Choosing Windows by California Climate Zone

Window advice written for the whole country sorts the United States into a handful of bands and puts all of California in two of them. The state's own Energy Code is much finer than that. It divides California into 16 climate zones, and the figure a replacement window has to meet depends on which one your house is in. This page is the statewide map in words. It is part of our California window guide.

What the zones are

The California Energy Commission (CEC) describes the zones as a response to the state's "diversity of climates," with boundaries fixed by geographic description. Several parts of the Energy Code, including the requirements for windows, change with the zone a building is in.

Two points that save confusion:

• The numbers are labels, not a temperature scale. Zone 1 is the cool north coast and Zone 16 is the mountains. Zone 15 is the hottest.
• They are not the zones on window marketing. ENERGY STAR and the national model energy code use their own, much coarser maps. Only the CEC's 16 zones decide what a California permit requires.

How to look up your zone

The authoritative answer comes from the CEC's EZ Building Climate Zone Search Tool. Type the street address and it shows where the property falls against the zone boundaries.

The CEC also publishes a spreadsheet of zone assignments by ZIP code. It is quick, and for most addresses it gives the right answer, but the Commission offers it "for the convenience of local jurisdictions." ZIP codes do not follow climate boundaries. Where one ZIP takes in more than one zone, the local jurisdiction may designate a single zone for the whole ZIP. So if the ZIP list and the address tool disagree, ask your building department which it uses. In the foothills east of Sacramento and around the edges of the Bay Area, that question is worth asking.

All 16 zones

The cities below are from the CEC's ZIP code list. The two right-hand columns are the prescriptive maximums for replacement windows in an existing home under the 2025 Energy Code, which applies to permit applications submitted on or after 1 January 2026.

The one-line climate descriptions are ours, written to orient you. The CEC is careful to say the zones are based on energy use, temperature, weather and other factors rather than being simple labels like "desert" or "alpine," and the city lists are examples, not boundaries. Note the surprises: Escondido and El Cajon are Zone 10 while the city of San Diego is Zone 7, and Concord, Walnut Creek and Livermore share a zone with Sacramento, not with Oakland.

The two numbers, and which one your climate cares about

U-factor measures how readily heat passes through the whole window. Lower is better, and it matters whenever there is a temperature difference between inside and outside: cold mornings, and also a 105-degree afternoon with the air conditioning on.

SHGC, the solar heat gain coefficient, measures how much of the sun's heat comes through the glass. Lower blocks more. It matters when the sun is on the window and you are paying to cool the house.

You can read the state's judgment about each climate straight off the table:

• Zones with no SHGC requirement (1, 3, 5, 16) are the places where cooling is a small part of the energy picture. Insulation is the priority, so U-factor is the only number set.
• Zones with U-factor 0.30 (6 to 10, and 15) are Southern California from the beaches to the low desert. Winters are mild, so the code asks a little less of insulation there and holds the line on solar gain.
• Zones with both 0.27 and 0.23 (2, 4, 11 to 14) have a real winter and a real summer. This is where most of inland Northern California sits, and it is the most demanding combination.

What that means for the glass, region by region

The coast: Zones 1, 3, 5, 6 and 7

On the coast the window's job is mostly to hold heat in and keep condensation off the glass on foggy mornings. A low U-factor does both. Solar gain is not the enemy it is inland, and in Zones 1, 3 and 5 the prescriptive standard sets no SHGC limit, which leaves you free to choose glass that lets more winter sun in if you want it. Many coastal homeowners still pick a standard low solar gain package because it is what is stocked and it cuts fading and glare; that is a reasonable choice, not a required one.

The coast's bigger window problem is not on the label at all: salt air and constant damp are hard on hardware and finishes. That is covered in windows for coastal salt air and fog.

The valleys: Zones 2, 4, 11, 12 and 13

This is where we do most of our work, and it is the two-season climate: weeks of triple-digit afternoons, then cold, foggy winter mornings. Both numbers matter and both are set at their strictest. For most Sacramento Valley houses the glass package that makes the biggest difference to comfort is a dual-pane unit with a low solar gain Low-E coating, because the cooling season is long and the west sun is brutal. Our page on the best windows for cooling goes into the glass options, and the best energy-efficient windows for Sacramento applies them locally.

Triple-pane glass is rarely needed to meet the standard in these zones. Whether it is worth having for comfort or noise is a separate question, answered in are triple-pane windows worth it in Sacramento.

Inland Southern California: Zones 8, 9 and 10

Mild winters and hot summers put the weight on SHGC. The U-factor maximum is the more relaxed 0.30, but solar gain is capped at the same 0.23 as the Central Valley. West and south glass with no shade is where low solar gain glass does the most.

The deserts: Zones 14 and 15

The high desert (Zone 14) has the valley's problem with more extreme swings: hot days, cold nights, and winter cold that makes insulation count. It gets the stricter 0.27. The low desert (Zone 15) is about solar control first; winters are mild enough that the U-factor maximum is 0.30.

The mountains: Zone 16

Truckee, South Lake Tahoe, Mammoth and the rest of the high country are heating climates. There is no prescriptive SHGC limit, and winter sun through south glass is useful. U-factor is the number to push, and this is the one California climate where triple-pane glass is commonly worth pricing. Mountain houses also face snow load, ice at the sills and, in most of the zone, wildfire exposure; see our page on Tahoe and mountain homes.

Orientation still matters inside every zone

The prescriptive figures for replacement windows are the same for every wall of the house. The sun is not. A few rules of thumb that hold across California:

• West glass takes the worst of it. Low afternoon sun arrives when the outdoor air is hottest, and roof overhangs do little to stop it. If a room is unlivable at 5 p.m. in July, the west windows are usually why.
• South glass is the easiest to shade. The summer sun is high, so an overhang or awning blocks much of it, while the lower winter sun still gets in.
• North glass gets little direct sun. There, U-factor and daylight matter more than SHGC.
• Shade counts. A window under a deep porch or behind a mature tree sees far less solar gain than the same window on a bare wall.

None of that changes what the permit requires. It changes where an upgrade beyond the minimum pays you back in comfort.

The fine print, briefly

The figures in the table are the prescriptive route for alterations, which is how nearly all residential replacement projects comply. Three things to know, each explained in full on our page about what Title 24 requires of your replacement windows:

• There is a small-job exception. Replacing 75 square feet or less of windows may be done at a maximum U-factor of 0.40 and a maximum SHGC of 0.35.
• There is a performance path for projects where a particular window misses the prescriptive number.
• The figures are checked against the NFRC label on the actual product. Our guide to reading an NFRC label shows where to find them.

Whether your project needs a permit at all is covered in do you need a permit to install retrofit windows in California.

Sources

• California Energy Commission: Climate Zone Tool, Maps, and Information Supporting the California Energy Code
• California Energy Commission: EZ Building Climate Zone Search Tool
• California Energy Commission: Building Climate Zones by ZIP Code (spreadsheet)
• California Energy Commission: 2025 Building Energy Efficiency Standards (Title 24, Part 6), Section 150.2(b)1
• National Fenestration Rating Council: The Energy Performance Label

Working with Heritage Exteriors

We work from the Bay Area to the Sacramento Valley and Lake Tahoe, which covers climate zones 2, 3, 4, 11, 12 and 16, and a quote from us states the NFRC U-factor and SHGC of the window quoted so you can check it against the table yourself. If your address sits near a zone boundary, ask us to confirm the zone before anything is ordered. Licensed in California, CSLB #1055919. Call (916) 546-2991.