Climate zone 3B: warm-dry
Inland California, southern Nevada, west Texas and New Mexico lowlands: dry heat and cool winters.
Zone 3B covers 110 counties in 6 states, home to about 35,533,272 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 3B
| 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 3B counties fall in the hot-dry Building America region. Intense sun and heat, little rain, monsoon storms in some areas, and hard water.
The “B” (dry) regime means low humidity, high sun and often hard water: sealants, roof coverings and water-using appliances wear faster.
Largest counties in zone 3B
States in this zone: Texas (70), California (25), New Mexico (8), Arizona (5), Nevada (1), Utah (1).