What R-value attic insulation is required in California?

For a new home, R-30 or R-38 at the ceiling depending on zone and attic design, and never less than the R-22 mandatory minimum. When an existing vented attic is altered, R-49 applies in most zones unless the attic already has R-38.

Is a radiant barrier required under Title 24?

On the prescriptive path, in most zones: zones 2 to 15 when the ducts are in conditioned space, and zones 2, 3 and 5 to 7 with a high-performance attic. A performance design can trade it against other measures.

What applies if I add insulation to my attic?

Adding or replacing attic insulation counts as altering the ceiling, so §150.2(b)1J applies: R-49 in most zones, plus air sealing and covered recessed lights depending on zone and existing insulation. An attic that already has R-38 is exempt.

What wall insulation does Title 24 require?

At least R-15 in 2x4 walls and R-21 in 2x6 walls as a mandatory minimum. A new home on the prescriptive path needs a wall U-factor of 0.048 in most zones, or 0.065 in zones 6 and 7.

Title 24 Insulation Requirements for Homes

There are two layers. Mandatory minimums in §150.0 apply to every home: R-22 at the ceiling, R-15 in 2x4 walls, R-21 in 2x6 walls and R-19 under raised floors. The prescriptive package in Table 150.1-A then raises the bar by climate zone, and altering an existing vented attic can trigger R-49.

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The mandatory floor

Section 150.0 sets minimums that apply whichever compliance path is used. A design can trade above them, never below.

Ceilings and rafter roofs: R-22 between wood framing, or an assembly U-factor of 0.043. In a vented attic it goes at the ceiling. In an alteration the minimum is R-19 (U-0.054).
2x4 walls: U-0.095, met with R-15 at 16-inch spacing. 2x6 walls: U-0.069, met with R-21. Both rose in 2025, from R-13 and R-20. Existing walls already insulated to R-11 are excepted from the 2x4 minimum.
Raised floors: U-0.037, or R-19 in a wood-framed floor. A controlled ventilation or unvented crawlspace can omit it if the foundation walls are insulated and the other listed conditions are met.
Attic access: insulation permanently attached to the access door, and the opening gasketed.

There is also a mandatory roof-deck requirement. In zones 4 and 8 to 16, the roof deck of a newly built attic above conditioned space must reach U-0.184, unless the air handler and ducts sit inside conditioned space, the system is ductless, or another listed exception applies.

Prescriptive values by climate zone

New homes on the prescriptive path meet Table 150.1-A. The attic varies most, because the table offers two attic designs.

Option B, the high-performance attic, keeps ducts in a ventilated attic but insulates the roof deck as well as the ceiling. It calls for R-19 below the roof deck in zones 4 and 8 to 16, and R-38 at the ceiling except in zones 3, 5, 6 and 7, where it is R-30.

Option C puts the ducts and air handler in conditioned space and insulates the ceiling below a vented attic: R-38 in zones 1 and 8 to 16, R-30 in zones 2 to 7. A cathedral ceiling needs R-38 in every zone.

For framed walls the maximum is U-0.048 in every zone except 6 and 7, where it is U-0.065. That is roughly half the U-factor of an R-15 2x4 wall. The table allows cavity insulation, continuous insulation or both, as long as the assembly reaches the figure.

Raised floors stay at R-19 in all zones. Slab-edge insulation is required only in zone 16, at R-7, to a depth of 16 inches or the depth of the footing if that is less. Mass walls have their own interior and exterior values. You can look up your zone on the climate zone page.

Radiant barriers

A radiant barrier is a low-emittance layer on the underside of the attic roof. It must have an emittance of 0.05 or less, be certified to the Department of Consumer Affairs, and be installed to the criteria in the Reference Appendices.

The prescriptive table requires one with Option C in zones 2 to 15, and with Option B in zones 2, 3 and 5 to 7. Additions of 700 square feet or less need one in zones 2 to 15. In a reroof, an attic radiant barrier is one of the accepted alternatives to a cool roof on a steep-sloped roof.

When an alteration triggers insulation

An alteration brings in the insulation rules only for what it touches. An altered ceiling or rafter roof must reach at least R-19, and the compliance manual treats a rafter roof as altered once the space between the framing becomes accessible. Altered framed walls meet the wall minimums, with the R-11 exception for walls that are already insulated.

A vented attic is the bigger trigger. The attic rules in §150.2(b)1J apply when the ceiling below a vented attic is altered, which includes adding or replacing attic insulation, or when an entirely new duct system is installed there. Then:

R-49, or U-0.020, at the ceiling in zones 1 to 4, 6 and 8 to 16; in zones 1, 3 and 6 a home that already has R-19 is excepted
air sealing of the ceiling plane in zones 2, 4 and 8 to 16, unless there is already R-19 or atmospherically vented combustion appliances are inside the home
recessed lights covered to the same depth in zones 1 to 4 and 8 to 16, with fittings not rated for insulation contact replaced or covered; zones 1 to 4 and 8 to 10 are excepted where R-19 is already in place
attic ventilation brought up to the residential code

None of this applies if the attic already has R-38, if the work would disturb asbestos, if there is knob-and-tube wiring in the attic, or if the attic is shared with units whose ceilings are not being altered. The building inspector checks these items; an ECC-Rater does not.

Additions

An addition of 700 square feet or less has its own set: R-38 in a vented attic in zones 1, 2, 4 and 8 to 16, R-30 in zones 3 and 5 to 7, and a radiant barrier in zones 2 to 15. Extended walls may keep their existing depth with R-15 in 2x4 or R-21 in 2x6 framing. Larger additions follow the new-home table with the same wall allowance. More on additions is on does a room addition need Title 24.

Quality Insulation Installation

The 2025 prescriptive package marks Quality Insulation Installation as required in all 16 climate zones for a new home, and §150.1(c)1E applies it. Additions of 700 square feet or less are excused. QII concerns how the insulation is installed rather than its R-value, and it is verified during construction. How that works is on our QII page.