Duct Replacement 25-Foot Rule Under Title 24
Installing more than 25 feet of new or replacement duct brings Section 150.2(b)1D: new ducts in unconditioned space need R-6 or R-8 insulation, and the whole duct system is sealed and verified by an ECC-Rater. Ducts in a garage skip the length test. A nearly all-new system must reach 5 percent leakage.
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Where the 25 feet comes from
Section 150.2(b)1D says "more than 25 feet of new or replacement space-conditioning system ducts". Exactly 25 feet does not trigger it. The text does not say whether separate runs are added together, so ask the plan checker instead of assuming.
What happens at that length
- Insulation. New ducts in unconditioned space get R-6 in zones 3 and 5 to 7, and R-8 in the other zones (Table 150.2-D).
- Sealing. The altered duct system is sealed and tested wherever it runs, except where the existing duct contains asbestos.
- Garage. Altered ducts, air handlers, coils or plenums there need 6 percent leakage or a smoke test at any length. The manual's Example 9-44 applies it to 20 feet.
Below 25 feet is not a free pass
Any new duct must still meet the mandatory duct requirements of Section 150.0(m)1 to 10 on insulation, labeling and dampers, and the manual's Example 9-40 shows a raised platform barred as a return plenum.
How much you replace sets the pass number
A system built of at least 75 percent new duct material, with up to 25 percent reused accessible parts, is an entirely new system: leakage at 5 percent of airflow, plus MERV 13 filtering and the airflow and fan efficacy checks. Replace less and the combined system must test at 10 percent total or 7 percent to outside, or have its accessible leaks sealed and smoke-tested. An air handler and ducts in a vented attic also bring the attic rules of Section 150.2(b)1J.
The test covers the whole connected system, not the new run alone, which is why a 30-foot reroute can expose leaks in old trunk lines. Example 9-39 applies the rule to ducts replaced for failed insulation. Details are on duct leakage, duct insulation, extensions to an addition and every test trigger. Order from residential.
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 from the 25-foot rule alone, but a new air handler, outdoor unit or coil would trigger one. The new duct still meets the mandatory insulation, labeling and damper rules.
No. The leakage target applies to the whole duct system connected to the air handler, old sections included.
Explore Ducts
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