California building climate zones

Sixteen zones, and the one you build in sets part of the requirements

California is divided into 16 building climate zones, numbered 1 through 16, and the one your project sits in changes what the Energy Code asks of it. The lookup below returns the zone the California Energy Commission assigns to a ZIP code — the same number that goes into the compliance software and prints on the CF1R.

The term is defined in the code itself. Section 100.1(b) of the 2025 standards calls climate zones the 16 geographic areas of California for which the Commission has established typical weather data, prescriptive packages, and energy budgets. Three things, and all three move when the zone moves: the hourly weather file the model runs against, the prescriptive checklist the design has to satisfy, and the budget it has to beat.

The same house in two zones is not the same job

Take one set of drawings, change nothing on them, and put the building in a different climate zone: the compliance answer changes. Section 150.1(c) says a building on the prescriptive path meets all of the requirements for the appropriate climate zone in Table 150.1-A, where NA means the feature is not permitted in that zone and NR means there is no requirement for it there. A few rows of that table, and one requirement it points to:

• Windows. Maximum U-factor of 0.27 in climate zones 1 through 5, 11 through 14 and 16; 0.30 in zones 6 through 10 and 15.
• Solar heat gain. Maximum SHGC of 0.23 in zones 2, 4 and 6 through 14, and 0.20 in zone 15. In zones 1, 3, 5 and 16 there is no prescriptive SHGC requirement at all.
• West glass. A separate west-facing cap at 5 percent of conditioned floor area in zones 2, 4 and 6 through 15, and no separate west-facing cap in zones 1, 3, 5 and 16. The 20 percent total fenestration cap applies in all 16 zones either way, and west-facing glazing counts toward it.
• Whole-house fan. Prescriptively required in zones 8 through 14, and in no other zone.
• Framed walls. U-0.048 in every zone except 6 and 7, where it is U-0.065.
• Zone 16 alone. The only zone with a prescriptive slab perimeter floor requirement, and the only one that takes below-grade exterior walls to R-19 — the table reads R-5.0 in zones 1 through 13 and R-10 in zones 14 and 15. On the heat pump water heater options in Section 150.1(c)8, zone 16 is also the only zone that has to add a field-verified drain water heat recovery system.

That is one table, read down a different column. The component-by-component requirements page walks the rest of it.

The performance path does not get around it

Most of what I build runs the performance path, and the zone follows it there. The standard design a proposed building is measured against is the same building with the prescriptive requirements minimally met, and the basis of that standard design is Table 150.1-A — the column for your zone. So the target itself moves. The long-term system cost factors the 2025 budget is denominated in are set separately for each climate zone and each fuel type, the peak-cooling metric applies only in zones 4 and 8 through 15, and the load-shifting credit for a JA13 heat pump water heater runs from 3.1 percent in zone 14 to 22.7 percent in zone 16. Same water heater, very different value. In the compliance software the zone is entered from the project ZIP code, and the ZIP and the zone are both reported on the CF1R.

A zone can also add an inspection

ECC field verification — HERS on 2019 and 2022-cycle permits — is triggered by which measures are installed, not by the building type. So a zone that pushes a project onto a verified measure adds a site visit the identical house in another zone would not need. Refrigerant charge is the clearest case: for air conditioners it is required in zone 2 and zones 8 through 15, but under the 2025 code it is required for heat pumps in all 16 zones. An ECC-Rater does that work, an independent third party certified through a CEC-approved provider. Separate trade, not mine.

How the zone is actually determined

Two primary sources say it slightly differently. Section 100.1(b) says climate zones are defined by ZIP code and listed in Reference Joint Appendix JA2, and that the map at Figure 100.1-A is only approximate. JA2 itself, at section JA2.1.1, says the climate zone applicable to a building project is determined based on its physical location, as it relates to the determinations of climate regions found in the Commission publication California Climate Zone Descriptions. JA2 raises ZIP codes in exactly one sentence, and only to say that where a ZIP code contains more than one climate region, local jurisdictions may, at their discretion, designate a single climate zone for the entire ZIP code.

So the ZIP assignment is the working answer — the software takes it from the ZIP, the certificate reports it — while the zones underneath are geographic. The CEC describes its own address search app as showing locations in relation to the geographic metes and bounds that determine the climate regions, and says the ZIP list is published for the convenience of local jurisdictions that elect to designate one zone for a whole ZIP code.

How to read the lookup below

The lookup is that CEC file, and it is unambiguous. I downloaded and parsed it: 2,694 California ZIP codes, each mapped to exactly one zone, with no duplicates. No ZIP comes back with two answers.

The real caveat is not ambiguity in the list. It is that a ZIP-level list sits on top of boundaries drawn geographically, and the CEC says so in its own compliance-manual commentary: cities may occasionally straddle two climate zones, and in those cases the exact building location and correct climate zone should be verified before any calculations are performed. That is the sentence to remember when a parcel sits near a city line. The code carries one boundary provision of its own, at tract scale rather than lot scale — where a single contiguous subdivision or tract falls in more than one climate zone, all buildings in it may be designed to the zone containing 50 percent or more of the dwelling units, with the same allowance on the multifamily side. A subdivision-wide allowance, not a choice a designer makes on one lot.

What I do with it on a real job

I set the zone from the project ZIP code, and it prints on the certificate a plan checker reads. If the site is near a boundary, or the city straddles two zones, I check the address against the CEC tool before running anything — rebuilding a model in the right zone after the numbers are done is the avoidable kind of plan-check problem. Send the address with the plans and I will confirm the zone first.

Find a ZIP code’s climate zone

Straight from the California Energy Commission’s own ZIP-code list — all 2,694 California ZIP codes, each mapped to one of the 16 building climate zones. This runs entirely in your browser; nothing is sent anywhere.

Source: California Energy Commission, Building Climate Zones by ZIP Code. The ZIP list is a convenience mapping. Reference Joint Appendix JA2 raises ZIP codes in a single sentence, and only to say that where a ZIP code contains more than one climate region, local jurisdictions may, at their discretion, designate one climate zone for the whole ZIP code — which is why a list like this can show one zone per ZIP at all. Send me the project address and I will confirm the zone against the model rather than the ZIP.

How the 16 zones divide up California

Prices

Flat fees, in writing before I start. No hourly, no bid package, no change-order games — the number on the quote is the number on the invoice.

One honest note

Climate zone questions

If yours isn’t here, see the full FAQ or call and ask.