9 min read · Cost
Replace enough window area in a California home and the state's energy code — Title 24, Part 6 — stops being background noise and becomes the spec sheet for your project. The 2025 Building Energy Efficiency Standards took effect January 1, 2026, and they tightened the window numbers, so advice written for the old cycle is now out of date. This guide covers the code mechanics: when a replacement triggers Title 24, what the prescriptive targets are, how the paperwork and inspection actually work, and how the energy rules stack with the wildfire glazing rules in WUI parcels. For what U-factor and SHGC actually mean on the label, start with our U-factor and SHGC explainer — this page is about compliance, not ratings.
When a window replacement triggers Title 24
Any window replacement in California is an 'alteration' under Title 24, Part 6, and it needs a permit — that part is not size-dependent. What the size changes is which requirements apply. The commonly used dividing line is 75 square feet of replaced window area: at or above it, the full prescriptive fenestration requirements apply to the new units; below it, the code has historically allowed substantially relaxed values for small alteration areas (a maximum U-factor of 0.40 and SHGC of 0.35 under the prior cycle's tables — confirm the current-cycle row with your building department before relying on it). In practice, a whole-house window replacement — the project most homeowners are actually pricing — lands squarely in full-compliance territory, and the smart move is to spec code-level glass everywhere anyway: the cost difference between a 0.40 unit and a compliant one is small next to the labor, and mixing glass packages across one elevation is a false economy. Our window replacement guide covers the rest of the scoping decisions.
The current prescriptive targets: U-factor and SHGC
Under the 2025 standards, the prescriptive path for residential replacement windows centers on a maximum U-factor of 0.27 — tightened from the 0.30 that defined the 2022 cycle. On solar heat gain, the prescriptive SHGC cap published by the California Energy Commission is 0.23 in the cooling-dominated climate zones — which is most of inland Northern California, including the Sacramento Valley — while the mildest coastal zones and the coldest mountain zone carry no prescriptive SHGC cap at all, because solar gain there is free heating rather than a cooling load. Two practical notes: first, these are prescriptive targets, and the exact row for your climate zone is what governs — look your address up with the CEC climate zone tool and confirm the value with your building department rather than a blog, including this one. Second, hitting U-0.27 is not exotic: mainstream dual-pane vinyl and fiberglass lines reach it with the right low-e package, so the code is less a cost bomb than a floor under glass quality.
Prescriptive vs. performance: the two ways to comply
Title 24 always offers two compliance routes. The prescriptive path is the simple one: every replacement unit individually meets the published U-factor and SHGC targets for your climate zone, you list the NFRC numbers on the permit documents, and you are done — no modeling, no trade-offs. The performance path instead models the whole building's energy budget, which lets one component miss its prescriptive target if others overshoot theirs — useful when a design forces a big glass wall, a specialty shape, or a historic-profile window that cannot hit 0.27. For a straightforward window swap, prescriptive is almost always the right route, because performance modeling costs money (an energy consultant runs it in approved software) that a normal replacement project has no reason to spend. Where performance earns its fee is combined projects — a re-side with added insulation, for example, can generate the modeled headroom that buys flexibility on glazing, which is one more argument for scoping windows and siding together. Title 24's wall rules are a separate topic we cover in the Title 24 siding guide.
Permits, paperwork, and what the inspector actually checks
The compliance workflow is more clerical than technical, and knowing it demystifies the permit. Your contractor files for a permit and submits energy compliance documentation identifying the U-factor and SHGC of the specified units. The windows arrive with temporary NFRC labels on the glass — the certified ratings for that exact unit, not the product line's best case — and here is the detail that catches people: the inspector wants to see those labels at final inspection, so do not peel them until the window is signed off. The inspector verifies the labeled numbers match the compliance documents, checks that egress openings were not compromised (our egress requirements guide covers that separate body of rules), and looks at flashing and installation on visible openings. Unpermitted window work surfaces at sale time — disclosure forms and buyer inspections both flag it — so the permit is cheap insurance. What a jurisdiction charges varies; our permit cost guide covers how NorCal jurisdictions price exterior permits generally.
