Spray Foam at the Roof Deck: Title 24 Requirements
Title 24 allows spray foam under the roof deck, but an unvented attic insulated that way cannot use the prescriptive package and goes to the performance method. Foam under the deck of a vented attic, with an air-gapped roofing product, can meet Option B at R-19 in climate zones 4 and 8 to 16.
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Where foam fits the prescriptive package
Table 3-6 of the compliance manual lists spray foam among the ways to install Option B's R-19 below the roof deck, with batts or netted cellulose. The attic stays ventilated, the roofing has an air space under it, as tile does, and the air barrier is at the ceiling. Three designs fall outside the package: a sealed attic with deck insulation (Example 3-14), insulation above the deck (3-15), and asphalt shingles laid without an air gap over below-deck R-19 (3-16).
How thick is R-19
JA4 Table 4.1.7 gives default thicknesses. Closed-cell foam, 1.5 to under 2.5 pounds per cubic foot, counts R-5.8 per inch. Open-cell, 0.4 to under 1.5, counts R-3.6.
- R-19: 3.50 inches closed-cell, 5.3 inches open-cell.
- R-30: 5.25 and 8.3 inches.
- R-38: 6.75 and 10.6 inches.
A higher value per inch is allowed only with a tested figure certified by the Department of Consumer Affairs, kept on site and noted on the certificate of installation. An ECC-Rater then verifies the installation.
Foam as the air barrier
Closed-cell foam serves as an air barrier at 2.0 inches or more, open-cell at 5.5 inches or more. Either may instead be sprayed to the thickness that tests at 0.02 L/s per square meter or lower at 75 Pa. Surface depressions in closed-cell foam may be no deeper than 1/2 inch.
A modeling note
JA4 says its attic U-factor table is not for insulation located at the roof, such as foam sprayed to the underside of the deck in a sealed attic. That design is modeled in compliance software. Sealed attics are treated on unvented attic requirements, and the overall values on insulation requirements. Rafter-depth limits are on cathedral ceiling insulation. See 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 with Option B. The manual says a radiant barrier is not required when insulation is installed below the roof deck, although a draped one may earn performance credit.
An ECC-Rater verifies spray foam whenever the R-value per inch used differs from the default in Table 4.1.7, using the procedures in Residential Appendix RA3.5.6.
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