The IECC is the model code most U.S. jurisdictions adopt as law, with chapters covering both residential and commercial buildings, while ASHRAE 90.1 is the consensus technical standard that governs commercial buildings and often serves as an accepted alternate compliance path inside the IECC. Before you set a design basis, confirm which edition your authority having jurisdiction (AHJ) has adopted and which compliance path applies. The U.S. Department of Energy (DOE) and the International Code Council (ICC) both publish guidance that tracks these adoptions state by state.
TL;DR:
- Confirm the adopted edition and local amendments with the AHJ early; jurisdictions may enforce versions one or more publication cycles behind.
- Once a project selects the IECC or ASHRAE 90.1, every discipline must follow that document throughout; mixing provisions can trigger plan review rejection.
- Choose the performance path early: ASHRAE 90.1’s Energy Cost Budget and Appendix G use different baselines, so switching later may require rebuilding the model.
- ASHRAE 90.1 commonly sets a whole building blower door limit near 0.40 cfm per square foot of envelope area; local IECC rules may be stricter.
- For NYC buildings above 25,000 square feet, Local Law 97 sets separate ongoing emissions limits, though code design models can support later reporting.
Table of Contents
- Quick side-by-side overview: scope, authorship, and update cycle
- How adoption and DOE determinations shape what teams must follow
- Key technical differences that change scope and cost
- Practical implications for project teams and the design basis
- Adoption timeline and where to check jurisdiction requirements
- How we help clients apply IECC and ASHRAE 90.1 on real projects
- Comparing energy efficiency impacts and operating cost implications
- Mandatory and optional provisions that steer design decisions
- Fitting IECC or ASHRAE 90.1 compliance into Local Law 97 planning in NYC
- Documentation and reporting requirements for NYC DOB permit filings
- How future code updates affect existing building upgrades
- What the conventional advice gets wrong
- Get code compliance built into your project from day one
- FAQ
- Sources
Quick side-by-side overview: scope, authorship, and update cycle
The IECC comes from the ICC on a three-year cycle and includes separate residential and commercial provisions, though many states adopt the whole code rather than splitting by chapter. ASHRAE 90.1 is published by ASHRAE jointly with the Illuminating Engineering Society (IES) under ANSI consensus procedures, and it focuses exclusively on commercial and high-rise residential buildings. The commercial chapters of the IECC frequently point back to 90.1 as an alternate compliance option, so a project team working under the IECC commercial provisions may legally comply by following 90.1 in its entirety instead.
- IECC: ICC publication, residential and commercial chapters, three-year update cycle.
- ASHRAE 90.1: ANSI/ASHRAE/IES standard, commercial and high-rise residential scope, updated on a similar multi-year cycle.
- Commercial IECC chapters often allow full substitution with 90.1 as an alternate path, but the choice must be made once and applied completely.
Knowing which document actually governs your building type is the first filter. A single-family home almost always falls under the IECC residential chapters; a mid-rise office building might comply under either the IECC commercial provisions or ASHRAE 90.1, depending on what the state or city has adopted.
How adoption and DOE determinations shape what teams must follow
DOE does not write either code, but it carries a statutory obligation to review each new edition and determine whether it improves energy efficiency over the prior version. That review process, grounded in the Energy Conservation and Production Act, feeds directly into state certification deadlines that determine when states must respond to a new edition.
- DOE reviews each updated IECC and ASHRAE 90.1 edition within a defined window after publication and issues a formal determination.
- An affirmative determination starts a statutory clock for states to certify that their codes meet or exceed the new edition.
- States frequently lag behind the national publication date, so the version enforced locally can trail the newest ICC or ASHRAE release by one or more cycles.
DOE's review window runs about a year after a new edition publishes, and recent determinations covering Standard 90.1-2022 and the 2024 IECC illustrate how that timeline plays out in practice. The practical lesson for design teams is simple: never assume the newest published edition is the one your jurisdiction enforces. Pull the AHJ's current adopted code citation before finalizing a design basis, and check whether local amendments have tightened or altered specific provisions. A permit reviewer working from an older adopted edition will reject drawings based on requirements that have not yet taken legal effect locally, which creates costly redesign cycles late in a project.
