Attic insulation R-value requirements under Title 24, zone by zone

The short version: the R-value your attic needs in 2026

The attic insulation R-value California asks for isn't one number for everyone. It depends on your climate zone and on what's already up there, and the zone sections below give it for each part of the Bay Area.

California's Energy Code is Title 24, Part 6. The 2025 edition applies to projects whose permit applications are filed on or after January 1, 2026; in the Energy Commission's words, "Buildings whose permit applications are applied for on or after January 1, 2026, must comply with the 2025 Energy Code."

For an attic, the rule that matters lives in one paragraph, Section 150.2(b)1J of the Standards. It applies to vented attics, and the Compliance Manual notes that "most single-family homes in California contain vented attics." Whether it asks for a little or a lot depends on two facts about your house: its climate zone, and how much insulation the attic has before anyone touches it.

One correction worth making early. You'll find pages saying the 2025 code "raised R-38 to R-49" for existing attics. It didn't. R-49 for altered vented-attic ceilings was already in the 2022 code, and the 2025 Compliance Manual still sends readers back to the 2022 manual for its attic-insulation detail. If a quote or an article leans on that claim, check the rest of it too.

When your ceiling counts as altered

The Compliance Manual puts it this way: "A ceiling may be considered altered under various conditions including when the existing attic insulation is replaced, new attic insulation is added, or the ceiling plane is replaced." A top-up counts. A tear-out and replace counts. Replacing the ceiling itself counts.

One less obvious trigger: an entirely new duct system. The Standards say that when a new or complete replacement duct system goes in and "the air handler and ducts are located within a vented attic, the requirements of Section 150.2(b)1J shall also be met." Replacing the whole duct system in a vented attic can bring the attic rules along with it, even if nobody planned to touch the insulation.

What doesn't count: adding recessed lights. The manual's Example 9-23 calls two new kitchen cans "a lighting alteration" that "does not constitute a ceiling alteration."

Once the ceiling is altered, there are four possible requirements. Section 150.2(b)1J lists them as items i to iv:

1. Insulation (i). R-49 or more at the ceiling level, "for the insulation alone," or a weighted U-factor of 0.020.
2. Air sealing (ii). "Air seal all accessible areas of the ceiling plane between the attic and the conditioned space," following Section 110.7.
3. Recessed lights (iii). Recessed downlights "shall be covered with insulation to the same depth as the rest of the ceiling." Fixtures not rated for insulation contact are replaced or fitted with a fireproof cover that insulation can go over; see insulation over recessed lights.
4. Ventilation (iv). Attic ventilation must comply with the California Residential Code, Section R806.

Which of the four apply to you is the job of the table below. It reproduces the Compliance Manual's Table 9-8 for the four zones our 30 cities sit in.

Climate zone 2: Napa, San Rafael and Santa Rosa

Zone 2 is the North Bay's inland valleys, warmer in summer than the bay shore. The code treats it strictly. R-49 is required whatever the attic had before, unless one of the exemptions further down applies. If the existing insulation is below R-19, air sealing and recessed-light coverage come along too. At R-19 or more, it's R-49 plus a ventilation check.

Not every Marin or Sonoma town is in zone 2, which is why the zone of the next town over tells you nothing about yours. Look up your address rather than your county.

Climate zone 3: San Francisco and the bayside cities

Zone 3 covers 15 of our cities: San Francisco, Daly City, San Mateo and Redwood City on the Peninsula; Oakland, Berkeley, Emeryville, Alameda, Richmond, San Leandro, Castro Valley, Hayward, Union City, Fremont and Newark in the East Bay. Cool summers and a long, mild heating season.

It's also the gentlest zone in the table. Air sealing is never required here for an altered ceiling. If the attic starts below R-19, the job takes it to R-49, covers the recessed lights and checks the ventilation. If the attic already has R-19 or more, the R-49 and recessed-light items fall away under their own exceptions, and the requirement is the ventilation check alone.

Zone 3 is also where the main open choice on this page lives, covered in the R-38 section below.

Climate zone 4: San Jose and the Santa Clara Valley

San Jose, Santa Clara, Sunnyvale, Mountain View, Cupertino, Milpitas and Palo Alto. Hot summer afternoons, mild winters, and plenty of air conditioners working against a hot attic.

The attic rules match zone 2: R-49 at any starting level unless an exemption applies, with air sealing and recessed-light coverage added when the attic starts below R-19. Zone 4 also picks up a rule zones 2 and 3 don't have. Replacing more than half of a steep-sloped roof brings in cool-roof requirements under Section 150.2(b)1Ii, and a ceiling with at least R-38 is one of the listed equivalents. That's the reroof connection in one sentence; our reroof guide covers the order of work.

Climate zone 12: Concord, Walnut Creek and the Tri-Valley

Concord, Walnut Creek, Dublin, Pleasanton and Livermore: inland valleys behind the hills, where the fog rarely reaches and summer afternoons run hot.

