Can a California building comply with ASHRAE 90.1 instead of Title 24?

No. Part 6 is the code building departments enforce here, and the Building Standards Commission says model codes cannot be used in place of Title 24.

Is Title 24 more stringent than ASHRAE 90.1?

Compare them measure by measure rather than overall. The Energy Commission says the 2016 Energy Code was current with 90.1, and DOE records California certifications for the 2013 and 2016 editions.

Does 90.1 treat apartments differently from Title 24?

Yes. Under the national codes a multifamily building over three stories is commercial. Part 6 keeps it in the multifamily sections, though from four habitable stories it uses NRCC forms.

Why would a California team look at 90.1 at all?

For LEED v4, whose energy benchmark is 90.1-2010, for national clients, and for control sequences such as ASHRAE Guideline 36, which has separate versions for Title 24 and 90.1-2019.

Title 24 vs ASHRAE 90.1: How They Relate

ASHRAE 90.1 is the national model energy standard for commercial buildings, and federal law asks each state to show that its commercial code meets or exceeds the latest edition. California answers with its own Title 24 Part 6: a nonresidential project here is designed and permitted to Part 6, not to 90.1, and the two differ in scope and in detail.

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The federal link

Under the Energy Conservation and Production Act, each new edition of Standard 90.1 triggers a Department of Energy determination within 12 months. If DOE finds that the edition saves energy, every state has two years to certify that its commercial code meets or exceeds it. DOE issued its determination on 90.1-2022 on 6 March 2024, estimating national site energy savings of about 9.8 per cent, with state certifications due on 6 March 2026.

DOE's California profile records certifications submitted for 90.1-2013 and 90.1-2016. The Energy Commission's own manual says the 2016 Energy Code "was current with ASHRAE 90.1 national consensus standards". The relationship runs through certification: California keeps writing its own text and reports how it measures up.

Different ideas of what is commercial

Part 6 defines Standard 90.1 by its title, which covers buildings "except low-rise residential buildings". DOE, consistent with ASHRAE and the International Code Council, treats multifamily buildings over three stories, and hotels of any height, as commercial. Part 6 agrees about hotels and motels, which sit in its nonresidential sections, but keeps multifamily buildings of every height in their own sections. A seven-storey apartment block is a commercial building under 90.1 and a multifamily building under Part 6, filed on NRCC forms. The residential and nonresidential split explains the categories.

Part 6 also reaches into processes. Since the 2013 code it has covered parking garage ventilation, process boilers, compressed air, commercial refrigeration, laboratory exhaust, data centre cooling and commercial kitchens; see covered processes.

One measure, two approaches: CO2 ventilation control

Demand-controlled ventilation shows how a familiar 90.1 design can miss. Section 120.1(d)4C requires the controls to hold CO2 at no more than 600 ppm above the outdoor level, which works out to a 1,000 ppm setpoint when outdoor air is at 400 ppm. The Nonresidential Compliance Manual notes that this is not the approach of current ASHRAE 62.1 or 90.1, which set no fixed CO2 target for all spaces and lower ventilation rates per person. ASHRAE Guideline 36-2021 publishes separate control sequences for Title 24 and for 90.1-2019. A sequence copied from a project in another state needs checking against Part 6.

Other habits that do not transfer

Solar and batteries. Part 6 sets photovoltaic and battery requirements for new nonresidential buildings, which the 2025 code resized around solar access roof area; see solar PV and battery storage.
Acceptance testing. Lighting controls and mechanical systems are tested and recorded on NRCA forms before final; see acceptance testing.
Software. Performance compliance runs in Energy Commission-approved programs, not in a 90.1 modelling method.
Rooftop units. When an end-of-life rooftop unit of a certain size is replaced in an existing retail, school, office or library building, the 2025 code calls for a high-efficiency system, including heat pumps.

Where 90.1 still matters here

California teams still meet 90.1 through national clients' standards, control sequences such as Guideline 36, and green ratings: LEED v4 used 90.1-2010 as its energy benchmark, and USGBC published a path for California projects, covered in Title 24 vs LEED.