ADU Wall Insulation Requirements
The rule depends on how the ADU is built. A new detached ADU needs framed walls at U-0.048, or U-0.065 in zones 6 and 7, which cavity batts alone do not reach. An addition that keeps its siding needs R-15 in 2x4 walls or R-21 in 2x6 walls, with no continuous insulation. Altered walls meet the mandatory minimum.
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New detached ADU
Table 150.1-A sets framed walls at U-0.048 in every zone except 6 and 7, where the limit is U-0.065. The figure describes the whole assembly, so R-21 batts in a 2x6 wall reach only 0.069 in Joint Appendix Table 4.3.1 and need continuous insulation too. Quality insulation installation, verified by an ECC-Rater, applies in all 16 zones. The wall assemblies page shows which combinations pass.
Attached ADU or garage conversion
These are additions. Extending an existing wood-framed wall may keep its depth with R-15 in 2x4 framing or R-21 in 2x6, and so may a wall whose siding is not being removed; continuous insulation is not required. A new wall, or one that loses its siding, returns to the package value. Existing walls around newly conditioned space count as new, so an ECC-Rater cannot verify their existing insulation for performance credit. Quality insulation installation does not apply at 700 square feet or less. Above that it does, though converted space needs no header insulation and no air sealing where the existing air barrier stays.
Conversion of a conditioned room
A pool house or studio that is already conditioned is an alteration. Altered framed walls meet Section 150.0(c), U-0.095 in 2x4 framing and U-0.069 in 2x6, and an existing 2x4 wall already at U-0.110 or R-11 is excepted. In the performance method an ECC-Rater may verify the existing R-value, which then replaces the current requirement as the standard design.
Block walls and cold zones
A masonry or concrete ADU with heat capacity of at least 7.0 follows the mass-wall rows: interior R-13 or exterior R-8 in zones 1 to 15, and R-17 or R-13 in zone 16. In zones 14 and 16 a vapor retarder goes on the conditioned side of the insulation in every exterior wall.
Compare the unit types on alteration versus addition, the garage case on the garage conversion checklist, and the full envelope on insulation requirements. Start a report from the ADU page.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
It meets the mandatory U-0.069 but not the prescriptive U-0.048 or U-0.065. Continuous insulation, R-5 in most zones or R-2 in zones 6 and 7, closes the gap in Joint Appendix Table 4.3.1.
No. If the siding stays, the prescriptive requirement for the addition is R-15 cavity insulation in 2x4 walls or R-21 in 2x6 walls, without continuous insulation.
Explore ADU and JADU
- ADU Whole-House Ventilation Requirements
- ADU Window SHGC Requirements
- ADU Window U-Factor Requirements
- Attached ADU Title 24 Checklist
- California ADU Rules and Title 24
View all ADU and JADU guides → · Permits and Compliance · Title 24 guide