Considerations for building energy modeling

Here’s a review of initial energy modeling performed during a project from the interview phase to development of final models used for documenting compliance to energy codes or for LEED certification, through a client’s survey one year after occupancy. Here’s what works, what doesn't, and what value can be achieved from each step in the process
By Patrick Dempsey, LEED AP, EIT, BEMP, CannonDesign, Chicago April 16, 2014

Figure 1: This is an example of the output of an energy modeling exercise for a project at the proposal phase. It shows the projected annual energy use intensity (EUI) and an estimate of annual energy cost (in dollars) for three different options based on an increase in project scope for a renovation and addition project. All images courtesy: CannonDesignWhen a customer arrives at a car lot, typically the first question posed is about the vehicle’s gas mileage and, relatively quickly, a rigorously EPA-tested fuel economy rating is provided. The customer can use this information to calculate the car’s annual energy cost using simple arithmetic. In August 2013, the average new car cost was $31,252 according to an article in USA Today with an average length of ownership of 71.4 months per Polk. The annual operating cost of a vehicle can be tabulated with minimal effort.

A 10,000- to 15,000-sq-ft renovation or new construction project may cost $3 million or more, and is expected to last 15 years without a major upgrade. It is unlikely that any contractor or designer, prior to signing a contract, would provide a client with a projected annual operating cost or an energy use estimate on the spot.

How can a purchase that’s 100 times cheaper, with a lifecycle roughly five times smaller, provide such detailed energy consumption information by comparison? The simple answer is that buildings aren’t cars, as the design of a building is much more customized. However, that answer is quickly becoming inadequate. As clients, designers, team members, contractors, and municipalities require more information regarding building energy consumption, the feedback on energy consumption related to potential configuration and proposed systems must follow the same fast pace as in other industries. The challenge is establishing a process for developing information useful for decision making in a timely fashion without overanalyzing or getting lost in the details.

Energy code history, modeling origins

Energy modeling is rooted in the development of standardized energy codes. Prior to the development of a model or state energy code, ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings was created. The first recorded ASHRAE energy standard was 90.1-1975. Developed and amended several times in response to the energy crisis of the late 1970s, ASHRAE 90.1-1975 was the standard for state energy code development for over a decade; subsequent versions of 90.1 were issued in 1989 and 1999.

Beginning in 1998, the 2000 International Energy Conservation Code (IECC) was developed from the Model Energy Code, which was first published in 1983 by the Council of American Building Officials (COBA) and Building Officials and Code Administrators (BOCA). The two primary energy codes, ASHRAE 90.1 and the IECC, were so similar that they achieved equivalency. The IECC references ASHRAE, and with a two-year lag in equivalency, IECC 2012 is equivalent to 90.1-2010. Both codes include a compliance path allowing whole building energy modeling if the prescriptive or mandatory requirements can’t be met.

The process for demonstrating code compliance by energy modeling requires building a separate model for the building as designed and comparing the model to a code minimum baseline. As energy-efficiency requirements dictated by model codes increase, the need to use modeling to demonstrate code compliance will become more common on all but the most basic building types.

Figure 2: This graphical example shows the output of an energy modeling exercise for a project at the proposal phase. The graphic is intended to show the amount of photovoltaic arrays necessary to offset the energy usage of three different scales of project for a renovation and addition project.The energy modeling process

In its infancy, building energy modeling was a comparative exercise used to validate a design after major decisions were made. Energy-efficient design was based on general best practices, determined by lessons learned, and from minimum energy code standards for insulation, lighting levels, and ventilation. An energy-efficient design charette could examine reducing glazing, improving envelope performance, or optimizing the building’s orientation or the performance of building systems. The charette’s results would be incorporated in design documents.

The finished design might be modeled in a Dept. of Energy 2.0-based hourly simulation program providing some basic information on energy consumption and annual energy cost. This post-design processing allowed energy modelers to limit deliverables and avoid repetitive modeling. The major weakness in this process was that the energy modeling was being used to validate design as opposed to influencing it, which restricted the ability to achieve substantial improvements in building performance.

Today’s energy modeling is driving an interactive design process providing guidance for teams, from before a project is awarded through construction. For larger project pursuits in the proposal stage, design teams are coming to the interview table with a fleshed-out design concept and energy consumption estimates based on preliminary modeling/benchmarking. In some cases, multiple design concepts have been modeled and comparative data is provided—an activity that previously would have occurred at the design development phase has now moved into the marketing phase. The benefit of incorporating energy information at the proposal phase goes beyond positioning to win the project. From the onset, the design team shows awareness of the building’s energy impact and will consider it prior to decision making. At the very least the team is cognizant of the issues and will be more likely to communicate their concerns as the project moves forward.

To facilitate extremely early energy analysis, there are numerous tools for gathering quick and reliable energy performance information. Benchmarking tools such as Energy Star Target Finder or the Labs 21 Energy Benchmarking Tool offer useful data for early analysis. Taking it a step further, a quick energy simulation using the eQUEST schematic or design development wizard can generate a rough estimate for the concept’s relative energy conception. As with any energy modeling software, the output is only as useful as the input. The more time dedicated to accurately identifying the operating schedule and upfront usage, the better the generated information will be. Using similar projects or existing facility utility data can also be a useful tool for an interview when working in the initial stages.

