California Title 24 and Your Window or Siding Project

Replacing windows in a California home is an "alteration" under the Title 24 Energy Code, so the new windows must meet maximum U-factor and, in some climate zones, maximum solar heat gain limits. Under the 2025 Energy Code, which applies to permit applications filed on or after January 1, 2026, replacement windows in most single-family homes must have a U-factor of 0.27 or lower, with a looser limit for small jobs of 75 square feet or less. Re-siding by itself does not trigger a wall insulation requirement in the Energy Code, but any insulation you add to opened walls has to meet the code minimums.

Below is what Title 24 is, how it applies to windows and siding, how to read an NFRC label, and what to expect from the permit process in the Bay Area.

What Title 24 is

Title 24 of the California Code of Regulations is the California Building Standards Code. It has several parts, and a home exterior project usually touches three of them:

• Part 2.5, the California Residential Code, which covers structure, weather barriers, flashing and emergency escape openings.
• Part 6, the Energy Code (formally the Building Energy Efficiency Standards), adopted by the California Energy Commission. When people say "Title 24" about windows, this is the part they mean.
• Part 7, the California Wildland-Urban Interface Code, new for 2026, which applies in fire hazard zones.

The state updates the code on a three-year cycle. The 2025 edition took effect January 1, 2026, and replaced the 2022 edition. Your project follows the edition in effect when the permit application is filed, so a job permitted in late 2025 may have been reviewed under the older rules.

When replacement windows must meet Title 24

The Energy Code treats work on an existing home as an alteration. For single-family homes, Section 150.2(b) of the 2025 code sets these rules for windows:

• Replacement windows (new manufactured windows replacing existing ones of the same total area) must meet the U-factor and SHGC values for new homes in your climate zone.
• Small replacements of 75 square feet of window area or less can use a looser limit: a U-factor of 0.40 or lower in every climate zone, and an SHGC of 0.35 or lower in climate zones 2, 4 and 6 through 15.
• Added window area must meet the same performance values and the limits on total and west-facing window area, with an exception for adding up to 16 square feet.
• Repairs are different. Replacing glass in an existing sash, or a sash in an existing frame, counts as a repair as long as the replacement performs at least as well as the original.

All new windows also have to meet the mandatory requirements, including a maximum area-weighted average U-factor of 0.40 across new and replacement windows (with small exceptions) and caulking and sealing around the frame.

2025 Energy Code limits for replacement windows

These values come from the 2025 Building Energy Efficiency Standards and the Energy Commission's 2025 Single-Family Residential Compliance Manual. They apply to single-family homes using the prescriptive path. Multifamily buildings use a different table (see below).

California has 16 building climate zones. The Energy Commission uses Oakland as the reference city for climate zone 3 and San Jose for zone 4, but zone lines don't follow city limits, and North Bay valleys and inland East Bay towns can fall in other zones. Look up your address with the Energy Commission's climate zone tool or ask your building department. If your project can't meet the prescriptive values, an energy consultant can model the whole house under the performance path instead.

U-factor, SHGC and the NFRC label

Manufactured windows installed in California homes have to be certified and labeled. The National Fenestration Rating Council (NFRC) label is the sticker on the glass that shows the ratings the inspector checks.

• U-factor measures how fast heat passes through the whole window, frame and glass together. Lower is better insulating. Most windows rate somewhere between about 0.20 and 1.20.
• Solar heat gain coefficient (SHGC) is the share of the sun's heat the window lets in, on a scale of 0 to 1. Lower means less solar heat, which matters on south- and west-facing glass in hot inland areas.
• Visible transmittance is how much daylight passes through. Low-SHGC glass can look slightly darker, so compare samples if daylight matters.

The 2025 code requires the temporary label to stay on the window until the building inspector has seen it. Ask your installer not to peel them off at installation. Our fiberglass vs vinyl windows guide covers how frame material affects these ratings.

