Climate zone 4C: mixed-marine
Western Oregon and Washington lowlands: cool, wet winters and mild summers.
Zone 4C covers 34 counties in 3 states, home to about 9,388,257 people (FEMA NRI population). ENERGY STAR’s target for an uninsulated attic here is R60; with 3–4 inches already in place, add to R49.
Insulation targets for zone 4C
| Where | Guidance |
|---|---|
| Attic, uninsulated | R60 |
| Attic, 3–4 inches already | R49 |
| Floor over unheated space | R30 |
| Walls | When siding is removed: blow insulation into empty cavities and add R5 to R10 insulative sheathing; on an insulated 2x4 wall, add R10 sheathing. |
| Basement or crawlspace walls | Add R15 insulative wall sheathing or an R19 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 4C 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 4C
States in this zone: Oregon (18), Washington (14), California (2).