Analysis to provide answers and insights

BEM can be extremely effective at providing guidance on design decisions throughout all phases of a project, and at developing strategies to meet performance targets. An overview is provided below, but for much more detail and guidance on using BEM to inform specific design tasks, please visit the Learn by Design Task section of this site.

Compare results to targets

Perhaps the most common performance goals are EUI and carbon emissions.

• Energy use intensity EUI (kBtu/ft2-yr) - Can be based on site or source energy.
• Carbon emissions (tons of CO2 per year) - May require a calculation outside of the simulation tool using carbon emission factors (lb CO2/MWh or kg CO2/MWh) for local electricity source mix.

Other performance targets may also be appropriate.

• Peak thermal zone cooling load intensity (Btu/hr-ft2) - This may be an appropriate metric if a cooling system type with limits on cooling capacity, such as a radiant cooling system, is being considered.
• Peak electric demand (kW) - A project might seek to limit the size of electrical service required.
• Peak cooling load (tons) - There are several reasons to try to limit peak cooling load. Some buildings might be connecting to an existing cooling plant with limited capacity. In most cases, reducing peak cooling load will also reduce HVAC system cost.

Compare the performance of design alternatives

Probably the most important role for a simple box model is to compare different designs and answer design questions. These alternatives can be compared based on energy cost, EUI, carbon emissions or other metrics. Consider also comparing performance in terms of peak cooling and heating load and HVAC equipment size for alternatives that affect loads. A reduction in HVAC equipment cost can often offset the cost of load reduction measures.

Identify design priorities and savings opportunities

Another valuable use of a simple box model is to evaluate the results of a model to gain insights into savings opportunities.

• Estimates of energy consumption by end use may highlight end uses that offer the greatest savings opportunities.
• Peak electricity demand estimates by end use may also be useful in identifying strategies to reduce demand.
• Estimates of peak heating and cooling load intensity by zone or by orientation can help the design team prioritize where to apply load control strategies. Component sizing for an HVAC distribution system is based on these peak zone loads. Some simulation software and visualization tools provide useful images that display results such as peak heating or cooling loads for each thermal zoning using a color scale.  
• A breakdown of peak cooling and heating loads by component is very valuable, but its availability varies among simulation tools. When it’s available, this information highlights where to focus attention to reduce loads, e.g. walls, roof, window conduction, window solar, lighting or receptacle loads.

Additional Resources

• Architect’s Guide to Building Performance (American Institute of Architects) - This report outlines the case for using building performance simulation to inform decisions throughout the architectural design process. It provides an overview of the modeling process, guidance on how to integrate modeling in a firm, and numerous examples from case study projects that demonstrate how modeling informed the design.

• See also:
  • Learn by Design Task
  • Presenting results visually

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