Do ADU ducts have to be inside conditioned space under the 2025 Energy Code?

Not always, but it is one of only two prescriptive options. Section 150.1(c)9 says ducted systems must either use a "high-performance attic" or have the ducts and air handler located entirely in conditioned space, confirmed by an ECC rater (formerly HERS rater). If your ADU uses only ductless mini-split heads, this requirement does not come into play.

Option 1: high-performance attic

Ducts and air handlers may sit in a ventilated attic when the roof and ceiling insulation meet Option B in Table 150.1-A, with duct insulation also meeting that table. In climate zones 4 and 8 through 16, Section 150.0(a)1 separately requires roof deck insulation for new attics above conditioned space, unless an exception applies, such as a ductless system or an air handler and ducts located entirely in conditioned space.

Option 2: ducts in conditioned space

• All ducts and the air handler must be inside conditioned space and verified by the rater under Reference Appendix RA3.1.4.3.8.
• If the ADU has an attic, the ducts and air handler must be below the ceiling that separates the living space from the attic, typically in dropped soffits, a hallway chase or a mechanical closet.
• Under Section 150.0(m)1B, ducts verified to be entirely in conditioned space do not need duct insulation. Ducts elsewhere generally need R-6.0, with R-4.2 allowed in certain unvented attics.
• CEC ADU training materials pair this option with vented-attic ceiling insulation of R-38 in climate zones 1 and 8 through 16 and R-30 in zones 2 through 7.

Leakage still matters

Ducts in conditioned space still have to be sealed and tested. Section 150.0(m)11 limits total leakage of a new single-family duct system to 5 percent of air handler airflow when the air handler is installed. The test measures total leakage, so every joint and boot counts.

The performance path

On the performance path, other duct locations can be modeled, and the CEC's Compliance Manual describes credits for a verified duct design with shorter runs and ducts in beneficial locations. The energy model then has to make up for any losses elsewhere.

Why it matters for ADUs

Many ADUs are single-story with modest ceiling heights, so finding room for soffits takes coordination with the architect. A compact air handler in a central closet with short duct runs can fit more easily than an attic layout.

What it means for your mechanical plan

• Show where the air handler and every duct run sit relative to the insulated ceiling and walls.
• Coordinate soffit and chase sizes with the architectural sections.
• Make sure the duct location in the Title 24 report matches the drawings.

Compare ducted and ductless approaches in HVAC for multi-bedroom ADUs, and see prescriptive vs. performance compliance.

Sources: energy.ca.gov, energy.ca.gov, energy.ca.gov.