How Title 24 applies to re-siding

This is where homeowners get mixed messages. Here is what the 2025 code and the Energy Commission's manual actually say:

• Re-siding alone is not listed as an alteration that forces wall insulation. The Energy Commission's compliance manual describes opened exterior walls as an opportunity to insulate, not a requirement.
• If you add insulation to exterior framed walls, it must meet the mandatory minimums: R-15 in 2×4 framing or R-21 in 2×6 framing. Existing 2×4 walls already insulated to R-11 are exempt from the upgrade.
• If you replace windows during the re-side, those windows follow the replacement window rules above.
• Additions and newly conditioned space, such as converting a garage into an ADU, have their own wall insulation requirements.

Even when it isn't required, a re-side on an older, uninsulated Bay Area house is the cheapest time to add insulation. With the siding off, the sheathing can be inspected, and insulation can often be blown into the cavities through small holes in the sheathing before the new weather barrier goes on. Local building departments interpret the scope differently, so ask yours at the permit stage.

Fire zones and Title 24

If your home is in a Fire Hazard Severity Zone, windows may also need to meet the wildfire-zone code, usually with dual-pane glass that includes at least one tempered pane. The 2025 Energy Code recognizes this: its mandatory 0.40 U-factor cap includes an exception for windows in buildings that meet the California Wildland-Urban Interface Code in fire hazard areas. When you order, check the NFRC ratings for the tempered version of the window you picked, since glass options can change the numbers. Our fire-resistant siding and vents guide covers the rest of the wildfire-zone requirements.

Apartments and condos

Multifamily buildings follow Section 180.2 of the 2025 Energy Code, which uses a separate table of window values that varies by window type and climate zone. It includes an exception from the solar heat gain and visible light requirements when 150 square feet or less of the building's windows are replaced. Our apartment window replacement guide covers how those projects are planned.

Permits and paperwork

Window replacement usually needs a building permit in Bay Area cities, and the energy rules are enforced through that permit. Expect the following:

1. Permit application with the window schedule (sizes, locations and ratings), filed by the contractor or owner. Your contract should say who pulls it.
2. Energy compliance documentation showing the windows meet the prescriptive values, or a performance calculation if they don't.
3. Inspection, where the inspector checks the NFRC labels against the documents and looks at sealing and flashing.
4. Bedroom windows are also checked for emergency escape size. See our egress window guide.

Ask for any window quote to list the U-factor and SHGC of each unit so you can check them against your zone before you sign. You can see a combined siding and window project in our Alameda Hardie siding and Andersen A100 window project, or read about our window replacement service.

Frequently asked questions

Does Title 24 apply to replacement windows?

Yes. Replacing windows is an alteration under the California Energy Code, so the new windows must meet the U-factor and SHGC requirements for your climate zone. Small projects of 75 square feet or less have a looser limit, and replacing only glass or sashes in existing frames counts as a repair.

What U-factor do I need for replacement windows in the Bay Area?

Under the 2025 Energy Code, replacement windows in single-family homes in climate zones 1 through 5, 11 through 14 and 16 need a U-factor of 0.27 or lower. Those zones cover much of the Bay Area. Confirm your climate zone by address before ordering.

Do I have to add insulation when I replace my siding?

The Energy Code does not list re-siding alone as triggering a wall insulation upgrade. If you do add insulation to exterior framed walls, it must meet R-15 in 2×4 walls or R-21 in 2×6 walls. Check your building department's interpretation for your project.

Can I remove the stickers from new windows?

Not until the inspection is done. The 2025 Energy Code says the temporary NFRC label must stay on until the building inspector has seen it.

Which version of Title 24 applies to my project?

The version in effect when your permit application is filed. The 2025 Energy Code applies to applications filed on or after January 1, 2026.

Sources

• California Energy Commission: 2025 Building Energy Efficiency Standards
• California Energy Commission: 2025 Energy Code for Residential and Nonresidential Buildings (PDF)
• California Energy Commission: 2025 Single-Family Residential Compliance Manual (PDF)
• California Building Standards Commission: California Building Standards Code (Title 24)