Applying building energy modeling

Whole building energy modeling techniques and software are crucial for designing code-compliant buildings. At the same time, this near-mandatory exercise introduces opportunities for engineers to add significant value to projects.


Learning objectives
This article has been peer-reviewed.

  • Understand what building energy modeling is and why it is used.
  • Recognize how energy modeling tools and techniques can be used throughout the entire design process and the value they can provide.
  • Learn how to use energy modeling to maintain and improve building performance over the life of the building.

Driven by increasingly stringent building energy codes and certification programs, owners' operating cost needs, institutional sustainability imperatives, and growing regulatory legislation, high-performance building design has become a fundamental component of the overall building design process. The high-performance building design process integrates building system design to optimize overall building energy and water use, operability, function, cost, and resilience over the intended life of the building.

With the desire to reduce carbon emissions accelerating the pace of energy code and standards development, prescriptive design of compliant building systems is more and more challenging. At the same time, participation in building certification programs that include energy-performance improvements over energy codes is on the rise.

To meet these needs, engineering-led building energy modeling provides design teams with a powerful tool for predicting how design characteristics will affect energy use and cost, assisting in the creation of integrated building systems that can exceed building performance targets associated with project program and budget goals. But energy modeling not only provides owners with the knowledge necessary to make informed decisions in satisfying energy code requirements, it also provides the data necessary to continue improvement of building operation over the life of the facility.

Code compliance

Ten years ago, energy modeling executed during the design phases of projects was generally still relegated to the role of documentation, whether for code compliance, the U.S. Green Building Council's (USGBC) LEED rating system, or possibly a few lifecycle cost scenarios. Compared to today, there were very few modeling practitioners, the budget for such activity was often limited, and many of the tools were not nearly as user-friendly.

The rapid growth of the LEED rating system and other certification programs that require energy performance improvements over energy code has had a significant impact on the role of energy modeling in design, both directly and indirectly. The direct impact, attributable to the need or preference of many projects to complete modeling, has led to development of energy modeling talent within architecture, engineering, and construction (AEC) firms and has created a large contingent of firms that provide LEED, energy modeling, and related consulting services.

Indirect impacts are largely the result of heightened awareness of energy issues attributable to the USGBC's efforts, manifested in the form of institutional policy or legislation. One example is the decision by many organizations to require that all new projects achieve particular energy savings targets as compared to code because their LEED projects were not necessarily achieving high scores in the energy savings credit. The University of California, the University of Michigan, and the State of North Carolina are such entities.

Figure 1: The status of state energy code adoption is shown, based on information available at All graphics courtesy: Affiliated Engineers Inc.Energy modeling for code compliance

While some states were still using the 1989 version of ASHRAE Standard 90.1 as recently as 5 years ago, today most states have adopted recent versions of ASHRAE 90.1 (2013, 2010, or 2007) or the International Energy Conservation Code (IECC) (2012 or 2009). (See Figure 1.) This reflects the importance being placed on energy and energy performance in buildings and has in part been driven by federal legislation advancing the energy codes on a more regular basis. It also has placed a challenging requirement on project teams to demonstrate compliance. In the past many teams used a "prescriptive" path within these standards to demonstrate compliance (e.g., limit window area, use a certain level of insulation, select an air conditioning unit with a certain efficiency or better), but with performance requirements trending higher for code, many are forced to use a "performance" path to demonstrate compliance.

Energy modeling is used to support the performance path, allowing project teams to make trade-offs (e.g., perhaps the window area and fan horsepower don't comply with code requirements, but improved glass and more efficient HVAC system types can provide counterbalance). The energy model for the prescriptive path takes all of these factors into account to demonstrate that the proposed building will perform as well or better than the code minimum building.

Mandatory energy targets, lifecycle cost

States, universities, the federal government, and other organizations across the country have been incorporating energy savings targets into their design standards, many using a target of 30% savings as compared to code (e.g., the federal government, the states of Iowa and North Carolina, the University of Michigan, the University of Texas, and Cornell University). The University of California's Office of the President instituted a 20% better-than-code target. Regardless of the savings target, this requirement is being applied to all of the institution's projects as part of its design standards. These and other entities are also applying more aggressive targets on individual projects. In many cases this takes the form of energy use intensity (EUI) targets and even net-zero energy. Cornell University's approach is an example of considering percent energy savings reductions to develop EUI targets for various building types.

All of these have contributed to the increased demand for energy modeling. However, to evaluate the various possible pathways for achieving the targeted savings in the most cost-effective manner, energy modeling functions more as a complex, dynamic platform than as a simple verification tool. Cost effectiveness is paramount to implementing truly efficient design strategies; many organizations allow exemptions if reaching the target cannot be justified based on economics.

The private sector typically uses return on investment as the basis for decisions, often looking for returns that essentially provide payback in the 1- to 5-year range. Institutional clients tend to think in terms of total cost of ownership over a 20- to 30-year life. Lifecycle cost (LCC) is generally used to evaluate the economics. The Dept. of Energy's BLCC tool lifecycle cost analysis (LCCA) is one of several that are freely available and it, like others, may be the required tool depending on the client.

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