Key technical differences that change scope and cost
Several provisions routinely trip up project teams moving between the two documents, and the differences are rarely cosmetic. Definitions matter first: what counts as an "entrance" versus a non-entrance door, or whether a space qualifies as "semi-heated," changes whether vestibule requirements, continuous air barriers, or additional insulation apply.
- Envelope performance. R-values, U-factors, and window-to-wall ratio limits differ between the two documents and by climate zone, and exceeding the allowed glazing percentage can force a performance-path analysis instead of prescriptive compliance.
- Air leakage testing. ASHRAE 90.1 commonly sets a threshold near 0.40 cfm per square foot of envelope area for whole-building blower-door testing, while some IECC implementations and state amendments mandate stricter results, which can require additional sealing details or retesting.
- Lighting and mechanical mandatory provisions. Interior lighting power density limits, mechanical equipment efficiency minimums, and economizer requirements diverge in specific climate zones and building types, so a design that passes under one document can fail under the other.
- Performance paths. Both documents offer whole-building modeling alternatives to prescriptive compliance. ASHRAE 90.1 provides the Energy Cost Budget (ECB) method and the Performance Rating Method (Appendix G), and Appendix G generally uses a more stable baseline that can credit design features ECB does not recognize.
- Renewable energy provisions. Recent 90.1 cycles introduced prescriptive requirements tied to on-site energy generation in some climate zones and building types, and DOE's own modeling separates gross impacts, which exclude on-site generation, from net impacts, which include it.
Pro Tip: Decide early whether your energy model will run under ECB or Appendix G, since the baseline assumptions differ enough that switching mid-project usually means starting the model over.
Practical implications for project teams and the design basis
The compliance path belongs in the contract documents, not just in a memo. Spell out the adopted code edition, the chosen compliance path (prescriptive, ECB, or Appendix G/PRM), and the responsible party for energy modeling directly in the drawings and specifications. Project teams cannot mix provisions from the IECC and ASHRAE 90.1: once a path is selected, the entire building must comply under that single document, start to finish. Mixing triggers plan review rejections and can force a full re-submission.
- Cite the exact adopted code edition and year in the stamped basis-of-design narrative.
- State the chosen compliance path explicitly on the drawings, not just in the specifications.
- Include the blower-door testing plan and acceptance criteria in the contract documents.
- List the energy model software and baseline assumptions where a performance path applies.
- Confirm the AHJ's current adopted edition in writing before the design development phase closes.
Pro Tip: Ask the AHJ plan reviewer to confirm the adopted edition and any local amendments in writing early in design development, not at permit submission, when a conflict is far more expensive to fix.
The engineer of record typically signs off on the code basis, but the architect and owner need to agree on the compliance path before mechanical, electrical, and plumbing drawings proceed, since a late change cascades through every discipline's documentation.
Adoption timeline and where to check jurisdiction requirements
Both the IECC and ASHRAE 90.1 move on roughly three-year publication cycles, with DOE's statutory review following each release. States rarely adopt on the same calendar; many run one or two cycles behind the national publication, and some apply amendments that make local requirements more stringent than the base document.
- Check the DOE Building Energy Codes Program's state certification pages for your state's current certified edition.
- Confirm the AHJ's adoption language, since some jurisdictions adopt the IECC with amendments that override specific sections.
- Request the adopted code citation directly in your permit application cover letter to create a documented record.
Local more-stringent amendments are common in major metropolitan jurisdictions, so never assume the state-level adoption automatically describes what a city or county enforces.
How we help clients apply IECC and ASHRAE 90.1 on real projects
We build the code compliance path into the design basis from day one rather than treating it as a late-stage checklist item. We coordinate to confirm the adopted edition with the AHJ, set the compliance path in the contract documents, and carry blower-door testing and commissioning requirements straight through to permit filing. That coordination matters most on projects where mechanical equipment selections, envelope detailing, and energy modeling assumptions all have to agree before a permit set goes out the door.
Comparing energy efficiency impacts and operating cost implications
National modeling from DOE and Pacific Northwest National Laboratory (PNNL) consistently finds that successive IECC and ASHRAE 90.1 editions deliver measurable site energy savings over the editions they replace. Because the two documents share much of their technical basis, commercial buildings built to a given IECC commercial edition and buildings built to the corresponding ASHRAE 90.1 edition tend to land in a similar efficiency range on a national basis, even though specific provisions differ project by project.
