ADU Attic and Roof Insulation Requirements
A new detached ADU needs ceiling insulation of at least R-22 and, in zones 4 and 8 to 16, roof deck insulation reaching U-0.184 unless its system is ductless. The prescriptive package then sets R-30 or R-38 at the ceiling by zone. An addition of 700 square feet or less uses R-38 or R-30 in a vented attic.
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Two mandatory items first
A ceiling or rafter roof must reach U-0.043, or R-22 in wood framing. In zones 4 and 8 to 16 the roof deck of a new attic over conditioned space must also reach U-0.184, which the manual equates to about R-3 or R-4. The deck item is excused when ducts and air handler sit entirely in conditioned space, when 12 feet or less of supply duct is outside it, or when the system is ductless, as in a mini-split ADU (ducted versus ductless).
The two prescriptive attics
Option B keeps ducts in a ventilated attic: R-19 below the roof deck in zones 4 and 8 to 16, plus R-38 at the ceiling (R-30 in zones 3 and 5 to 7), and a radiant barrier in zones 2, 3 and 5 to 7. Option C puts ducts in conditioned space, verified by an ECC-Rater: R-38 at the ceiling in zones 1 and 8 to 16, R-30 in zones 2 to 7, a radiant barrier in zones 2 to 15, and R-38 in a cathedral ceiling in every zone.
Roofs that need a performance model
The manual sends three designs to the performance method: an unvented attic insulated at the roof deck, insulation above the deck, and asphalt shingles with no air gap over below-deck R-19.
Additions and conversions
An addition of 700 square feet or less with a vented attic needs R-38 (U-0.025) in zones 1, 2, 4 and 8 to 16 or R-30 (U-0.031) in zones 3 and 5 to 7, plus a radiant barrier in zones 2 to 15. Larger ones follow Option B or C. Adding insulation to the attic of an already conditioned structure brings R-49 in zones 1 to 4, 6 and 8 to 16 unless an exception, such as existing R-38, applies.
The roofing surface has its own rule on the ADU cool roof page, existing attics on insulation requirements, and equipment on mini-split ADUs. Order 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
Not under the mandatory roof deck rule in Section 150.0(a)1, which excepts ductless systems. The ceiling minimum and the prescriptive package are separate tests, so confirm the package with the report preparer.
For an addition of 700 square feet or less with a vented attic, R-30 at the ceiling and a radiant barrier, since zone 3 sits in the radiant barrier range of zones 2 to 15.
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View all ADU and JADU guides → · Permits and Compliance · Title 24 guide