ADU Slab Edge and Floor Insulation Requirements
Prescriptive slab edge insulation applies in climate zone 16 only: R-7.0 or F-0.58, to 16 inches or the footing depth if that is less. Zones 1 to 15 have none unless the slab is heated. Wood-framed raised floors need R-19 or U-0.037 in every zone, and concrete raised floors follow a zone table.
- Choose your report
- Upload your plans
- Pay securely
- Permit-ready report by email
Slab edge, zone by zone
Table 150.1-A lists slab perimeter insulation as no requirement in zones 1 to 15 and F-0.58 or R-7.0 in zone 16, installed to 16 inches or the footing depth, whichever is less. The manual gives additions the same split in every size band.
A heated slab changes the answer
A radiant slab triggers Section 110.8(g) in every zone. Vertical insulation runs from the top of the slab down 16 inches or to the frost line, whichever is greater, and may stop at the top of the footing where that is less: R-5 in zones 1 to 15, R-10 in zone 16. Insulation placed between slab and foundation wall instead runs to the footing and then 4 feet inward, R-5 in zones 1 to 15 and in zone 16 R-10 vertical with R-7 horizontal. A solid guard and an insect plate are also required.
The product and the check
Section 150.0(f) limits slab edge material to 0.3 percent water absorption (ASTM C272) and 2.0 perm per inch (ASTM E96), with protection from damage and sunlight. The ECC-Rater verifies it alongside the air barrier under quality insulation installation.
Floors above the ground
A wood-framed floor over unconditioned space or outside air needs R-19 or U-0.037 in all 16 zones. Table 3-7 of the manual lists R-19 at U-0.037 over a crawlspace but 0.049 with none, yet Section 150.1(c)1C accepts R-19 between wood framing by R-value, so state which route your ADU uses. Concrete raised floors follow Table 3-8: U-0.092 with R-8 in zones 1, 2, 11, 13, 14 and 16, U-0.138 with R-4 in zones 12 and 15, and no requirement in zones 3 to 10. The over-a-garage case has its own floor insulation page.
Walls are on ADU wall insulation, the verification step on quality insulation installation, and the zone on the zone 16 page. Start 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 prescriptively. Zones 1 to 15 have no slab perimeter requirement for new buildings or additions. A heated slab is the exception, because Section 110.8(g) applies in every zone.
To 16 inches below the top of the slab or the depth of the footing, whichever is less, at R-7.0 or F-0.58.
Explore ADU and JADU
- ADU Solar Exceptions and Exemptions Under the 2025 Energy Code
- Does an ADU Need Solar Under Title 24?
- What If Plan Check Returns Title 24 Corrections on an ADU?
- How Much Does an ADU Title 24 Report Cost?
- What Plans Are Needed for an ADU Title 24 Report?
View all ADU and JADU guides → · Permits and Compliance · Title 24 guide