California's insulation requirements come from the Energy Code (Title 24, Part 6). The 2025 edition applies to buildings whose permit applications are filed on or after January 1, 2026. It sets mandatory minimums for every single-family project and higher prescriptive attic levels that vary by climate zone.
Mandatory minimums apply whichever compliance path a project uses. Here's what the California Energy Commission's own summaries say.
What Are the Mandatory Insulation Minimums in California?
The California Energy Commission's 2025 summary lists mandatory minimums of R-22 for wood-frame ceilings, R-15 for 2x4 wood-frame walls, R-21 for 2x6 walls, and R-19 for wood-framed raised floors. Each can instead meet a maximum U-factor.
| Assembly (2025 Energy Code, mandatory) | Minimum R-value | Or maximum U-factor |
|---|---|---|
| Wood-frame ceiling and rafter roof | R-22 | 0.043 |
| Rafter roof alterations | R-19 | 0.054 |
| 2x4 wood-frame wall | R-15 | 0.095 |
| 2x6 wood-frame wall | R-21 | 0.069 |
| Wood-framed raised floor | R-19 | 0.037 |
The CEC's summary says single-family buildings subject to the Energy Code must meet all applicable mandatory measures, regardless of the compliance approach used. It also says loose-fill insulation must meet the manufacturer's required density for the labeled R-value.
What Changed in the 2025 Code?
Wall minimums went up. The CEC's "What's New for 2025" sheet says 2x4 walls now need a maximum U-factor of 0.095 (minimum R-15) and 2x6 walls 0.069 (minimum R-21). The CEC's 2022 summary listed mandatory minimums of R-13 and R-20.
The same sheet says prescriptive Option C of Table 150.1-A now calls for ventilated attics of at least R-38 in climate zones 1 and 8–16 and at least R-30 in zones 2–7. It adds a minimum of R-38, with no radiant barrier, for cathedral ceilings in all climate zones.
Which Attic R-Value Applies to My Home?
It depends on your California climate zone and the compliance path your project uses. Under prescriptive Option C, a ventilated attic needs at least R-38 in zones 1 and 8–16 and at least R-30 in zones 2–7.
California uses its own 16 building climate zones. Your building department or energy consultant can confirm your zone and which path your project follows. Whichever path a project uses, the mandatory minimums above still apply.
How Much Insulation Does ENERGY STAR Recommend for Existing Homes?
For retrofits, ENERGY STAR's guidance runs higher than the code minimums: R-30 to R-60 for an uninsulated attic, depending on the zone. But ENERGY STAR uses the national climate zones from the International Energy Conservation Code, which differ from California's 16 zones.
| ENERGY STAR zone (national, not California's) | Uninsulated attic | Attic with 3–4 inches already |
|---|---|---|
| 1 | R30 | R25 |
| 2 | R49 | R38 |
| 3 | R49 | R38 |
| 4A and 4B | R60 | R49 |
ENERGY STAR notes that its guidance is based on the 2021 International Energy Conservation Code residential provisions. For rebates that help pay for added attic insulation, see our California insulation rebates guide.
Official Sources
Code requirements come from the California Energy Commission, and retrofit recommendations from ENERGY STAR, all checked on October 10, 2026.
- CEC 2025 Building Energy Efficiency Standards, with the January 1, 2026 permit-application date
- CEC 2025 Single-Family Mandatory Requirements Summary, with the mandatory R-values
- CEC What's New for 2025 Single-Family, with the wall and attic changes
- ENERGY STAR recommended home insulation R-values
Frequently Asked Questions
What R-value is required for attic insulation in California?
The 2025 Energy Code's mandatory minimum for a wood-frame ceiling is R-22. Under prescriptive Option C, a ventilated attic needs at least R-38 in climate zones 1 and 8–16 and R-30 in zones 2–7.
What R-value is required for walls in California?
At least R-15 in 2x4 wood-frame walls and R-21 in 2x6 walls, or a maximum U-factor of 0.095 and 0.069 respectively, under the 2025 Energy Code.
When did the 2025 Energy Code take effect?
The California Energy Commission says buildings whose permit applications are filed on or after January 1, 2026, must comply with the 2025 Energy Code.
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