Climate Zone 10
What the energy code asks of your windows
The two numbers that decide whether a replacement window passes in Riverside County, and how to read them off the label.
What it means for you
Two numbers, printed on the glass
Riverside County sits in Title 24 Climate Zone 10: inland, hot, and rated on how much sun your glass lets in rather than how much heat it lets out. On a standard replacement job, this is what a window has to hit.
You do not have to take anyone's word for it. Both numbers are certified by the National Fenestration Rating Council and printed on a label stuck to every new window. If someone describes performance without producing an NFRC number, they are describing a feeling, not a specification.
Maximum U-factor
0.30
How much heat conducts straight through the whole assembly. Lower is better insulated.
Maximum SHGC
0.23
The fraction of the sun's heat that gets through the glass. Lower means less solar gain.
The sun is the problem here
SHGC is the number that decides whether a west-facing room is usable at four o'clock. At 0.23, less than a quarter of the sun's energy hitting that glass gets into the room.
Clear glass will not pass
A window that meets both limits is a dual-pane unit with a spectrally selective low-E coating. There is no way to hit 0.23 SHGC with clear glass, whatever the frame is made of.
Small jobs get an easier limit
Replace 75 square feet of glass or less and the limits are 0.40 and 0.35 instead. Cross that line and the whole job is held to the tighter pair, which changes what you can order.
Leave the labels on
The NFRC sticker is the compliance document the inspector reads. Nobody should peel it off before the job has passed inspection.
In plain English
Why SHGC matters more than U-factor in Riverside
U-factor is insulation
U-factor measures conduction: heat moving through the frame, the spacer and the glass because it is hot on one side and cool on the other. It is the reciprocal of R-value, so where insulation counts up, U-factor counts down. A U-factor of 0.30 is roughly an R-3 assembly.
It governs winter losses and the slow soak of heat on a brutal afternoon. It matters. It is not what makes a west-facing room unusable at four o'clock.
SHGC is sunlight as heat
Solar heat gain coefficient measures radiation: sunlight passing through the glass and landing in the room as heat. An SHGC of 0.23 means 23 percent of the sun's energy hitting that window gets in. Old clear dual-pane glass runs around 0.70.
That difference is not incremental. On the same square footage of west glass, dropping from 0.70 to 0.23 removes roughly two-thirds of the solar load on that elevation.
The Inland Empire version
In a coastal climate the heating season dominates and U-factor is the number that pays. Inland, the cooling season dominates and the sun does most of the damage. That is exactly why the code allows a higher U-factor in Zone 10 than it does in the mountains and the north, while holding SHGC tight at 0.23. The standard is telling you where the money is.
Which limit applies to your job
Four thresholds, and the size of the job decides
| Job type | When it applies | Max U | Max SHGC |
|---|---|---|---|
| Standard replacement job The prescriptive Climate Zone 10 package. This is the number most whole-home jobs in Riverside are held to. | More than 75 sq ft of vertical fenestration replaced | 0.30 | 0.23 |
| Minor alteration Exception 1 to Section 150.2(b)1B. Roughly three to five average windows, depending on size. | 75 sq ft of vertical fenestration or less | 0.40 | 0.35 |
| Replacement skylight Exception 2 to Section 150.2(b)1B. | Any skylight swapped out | 0.55 | 0.30 |
| Mandatory floor The weighted-average maximum U-factor that applies regardless of which compliance path you use. | Every job, no exceptions in Zone 10 | 0.40 | Not set |
From Table 150.1-A of the 2025 standards and Section 150.2(b)1B with its exceptions. Full citations are at the foot of this page.
Setting the record straight
Is the 2026 limit 0.30 or 0.27?
In Riverside County it is 0.30. The confusion is real and it has a simple cause: the 2025 standards did tighten the prescriptive window U-factor, but only in some climate zones, and Zone 10 was not one of them.
The California Energy Commission 2025 Single-Family Residential Compliance Manual states it directly: the prescriptive maximum U-factor for window assemblies was reduced to 0.27 from 0.30 in Climate Zones 1 to 5, 11 to 14 and 16, and the prescriptive maximum U-factor in Climate Zones 6 to 10 and 15 remains at 0.30. The adopted Table 150.1-A carries 0.30 and 0.23 on the Climate Zone 10 row.