Figure 3: This shows the output of an energy modeling exercise for a project at the proposal phase. The graphic's intent is to compare three different levels of energy-efficient design to existing benchmarked projects in terms of annual energy utilization intensity (EUI). The circles represent the EUI of three proposed levels of energy performance for the proposed project and compares them to four similar projects executed by the interviewing firm. The smaller the circle, the smaller the energy footprint of the building.Step 1: Benchmarking

Regardless of whether energy information was analyzed in the pre-award phase, most projects now incorporate an “absolute” goal-setting activity during the initial phase, called energy benchmarking. Energy benchmarking compares the program or building shell design to existing building metrics. The most readily available form and tool for benchmarking is the Energy Star Target Finder calculator; it is web-based, free, and easy to use.

Target Finder uses the 2003 Commercial Buildings Energy Consumption Database to score buildings between 1 and 100. An average building comparison to existing building stock would have a score of 50. A project with an energy performance goal of 25% better than ASHRAE 90.1-2007 typically would have an energy score between 80 and 85. The score is determined by evaluating the energy utilization intensity (EUI) in kBtu/sq ft. The location, building type/usage, and gross square footage are needed to obtain a score. The tool has an input for projected/actual energy use, so later modeling results can be input and the “simulated score” developed. The final output includes a printable summary page and a statement of design intent, documenting the projected energy performance.

The benchmarking process provides an alternative starting point to U.S. Green Building Council LEED energy goals based on ASHRAE 90.1-2007. Not all projects pursue LEED certification, and the LEED energy targets are based on a theoretical baseline, annual energy cost. EUI for a LEED target is not final until the baseline model is complete. Energy Star data supports teams that have an early, more tangible alternative to an absolute target to be working toward.

Energy Star uses a publicly available database for benchmarking buildings. An alternative to benchmarking against a composite index of existing buildings is to benchmark against similar projects recently designed or currently designed by entity doing the analysis. Most large architectural and engineering firms have signed the American Institute of Architect’s 2030 Challenge, which requires reporting energy consumption for all projects to the AIA for compilation and comparison. The reporting tool for this process requires that partner firms track the energy consumption, cost, type, and square footage. The information can then be built into a usable database for tracking project energy performance. A project benchmarking exercise can thus compare similar projects using similar, current, efficient design technologies and engender friendly competition within a firm or group of firms. Typically, benchmarking summaries list the project name, location, square footage, and EUI along with those of similar peer projects. By using existing projects, the benchmarking phase engages the design team to ask questions about why one project is projected to perform better or worse than another, thereby influencing design at an early stage instead of just validating compliance.

Step 2: Analyzing design decisions

Design and energy modeling teams must adjust and provide more information and through efforts in the early project stages. The effort associated with providing additional early analysis should not detract from a final deliverable that holds up to the expectation of building owner paying the utility bills for 40 years. Energy modeling must be tailored to the overall process and each particular project stage. Spending significant time on the appearance or absolute model accuracy at the initial stages is wasteful. Attempting to build on an initial, expedited interview-level model and polish it through a potentially multi-year design process may also be ineffective. It is often easier to build updated models as the design evolves, or use the original shell to inform the design team about general efficiency strategies. If floor-to-floor heights drastically change or if the footprint expands, it likely will not impact the desired orientation from an energy perspective.

During development of initial design decisions, it’s important to understand whether a decision or strategy saves energy and the magnitude of those savings. Modeling in early phases should focus on relative comparisons such as orientation or comparing glazing on alternate facades. Absolute savings aren’t as important now as providing positive or negative feedback for decision making. Energy deliverables are typically provided at major project milestones, including 100% design development and construction documents issuance, and include energy usage estimates, annual energy cost information, project assumptions, and systems data. The assumptions and systems data should be reviewed by team members for accuracy, and changes tracked throughout the project. Many government and environmentally minded private entities currently require energy deliverables at various milestones. Proactive and forward-thinking design teams already institute these actions and tools internally.

Table 1: Example of internal benchmarking exercise The table shows internal reporting on energy model accuracy and utility reporting for a completed project. The report allows the project to be quickly reviewed and a determination to be made as to whether further model investigation or site investigation may be necessary based on the predicted energy utilization intensity (pEUI). The new project being compared to other current projects is shaded.Reacting to change

After the issuance of construction documents and development of a final energy performance summary, the interactive energy modeling process becomes reactive. A set of documents has been provided, a performance goal established, and measurement tools and strategies provided. Procurement and construction begins and aspects of the project may change.

For a very energy-conscious client, questions may be asked on a monthly, weekly, or daily basis. If a fan goes from variable to a constant speed, what is the energy impact? If the insulation level of the roof increases by 2 in., what are the cost savings? The energy modeler and the design team are called on to provide quick responses. Often delays in the construction process can cost more than the option being considered. The emphasis should not be on updating a modeling report for every change or proposed change.