The WUI overlay: energy code and fire code stack
In a Wildland-Urban Interface parcel, Title 24's energy rules and the state's wildfire glazing rules apply simultaneously — they are separate chapters of the same Title 24 umbrella, and satisfying one does nothing for the other. The 2025 California Wildland-Urban Interface Code requires exterior glazing in designated fire-severity zones to be dual-pane with at least one tempered pane (or meet one of three alternate paths), while Part 6 independently requires the U-factor and SHGC targets above. The good news is the specs are fully compatible: tempering a pane does not hurt its energy ratings, and every major manufacturer builds tempered low-e dual-pane units that satisfy both chapters in one product. The failure mode is contractual, not technical — a quote that says 'Title 24 compliant' has said nothing about tempering, and vice versa, so in a WUI parcel make the contract name both: the NFRC numbers and the tempered glazing. Our fire-rated windows guide covers the wildfire side in full.
Climate-zone reality across Northern California
The code reads differently depending on where you stand. Sacramento, Roseville, and Folsom sit in climate zone 12 — hot-summer valley — where the SHGC cap does the heavy lifting: low-solar-gain glass is what keeps a west-facing room livable in July, and the code is effectively mandating the glass you would want anyway. The foothills through Auburn and El Dorado Hills share the cooling-dominated logic with added WUI overlap, so most foothill replacements carry both specs. Truckee and Tahoe sit in climate zone 16 — the cold-mountain zone — where the calculus inverts: U-factor is everything, winter solar gain is welcome, and the prescriptive SHGC cap does not apply. The Bay Area's mild coastal zones lean on U-factor with little or no SHGC requirement. None of this changes what you order so much as which number deserves your attention when comparing bids — and it is why a quote imported from a different region can spec exactly the wrong glass. Our window frame materials guide and best window brands guide cover how to pair the frame and the line with the glass package your zone demands.
Key takeaways
- Window replacement is a permitted alteration under Title 24, Part 6 — the 2025 standards (effective January 1, 2026) govern current projects
- The prescriptive targets: maximum U-factor 0.27, with an SHGC cap of 0.23 in cooling-dominated zones — confirm your climate zone's exact row with the CEC or your building department
- Replacements below roughly 75 sq ft of aggregate area have historically qualified for relaxed values; whole-house projects face full compliance
- Inspectors verify the temporary NFRC labels against the permit documents at final — don't peel them early
- In WUI parcels the energy code and fire code stack: the contract should name both the NFRC numbers and the tempered glazing
FAQ
Quick Answers
Yes — window replacement is an alteration under Title 24, and jurisdictions require a permit regardless of project size. The permit is also how compliance is documented: the energy paperwork lists the units' U-factor and SHGC, and the inspector verifies the NFRC labels against it at final inspection.
Under the 2025 standards effective January 1, 2026, the prescriptive target for replacement windows is a maximum U-factor of 0.27 — tightened from 0.30 in the prior cycle. Small alteration areas below the 75-square-foot threshold have historically qualified for relaxed values; confirm the current row for your climate zone with your building department.
It's the commonly applied threshold separating small alterations from full prescriptive compliance. Replace 75 sq ft or more of window area and each new unit must meet the full prescriptive targets; below it, the code has allowed substantially relaxed values. Most whole-house replacements exceed the threshold easily.
Different part of Title 24. The energy rules are Part 6; the wildfire glazing rules — dual-pane with at least one tempered pane in WUI zones — live in the 2025 California WUI Code, Title 24, Part 7. Both apply independently in a fire-severity zone, and one product can satisfy both if the contract specs both.
Sources
Authoritative references
- CEC 2025 Building Energy Efficiency Standards (effective Jan 1, 2026)
- CEC — California's 16 Building Climate Zones (maps & address finder)
- National Fenestration Rating Council (NFRC) — window performance ratings
- 2025 California Wildland-Urban Interface Code (Title 24, Part 7) — full text
- ENERGY STAR — Residential Windows, Doors & Skylights
External links to government, code, and manufacturer sources. Sierra Siding is not affiliated with these organizations; references are provided for verification.