Insulating Existing Ducts in the Attic: Required or Optional?
Optional. Title 24 does not make you re-insulate existing attic ducts you leave alone, because Table 150.2-D applies to new ducts in unconditioned space. If you do add insulation to an existing duct, it must comply with Section 605.0 of the California Mechanical Code, and a contractor must certify that in writing.
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What you can leave alone
Table 150.2-D sets R-6 in zones 3 and 5 to 7 and R-8 in zones 1, 2, 4 and 8 to 16, but Section 150.2(b)1Di aims it at new ducts located in unconditioned space. A thin or aging run that stays in place is not pulled up to that table.
When the work turns into an alteration
Replacing a duct, as opposed to wrapping it, changes the answer. The manual's Example 9-39 describes a duct replaced because its insulation had deteriorated: it is an alteration, and once more than 25 feet is altered, the sealing test, ECC verification and prescriptive insulation of Section 150.2(b)1D all apply. A shorter replacement piece still has to meet the mandatory construction rules in Section 150.0(m). The comparison with sealing is on the replace-or-seal page.
Adding wrap to an existing duct
Section 110.8(d)3 covers this directly: insulation installed on an existing space-conditioning duct must comply with Section 605.0 of the California Mechanical Code. Under 110.8(d), a contractor who installs it certifies to the customer, in writing, that it does. Keep that paper with your records.
Protect what you add
Insulation must be shielded from sunlight, moisture, equipment maintenance and wind, and exposed cellular foam must be protected or coated against solar degradation (Section 150.0(m)9).
Credit in the performance method
Table 150.2-G sets the standard design for duct insulation at the proposed level when existing conditions are not verified, and at the existing level when they are. Upgrading old duct insulation therefore earns credit only if the existing condition is verified by a certified ECC-Rater, the role formerly called a HERS rater. Without that verification, the upgrade is neutral on paper.
Values by zone are on the duct insulation page, the length trigger is in the 25-foot rule, and attic work above the ducts is in attic insulation for remodels. Orders start at 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
The permit requirement is decided by your building department. What the Energy Code adds is the Mechanical Code standard for the insulation and the contractor's written certification.
Not as a general retrofit method. Buried ducts are a performance-method credit with markers and verification, covered on the buried ducts page.
Explore Ducts
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- Burying Attic Ducts Under Blown-In Insulation: What Title 24 Says
- Can You Use Duct Tape on HVAC Ducts? What Title 24 Allows
- Do I Have to Replace My Old Ducts When I Replace My AC or Furnace?
View all Ducts guides → · HVAC and Water Heating · Title 24 guide