Title 24 Duct Insulation Requirements
The level is R-6 or R-8, set by climate zone. In an existing home, once more than 25 feet of new or replacement duct goes into unconditioned space, Table 150.2-D requires R-8 in zones 1, 2, 4 and 8 to 16 and R-6 in zones 3 and 5 to 7. R-6 is the floor for any supply or return duct, and ducts entirely inside the conditioned space need none.
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The R-6 floor
Section 150.0(m)1B sets R-6 as the minimum for ducts and plenums, new construction and alterations alike. Three cases are excused or eased. Ducts entirely in conditioned space need no insulation, which an ECC-Rater must confirm. Uninsulated runs below the ceiling, inside the thermal envelope, are allowed if the penetrations are draft-stopped and air-sealed and the connections in unconditioned space are R-6. Ducts in an unvented attic may drop to R-4.2 where the roof deck has at least R-30 in contact with it, the gable ends meet the wall insulation requirements and the home tests at 3.0 ACH50 or less.
New homes
Table 150.1-A sets R-6 or R-8 by climate zone and by attic option, and the compliance manual puts R-8 in the high-performance attic of zones 4 and 8 to 16. The prescriptive route is explained on the new home page.
Replacing or extending ducts
Every new duct meets the mandatory rules, including labelling and dampers. Past 25 feet, new ducts in unconditioned space follow Table 150.2-D and are sealed and tested, whatever their location. An addition that extends an existing duct system by any length triggers the same insulation and sealing rules (Exception 4 to Section 150.2(a)). Swapping only the equipment brings sealing, not insulation; see HVAC and Title 24 and duct leakage testing.
Wrapping ducts already in place
Insulation added to an existing duct must comply with Section 605.0 of the California Mechanical Code (Section 110.8(d)3). A contractor who installs it must certify in writing to the customer that it does.
How the R-value is counted
- Only the insulation counts. Air films, vapor retarders and other duct components are left out, and the value is tested at 75 degrees F mean temperature.
- Duct wrap is assumed to be compressed by 25 percent, so its installed thickness is 75 percent of nominal.
- For flexible duct, thickness is half the difference between the outside and nominal inside diameters.
- Insulated flex duct carries a label showing its R-value at least every 3 feet.
- Insulation exposed to weather must be suitable for outdoor service, and all of it is protected from sun, moisture and wind.
Ducts, air handlers or coils placed in a garage meet an extra sealing test at any length (Section 150.2(b)1D). What counts as unconditioned space is explained separately.
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 the Energy Code. Replacing equipment brings duct sealing and testing, not new insulation. Insulation levels apply to new or replacement ducts, and to ducts you choose to wrap, which must meet the Mechanical Code.
It is the thermal resistance of the insulation alone, at the installed thickness, tested at 75 degrees F mean temperature. Flex duct must be labelled with it at least every 3 feet.
Down to R-4.2, if the roof deck has at least R-30 in contact with it, the gable ends meet the wall insulation requirements and the house tests at 3.0 ACH50 or less.
Explore Ducts
- Duct Leakage Testing: When Is It Required Under Title 24?
- When Is Duct Leakage Testing Required?
- Duct Replacement 25-Foot Rule Under Title 24
- Ducts in Soffits and Conditioned Space: Do They Need Insulation?
- Extending Existing Ducts to an Addition: Title 24 Rules
View all Ducts guides → · HVAC and Water Heating · Title 24 guide