California Climate Zone 4

Where zone 4 is

The Energy Commission assigns 138 ZIP codes to zone 4 — about 5.1 per cent of every ZIP code it lists, and the 9th largest of the sixteen. Places in it include San Jose, Palo Alto, Sunnyvale, Gilroy, Paso Robles, Atascadero and King City.

The zone falls in two separate areas rather than one block, which is why a project in one part can feel unrelated to a project in the other. The ZIP codes fall in the prefix ranges 932xx, 934xx, 939xx, 940xx, 943xx, 950xx, 951xx.

What drives compliance in zone 4

Warmer summers than the bay shore and mild winters put the emphasis on cooling without making it extreme. Glazing orientation matters more here than one zone west, and shading on south-facing and west-facing elevations starts to earn its place in the calculation.

Where zone 4 ends

Zone 4 covers every ZIP in Santa Clara County without exception, which is unusual for a county of that size, and it has a second southern arm running through the inner Salinas Valley into San Luis Obispo County. Around the bay, the boundary is the county line.

Boundaries follow conditions rather than administrative lines, so the assignment is resolved from the project ZIP code rather than inferred from the city on the drawings.

The 2025 Standards in zone 4

Zone 4 is the lowest-numbered zone under the roof-deck rule: Section 150.0(a)1 asks for a U-factor of 0.184 on the deck of a newly built attic in zones 4 and 8 through 16. Option B adds R-19 below the deck to R-38 at the ceiling.

Cities we cover in zone 4

2 of our city guides cover places in this zone.

• Title 24 in San Jose
• Title 24 in Sunnyvale

County guides: Title 24 in Santa Clara County, Title 24 in Monterey County, Title 24 in San Luis Obispo County.