Energy Modeling

What Is An Energy Model?

In essence, an energy model is a set of computer-generated calculations that help designers compare design options and predict anticipated energy consumption of a building and its systems. As technologies have advanced, so has the ability to analyze various aspects of building performance, either with general purpose “whole building” energy modeling packages or software developed for a specific purpose be it daylighting, ventilation, thermal bridging, occupant comfort, or another parameter. The modeler inputs data describing aspects of the building and external design conditions and the “calculation engine” drives the output. Building professionals can then analyse the output to make meaningful comparisons of different design options or compare whole building performance against a benchmark or targeted energy use intensity (EUI).

Why Is An Energy Model Important For Design Excellence?

Many architecture and engineering firms consider modeling a necessity in the design process. Performance modeling increases the ability to make informed decisions to improve energy performance, increase thermal and visual comfort, and improve building durability, among other design issues key to design excellence.

Some practitioners’ experience with modeling is solely as a tool used in the performance approach to document compliance with an energy code, or as a tool used for third-party certification such as USGBC LEED. It is important to understand that a compliance model is not intended to be a precise, predictive model of post-occupancy energy performance. Codes typically set a floor for allowable energy performance and a compliance model documents that the minimum level of performance has been achieved. This approach usually brings the energy model into the process at a point where it is too late to provide much design feedback. Thus, the model does not help designers understand scenario interactions with potentially positive or negative consequences. While compliance modelling is valuable for tracking certain energy metrics and achieving project milestones, modeling for design excellence goes beyond compliance.

Who Does the Performance Modeling?

Modeling can get complicated, but it doesn’t have to be! Make a plan and incorporate modeling in the project timeline. Effective use of modeling requires the architect or team to identify questions you need to ask, the level of detail needed to answer them, and the point in the process when the answers are required to facilitate a design decision. Identify who on the design team has the required expertise and is responsible for each of the different analyses needed to answer key questions. The team should agree upon the software tool, or in some cases tools, that will be used by the responsible parties.

The architect’s role is often to lead the process by knowing enough to have useful and collaborative discussions with those with expertise in specific areas, rather than doing their own energy modeling. However, this isn’t always the case. The AIA publication The Architects Guide to Building Performance draws a distinction between ‘single aspect’ and ‘whole building’ simulation. Single aspect analysis such as a massing or window shading study can often be done by a knowledgeable architect. Whole building analysis very early in design can also sometimes be performed by an architect. Certain software permits an architect to describe the enclosure while choosing among general approaches to electric lighting and HVAC systems that include default assumptions. The results are best thought of as providing a ranked order of the performance of different façade design options under consideration than as an accurate prediction of performance. This is still very valuable information!

When greater precision and information on HVAC and lighting systems is needed, an engineer or outside modeling consultant is needed. It is important to discuss the various assumptions the modeler must make about plug loads, occupancy patterns, and building operations.

What Are The Impacts Of Energy Modeling?

Energy models allow for optimization without compromising design. Early energy analysis can identify how even small changes in building massing, orientation, window-to-wall ratios, glazing types, insulated wall assembly, and more can have large impacts on building heating and cooling loads. These loads directly impact the size of the mechanical equipment, the size of the mechanical rooms, and the size of the utility bill. Façade appearance iterations including external shading elements can be studied along with their efficiency implications. These are just two examples of how results from an energy model can affect design.

Energy modeling also supports project cost management. A model improves the ability to compare first cost and operational cost payback periods for alternative façade or wall assembly options and HVAC, structural, and lighting systems. All clients are likely to choose a slightly more expensive and more efficient system when the payback is quick. Models make it possible to better understand the cost, or savings, of improved indoor air quality and occupant comfort.

Energy modeling also allows designers to push the envelope with less risk, adding an element of predictability to the process. Engineers can right-size building systems and equipment, reducing material resources and energy use while managing construction cost. Through energy modeling designers can identify specific best practices, compare a design to other projects in or outside of a firm, and compile and track firm progress on climate action through programs such as AIA’s 2030 Commitment. If building information modeling (BIM) software was used to develop the design the same digital model can sometimes be used for energy modelling by consultants. If architects plan to share their model, they should discuss model construction requirements with the consultant beforehand. This will reduce the need to revise or recreate the model for performance analysis.

What Is The Cost Of Energy Modeling?

The cost of a performance energy model can be split into three different categories: software, labor, and timing. The energy modeling world is full of both free-to-use and paid software, with the main difference being the support available. High quality free software is available from the US Department of Energy and university researchers, but with little support. Commercial software often includes support or support can be purchased for a fee. Whether free or paid, whole building energy software takes time to master. If you don’t have that time, work with a consultant.

The timing and the precision required of an energy model is part of the cost equation. Analysis performed by a skilled modeler early in schematic design can have a big impact with very little cost. Comparing the energy implications of different approaches to siting, massing, and window-to-wall ratio can often be done in an hour or two. More precise modeling later in the process takes more time. Even then, there are numerous examples where the cost of energy modeling was paid back in savings in first cost of equipment or through operational savings captured in a matter of months.

The old adage “Garbage In = Garbage Out” applies here. Models are only as good as the data that is provided, which means the designer and modeler must be in sync if they are not the same person. Base knowledge, clear communication, patience, and perseverance are mainstays of the energy modeler, whether inside an architect’s firm or hired as a consultant.

What You Can Do Now

  • If you’ve never used an energy model before, the first step is to commit to using one on your next project. It’s ok to start simple, with one key variable to test.
  • Add an energy model to your list of deliverables (and into your fees), if not for your client then for yourself.
  • Integrate energy modeling early in the design process when it need not be expensive and when you can get a lot out of it.
  • Set project performance goal and benchmarks: a certain EUI, a certain % better than code, etc. Use these to decide what questions you want the energy model to answer about your design. See AIA’s 2030 Commitment.
  • Treat your engineers and modelers as integral members of the design team. Ask them for recommendations to improve performance. They may see an opportunity that you do not. Your job isn’t to follow every recommendation but to meet the various design requirements while integrating excellent performance.
  • If you are currently working with an energy modeler to document compliance, ask if they could provide additional non-compliance modeling services earlier in a project.


Tools and Resources

  1. Architect’s Guide to Building Performance: Integrating Simulation into the Design Process

  1. Nonresidential Focus: The Shockingly Short Payback of Energy Modeling

  1. Climate Consultant Software, Energy Design Tools,
  2. Building Energy Software Tools Directory – An extensive list for knowledgeable practitioners.

Colin Skinner is a senior energy modeler at Buro Happold’s Los Angeles office.

Leigh Christy, FAIA, LEED AP BD+C, is a Principal in the Los Angeles office of Perkins & Will.

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