The State of Technology: Outcome-Based Energy Codes and zEPI

Two emerging code developments to targeting improved building performance

Matt Lockwood, ERS, for Zondits

This month, our State of Technology series focuses on energy codes. Let’s talk about two innovations in development over the last decade in energy codes: outcome-based energy codes and zEPI. Both started as dreams of energy code wonks to address the problems with existing code and benchmarking solutions, and both have been gaining prominence and momentum in the energy codes world.

First, let’s touch on existing energy codes. Energy codes define the minimum requirements that all buildings must achieve regarding how a building’s envelope, heating and cooling systems, and lighting should be designed and installed. Energy codes are used by state and local jurisdictions to mandate and enforce standards. The two most prominent model codes, the International Energy Conservation Code (IECC) and the American Society of Heating, Refrigerating, and Air-Conditioning Engineers 90.1 (ASHRAE 90.1), are updated in three-year cycles to integrate new technologies and to gradually move toward more stringent energy efficiency requirements. It is then up to state and local jurisdictions to adopt, implement, and enforce some, all, or even more aggressive requirements than the model’s codes. Energy codes are one of the primary ways in which we achieve increased building energy efficiency.

Existing Code Compliance Methods

There are two primary methodologies for compliance and enforcement in use today: prescriptive and performance modeling-based code systems.

[mks_pullquote align=”left” width=”300″ size=”20″ bg_color=”#81d742″ txt_color=”#ffffff”]The performance modeling compliance method allows for greater flexibility in design within the compliance path.[/mks_pullquote]Prescriptive energy codes provide required performance levels that individual equipment and building systems must achieve. Prescriptive models are attractive because they’re easy to follow as they clearly lay out what is acceptable and require little analysis to ensure compliance. However, as energy efficiency requirements become increasingly stringent, codes become more complicated, verging on byzantine. It is a constant struggle to train code officials on a revolving door of energy code requirements, and poor enforcement of prescriptive code compliance has plagued the industry. Also, the prescriptive method by design rarely accounts for the interactive effects of individual building systems. Finally, many building owners are only interested in meeting minimum code compliance – this biases design toward the least expensive initial investment rather than more comprehensive design decisions that might achieve significantly higher savings over time.

The performance modeling compliance method allows for greater flexibility in design within the compliance path. Performance-based codes contain broad energy efficiency goals that require computer modeling to verify compliance, typically expressed in terms of percent better than code in comparison to a baseline code-compliant building. There remain drawbacks, however. Modeling results are only as good as the inputs; a modeler must take great care that their model is a good representation of the building that will be built. Significantly, performance modeling contains an implicit assumption that the actual performance of a building’s systems will meet the manufacturer’s stated specifications without issue. As we all know, equipment does not always perform up to spec, and even the best designed system sometimes breaks down and requires maintenance.

Therein lies the chief issue with traditional code compliance methods; codes use as-built requirements as a proxy for building performance, but they do not mandate actual performance levels. Neither prescriptive nor modeled performance codes require that building systems actually function over time, nor have they historically required commissioning. The graphic below, from the New Buildings Institute’s “Energy Performance of LEED for New Construction Buildings,” highlights this problem – a building’s model may not guarantee actual energy performance over time.

State of Technology

Additionally, most current code systems prescribe only large building systems and lack accounting for building plug loads – computers, printers, toasters, water coolers, televisions, etc. – that can make up a significant portion of a building’s overall energy use. As building energy codes become more and more stringent, these unregulated loads can make up a larger share of the total building energy use (between 30% and 75%, depending on industry) that are untouched by code requirements.

Enter Outcome-Based Codes

Outcome-based codes are an alternate approach to compliance that have been gaining interest and momentum in the energy code industry. An outcome-based code looks at a whole building’s energy use (including otherwise undocumented plug-loads) measured over a period of time post-occupancy, typically a consecutive 12-month period. An outcome code would require that the building’s actual energy use during that period not exceed a predetermined maximum amount. This code path guarantees that energy efficiency is achieved in practice over time, not just in theory. Additionally, outcome-based codes allow for a maximally flexible regulatory compliance path – as long as the building performs, there are relatively few prescriptive burdens which would dictate the design process.

[mks_pullquote align=”left” width=”300″ size=”20″ bg_color=”#81d742″ txt_color=”#ffffff”]There are, of course, a number of challenges to the implementation of outcome-based codes, not the least of which is that the enforcement power of authority of current code compliance agencies typically ends when a certificate of occupancy is issued.[/mks_pullquote]There are, of course, a number of challenges to the implementation of outcome-based codes, not the least of which is that the enforcement power of authority of current code compliance agencies typically ends when a certificate of occupancy is issued. There have been a number of proposals to address this challenge that are currently being tested in different localities across the United States, including the post of a performance bond (Seattle), performance licensing (Boulder), required period retrocommissioning (NYC), or a conditional occupancy permit (recommended by the international green construction code, 612.3.1).

Another significant issue is choosing how to set the energy performance targets for a diverse set of building stock. Typically, energy performance on a whole-building scale is reported as the building’s energy consumed, per square foot, per year – this is called energy use intensity (EUI). EUI measures are useful for tracking an individual building’s energy over time, but they are not equipped to address different building types in different climate zones all with different use cases and variable levels of thermal gain.

Here is where zEPI comes in.

[mks_pullquote align=”right” width=”300″ size=”20″ bg_color=”#81d742″ txt_color=”#ffffff”]zEPI stands for Zero Energy Performance Index and represents a new way of viewing holistic energy use in buildings. [/mks_pullquote]What is zEPI? zEPI stands for Zero Energy Performance Index and represents a new way of viewing holistic energy use in buildings. Rather than viewing the performance as percentage better than code, zEPI, creates an absolute scale of building energy performance with net zero energy (NZE) as the ultimate goal. By creating a methodology for ranking building energy performance, zEPI allows for intuitive application of outcome-based building energy codes and offers multiple benefits over current prescriptive-based codes.

Described simply, zEPI is a scale used to benchmark the energy use performance of buildings where a lower score indicates less energy use. The scale is benchmarked at two points: zero, which is a net zero energy, and 100, which is a building with average energy use intensity for a building of that type in the year 2000. Buildings that are net energy producers can achieve a negative score, while buildings that have a higher energy use intensity than the 100 score benchmark will have a score greater than 100.

[bctt tweet=”zEPI creates an absolute scale of building energy performance with NZE as the ultimate goal. ” username=”ZonditsEE”]

What are the benefits of zEPI to outcome-based code frameworks? zEPI allows different individual buildings, energy code baselines, and even different building portfolios to all be compared on the same easily comprehensible scale. It also gives policymakers and code enforcement officials a simple tool to both set future energy efficiency goals and to ensure compliance.

zEPI and outcome-based energy codes are both tools that can be employed to significantly simplify energy efficiency goalsetting and demonstrated code compliance. Both enable an understanding of actual building performance and actual building energy use over time, rather than a best guess from a theoretical framework. Finally, they both include the plug loads of a building as a critical component in a building’s energy consumption.

ERS, “Commercial Building Codes: Finding the Path to Improved Compliance.”
Alaska Housing Finance Corporation, “Comparative Analysis of Prescriptive, Performance-Based, and Outcome-Based Energy Code Systems,” May 2011.
New Buildings Institute, “Getting to Outcome-Based Building Performance: Report from a Seattle Summit on Performance Outcomes,” May 2015.
New Buildings Institute, “Outcome-Based Energy Codes on the Way to Net Zero.”