Are aluminum windows allowed in California?

Yes. The Energy Code sets U-factor and SHGC limits for the whole window and does not prohibit any frame material. An aluminum window complies if its NFRC-rated values meet the limits that apply to the project.

Do thermally broken aluminum windows meet Title 24?

Some do. A thermal break reduces heat loss through the frame, but compliance depends on the NFRC-rated U-factor of the whole unit. Without a rating, a double-pane operable thermally broken window defaults to U-0.66, above the 0.40 mandatory cap.

What if my aluminum windows have no NFRC label?

Manufactured windows without a rating take the default values in Tables 110.6-A and 110.6-B, and for metal frames those defaults fail the mandatory U-factor cap. Field-fabricated windows must be shown to comply using the same default tables.

Does the fire-area exception mean any aluminum window is acceptable?

No. It removes the 0.40 mandatory cap for buildings built to the wildland-urban interface provisions in designated fire areas. The prescriptive values still apply unless the project uses the performance path.

Title 24 Aluminum Windows: Do They Comply?

Q: Can I keep my old aluminum frames and only replace the glass?

Replacing glass in an existing sash and frame, or a sash in an existing frame, is treated as a repair provided the replacement performs at least as well as the original. Replacing the whole window is an alteration under §150.2(b)1B.

They can. Title 24 limits the whole window’s U-factor and SHGC; it does not regulate what the frame is made of. The catch is the rating: a double-pane operable aluminum window with no NFRC rating is assigned a default U-factor of 0.79 (0.66 with a thermal break), and the 2025 code caps a home’s windows at a weighted average of 0.40.

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The code regulates the window, not the frame

There is no list of permitted frame materials in the Energy Code. What it controls is the performance of the whole fenestration product: the U-factor, which measures heat flow, and the solar heat gain coefficient (SHGC), which measures how much of the sun’s heat comes through. Under §110.6(a), the U-factor is rated to NFRC 100 and the SHGC to NFRC 200, or the product takes a default value from the code’s tables.

Frame material matters because it feeds into those ratings. The Energy Commission’s compliance manual puts it plainly: the U-factor covers the entire product, frame included, and for metal-framed products the frame losses can be significant. Wood, vinyl and fiberglass are better insulators than metal.

What happens without an NFRC rating

A product with no NFRC rating takes its U-factor from Table 110.6-A. For double-pane metal frames the defaults are:

operable aluminum window: U-0.79
fixed aluminum window: U-0.71
operable aluminum window with a thermal break: U-0.66
fixed aluminum window with a thermal break: U-0.55

For comparison, the same operable window in a nonmetal frame defaults to U-0.58. Every metal figure is above the 0.40 mandatory cap, and 0.05 is added for true divided lites or for dividers between the panes with a narrow spacer. An unrated aluminum window therefore cannot comply on its own. It needs an NFRC rating, or it has to be a small enough share of the glazing for better windows to carry it in the weighted average.

The rating also has to be visible on site. The temporary label on a manufactured window must not be removed before the building inspector has seen it.

Thermally broken frames

A thermal break frame is a metal frame that is not solid metal from inside to outside. The two halves are separated by a material with much lower conductivity, which interrupts the main heat path through the frame. That is why the default for a thermally broken operable window, 0.66, is lower than the 0.79 for plain metal.

The frame is still only part of the rating. The compliance manual notes that windows meeting the prescriptive values are likely to have at least double glazing with a low-emissivity coating, argon fill and an improved spacer. Whether a particular thermally broken aluminum unit gets there is answered by its NFRC label, not by the product name.

The numbers an aluminum window has to meet

There are two tests, and they work at different levels.

The mandatory test in §150.0(q) applies to new homes, additions and alterations. The weighted average U-factor of the fenestration must be 0.40 or lower. Up to 10 square feet, or 0.5 per cent of the conditioned floor area if that is greater, is exempt.

The prescriptive test in Table 150.1-A is tighter. For a new home the maximum U-factor is 0.27 in climate zones 1 to 5, 11 to 14 and 16, and 0.30 in zones 6 to 10 and 15. The 0.27 figure is new in the 2025 cycle; it was 0.30. The maximum SHGC is 0.23 in most zones and 0.20 in zone 15, with no SHGC limit in zones 1, 3, 5 and 16. You can find your zone on the climate zone page.

Homes in fire hazard areas

The 0.40 mandatory cap has an exception for fenestration in buildings that meet Part 7 of the California Building Code, the wildland-urban interface provisions, where the building is in a Fire Hazard Severity Zone or a Wildland-Urban Interface Fire Area designated by the local enforcement agency.

That exception is written against the mandatory cap only. The prescriptive values in Table 150.1-A carry no matching exception, so a home in a fire area whose windows cannot reach them complies through the performance path instead.

Prescriptive or performance

On the prescriptive path the U-factor and SHGC are applied as an area-weighted average across the windows and glazed doors. That leaves some room: a small aluminum unit can be offset by windows that beat the target, as long as the average holds.

On the performance path the home is modelled against a standard design that uses the prescriptive values. Windows that miss them cost energy in the model, and the shortfall is made up elsewhere, with better glazing on other elevations, more insulation or more efficient equipment. That is the route for a design built around large aluminum sliding doors. The general comparison is on prescriptive vs performance.

Replacement aluminum windows

Replacing windows in an existing home is an alteration under §150.2(b)1B. New windows of the same total area must meet the U-factor and SHGC in Table 150.1-A, with one useful exception: up to 75 square feet of replacement vertical glazing may instead meet a U-factor of 0.40 in every zone and an SHGC of 0.35 in zones 2, 4 and 6 to 15. Zone 15 has a separate exception capping replacement glazing at an SHGC of 0.23.

Two points matter for older aluminum windows. Replacing only the glass in an existing sash and frame, or a sash in an existing frame, is treated as a repair, provided the replacement performs at least as well as the original. And replacing every single-pane metal window in a house is a prescriptive alteration under the same section, with the existing-plus-alteration performance method available if the new units fall short.

Whether a window project needs a report at all is covered on do new windows need Title 24.

What we need to check an aluminum window

The NFRC U-factor and SHGC for each product, its size, the direction it faces, and whether it replaces an existing window or adds new glazing. A manufacturer’s sheet showing the rated values is enough to start. If the products are not chosen yet, we model the ones you are considering and show which of them the design can carry.