IECC 2021 climate zone
Climate zone 3C: warm-marine
Coastal California: mild year round, with marine moisture.
Zone 3C covers 14 counties in 1 state, home to about 8,574,696 people (FEMA NRI population). ENERGY STAR’s target for an uninsulated attic here is R49; with 3–4 inches already in place, add to R38.
Insulation targets for zone 3C
| Where | Guidance |
|---|---|
| Attic, uninsulated | R49 |
| Attic, 3–4 inches already | R38 |
| Floor over unheated space | R19 |
| Walls | When siding is removed: blow insulation into empty wall cavities, and add R5 insulative sheathing beneath new siding. |
| Basement or crawlspace walls | Add R5 insulative wall sheathing or an R13 batt. |
Homeowner retrofit guidance, not your local code. Your jurisdiction may enforce an older or amended IECC; ask the building department before a permit job.
What it means for upkeep
Most zone 3C counties fall in the marine Building America region. Mild and wet: moisture, moss, gutters and ventilation matter more than extreme temperatures.
The “C” (marine) regime means mild temperatures and long wet seasons: moss, gutters and ventilation matter more than extreme cold or heat.
Largest counties in zone 3C
Santa Clara County, CA1,934,625Alameda County, CA1,678,284San Francisco County, CA873,808Ventura County, CA843,136San Mateo County, CA763,661Sonoma County, CA488,330Santa Barbara County, CA447,998Monterey County, CA438,599San Luis Obispo County, CA280,370Santa Cruz County, CA270,408Marin County, CA262,172Napa County, CA137,956Mendocino County, CA91,247San Benito County, CA64,102
States in this zone: California (14).
PNNL county zone fileFEMA NRI population