ASHRAE 90.1 vs Title 24: Which Standard Governs Your Project?

The question arrives in one of two forms. An owner with buildings in Texas, Arizona and California asks why the California project needs a completely different energy analysis from the other three. Or a California developer chasing LEED asks why the energy consultant has quoted for two models when the project only has one building. Both questions have the same answer, and it is worth understanding before the fee proposals arrive rather than after.

ASHRAE 90.1 and Title 24 Part 6 are not competing versions of the same document. They are different kinds of instrument, written by different bodies, for different purposes, and on any given California project both can be live at the same time for entirely different reasons. This article sets out what each one is, which one legally governs, why the other still shows up on the invoice, and how to structure the work so the two analyses share as much as they usefully can. Budlong provides MEP engineering services across California and on multi-state portfolios, and the pattern below comes from doing both submissions on the same buildings. You can see the breadth of that work on our expertise overview.

1. What Is ASHRAE 90.1?

The full title is ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings. It is published jointly by ASHRAE and the Illuminating Engineering Society, approved as an American National Standard by ANSI, and maintained under continuous maintenance so that individual addenda are approved and published as they clear the committee.

A full edition is issued roughly every three years. The current published edition is 90.1-2025, which rolls up 105 addenda approved since the 2022 edition. The editions before it, in order, are 90.1-2022, 90.1-2019, 90.1-2016, 90.1-2013 and 90.1-2010. Those older editions matter more than you would expect, because rating systems and federal rules keep pointing at them for years after they are superseded.

The critical point for an owner is what 90.1 is not. It is not a law, and it is not a code. It is a voluntary consensus standard that becomes enforceable only through one of three routes: a jurisdiction adopts it into its building code, a federal regulation cites it, or a contract obliges the design team to meet it. Absent one of those, publishing a new edition changes nothing on any live project.

2. How a Standard Becomes Law

The mechanism that turns 90.1 from a book into an obligation is a federal one, and understanding it explains why the map of American energy codes looks the way it does.

The DOE determination process

Under federal energy policy legislation, whenever ASHRAE publishes a new edition of 90.1, the U.S. Department of Energy must analyse it and issue a determination as to whether the new edition would improve energy efficiency in commercial buildings. If the determination is affirmative, every state is then required to review the energy provisions of its commercial building code, update it to meet or exceed the new edition, and certify to DOE that it has done so.

The determination for 90.1-2022 was published on 6 March 2024. DOE found that buildings meeting the 2022 edition would achieve roughly 9.8 percent site energy savings, 9.4 percent source energy savings, 8.9 percent energy cost savings and 9.3 percent carbon savings compared with 90.1-2019. That triggered a two year clock, with state certifications due by 6 March 2026. The supporting analysis and the state by state status are published by the DOE Building Energy Codes Program.

What states actually do with it

States respond in three broad ways. Most adopt either 90.1 directly or the commercial provisions of the International Energy Conservation Code, which itself allows 90.1 as an alternative compliance path, meaning 90.1 sits behind the code even where the code is nominally the IECC. A smaller group operates on home rule, leaving adoption to individual jurisdictions. A handful have no mandatory statewide commercial code at all.

California takes the remaining route. It writes its own.

3. How the Two Documents Are Structured

The internal architecture of each document tells you a great deal about how it behaves on a project.

The structure of 90.1

90.1 runs on numbered sections. The technical requirements sit in the sections covering building envelope, heating ventilating and air conditioning, service water heating, power, lighting and other equipment. Each of those sections contains mandatory provisions that always apply plus prescriptive provisions that apply if the prescriptive route is chosen. A section near the end sets out the Energy Cost Budget Method, a simulation based alternative. Normative Appendix G then contains the Performance Rating Method, which is a separate simulation procedure with a different purpose.

That last distinction trips people up constantly. The Energy Cost Budget Method is a compliance tool: it proves the building meets the standard. Appendix G is a rating tool: it measures how far beyond a fixed reference point the building goes. LEED and most beyond-code programmes use Appendix G, not the Energy Cost Budget Method.