Zone 12 asks for the most. R-49 at any starting level, air sealing when the attic starts below R-19, and recessed lights covered to full depth even when the attic already has R-19 or more. That last point is where Section 150.2(b)1Jiii differs from zones 2 to 4: its R-19 exception names "Climate Zones 1 through 4 and 8 through 10," and zone 12 isn't in that list. One worked example in the manual (Example 9-21) reads as if the recessed-light item drops away at R-19 everywhere. The Standards text and Table 9-8 agree with each other and not with the example, so we follow them, and your building department has the final word. Cool-roof rules on a reroof apply here as in zone 4.

When none of it applies

Section 150.2(b)1J has five exceptions to the whole paragraph. Four of them switch off all four requirements:

• R-38 or more already there. "Dwelling units with at least R-38 existing insulation installed at the ceiling level."
• Asbestos. Where the alteration "would directly cause the disturbance of asbestos." The manual adds that this covers asbestos that isn't being remediated as part of the job. Vermiculite is the common case: see our vermiculite guide, and leave it undisturbed.
• Knob-and-tube wiring in the vented attic. The manual reads this as wiring "which is not being removed as part of the scope of work." Our knob-and-tube guide covers the electrical side, which has rules of its own.
• A shared attic, where the attic over your unit is shared with other units whose ceilings aren't being altered.

The fifth is a space exception. Where the accessible attic space can't hold the required R-value, "the entire accessible space shall be filled with insulation," as long as that doesn't break the ventilation rules in the Residential Code.

Air sealing has one exception of its own. Homes with "atmospherically vented space heating or water heating combustion appliances located inside the pressure boundary" are exempt from item ii only. The manual is explicit that this doesn't cover appliances in a vented attic, garage or crawlspace. Our air sealing page explains why that exception exists.

If you claim an exemption, the manual says the existing conditions are documented on the Certificate of Compliance, which "can be completed and signed by the homeowner, contractor, energy consultant, or any other party taking responsibility for the documentation," and the building official verifies them.

R-38 vs R-49

People ask this as if it were a free choice. In the Bay Area the code makes it for you most of the time:

• Zones 2, 4 and 12, attic below R-38: an altered ceiling goes to R-49. Stopping at R-38 isn't the prescriptive option.
• Any zone, attic already at R-38 or more: you're exempt, so going further is voluntary.
• Zone 3, attic between R-19 and R-38: R-49 isn't required either. This is the main case where you decide.

When the choice is yours, the questions are practical. How much is up there now, and is it clean? Does the house get hot upstairs, or only cold? Is there room under the roof at the eaves for more depth without blocking ventilation? We don't attach a savings figure to the extra inches, because no source we trust gives one for your house. Our self-check guide shows how to read the depth you have, and the cost page has Bay Area market ranges for the work.

Find your climate zone

The table names the zone for each of our 30 cities, taken from the Energy Commission's ZIP-code list. Every ZIP code we checked for each city falls in a single zone.

Two caveats from the CEC itself. Its map shows locations against "the geographic metes and bounds that determine California's climate regions," so the boundary, not the ZIP code, is the legal line. And "where a ZIP code contains more than one climate region, local jurisdictions may, at their discretion" assign a single zone to the whole ZIP code. If your address sits near a zone edge, check it in the CEC's EZ Building Climate Zone Search tool (opens in a new tab), and if it still looks borderline, ask the building department which zone it uses.

Paperwork, and who checks the work

The Compliance Manual's Appendix A lists the forms. For a prescriptive alteration with no field verification, the Certificate of Compliance is CF1R-ALT-05-E and the Certificate of Installation is CF2R-ALT-05-E. Insulation work also has its own installation certificate, CF2R-ENV-03-E (Insulation Installation).

On verification the manual is direct: "An ECC Rater is not required to verify any of these prescriptive requirements. All requirements will be verified by an official from the building department." ECC stands for Energy Code Compliance, the 2025 code's name for field verification and diagnostic testing. Whether your city wants a permit for a given attic job is a question for its building department, and the answer varies by city.

Next door to the attic

This page stops at the ceiling. The neighbors, in one line each:

• Ducts have their own alteration rules, including leakage testing; see ductwork.
• Walls opened during a remodel have their own insulation rules; batt insulation and wall insulation cover them.
• Floors over a crawl space or a garage are covered on the crawl space and garage pages.
• Ventilation under Section R806 is its own subject on our attic ventilation page.

A question we won't guess at

If old insulation comes out completely before new insulation goes in, is a zone 3 attic judged on what was there before the removal, or on the bare ceiling? The Standards speak of "existing insulation installed at the ceiling level" and don't address the sequence. The difference matters in zone 3, where R-19 existing means ventilation only and an empty ceiling means R-49 plus recessed-light coverage. Ask the building department before the removal starts, not after.

When you book a free inspection, we note the insulation that's there now and your climate zone. You get a written quote, and nothing is due before you sign.