official Zone 4C design temperatures (29°F winter / 85°F summer) and IECC requirements for pinpoint accuracy. First report is FREE.
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