The structure of Title 24 Part 6

Title 24 Part 6, the California Energy Code, is written from scratch by the California Energy Commission and adopted into the California Building Standards Code through the California Building Standards Commission. There is no national model behind it. Its sections run in the 100 series for mandatory measures applying to all buildings, the 120 series for mandatory nonresidential systems requirements, the 140 series for prescriptive nonresidential requirements, and the 141 series for additions and alterations. Compliance is demonstrated on a prescriptive path or a performance path, and the performance path is run in software approved by the Energy Commission.

Part 6 is only one of twelve parts of Title 24. The rest cover structural, mechanical, electrical, plumbing, fire and green building requirements, and it is Part 11, CALGreen, that carries the water, materials and indoor environmental quality provisions that 90.1 does not address at all.

4. Which One Governs Your Project

Strip away the nuance and the legal answer in California is a single sentence. Title 24 Part 6 governs, always, on every permitted project, and ASHRAE 90.1 never governs by itself.

In California

The code edition in force on the date the permit application is submitted controls the project. Applications submitted on or after 1 January 2026 fall under the 2025 Energy Code. There is no provision anywhere in Part 6 that lets a project substitute a 90.1 compliance demonstration for a Title 24 one. A plan checker wants Title 24 forms, produced by approved software, signed by a documentation author. Nothing else clears the counter.

California satisfies its federal obligation under the determination process through its own code, which DOE has recognised as meeting or exceeding the applicable edition of 90.1. That recognition is the reason California is never required to adopt 90.1 in the first place.

Outside California

Elsewhere the governing document is whatever the state or jurisdiction has adopted, which will be a specific edition of 90.1, a specific edition of the IECC, or an amended version of one of them. Two projects in adjacent states can sit three editions apart. The first question on any out of state project is not which standard applies but which edition of which standard, and whether the state has amended it.

When both are live at once

The common California case is a project that must comply with Title 24 to get a permit and must demonstrate 90.1 performance to satisfy something else: a LEED certification target, a federal design standard, a lender or tenant requirement, or a corporate sustainability commitment. Both are real obligations. Only one of them stops the building being built.

5. What Each One Asks of Each Building System

Both documents regulate the same list of systems. They differ in how hard they push, and in a handful of places they differ in what they regulate at all.

Envelope

Both set insulation levels, fenestration U factor and solar heat gain coefficient limits, air leakage requirements and cool roof provisions, indexed to climate zone. The climate zone systems are different: 90.1 uses the ASHRAE and IECC climate zone map, while Title 24 uses sixteen California climate zones defined by the Energy Commission. A single California city can sit in one ASHRAE zone and a quite different California zone, so envelope assemblies developed for one analysis cannot be assumed to satisfy the other.

Mechanical

Both set minimum equipment efficiencies, economizer requirements, fan power limits, duct sealing and insulation rules, zoning requirements and ventilation provisions. Title 24 in its 2025 cycle pushes considerably harder in two areas that hit budgets directly. It expands heat pump baselines into multi zone office and school systems, and it revises the minimum outdoor air calculation so that the ventilation rate takes the larger of an occupant based and an area based figure. It also now mandates ASHRAE Guideline 36 control sequences for variable air volume systems, economizers and supply air temperature reset on new and replacement direct digital control systems, which is a rare case of Title 24 importing an ASHRAE document wholesale. Getting the plant right starts with sound HVAC load calculation methods and a deliberate approach to HVAC system selection for commercial buildings, supported by our HVAC design services.

Service water heating

Efficiency minimums, pipe insulation and recirculation control appear in both. Title 24 carries the electrification trajectory much further, with heat pump water heating baselines that reshape central domestic hot water plant in multifamily and hospitality buildings. This is a frequent source of surprise for owners applying a national standard to a California site, and it is worth coordinating early with plumbing engineering services.

Lighting and power

Both set interior and exterior lighting power allowances and mandatory control requirements, and the two allowance tables are structured similarly enough that a design tuned to one is usually close to the other. The 2025 Title 24 cycle removed the Tailored Method that complex projects had relied on for additional allowance, which pushes more lighting-driven projects onto the performance path. Coordination between architectural lighting design and electrical engineering services now happens earlier than it used to.

