Kneewall Insulation Under Title 24: R-Value, Air Barrier and What QII Checks
Insulate a kneewall like an exterior wall. It counts as a demising wall, so it needs the mandatory minimum, U-0.095 for 2x4 framing (R-15 at 16 inches on center) or U-0.069 for 2x6 (R-21), not the tighter new-home wall value. Only a kneewall with attic space behind it needs insulation.
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Only the wall with attic behind it
Joint Appendix JA1 defines a knee wall as a sidewall separating conditioned space from attic space under a pitched roof, to be insulated as an exterior wall under the chosen compliance method. The spray foam procedure adds that kneewalls within conditioned space need no insulation. In a finished attic room, the short wall facing the cold eave void is the one to insulate.
The number to hit
The compliance manual says demising partitions and knee walls are not held to the prescriptive wall values in Table 150.1-A. They meet the mandatory minimum of Section 150.0(c), shown in wood framing as R-15 in a 2x4 wall or R-21 in a 2x6 wall at 16 inches on center. The Certificate of Compliance value is what the rater compares with the installed batt.
Where installations go wrong
Where QII applies, workmanship counts as much as the R-value. A batt that slumps, is squeezed or leaves a void fails, however high its label. Behind the wall, the attic side of any air-permeable batt must be covered by a continuous air barrier. In front, the batt must press on the drywall. It also needs support so it cannot drop, and framing plates above and below. Steel studs take continuous insulation on their outer faces, unless the report uses the JA4 Table 4.3.4 values.
Does QII reach your project
QII is prescriptive for new homes and additions over 700 square feet. Smaller additions are excused from it and the alteration section lists no QII item, so a kneewall in a small addition still takes R-15 or R-21 but is not rater-inspected under QII. The manual also lists kneewalls among the leaks to seal when air sealing an attic floor.
A converted attic is covered on the attic conversion page and its living-space version. See also wall insulation, skylight shaft insulation, the QII page and our residential 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
No. The manual excludes demising partitions and knee walls from the Table 150.1-A wall values and sends them to the mandatory minimum of Section 150.0(c).
The QII protocol for spray foam says kneewalls within conditioned space do not need to be insulated.
Explore Walls
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- Commercial Air Barrier Under Title 24: The 25 Percent Wall Alteration Rule
- Advanced Framing and 24-Inch Stud Spacing: How Title 24 Counts Wall Insulation
- Brick, Block and Concrete Walls: Mass Wall Insulation Under Title 24
View all Walls guides → · Windows and Building Envelope · Title 24 guide