Title 24 Window Replacement Rules: U-Factor and SHGC

Replacement windows in California must meet a maximum U-factor and, in most climate zones, a maximum SHGC set by the 2025 Energy Code. If you replace 75 square feet of windows or less, a looser limit applies: U-factor 0.40 and, where required, SHGC 0.35.

This page covers single-family homes, including ADUs. It uses Section 150.2(b) of the 2025 Energy Code, which applies to permit applications submitted on or after January 1, 2026.

Two numbers that matter

U-factor is how fast heat moves through the whole window. Lower insulates better.
SHGC (solar heat gain coefficient) is how much of the sun's heat comes through. Lower blocks more heat.

Both numbers are on the window's NFRC label and on the manufacturer's spec sheet. Frame material does not matter to the code by itself. Only the rated numbers do.

Repair, replacement or added window?

The code treats these three jobs differently.

Job	How the code sees it	Rule
New glass in an existing sash and frame, or a new sash in an existing frame	Repair	Not covered, if the new part performs at least as well
New windows replacing old ones, same total area	Replacement fenestration	U-factor and SHGC limits for your zone
New openings, or bigger windows	Added fenestration	U-factor and SHGC limits, plus total and west-facing area limits

Moving a window to a new spot in the same wall, without adding area, counts as a replacement.

Replacement window limits by climate zone

These are the prescriptive maximums for windows in an alteration under the 2025 code.

Climate zone	Max U-factor	Max SHGC
1	0.27	No requirement
2	0.27	0.23
3	0.27	No requirement
4	0.27	0.23
5	0.27	No requirement
6, 7, 8, 9, 10	0.30	0.23
11, 12, 13, 14	0.27	0.23
15	0.30	0.23
16	0.27	No requirement

In zone 15, new homes have a stricter 0.20 SHGC, but the code allows 0.23 for windows added or replaced in an alteration.

Not sure of your zone? Look up your address in the CEC's climate zone tool or see California climate zones. The city of Los Angeles alone spans zones 6, 8, 9 and 16, so check the address, not the city.

The small-job exception: 75 square feet or less

If the replacement windows total 75 square feet or less, they can meet these looser limits instead:

U-factor of 0.40 or lower, in every climate zone.
SHGC of 0.35 or lower, in climate zones 2, 4 and 6 through 15.
In zone 15, the SHGC limit stays at 0.23.

As a rough guide, 75 square feet is about five 3-by-5-foot windows.

Skylights

Replaced skylights may have a U-factor up to 0.40 and an SHGC up to 0.30.

Adding window area

New windows that add area must meet the same U-factor and SHGC limits. They also count toward the limits on total window area (20% of conditioned floor area) and west-facing window area (5%, in zones 2, 4 and 6 through 15).

There is a small allowance: adding up to 16 square feet of new window or skylight area does not have to meet the total and west-facing area limits.

If the windows are part of a room addition, the addition rules apply instead. See Title 24 for home additions.

Shading can count toward SHGC

Where the code sets a maximum SHGC, you can meet it with the glass itself, with a permanent exterior shading device that achieves the same result, or with a mix of both. Exterior shading must be fastened so it is not meant to be removed.

Mandatory rules for every window

These apply no matter which compliance path you use:

Weighted average U-factor of 0.40 or lower across the fenestration (Section 150.0(q)). Up to 10 square feet or 0.5% of floor area, whichever is greater, is exempt. Up to 30 square feet of dual-glazed garden windows is exempt. Buildings built to the California Wildland-Urban Interface Code in a Fire Hazard Severity Zone or WUI fire area are also exempt.
Labels. Each window needs a temporary label with its certified U-factor, SHGC and visible transmittance. The label must stay on until the inspector sees it.
Air sealing. Gaps around new windows must be caulked, gasketed or weatherstripped.

Prescriptive or performance?

Most window jobs use the prescriptive path: every new window meets the table above. It is fast and needs no modeling.

If a window you want does not meet the table, the performance path models the whole house. Better features elsewhere, such as added attic insulation, can make up the difference. That needs more information about the existing house.

What to have ready

Your address, so the climate zone is right.
A list of each window: room, size, which way it faces, and whether it is a replacement or new opening.
The NFRC U-factor and SHGC for each product. A quote or spec sheet is fine.
Photos or a simple floor plan showing where the windows are.

Ask your building department which form it wants with the permit. The CEC's manual says prescriptive alterations use a CF1R-ALT form, submitted with the plans or with the permit application when no plans are required.

Need a CF1R for a window job, an addition or an ADU? See our flat prices.

Frequently asked questions

What U-factor do replacement windows need in California?

Under the 2025 Energy Code, 0.27 or lower in climate zones 1-5, 11-14 and 16, and 0.30 or lower in zones 6-10 and 15. If you replace 75 square feet of windows or less, a U-factor up to 0.40 is allowed instead.

Do I need a Title 24 form to replace windows?

Replacing windows is an alteration under the Energy Code. The CEC's compliance manual says prescriptive alterations use a CF1R-ALT form, submitted with the plans or with the permit application when no plans are required. Your building department sets the exact submittal.

Is replacing just the glass a Title 24 alteration?

No. The 2025 Energy Code treats glass replaced in an existing sash and frame, or sashes replaced in an existing frame, as a repair, as long as the replacement performs at least as well as the original.

Where do I find a window's U-factor and SHGC?

On the window's NFRC label and on the manufacturer's spec sheet or quote. The 2025 Energy Code requires a temporary label listing the certified U-factor and SHGC to stay on the window until the inspector has seen it.

What if my windows do not meet the prescriptive values?

You can use the performance method instead, which models the whole house and lets better features elsewhere make up the difference. Every window must still meet the mandatory maximum weighted average U-factor of 0.40, unless an exception applies.

Title 24 rules for replacing or adding windows