Radiant Barrier Requirements Under Title 24
A radiant barrier is required only inside certain packages: Option C ducts in conditioned space in zones 2 to 15, Option B in zones 2, 3 and 5 to 7, and additions of 700 square feet or less in zones 2 to 15. The product needs an emittance of 0.05 or less and goes in shiny side down.
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Where the code asks for one
- Option B attics (ducts stay in the attic): zones 2, 3 and 5 to 7. In zones 4 and 8 to 16, R-19 under the deck is the measure and a radiant barrier is not required, though a draped one can earn performance credit.
- Option C attics (ducts verified in conditioned space): zones 2 to 15.
- Additions of 700 square feet or less with a ventilated attic: zones 2 to 15, but not over cathedral ceilings, per Table 9-1.
- Re-roofs: a qualifying attic radiant barrier, one not placed directly above spaced sheathing, lets a steep roof skip the cool roof product.
What qualifies as the product
Section 110.8(j) requires an emittance of 0.05 or less, tested to ASTM C1371 or E408, and certification to the Department of Consumer Affairs.
How it must be installed
Appendix RA4.2.1 asks for a permanent installation, shiny side down. It lists four methods: draped over the top chords, stapled between them, stapled under them, or factory-laminated to the underside of the deck. A foil stapled under the top chords needs at least 1.5 inches of air space at mid-span. Gable ends and other vertical surfaces in the attic are covered too, though on an addition the barrier goes only on that addition's roof underside and gable ends.
The attic has to breathe
For prescriptive credit, RA4.2.1.1 asks for 1 square foot of vent area per 300 square feet of attic floor, with at least 30 percent of it high on the roof. The barrier is listed under special features on the certificate of compliance.
A common mistake
Adding a barrier to the existing attic as a trade against an addition earns nothing: once the attic is altered, the standard design matches Table 150.1-A (Example 9-55). The ceiling values are on insulation requirements, the living-space case on attic conversions, and your list on climate zones. The re-roof exception is on cool roof requirements for a re-roof. Start with residential Title 24 reports.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
Not for the ceiling. In Option B it is the roof-level measure in zones 2, 3 and 5 to 7, where no below-deck insulation is listed, and the ceiling R-value is still required.
Not under the prescriptive package. Zones 1 and 16 appear on none of the radiant barrier lists in Table 150.1-A.
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View all Insulation guides → · Windows and Building Envelope · Title 24 guide