Ducted vs Ductless Mini-Split for an ADU: Title 24 Differences
Ductless removes the duct rules, not the heat pump rules. A ducted ADU system adds duct location and insulation limits, a 5 percent leakage test, 350 cfm per ton of airflow and a fan efficacy cap; a ductless one skips the duct and fan-efficacy tests, though a minimum airflow check can still apply. Both still need refrigerant charge verification and the same sizing and control rules.
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Where ducts may sit in a new detached ADU
Section 150.1(c)9 allows ducts and the air handler in a ventilated attic only when the roof and ceiling insulation meets Option B in Table 150.1-A. Otherwise the whole duct system sits in conditioned space and is verified under RA3.1.4.3.8. A high-performance attic also needs R-6 ducts in zones 3 and 5 to 7, R-8 elsewhere, and 5 percent total leakage.
Roof-deck insulation
In zones 4 and 8 to 16, a roof deck over conditioned space must reach U-0.184. Exception 1 to Section 150.0(a)1 lifts that if the handler and ducts are all in conditioned space, or the handler is outside it with 12 linear feet or less of supply duct there, or the system is ductless.
Tests that only ducted systems face
- Total duct leakage of no more than 5 percent of air-handler airflow (Section 150.0(m)11).
- Airflow of at least 350 cfm per ton in a new system, 300 in an altered one, with fan efficacy of 0.58 W/cfm or less for air handlers that are not gas furnaces.
The Energy Commission's list of ECC-Rater checks marks duct leakage and fan efficacy as ducted-only and lists minimum airflow without that label, so an airflow check can still reach a ductless unit.
Additions and capacity
Under Section 150.2(a)1Eii an addition's system has no maximum capacity if airflow is verified at 350 cfm per ton or more. Without that, Tables 150.2-A and 150.2-B cap it. Ductless systems are excepted from those limits.
What stays the same
Refrigerant charge verification for heat pumps in all zones, the Section 150.0(h) controls and sizing, and a setback thermostat. Attic ducts under a roof without Option B insulation are the combination the ductless exception avoids.
See can an ADU use a mini-split, duct leakage testing, duct insulation, the detached ADU report and the ADU page.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
No. A ductless heat pump still needs refrigerant charge verification in every climate zone and any credited SEER2, HSPF2 or capacity checked. Only the duct tests and fan efficacy fall away.
In zones 4 and 8 to 16 a ductless system is one of the exceptions to the U-0.184 roof-deck requirement for a new ADU attic above conditioned space.
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View all ADU and JADU guides → · Permits and Compliance · Title 24 guide