So if you have read that the 2026 code requires 0.27, that source was almost certainly describing a different climate zone, or quoting the code-change proposal rather than what was adopted. Zone 15, the desert, is the one that got genuinely stricter treatment for alterations, and it is not us.
The exemption that may not exist
A 10 square feet or 0.5 percent of floor area exemption circulates widely in window marketing. We went looking for it in the adopted single-family provisions and could not find it in Section 150.2(b)1B or any of its exceptions. We are not going to repeat it as fact on a page that people may rely on.
What is unambiguously in the code are two much more useful things: the 75 square foot minor-alteration exception, and the rule that re-glazing an existing sash or replacing a sash in an existing frame is a repair rather than an alteration when performance is at least equivalent. Between them they cover most genuinely small jobs. If a permit technician tells you otherwise, they are the authority having jurisdiction and they win.
Same zone, same numbers
Where Climate Zone 10 covers
California energy climate zones are not counties and not utility territories. Zone 10 runs across most of western Riverside County, so these cities are all held to the same window limits even though their utilities and permit rules differ completely.
- Riverside
- Corona
- Jurupa Valley
- Moreno Valley
- Norco
- Eastvale
- Temecula
- Murrieta
- Menifee
- Perris
Confirm your own address against the Energy Commission climate zone lookup before relying on it. Zone boundaries follow ZIP code assignments, and a handful of addresses on the county edges fall into Zone 9 or Zone 15.
Questions
About the energy code
Does replacing three windows trigger the full 0.30 requirement?
Usually not. Exception 1 to Section 150.2(b)1B allows replacement of 75 square feet or less of vertical fenestration at a U-factor of 0.40 and an SHGC of 0.35. Three to five typical windows will usually sit under that line. Cross it and the whole job is held to 0.30 and 0.23.
Is re-glazing the same as replacing a window?
No, and the code is explicit about it. Glass replaced in an existing sash and frame, or a sash replaced in an existing frame, is a repair rather than an alteration, provided the replacement is at least equivalent to the original in performance. Repairs do not trigger the alteration requirements.
What is an NFRC label and why does it matter?
It is the sticker on the glass carrying the certified U-factor and SHGC for that specific product. It is what the inspector reads. Do not peel the labels off before the job passes inspection.
Can I use a window that misses 0.23 SHGC?
Sometimes. The prescriptive package is the simple path, not the only path. A performance approach models the whole house and can trade a weaker window against something stronger elsewhere, which needs a compliance document prepared for the permit. If your heart is set on clear glass on a north elevation, that is the conversation to have before ordering.
Do I need a permit at all?
It depends on the city, not on the code. The City of Riverside exempts a true like-for-like replacement that does not alter the frame or the weatherproofing. Moreno Valley requires a permit for most window replacements. Confirm with the building department that will actually issue it, or let us, since we pull the permits on the jobs that need them.
Sources
Where these code values come from
All retrieved September 1, 2026. Everything on this page is a description of a published standard, and the authority having jurisdiction is the building department that issues your permit.
- California Energy Commission, 2025 Single-Family Residential Compliance Manual (CEC-400-2025-005) June 2025. States that the prescriptive maximum U-factor was reduced to 0.27 in Climate Zones 1-5, 11-14 and 16, and remains at 0.30 in Climate Zones 6-10 and 15.
- California Energy Commission, What’s New for 2025, Single-Family Revised February 19, 2025. Prescriptive U-factors by climate zone, and the mandatory maximum weighted average U-factor of 0.40.
- 2025 Building Energy Efficiency Standards, Table 150.1-A component package Climate Zone 10 row: maximum U-factor 0.30, maximum SHGC 0.23.
- California Energy Code Section 150.2(b)1B and its exceptions The 75 square foot minor-alteration exception, the skylight exception, and the rule that re-glazing an existing sash or frame is a repair.
- ENERGY STAR, residential windows key product criteria ENERGY STAR window criteria are climate-zone specific and based on NFRC-certified ratings.
We spec to the code before we spec to the catalog
Every window we install carries an NFRC label with numbers you can check against this page, and we pull the permit where one is required.