01Key facts / TL;DR
Yes, the 2025 single-family prescriptive window criteria changed for additions and alterations; do not reuse a 2022 schedule without review.
It depends on climate zone and applicable conditions: the CEC summarizes maximum U-factor 0.27 in zones 1–5, 11–14 and 16, with 0.30 retained in zones 6–10 and 15.
No, those summary numbers are not every exception or the mandatory floor for every project pathway.
Yes, use the actual rated assembly and distinguish replacement glazing from added openings before selecting documentation.
02Read the two ratings as different design inputs
U-factor describes heat transfer through the rated window assembly. Lower U-factor indicates less transfer under the rating conditions. Solar heat gain coefficient, or SHGC, describes the fraction of incident solar heat admitted. A window can perform well on one measure and differently on the other; “energy efficient” is not a sufficient input.
Neither number is the insulation R-value of the wall around the opening. A glazing brochure may quote center-of-glass performance that is not the complete assembly value needed for the project. Supply supported whole-product ratings for the proposed configuration, including differences between fixed and operable products.
For a homeowner, the immediate task is simple: ask the supplier for the evidence behind the scheduled U-factor and SHGC before ordering. For an architect, align that evidence with opening IDs, dimensions and operation type. For the preparer, verify that the input belongs to the product the drawings actually specify.
03What the CEC says changed in 2025
Chapter 9 of the CEC 2025 Single-Family Residential Compliance Manual summarizes the prescriptive maximum U-factor reduction from 0.30 to 0.27 for additions and alterations in climate zones 1–5, 11–14 and 16. It says zones 6–10 and 15 remain at 0.30. These are single-family prescriptive summaries, not a universal rule for multifamily or commercial glazing.
The same summary notes that homes of 500 square feet or less retain a 0.30 U-factor requirement in climate zone 5. That condition illustrates why publishing only a statewide “all windows must be 0.27” slogan would be misleading. Check the actual provision and all conditions for the project rather than relying on a product salesperson’s shorthand.
Chapter 9 also identifies SHGC 0.23 in climate zone 15 and specifically discusses alterations adding fenestration area. Read the applicable §150.2 pathway and referenced table before applying that value to a particular opening. It is not a claim that every replacement window statewide needs SHGC 0.23.
| Single-family climate zones | Summary maximum U-factor | Important limitation |
|---|---|---|
| 1–5, 11–14, 16 | 0.27 | Scope-specific prescriptive criteria; Chapter 9 notes the small-home CZ 5 condition |
| 6–10, 15 | 0.30 | Not a statement that SHGC, area or other requirements disappear |
04Addition, replacement and added glazing are different branches
An addition creates a new scope boundary; an alteration changes an existing component. Replacing a window in the same opening is not identical to cutting a new opening or increasing glazing area. Show these conditions separately so the preparer can apply the relevant addition or alteration provisions.
The code contains particular small-area and other alteration exceptions. This guide deliberately does not turn them into a simplified universal area exemption. The applicable area, existing condition, product type and other conditions must be checked in the current provision. A small order is not automatically free of energy documentation.
Addition glazing can also involve area and orientation constraints under the selected prescriptive path. A compliant U-factor alone does not prove that the addition’s glazing package meets every requirement. Supply the opening schedule, addition area and orientation rather than asking the consultant to evaluate a window in isolation.
Does an addition need Title 24?Window replacement intent guide
05Confirm the location and edition before ordering products
Use the AHJ-recorded permit-application date. Applications on or after January 1, 2026 use the 2025 Energy Code; a valid earlier application can remain under an earlier cycle. A window quote date or a later installation date does not by itself decide the edition.
Confirm climate zone for the exact project location using the CEC map and reviewing agency. Our reference lookup distinguishes postal convenience assignments and weather-reference cities from a confirmed parcel determination. A city can span conditions; do not assume an entire county has one zone.
Keep the edition and location evidence with the model and schedule. If the product order changes, the architect and preparer need to review the actual replacement ratings before finalizing the package. An old passing report is not a blanket approval for later substitutions.
Climate-zone lookup and its limitsApplication-date decision guide
06Can performance modeling help?
Where a performance approach is allowed, the model can compare eligible combinations of glazing, assemblies and systems. That can be useful when a design does not fit the straightforward prescriptive package. Mandatory measures still apply; the model is not permission to use any window without limits.
Ask which assumptions are preliminary and what design changes are genuinely available. If the chosen product is fixed, the option study should identify feasible measures elsewhere rather than quietly inserting a better window value. Performance analysis is only useful when the final drawings can implement the evaluated design.
Keep single-family, multifamily and commercial rules separate. A condo-window project should first confirm its building family; this guide’s single-family change table should not be pasted into a multifamily submittal as if it were a universal schedule.
07Illustrative schedule review before the order is released
ILLUSTRATIVE ONLY: an architect designs a 450-square-foot single-family addition in a project location confirmed as climate zone 12. The supplier proposes a rated U-factor of 0.30. The team is considering the applicable prescriptive addition pathway, not claiming a particular exception.
The CEC change summary flags 0.27 for this zone group, so the architect requests a qualifying product option or asks the preparer to evaluate an allowed performance package. The team also checks glazing area, orientation and SHGC rather than declaring compliance from the U-factor alone. No model result is invented here.
The intake packet contains the dated opening schedule, supported ratings, addition boundary and existing/proposed plans. Once a direction is chosen, the energy package and drawing schedule are updated together. This avoids a plan-check mismatch caused by the model naming a different product from the purchase order.
08Six window-input questions
Does 0.27 apply to every California window replacement?
No. The cited change is a single-family prescriptive summary with climate-zone and other conditions. Review the actual alteration provision, applicable exceptions and project classification.
Is SHGC the same as U-factor?
No. SHGC concerns admitted solar heat; U-factor concerns heat transfer through the rated assembly. Provide both where required rather than treating one as a substitute for the other.
Can I use a center-of-glass brochure value?
Do not assume it is the required whole-product rating. Obtain supported assembly data for the actual product and configuration and coordinate it with the schedule.
Does a small replacement area mean no rules apply?
Not automatically. Specific small-area provisions have conditions. Ask the responsible preparer and AHJ to identify the applicable rule instead of claiming a blanket exemption.
Can an approved model cover a different window order?
Not without review. Changed ratings, area or operation type can affect the package. Send the revised schedule and supported product information before relying on the prior output.
What should accompany my window quote request?
Send existing/proposed opening IDs and sizes, orientations, supported U-factor/SHGC, application date, project scope and confirmed location information. Include any correction letter or prior report.
09Turn the question into a scoped documentation request
Title 24 Lab is a provisional brand for design-phase energy-document preparation. Field testing, acceptance testing, registry fees, CALGreen documentation and AHJ fees are excluded. This is educational information, not legal advice; confirm the application date, classification and requirements with the AHJ. Review the shared published pricing, then upload the current plans and identify unresolved inputs. Submission is not payment, production acceptance or permit approval.
See published residential and commercial pricingWhat to send with the plansGet a quote / upload plans privately
Sources
- CEC: 2025 Energy Code and January 1, 2026 effective date
- CEC: 2025 single-family manual, Chapter 9 — additions, alterations and repairs
- CEC: 2025 single-family CF1R, CF2R and CF3R forms
- CEC: climate zone tool, maps and location guidance
- CEC 2025 Energy Code: adopted standards, including §§110.3, 110.4, 150.0 and 150.2