On-site renewables and storage

This is the clearest divergence. Title 24 mandates solar photovoltaic systems on a long list of nonresidential occupancy types and now pairs them with battery energy storage sized by formula. 90.1 treats on-site renewable energy through credit and allowance mechanisms rather than through a blanket mandate of the same shape. An owner porting a national design into California will find the roof, the structure and the electrical service all need revisiting, which is why photovoltaic design belongs in concept design rather than in a later package.

6. Why 90.1 Still Matters in California

If Title 24 governs every California permit, why does 90.1 appear on so many California projects? Because four separate constituencies require it, and none of them is the building department.

LEED

This is the big one. The Energy and Atmosphere category of LEED is written against ASHRAE 90.1, and the reference edition depends on the LEED version the project registered under. LEED v4 referenced 90.1-2010. LEED v4.1 referenced 90.1-2016. LEED v5, ratified in March 2025 by the U.S. Green Building Council, allows either 90.1-2019 with addendum cr or 90.1-2022, with the 2022 edition becoming mandatory for projects registering on or after 1 January 2028. LEED v5 becomes the only version available for new commercial registrations from 1 July 2027.

The Minimum Energy Performance prerequisite has to be satisfied before any energy points are available at all, and the energy efficiency credit is scored on how far the proposed building beats the 90.1 baseline. LEED v5 also changed the metric, moving away from energy cost toward a future source energy basis with a 2.0 site-to-source conversion factor for electricity on United States projects, and it applies lower Building Performance Factors than v4.1 did. The practical effect is that a California project relying on a heavily electrified design now scores differently than it would have under the older cost-based method, and usually more favourably.

Federal projects

New federal commercial buildings are governed by 10 CFR Part 433, which sets ASHRAE 90.1-2019 as the baseline for buildings whose design for construction began on or after 7 April 2023, and requires energy consumption at least 30 percent below that baseline where the improvement is life cycle cost effective. A federal building on a California site therefore carries a 90.1 obligation regardless of what the state code says, and in practice the design team runs both analyses. Our federal MEP work is built around that double requirement.

Corporate and portfolio standards

Large owners write design standards once and apply them everywhere. Most of those standards are anchored to a 90.1 edition, because it is the only energy reference that means anything in more than forty states. Keeping that anchor is sensible. The mistake is treating it as a compliance route in California rather than as an internal performance floor sitting underneath a stricter legal requirement.

Financing, reporting and tenant requirements

Green loan covenants, tenant fit-out standards, ESG disclosures and utility incentive programmes frequently express performance as a percentage better than a stated 90.1 edition. Those are contractual obligations with real money attached, and they need the Appendix G analysis whether or not the project is chasing certification.

7. Healthcare, Laboratories, Data Centres and Federal Work

Four building types make the comparison harder than the general case, and all four are common in California.

Healthcare

Hospitals and skilled nursing facilities are reviewed by the state health facilities authority rather than a local building department, and they carry ventilation and pressure relationship requirements written for infection control rather than energy. Both Title 24 and 90.1 have to be applied around those constraints, and neither document was drafted with them as the starting point. On a LEED-seeking hospital the Appendix G baseline system assignment for the acute care areas deserves an early conversation, because getting it wrong distorts the entire result. Our healthcare facilities teams handle this on acute care, outpatient and behavioural health work.

Laboratories

The 2025 Energy Code removed the laboratory exemption that earlier California cycles carried, and laboratory exhaust systems now face prescriptive requirements. That is a material change for life sciences developers who had planned around the previous treatment. 90.1 has long addressed laboratory exhaust and energy recovery in its own way, so a lab project chasing LEED in California now has two sets of requirements on the same high-volume exhaust system, and they do not resolve identically. This directly affects our laboratory and industrial clients.

Data centres and computer rooms

Both documents treat computer rooms as a special case with their own efficiency, economizer and containment provisions, and both are chasing a technology that moves faster than a three year code cycle. Rack densities and liquid cooling have outrun the assumptions embedded in either standard. Projects in this space typically need the performance path in California, an Appendix G exceptional calculation for any LEED submission, and an early conversation with the authority having jurisdiction. See our mission critical capability for context.

Education and civic work

School districts and public agencies frequently carry both a Title 24 obligation and a board-level sustainability policy expressed in 90.1 or LEED terms, and the new multi zone heat pump baselines in the 2025 cycle land on classroom buildings directly. Our education sector work routinely runs both analyses on the same modernisation package.

