ASHRAE Standard 100: The Performance Standard for Existing Buildings

Almost every energy code you have heard of — ASHRAE 90.1, the IECC, Title 24 — applies when a building is designed, built or substantially altered. Once the certificate of occupancy is issued, they have nothing more to say. Yet the buildings that will exist in 2050 are mostly already standing. ANSI/ASHRAE/IES Standard 100 is the standard written for them: it sets measured performance targets for existing buildings and defines what an owner must do when a building misses them.

From guidance to an enforceable standard

Standard 100 has existed in various forms for decades, originally as guidance on energy conservation in existing buildings. The 2018 edition was a turning point: it introduced energy use intensity (EUI) targets by building type and climate zone, turning the document from a list of good practices into something a building could pass or fail.

The current edition, Standard 100-2024, now jointly published with the Illuminating Engineering Society, is titled Energy and Emissions Building Performance Standard for Existing Buildings. Two changes matter most. It adds greenhouse-gas emissions targets alongside energy targets, and it is written in mandatory, code-enforceable language so that a city or state can adopt it directly as a building performance standard (BPS) rather than drafting one from scratch.

How the targets work

Standard 100 is built around actual, metered performance, not modelled design intent. The owner collects at least twelve consecutive months of energy data for the whole building, calculates the site EUI (and, under the 2024 edition, an emissions intensity), and compares it with the target for that building's activity type and climate zone. Benchmarking is normally carried out in ENERGY STAR Portfolio Manager, the same tool most US disclosure ordinances already use.

Targets are set per building type because a hospital, a warehouse and an office use energy in fundamentally different ways. Mixed-use buildings get a target weighted by floor area across their activities. Getting that area split right is often the first thing to check when a building appears to fail: a data centre or commercial kitchen mis-classified as general office will make an efficient building look like a poor performer.

If the building meets the target

A building that already performs at or better than its target demonstrates compliance largely through its measured data. It still has to keep doing the things that sustain performance — the operational requirements below — but it is not forced into a capital retrofit programme it does not need. That is the central idea of a performance standard: reward outcomes, not checklists.

If it misses: audit, then implement

A building that misses its target follows a defined improvement route. In outline:

• Energy audit — a structured audit that identifies energy efficiency measures, with rigour consistent with ASHRAE Standard 211 audit levels;
• Measure selection — measures are evaluated for energy savings and cost-effectiveness, so the owner implements what makes economic sense rather than everything that is technically possible;
• Implementation — the selected measures are carried out within the compliance period;
• Verification — the building's post-implementation performance is tracked against the target, which is where measurement and verification methods such as ASHRAE Guideline 14 become relevant.

The detailed pathways, cost-effectiveness tests and timelines are set by the published standard and, more importantly, by the jurisdiction that adopts it. Adopting authorities routinely amend targets, deadlines and exemptions.

The operational requirements everyone has to meet

Whether a building passes or fails, Standard 100 expects it to be managed, not just equipped. The standard requires an energy management plan and an operations and maintenance programme — documented schedules, setpoints, maintenance tasks and responsibilities. This is deliberate. Many buildings fail their targets not because of their equipment but because of how it is run: schedules that drifted to 24/7, simultaneous heating and cooling, economisers stuck closed. An O&M programme is the cheapest energy-efficiency measure there is.

Readers familiar with ISO 50001 will recognise the philosophy: plan, measure, act, and keep doing it.

Where Standard 100 is used

The best-known example is Washington State's Clean Buildings Performance Standard, which was built on ASHRAE Standard 100 with state-specific amendments, requiring large commercial buildings to meet EUI targets or follow an energy management and audit route. Other states and cities developing building performance standards draw on it for structure and target-setting, and the 2024 edition's code-ready language was written precisely to make that adoption easier.

Not every BPS uses it. New York City's Local Law 97, for example, sets its own carbon limits per square foot and does not adopt Standard 100's targets. Always check which standard — and which edition, with which amendments — your jurisdiction actually references.

Where energy modelling fits

Compliance is measured from bills and meters, so why model at all? Because the question an owner actually faces is which measures will close the gap, in what order, and for what money. A calibrated whole-building model of the existing building lets you test retrofit packages, account for interactions between measures, and see the effect of electrification on both the EUI and the emissions target before committing capital. It is the same retrofit modelling process that supports investment-grade audits, aimed at a regulatory target instead of a lender.

The takeaway

Standard 100 shifts the question from "was this building designed to code?" to "does this building actually perform?" Start by benchmarking honestly, with correct activity types and floor areas. Then compare against the target that applies in your jurisdiction. If you miss it, treat the audit as the beginning of a plan, not a compliance document. The owners who do best under performance standards are the ones who started measuring before they were required to.

This article is general guidance and reflects information available at the time of writing. Requirements are defined by the published edition of ANSI/ASHRAE/IES Standard 100 and by the jurisdiction that adopts it, which may amend targets, pathways and deadlines — always confirm the edition and local rules that apply to your building.