I’m asked regularly about the discrepancies between modeled and measured building performance. At first blush, one might suspect that the building energy model (BEM) itself contains bad information. That notion could very well be correct! BEM inputs require a certain level of synthesis, interpretation, and assumption. But there are many other reasons why the BEM results may vary significantly from measured performance outcomes on a completed project. In basic terms:
- Actual weather in a given year could vary significantly from the climate normals used in the model.
- Occupancy schedules could have shifted in unexpected ways (perhaps more people use the attractive new facility).
- Behavioral patterns may not have been accurately anticipated (we still have much to learn about accurately predicting human behavior).
- Actual field conditions of the building construction may not fully align with the design documents.
- Mechanical systems may not be functioning as specified or expected.
- Facility may not be operated and/or maintained as specified / expected.
However, there are a few ways in which a project team may start to close the gap on the discrepancy between modeled and measured energy performance in buildings. I offer five below:
1. Use validated energy modeling programs.
In North America, there are three primary BEM programs used in the building design and construction industry for whole-building energy simulation: BLAST, DOE-2, and EnergyPlus. These programs have been validated and serve as the “simulation engines” working behind the scenes within many of the more user-friendly BEM software tools in the marketplace today. Outside of the US, there are other validated BEM programs that are also suitable. The point here is to make sure you understand the quality of the tool you choose to use.
2. Consider an ASHRAE energy modelers as part of an integrated design team.
As such with any professional service, there exists a range in skills and experience with regard to energy modeling. While not essential for achieving a high-quality model, some project teams may be well-served by engaging a dedicated energy modeler with ASHRAE’s Building Energy Modeling Professional (BEMP) Certification. Developed with the participation of the U.S. affiliate of the International Building Performance Simulation Association (IBPSA-USA) and the Illuminating Engineering Society (IES), BEMP certification (an ANSI-Accredited Personnel Certification Program) validates an energy modeler’s competency with regard to developing energy models and having full command of the results of the energy modeling software.
3. Introduce a integrated framework in accordance with ASHRAE Standard 209.
BSR/ASHRAE Standard 209: Energy Simulation Aided Design for Buildings except Low-Rise Residential Buildings defines the minimum requirements for providing design assistance using building energy simulation and analysis. The standard defines building energy modeling requirements as well as post-occupancy energy performance comparison criteria. Standard 209 applies to new buildings or major renovations of, or additions to, existing buildings utilizing BEM throughout the entire design and construction process – from “simple box” modeling during conceptual design through postoccupancy energy performance comparison. Among other benefits, the framework introduced by ASHRAE 209 better ensures that BEM is elevated as an essential component of the design process as it evolves and becomes more thorough as the building project develops.
4. Commit to third-party review and certification, such as LEED.
The U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating system provides a great deal of value to a design and construction project. Among other aspects of the certification process, LEED ensure that a knowledgeable, experienced third party reviews a project’s energy modeling inputs, platform, and results for consistency and completeness – better ensuring that the energy modeling accurately represents the balance of the project documents.
5. Collect and share the data
Simply put, we need more measured data. The more performance data that building owners commit to disclosing publicly – behind an veil of anonymity as appropriate – the better the design community will understand the suitability and accuracy of how it constructs and curates building energy models. This is a “big data” issue. The larger the sample size, the more accurate our energy modeling community will become.
Make no mistake, every energy model is wrong. They cannot predict the future. However, BEM does help us make better-informed design decisions. We are better design and construction professionals for appropriately and properly engaging building energy modeling.