8. Side by Side: Which Document Do You Actually Need?

The table below is the head-to-head comparison. Read it as a description of two different instruments rather than as a scorecard.

Which document each stakeholder needs

The second table is the one to put in front of a project team, because most confusion comes from assuming a single document satisfies every audience.

9. Running One Energy Model That Serves Both

The efficient answer to a dual obligation is one proposed building model and two baseline runs. Getting there requires the scope to be set up correctly at appointment, because retrofitting a second analysis onto a model built for one purpose costs more than doing both from the start.

What genuinely shares

The geometry, the envelope assemblies and their thermal properties, the equipment schedule with capacities and efficiencies, the lighting layout and power densities, the control sequences and the plant configuration all describe the same building. Build them once, in one tool, and keep them under one revision control. Every change made in design development then flows to both analyses instead of one quietly drifting out of date. This is the core discipline behind good energy modelling in MEP design.

What cannot share

Four things stay separate. First, the baselines: the Title 24 Standard Design is generated automatically by the compliance software from the proposed geometry, while the Appendix G baseline is constructed by the modeller with system types assigned by building type, size and fuel, and rotated through four orientations. Second, the fixed inputs: Title 24 dictates occupancy schedules, plug loads and other assumptions by rule so that submissions are comparable, whereas Appendix G lets the modeller use project-specific values in defined circumstances, so the same building can report different energy use in the two runs without either being wrong. Third, the metric. Fourth, the reporting package.

Software reality

A California compliance submission must come from software approved by the Energy Commission for the applicable code edition. Some of those tools can also produce Appendix G output, and where that is true the duplication is genuinely small. Where it is not, the practical approach is to build the proposed model in a general-purpose simulation tool, mirror it into the compliance tool, and reconcile the two at each design milestone. Agree which tool is authoritative before anyone starts modelling.

Sequencing it properly

Run the Title 24 screening first, at the end of schematic design, because it is the analysis that can stop the permit. Run the Appendix G baseline second, once the proposed design is stable, because it measures rather than gates. Then feed both into the commissioning scope so that the building commissioning process absorbs California acceptance testing and the LEED commissioning requirements as one field programme rather than two, coordinated through a single commissioning services appointment. Owners with wider decarbonisation ambitions should keep the same model live through sustainable design services rather than retiring it at certificate of occupancy.

Key Takeaways

• ASHRAE 90.1 is a consensus standard published by ASHRAE and IES. The current edition is 90.1-2025, and publication alone binds nobody.
• 90.1 becomes law through state adoption after a DOE determination, through federal rules, or through contract. California uses none of those routes for its own code.
• Title 24 Part 6 is written by the California Energy Commission, has no national model behind it, and governs every California permit by the application date.
• ASHRAE 90.1 never governs a California project on its own. There is no substitution provision in Part 6.
• 90.1 still applies contractually through LEED, federal design standards under 10 CFR 433, corporate portfolio standards, lenders and incentive programmes.
• Appendix G is a rating method with a stable baseline at roughly 2004 vintage. The Title 24 performance path uses a Standard Design that tightens every cycle.
• Passing Title 24 does not demonstrate the LEED Minimum Energy Performance prerequisite. Budget for a separate Appendix G run.
• One proposed model can serve both analyses if the scope is set up that way at appointment. Two baselines and two document packages remain separate deliverables.

10. Where Budlong Works

Both documents are applied by people, and people vary by jurisdiction. Plan check culture in Los Angeles is not the same as in San Francisco, local reach codes in several Bay Area cities push above the state baseline, and utility incentive programmes differ by service territory. Budlong maintains teams across California so that local review experience sits behind every submission, whether the project is answering to a California plan checker, a LEED reviewer, a federal agency, or all three. You can read more about how the practice is structured on our company story page.

Sectors where the dual obligation bites hardest

Healthcare carries competing ventilation mandates on top of both energy documents. Education faces the new multi zone heat pump baseline while working to board sustainability policies written in LEED terms. Commercial developers meet the solar and battery mandates first and often carry a tenant or lender performance target as well. Mission critical projects outrun the assumptions in either standard.

11. Related Reading

12. Frequently Asked Questions