Determine applicability of beyond-code programs
It is important to determine applicability of beyond-code programs early in the design as this will have bearing on design features that will need to be modeled, as well as many aspects of the energy modeling approach itself. Programs may include green building certification programs such as LEED or the WELL Building Standard, green codes or reach codes (local ordinances that require performance to exceed the minimum code requirements), or design standards such as ASHRAE 209 which require modeling to be applied to the design process.
Relevance to BEM Practitioner During Early Stages
Energy modeling to meet the requirements of a beyond-code program is a major driver for projects to use BEM. Programs such as LEED place heavy emphasis on high-performance design and use BEM as the primary means for determining the energy efficiency of a design. As a result, projects seeking green building certification will often set targets for energy performance and engage with a BEM practitioner to help assess design options and help determine how to meet the performance target.
Comparison to Baseline
Most beyond-code programs' energy modeling approaches are based on the concept of comparing a model of the proposed design to a baseline model. The baseline model is meant to represent that same building as if it had been designed to just meet the energy code requirements.
Modelers will generally start by defining the proposed design according to design drawings and specs, then create a second version of the model where they systematically replace components of the proposed design with different inputs as specified by the modeling rules (or "ruleset"). The rulesets can be complex to implement manually, but many BEM software tools now have the capability of automatically creating the baseline model.
Program-Specific Modeling Rules
Beyond-code programs typically require the proposed design to achieve a level of performance that meets or, in most cases, exceeds the baseline building's performance. The results of the two models are compared and a calculation is performed to show that the proposed design uses less energy - typically expressed as a percentage improvement value. A percent improvement must be demonstrated, and greater percentages may provide more credits or points, as discussed below. Refer to the Additional Resources section below for links on a variety of beyond-code programs and their modeling guidelines.
Version of the Energy Code or Standard
Many programs are based on ASHRAE 90.1 Appendix G, but be sure to check which version. LEED v4 is based on 90.1-2010, but older versions of LEED were based on different versions of 90.1. Newer versions of 90.1 or alternative calculation approaches may also be acceptable too but the program may require different percentage thresholds to be met. Also be sure to note that the version of the standard used for the beyond-code program may differ from the version used for demonstrating code compliance. For local reach codes, location-specific requirements may also come into play. If you are relying on a software tool with the capability to automatically generate baseline models, be sure that it is applying the correct ruleset(s).
Special Modeling Rules
Some programs may have created special rulesets for a particular feature of the building. For example, the LEED v4 Reference Manual provides detailed guidance on how to model features that are not a part of the base ruleset of ASHRAE 90.1 such as core and shell projects and district heating and cooling systems. Additionally, it provides a path to allow for alternative metrics to be used instead of energy cost (e.g. source energy, greenhouse gas emissions, or TDV for projects in California). LEED also allows for projects to use the newer ASHRAE 90.1-2016 Appendix G modeling rules instead of 2010 and provides special guidance on how to do so.
During the early stage analysis, the modeler can provide useful guidance to help the design team select between different design options. The models can predict energy impacts of anything ranging from facade design, HVAC system selection, lighting power, passive strategies, etc. This topic is discussed in more detail in the Conceptual Design section of this site.
During the early stages, decisions are made at a rapid pace, so if the modeling is to have any meaningful impact on the decision-making, it must be performed quickly. Often, there will need to be assumptions and simplifications made in the model, so be sure to communicate this to the design team along with some discussion on how performance may vary with a differing set of assumptions. Software tools with "wizards" or other features designed to quickly create a models, test energy efficiency measures, or test different HVAC systems can be very beneficial for early stage modeling.
Projects seeking LEED certification or other green building certifications may have identified a target number of points or credits they seek to achieve via the energy modeling process. As a result, the "options analysis" described above may be focused on how to achieve the desired number of points or credits. As noted, there is often a fair bit of uncertainty and assumptions required to model a building in the early stages, so be sure to communicate this. If the design features proposed by the team are not achieving the desired performance, or achieving the desired number of credits, this is your chance to help come up with some solutions! Of course, you'll want to make sure that any ideas you propose are suitable for the project, so your best bet may be to brainstorm a list of options, and bounce them off the design team before spending a lot of time running a bunch of options. On the flip side, if you feel that it may take too much time to get feedback from the design team, then go ahead and run the options quickly so you can provide timely advice. Just be sure to have an understanding of the design intent and try to select options that are not realistic.
- Green Building Standards And Certification Systems - The Whole Building Design Guide offers a nice list of many beyond-code programs (both U.S. and international) including links and descriptions.
- LEED - Minimum Energy Performance - Credit Details for LEED Energy Modeling credit. Includes links to numerous pilot credit options which allow for special rules to be applied as discussed above.
- California Reach Codes - A list of local reach codes adopted in California and details about how they're implemented.
Links to external websites are provided as a convenience for further research, but do not imply any endorsement of the content or the operator of the external site, as detailed in BEMcyclopedia's general disclaimers.