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In our latest guest blog, Patrick Pease, Building Performance Leader at Buro Happold, explores how applying the right level of detail in energy modeling, guided by the updated ASHRAE/IBPSA Standard 209-2024, helps modelers balance accuracy, efficiency, and purpose across all stages of design.
In the building design industry, energy modeling has become an essential tool for making informed decisions about building performance at all stages of design. The increasing application of energy modeling leads to challenges for practitioners who face increasingly larger workloads, tighter deadlines, and complex designs. To combat these challenges, energy modelers are applying new standardized approaches to Level of Detail (LoD) to right-size the effort, time, and costs for modeling. Detail is an important consideration for energy modeling: too little detail can compromise accuracy, while excessive detail wastes time and resources without meaningfully improving results. The newly revised ASHRAE/IBPSA Standard 209-2024 provides crucial guidance on this balance through its comprehensive approach to defining level of detail for building performance simulation.
The Challenge of "Right-Sizing" Modeling Effort
Energy modelers often face pressure to provide quick analyses early in the design process when building information is still changing daily. The need for speed often leads modelers to utilize more and more impressive automation tools that can import 3D geometry into energy modeling tools like IESVE Software. This creates what can be called the "automation trap” the temptation to add complexity simply because automation tools make it possible, rather than because the analysis requires it. Too much detail earlier, while it may be easy to create, can result in longer simulation times, or noise in the modeling results that do not support early design decision-making. By keeping models simple early and only adding detail when they meet a purpose, modelers can ensure each model is fit for purpose and is not causing uncertainty or an increase in running times.
The key insight is that the level of detail should be intentionally matched to both the design phase and the specific modeling objective. A conceptual design model comparing facade options requires fundamentally different inputs than an as-designed performance model prepared for code compliance. ASHRAE/IBPSA Standard 209-2024's Appendix I provides a structured framework for making these decisions.
Understanding Model Elements and Attributes
Standard 209-2024 identifies ten core model elements that form the foundation of any building energy model: Weather, Geometry, Zoning, Constructions, Fenestration, Lighting, Plug and Process Loads, HVAC, Service Water Heating, and Occupants. Each element contains multiple attributes that can be specified at varying levels of detail.
Consider lighting as an example. At the simplest level of detail, you might apply a single lighting power density with one operation schedule across all zones — sufficient for early concept studies comparing building massing options. At higher levels of detail, you would specify lighting power for each space based on actual fixture layouts, incorporate different schedules reflecting anticipated usage patterns, and account for automatic controls. The most detailed models might also specify the split between lighting heat gain to the space versus return air plenum based on specific fixture types.
The standard emphasizes that not all model elements require the same level of detail simultaneously. For HVAC system selection modeling (Cycle #4), thermal zoning and system representation require high detail, while lighting and plug loads may appropriately use simplified assumptions. This selective approach to detail allows modelers to focus effort where it matters most for the specific analysis objective.
Mapping Level of Detail to Modeling Cycles
One of Standard 209-2024's most practical contributions is defining eleven modeling cycles that correspond to common building performance simulation applications throughout the design and operation process. Each cycle has specific objectives, applicability, and recommended levels of detail for model inputs.
Cycle #1: Simple Box Modeling serves as the foundation, requiring only basic inputs like building use type, location, gross floor area, and number of stories. At this stage, a full weather file is needed, but perimeter-core zoning and simplified window geometry are appropriate. Software defaults can be used for many elements. This cycle is ideal for early feasibility studies and high-level comparisons.
Cycle #2: Conceptual Design Modeling introduces more geometric detail and begins evaluating design alternatives. This is where energy modeling actively informs design decisions about building orientation, window-to-wall ratios, and envelope performance. The model remains relatively simple — detailed HVAC system modeling isn't yet appropriate — but inputs should be sufficient to compare meaningful design options.
Cycles #3-7 cover the middle phases of design: load reduction modeling, HVAC system selection, design refinement, integration and optimization, and value engineering. These cycles progressively add detail as design decisions are finalized. By Cycle #5, models should include specific HVAC zoning, actual system types, and increasingly detailed schedules and controls.
Cycle #8: As-Designed Energy Performance represents the culmination of design phase modeling, typically occurring after construction documents are complete. This cycle requires the highest level of input detail, including sequence of operations, part-load performance curves, and thermal mass considerations. The outputs must support detailed analysis of hourly energy use by end use, unmet hours, and carbon emissions.
Practical Application: Bridging BIM and Energy Modeling
A common question that our Building Performance team at Buro Happold gets is how the BIM (Building Information Modeling) level of detail relates to the needs of Building Energy Modeling (BEM). While there is overlap, the relationship isn't one-to-one. BIM LOD 100 (schematic design) generally provides sufficient geometric information for modeling Cycles #1-2. BIM LOD 300-350 (detailed design) supports Cycles #3-7, though energy models require operational data—schedules, setpoints, sequences of operations—that don't reside in BIM models.
Moreover, not all BIM data is relevant to energy modeling. Detailed stair railings, plumbing drain layouts, and fire protection sprinklers appear in BIM, but they do not affect energy simulation whatsoever. Conversely, critical energy modeling inputs like utility rates, control sequences, and weather files exist outside BIM entirely, but they are critical to BEM.
Conclusion
The building industry increasingly relies on energy modeling to meet aggressive performance targets and regulatory requirements. As expectations for modeling increase, the need for clear guidance on the appropriate level of detail becomes more critical. ASHRAE/IBPSA Standard 209-2024's framework for level of detail provides that guidance, helping practitioners match modeling effort to project needs.
By understanding model elements, attributes, and how they map to specific modeling cycles, energy modelers can work more efficiently while maintaining necessary accuracy. This approach respects the reality that building design is an iterative process — projects do not need construction document precision in the model when the project is still in schematic design. The standard reminds modelers that the goal is not maximum detail, but appropriate detail matched to the questions at hand.
For those interested in learning more, the full ASHRAE/IBPSA Standard 209-2024 is available at ashrae.org, and the Standard Project Committee (SSPC 209) welcomes new members interested in advancing building performance simulation practices.