View the model itself as the living document and keep adjustments short and correspondence brief. If, hypothetically, every design report or study costs $1,000 a page, no one can afford a weekly updated energy report. However, if an e-mail or brief memo about the projected savings and impact is acceptable, then the process can work quite well. The reactive period typically can include finalizing of utility design incentives or submittal for third-party certification such as completion of energy documentation for the LEED process. The design review process for LEED still dictates somewhat of a retroactive process due to the nature of the energy modeling requirements.

LEED certification

Most industry energy modeling has been a direct result of the LEED certification process and its requirements. The current LEED 2009 or v3 energy requirements involve compliance with ASHRAE 90.1-2007 or California Title 24 as equivalent. Since LEED drives modeling, the majority of processes employed by design firms and energy modelers are centered on the LEED process, and a LEED scorecard is often issued for team review. The scorecard has projected targets for water efficiency, materials, energy efficiency, and so on. The energy modeler or modeling team is given a target goal, based on a percent better than ASHRAE 90.1, and then the design progresses and the energy modeler react to changes from the initial design. Systems selected based on the building‘s program dictate the baseline requirements, and the comparison begins.

Given the rules associated with ASHRAE 90.1 and specifically the Appendix G guidelines for energy modeling, it is often difficult to project multiple performance levels when systems and components are fluctuating dramatically. If the design uses natural gas instead of electric heat, that changes the baseline, even if no other design components have changed. The LEED certification process has required tools for early design collaboration; from an energy perspective, specifically the utilization of a basis of design (BOD) document, which records relevant goals and options. However, the BOD’s use, accuracy, and effectiveness are entirely at the design team, owner, and commissioning authority’s discretion.

The LEED energy modeling process is impacted by the LEED review process. For an energy model, the documentation level required for certification is substantial. Virtually all initial submissions receive LEED review comments that must be addressed. Changes may be required on both the proposed and baseline model affecting the points earned. Comments vary from the mundane, such as verifying the solar heat gain coefficient of a window type because it seems low, to very detailed, such as reviewing the district thermal energy guidelines for a building tied to a campus plant—justifying the “system loss” factor calculated for a university steam plant. The LEED review comments may ultimately impact the performance of the models and reduce or increase the performance in comparison to baseline.

Table 2: The table shows how the current proposed design compares to the IECC 2012 baseline building in terms of energy use intensity, measured in kBtu/sq ft/year, and is subdivided by energy-consuming process.Current energy code compliance

Virtually all of today’s required energy code compliance documentation uses a prescriptive Dept. of Energy tool called COMcheck. COMcheck allows the design team to input envelope, lighting, and HVAC data into a free tool that generates a design report. The design team signs the report and submits it with permit drawings to demonstrate energy code compliance. The software is regularly updated from each state’s current energy code.

Recent adoptions of the 2012 International Energy Conservation Code and the equivalent ASHRAE 90.1-2010 in some jurisdictions have significantly increased energy-efficiency requirements. In some instances, design teams have opted to use energy modeling to demonstrate code compliance in place of COMcheck to demonstrate whole building compliance and allow the design to exceed prescriptive glazing ratios or other envelope requirements that could not be met without significant hardship. As energy codes continue to update on a three-year cycle and energy-efficiency requirements become stricter, it will be more difficult for designers to be creative and still demonstrate code compliance without energy modeling. Adopting a process now to inform early design will pay dividends as modeling becomes a requirement in the future.

Energy modeling process and verification of performance

At the conclusion of the reactive energy modeling process, roughly a year after construction is complete and the building has been occupied, direct client interaction should be renewed through a review of building performance. Reviewing and verifying the performance of a recently occupied or new building is commonly known as measurement and verification (M&V). M&V requirements have been published by the Efficiency Valuation Organization under the International Performance Measurement and Verification Protocol (IPMVP). The IPMVP provides multiple options for verification of building performance to be administered by an M&V provider. Option D. Calibrated Simulation, outlines a methodology for comparing modeled building energy usage to actual utility consumption to verify performance.

Clients are becoming more aware of their energy consumption and are regularly analyzing building performance to decrease operating costs. More clients are requesting feedback on anomalous utility bills or energy consumption that exceeded expectations. Proactively reviewing the utility usage as part of an ongoing process, prior to a client reaching out, creates an opportunity for design teams to identify potential issues, examine potential saving opportunities, and better inform the next energy model/project. Measuring and verifying building performance is essential to the process. Clients and building owners will always be skeptical of simulated savings until backed up by actual performance data. Not only will the data improve the modeling going forward, but it also demonstrates that design entities are invested in the successful operation of the building.

A successful, interactive energy modeling process provides useful feedback to team members and introduces energy as a motivating factor early on—benchmarking to establish a goal, simple analysis of design impact decisions, practical reporting on progress, reactive feedback to change, and verification of actual results. Using a process with those steps will better inform clients, design teams, and entities reviewing the energy performance of projects.


Patrick Dempsey is a senior associate, mechanical engineering with CannonDesign