For an owner, the practical operating cost implications come less from which document you choose and more from which compliance path you follow within it. A prescriptive design that barely clears the minimums will typically run higher energy costs over the building's life than one modeled under Appendix G with efficiency measures layered in to offset a design tradeoff elsewhere. Mechanical system efficiency, envelope air tightness, and lighting power density all move the needle more than the choice between IECC and 90.1 alone.
Owners planning long hold periods should weigh the performance path even when prescriptive compliance would satisfy the permit, since the energy model built during design can double as a baseline for future retro-commissioning or capital planning work. That same model becomes useful again when a building later faces Local Law 97 or similar operational carbon reporting requirements, because the underlying system data has already been documented.
Mandatory and optional provisions that steer design decisions
Each document carries a baseline of mandatory provisions that apply regardless of compliance path, alongside optional or prescriptive requirements that a design team can trade off against each other.
Under the IECC, mandatory provisions typically include minimum envelope sealing, duct sealing and testing, and basic lighting controls, while prescriptive tables govern insulation values and window performance by climate zone. Under ASHRAE 90.1, mandatory provisions cover building envelope sealing, HVAC equipment efficiency minimums, service water heating equipment efficiency, and lighting controls, while the prescriptive path allows tradeoffs between envelope, lighting, and mechanical systems within set limits.
The optional performance paths in both documents, Appendix G and the Energy Cost Budget method, under ASHRAE 90.1 and comparable modeling routes under the IECC, matter most when a design pushes past a prescriptive limit, such as an oversized glazing area or an unconventional HVAC configuration. Choosing a performance path early lets engineers trade a less efficient envelope against a higher-efficiency mechanical plant, provided the whole-building model demonstrates equivalent or better performance than the code baseline.
Renewable energy provisions introduced in recent 90.1 cycles add a further layer: some climate zones and building types now carry prescriptive requirements tied to on-site generation, which changes how mechanical and electrical teams size systems and coordinate with the energy model from the earliest design phases.
Fitting IECC or ASHRAE 90.1 compliance into Local Law 97 planning in NYC
New York City building owners face a layered compliance picture: the NYC Energy Code (based on the IECC with local amendments) or ASHRAE 90.1 as an alternate path governs new construction and major renovations, while Local Law 97 imposes separate, ongoing carbon emissions limits on existing buildings above 25,000 square feet. The two frameworks are not the same requirement, but they share underlying data.
A building designed to a more efficient envelope, lighting, and mechanical package under either code compliance path will generally carry a lower baseline energy use intensity into its Local Law 97 reporting years, which can reduce the retrofit scope needed to avoid penalties later. For owners planning capital upgrades, aligning the energy code compliance path chosen during design or renovation with the building's Local Law 97 emissions trajectory avoids duplicating analysis: the same mechanical system data, envelope assumptions, and energy model can support both the permit filing and the ongoing emissions reporting obligation.
Teams should treat the energy code compliance documentation and the Local Law 97 compliance plan as linked workstreams rather than separate filings handled by different consultants at different times. When mechanical equipment replacements or envelope upgrades are being planned anyway for code compliance, folding in the emissions reduction analysis at the same stage typically costs less than running two separate studies months apart.
Documentation and reporting requirements for NYC DOB permit filings
The NYC Department of Buildings (DOB) requires a stamped energy code compliance statement on permit filings, identifying the adopted code edition, the compliance path chosen, and the responsible design professional. When a project uses ASHRAE 90.1 as the alternate commercial path rather than the NYC Energy Code's prescriptive provisions, the filing documentation must state that choice explicitly and carry it through every discipline's drawings, since DOB plan examiners check for internal consistency across mechanical, electrical, and plumbing sets.
Energy model documentation, when a performance path is used, needs to accompany the filing with the software, baseline assumptions, and compliance margin clearly stated. Blower-door testing plans and commissioning requirements belong in the specifications referenced by the filed drawings, not as a separate unfiled document, since DOB reviewers expect the permit set itself to establish how compliance will be verified during construction.
