ASHRAE updates mean more flexibility for designers, more uses for the same building energy models, more recognition of energy saving design strategies, and lower energy modeling costs.
What is ANSI/ASHRAE/IES Standard 90.1, and how does it effect how we measure energy efficiency? ASHRAE Standard 90.1 is the basis for nearly all commercial building energy codes in the U.S. Most states either adopt it directly, or adopt a code that allows it as a compliance option. You can comply with Standard 90.1 in two ways. The prescriptive path regulates design by establishing minimum criteria for energy-related characteristics of individual building components such as R-values of insulation, efficiency of HVAC equipment, maximum lighting power, and so forth. The performance path provides additional flexibility by allowing a designer to “trade off” requirements by not meeting some prescriptive requirements if the impact on energy cost can be offset by exceeding others, as demonstrated through energy simulation, thereby increasing design flexibility.
Standard 90.1 includes two performance paths: the Energy Cost Budget (ECB) method and Appendix G, the Performance Rating Method. Before the 2016 edition of Standard 90.1, only ECB was approved for demonstrating minimum compliance with the standard. Appendix G is familiar to those in the green building industry as the modeling method used to quantify building energy performance for above code programs such as the USGBC’s LEED rating system, commercial building federal tax credits, and the International Green Construction Code (IGCC).
Appendix G provides a significant advantage over the ECB performance path. The baseline in ECB, to which a proposed building design is compared, is essentially a clone of proposed design, with each parameter set at exactly the prescriptive code efficiency level. Appendix G on the other hand, uses a more independent baseline, where many of the parameters are set to standard practice, thus allowing credit for design features that go beyond standard practice. For example, Appendix G provides credit for optimized building orientation, optimized window area, “right sizing” of HVAC equipment, appropriate HVAC equipment type selection, and efficient use of building thermal mass. This helps to encourage integrated design and results in lower energy consumption in buildings.
Up until the 2016 edition, both ECB and Appendix G were updated in stringency every three years to match the increased stringency of the prescriptive path. Because state energy codes and various above code programs are not updated in unison, they end up referencing different vintages of the two performance paths, leading to confusion in the marketplace. Some states reference the 2007 version of ECB, some the 2010 version, and still others the 2013 version. LEED, the federal tax incentive program, and IGCC reference three different versions of Appendix G. Figure 1 shows the various uses for the different versions of the two performance paths in Standard 90.1. Given this, a single project that needs to achieve code compliance, LEED certification, and a federal tax incentive could potentially need three separate building models, leading to confusion and unnecessary expense.
Besides the addition of Appendix G as a third path for compliance, the Appendix G baseline will no longer become more stringent with each new edition of the standard. Instead, it will remain stable at approximately the 2004 level of stringency. Compliance with the 2016 standard will require the proposed building to achieve a performance cost index (PCI) less then targets that are building type and climate zone specific. A PCI of one represents a 2004 compliant building and a PCI of zero, a net-zero energy cost building. For subsequent editions of the code, the PCI target will simply be reduced while the baseline remains fixed. Using this new approach, buildings targeting compliance with any era of code or any beyond code program could theoretically use the same building model eliminating the costly need for multiple baselines previously described. The multiple uses and stable baseline will help encourage development of software tools that automate the creation of the baseline building since the tools will have a larger market and an extended useful life. This should lead to lower cost and more accurate building energy models. For these advantages to be realized, the new version of Appendix G will need to be referenced by codes and beyond code programs. Fortunately this process has already begun. In addition to compliance with Standard 90.1 2016, the new version of Appendix G has been adopted for compliance with the New York State Energy Code and incorporated in a pilot credit for LEED Version 4.
Michael Rosenberg’s participation on the ASHRAE Standard 90.1 committee is funded by the U.S. Department of Energy’s Building Energy Codes Program.