Unvented Attic Title 24 Requirements
An unvented attic is allowed but not prescriptive. The Standards let the mandatory insulation sit at the ceiling or at the roof, yet the manual sends a sealed attic insulated at the deck to the performance method. Ducts inside one may be insulated to R-4.2 only with R-30 against the deck, insulated gable ends and 3.0 ACH50 or less.
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What the Standards say
Section 150.0(a)2 requires ceilings and rafter roofs to reach U-0.043, or R-22 in wood framing. In a vented attic that insulation goes at the ceiling. In an unvented attic it may go at either the ceiling or the roof. In zones 4 and 8 to 16 the roof deck of a newly built attic over conditioned space must also reach U-0.184, unless an exception applies, such as ducts and air handler entirely in conditioned space or a ductless system.
Why it is modeled
The prescriptive package assumes a ventilated attic with the air barrier at the ceiling and ordinary wood trusses. Example 3-14 says a sealed attic with deck insulation must use the performance approach. The manual adds that ducts in an unvented attic do not satisfy Option C, which requires them in conditioned space verified by an ECC-Rater.
The duct exception
Section 150.0(m)1B asks for R-6 on ducts, but Exception 2 lowers it to R-4.2 inside an unvented attic when all three of these hold:
- At least R-30 insulation lies between the roof rafters in contact with the deck.
- The gable ends meet the wall insulation requirements of Section 150.1(c)1B.
- The home measures 3.0 ACH50 or less in field verification under RA3.8, tested with the attic hatch open.
Moisture and alterations
In zones 14 and 16, a Class I or II vapor retarder goes on the conditioned side of the insulation in unvented attics with air-permeable insulation (Section 150.0(g)2). The R-49 rule for altered attic ceilings covers vented attics only. An existing unvented or rafter assembly counts as altered once the space between framing is accessible, with R-19 as the minimum. The manual leaves its unvented attic chapter to the 2022 manual, so confirm details with the department.
Where to read more
Foam products and thicknesses are on spray foam at the roof deck, duct values on duct insulation and attic ceilings on attic insulation for remodels. Order a modeled report from 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 under Section 150.0(m)1B. The lowest value it lists for an unvented attic is R-4.2, and only when the R-30, gable end and 3.0 ACH50 conditions are met.
Not for Option C. The manual says locating ducts within an unvented attic does not meet that option.
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View all Insulation guides → · Windows and Building Envelope · Title 24 guide