Because DOB plan examiners reject filings that mix compliance provisions between the local energy code and ASHRAE 90.1, confirming the compliance path before the filing stage, not after a first-round rejection, keeps the review timeline predictable. A clear, single-path code narrative on the cover sheet of the drawing set, cross-referenced in the specifications, reduces the back-and-forth that otherwise stretches permit review by weeks.
How future code updates affect existing building upgrades
Buildings designed under an older IECC or ASHRAE 90.1 edition are not automatically required to meet every new provision as updates publish, but any future renovation, equipment replacement, or system upgrade typically has to meet the code edition in effect at the time of that later permit application. That creates a rolling compliance picture for engineering consulting work on existing buildings: a boiler replacement planned for next year gets evaluated against whichever edition the AHJ has adopted by then, not the edition the building was originally designed under.
This matters most for capital planning. An owner replacing mechanical equipment on a ten- or fifteen-year cycle should expect each replacement to trigger a fresh look at the then-current code requirements, including efficiency minimums that have generally tightened with each successive edition. Envelope work tied to a renovation can likewise trigger updated air leakage testing or insulation requirements that did not apply when the building was first constructed.
Tracking which provisions apply only to new construction versus which apply to any altered system is part of the ongoing consulting relationship on an existing building, since misreading that distinction leads to either over-scoping a simple equipment swap or under-scoping a renovation that actually triggers broader code compliance. Keeping a current record of the building's adopted code history, past energy models, and testing results makes each future upgrade faster to scope and permit.
What the conventional advice gets wrong
Most guidance on this topic treats IECC versus ASHRAE 90.1 as a binary choice to make once and forget. In practice, the more consequential decision is which compliance path to follow inside whichever document applies, since prescriptive, ECB, and Appendix G routes produce meaningfully different outcomes for the same building. Treating the code choice as the whole decision undersells how much the performance path selection affects both construction cost and long-term operating expense.
The other underrated point is timing. Teams that wait until permit submission to confirm the AHJ's adopted edition are gambling with the schedule, since a mismatch between the design basis and the locally enforced edition forces rework at the most expensive possible stage. Confirming the adopted edition and compliance path during early design development, in writing, costs almost nothing and prevents the majority of code-related permit delays we see on real projects.
Readers should prioritize documenting the compliance path in the contract and drawings before worrying about which national document sounds more rigorous. The code that matters is the one your AHJ has actually adopted and certified.
— Joseph
Get code compliance built into your project from day one
We set the energy code compliance path, confirm it with your AHJ, and carry that documentation through every mechanical, electrical, plumbing, and fire protection drawing we produce, so your permit set tells one consistent story. 
Our mechanical engineering team handles HVAC design and equipment sizing against whichever IECC or ASHRAE 90.1 path your project follows, our plumbing engineering group covers the systems that pair with envelope and mechanical compliance, and our NYC permit filing support keeps your DOB submission aligned with the adopted code edition. Request a consult through our services page to get your compliance path confirmed before design development locks in.
FAQ
Which code applies to residential versus commercial buildings?
The IECC's residential chapters govern most single-family and low-rise residential buildings, while commercial buildings typically fall under either the IECC commercial chapters or ASHRAE 90.1 as an alternate compliance path. High-rise residential buildings often fall under the commercial provisions of both documents depending on the jurisdiction's adoption language.
Can a project mix provisions from both codes?
No. Once a project selects a compliance path, every discipline must follow that single document in full, and mixing provisions between the IECC and ASHRAE 90.1 typically triggers a plan review rejection. Confirm the chosen path in the contract documents before drawings proceed.
How do I find which edition my jurisdiction has adopted?
Check your state's current certified edition on the DOE Building Energy Codes Program's state certification pages, then confirm the AHJ's specific adoption language and any local amendments. Many jurisdictions enforce an edition that trails the newest ICC or ASHRAE publication by one or more cycles.
What does Bazini Engineering charge for energy code compliance support?
Pricing for mechanical, electrical, plumbing, and fire protection design services, including energy code compliance support, depends on project scope and is available on request through our services page. We scope each engagement around the building type and the compliance path the project requires.
How does Local Law 97 relate to the energy code compliance path?
Local Law 97 sets ongoing carbon emissions limits for existing buildings, separate from the energy code requirements that apply to new construction and major renovations. A more efficient design under either code compliance path generally lowers a building's baseline energy use entering its Local Law 97 reporting years.