Ducts in Soffits and Conditioned Space: Do They Need Insulation?
No, when the entire duct system sits inside conditioned space and field verification proves it. The proof is a visual check of location plus no more than 25 cfm of leakage to outside under Appendix RA3.1.4.3.8. In a home with an attic, the ducts must run below the ceiling that separates living space from the attic.
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The ceiling line decides what counts
Section 150.0(m)1Bii excuses insulation only for a duct system located entirely within conditioned space. For dwelling units with attics, that means below the ceiling separating the occupiable space from the attic. A run in a dropped ceiling or chase under that ceiling is a candidate; a run in the attic is not.
The proof: look, then measure
Two checks come from the Reference Appendices. RA3.1.4.1.3 is a visual inspection that confirms every part of the system is inside conditioned space; one stretch outside and the system does not pass. RA3.1.4.3.8 then requires leakage to outside of 25 cfm or less, measured by the RA3.1.4.3.4 method with a blower door holding the house at 25 Pa. The limit is a flow in cfm, not a percentage of airflow.
With ducts in a basement, the setup changes: doors to a conditioned basement stay open, but doors to an unconditioned one are closed.
The soffit exception
A lighter route covers duct portions below the attic ceiling. Exception 1 to Section 150.0(m)1Bi leaves that portion uninsulated if all three conditions hold:
- The uninsulated portion lies entirely inside the thermal envelope, confirmed by visual inspection.
- Wherever it penetrates into unconditioned space, the opening is draft stopped and air-sealed to the materials around it.
- Every connection in unconditioned space is insulated to at least R-6.0, confirmed by visual inspection.
New homes and renovations
In a newly constructed house, Section 150.1(c)9B offers the same idea prescriptively: ducts and air handler entirely in conditioned space, verified under RA3.1.4.3.8, as the alternative to the high performance attic. In an alteration, Table 150.2-D insulation levels are written for new ducts in unconditioned space, so a run that stays inside the envelope is a different case from one that crosses into the attic. An ECC-Rater and the building department confirm which verification applies.
For the R-6 and R-8 values on ducts that do cross into unconditioned space, see duct insulation requirements. See also unconditioned space, buried attic ducts and new home 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 if the whole system is verified inside conditioned space, or if the soffit portion meets the three conditions of Exception 1 to Section 150.0(m)1Bi, including R-6.0 on connections in unconditioned space.
No. RA3.1.4.3.8 sets 25 cfm of leakage to outside, measured as a flow at 25 Pa.
Explore Ducts
- Extending Existing Ducts to an Addition: Title 24 Rules
- Flexible Duct Installation Rules Under Title 24
- Insulating Existing Ducts in the Attic: Required or Optional?
- Second-Story Addition: Ducts, Stairwell and Solar Under Title 24
- Airflow Verification: When Is It Required Under Title 24?
View all Ducts guides → · HVAC and Water Heating · Title 24 guide