ADU Over a Garage: Floor Insulation and Title 24
The floor between a garage and an ADU above it is a raised floor and must reach an assembly U-factor of 0.037, which R-19 in a wood frame is deemed to meet. Where Quality Insulation Installation applies, an ECC-Rater also checks how it is installed, including an insulated rim joist and a continuous thermal barrier.
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Choosing the assembly
Section 150.0(d) covers raised floors that separate conditioned space from unconditioned space or ambient air, so it reaches this floor and not the floor between two stories of the ADU. R-19 in wood framing needs no calculation. A deeper joist or continuous insulation is a U-factor exercise. Joint Appendix Table 4.4.1, written for wood floors over a crawl space at 16 inches on center, lists U-0.037 for a 2x6 with R-19, 0.028 for a 2x10 with R-30 and 0.024 for a 2x12 with R-38. Adding R-4 of continuous insulation to the 2x6 with R-19 gives 0.032. Floors exposed to open air have their own table, 4.4.2, which reads higher (0.049 for the 2x6 with R-19), so the report preparer picks the table that matches the assembly.
What the ECC-Rater checks
Quality Insulation Installation is a prescriptive requirement in all 16 zones for a new building, and for an addition over 700 square feet. The Reference Appendix has a protocol for floors over a garage:
- The separation between the ADU and the garage is insulated as a continuous thermal barrier.
- The rim joist is insulated; rim and band joists beside conditioned space are airtight.
- Batts touch the subfloor, are not doubled over or compressed, and are cut around wiring and plumbing.
- Faced batts face the living space and have continuous support, such as netting or stapled flanges.
Loose-fill and rigid board follow the same contact and rim-joist rules.
If the floor already exists
Adding insulation to an existing floor over a garage is an alteration. In a performance model, Table 150.2-G sets the standard design from the current mandatory requirement, unless an ECC-Rater verifies the existing insulation, when the existing R-value is used instead.
Show the framing depth, insulation type, R-value and rim-joist treatment on the section. Classification and the garage-to-house details are on ADU above a garage; insulation basics are on insulation requirements and QII, and the tester role on what an ECC-Rater does. Related: garage conversion ADUs and ADU Title 24 reports.
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 Section 150.0(d). That section covers raised floors separating conditioned space from unconditioned space or ambient air, and a floor between two conditioned stories does neither.
Where QII applies, yes. The ECC-Rater protocol for floors over a garage requires an insulated rim joist, with rim and band joists beside conditioned space made airtight.
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View all ADU and JADU guides → · Permits and Compliance · Title 24 guide