Simple Box Models

Example Simple Box Models, Pre-design (left) and Conceptual Design Massing (right)

Simple box models are a type of whole building energy model used to inform early design. A simplified model is used to test the impact of fundamental design decisions on energy consumption and peak heating and cooling loads. The appropriate level of detail in the model depends on the state of the design. During pre-design, the model may look like a simple box. During conceptual design, the model will typically represent a simplified version of one or more design concepts.

Simple box modeling is one of the modeling cycles that may be used for compliance with ASHRAE Standard 209-2018.

Video - Introduction to Simple Box Modeling

Common Use Cases for Simple Box Models

Architectural Design

Simple box models are most often used to analyze aspects of architectural design during Pre-Design and Conceptual Design. They may be used to analyze:

Building form

• Aspect ratio
• Massing
• Orientation
• Number of stories

Fenestration

• Window area
• Window performance
• Shading
• Skylights

Opaque envelope

• Insulation
• Thermal mass

End-use Breakdown

Example Energy End-Use Breakdown

A second purpose is to identify high priority strategies to meet energy performance goals. The simple box model can provide a preliminary estimate of the breakdown of energy consumption by end use, showing which end uses offer the greatest savings opportunities. The results such as those in the accompanying figure can show the design team the relative importance of HVAC energy, lighting energy and other end uses such as receptacle loads.

Performance Targets

Example Results Showing Efficiency Measures Contributing to Meet Performance Target

The simple box model can provide a preliminary estimate of energy use intensity (EUI) or other performance metrics to let the design team know if they are on track to meeting targets. When using this simplified model for this purpose, the design team should review model inputs to ensure that any defaults are reasonable based on what is known about likely building operation. The figure shows an example of how adding energy efficiency measures to the design leads to energy savings and contributes to meeting a project's performance target.

HVAC System Options

Simple box modeling is typically performed using the same HVAC system for all cases. However, with careful attention to model inputs, a comparative analysis of different HVAC system types is possible at an early stage. A comparison is especially valuable when considering system choices that affect architectural design due to different space or structural requirements. Some HVAC system types, such as typical variable-air-volume reheat systems, require space for ducts above the ceiling. Other system types, such as a radiant heating and cooling system, can get by with less space because they rely on pipes rather than ducts for distribution. Underfloor air distribution and displacement ventilation work best with higher ceilings and may also affect floor-to-floor height. A simple box model analysis may also be used to compare all-electric and mixed-fuel HVAC systems.

Timing for Simple Box Modeling

Example Evolution of Simple Box Model Complexity

Simple box modeling is most useful before significant building design decisions have been finalized, typically during Conceptual Design. It can also be applied during Pre-Design, when a sensitivity study can provide the design team with useful information to consider when developing design concepts. The results can show the team whether, for example, aspect ratio is a significant factor for their building type and location or whether window area has a significant impact on HVAC system size. This type of sensitivity analysis can be useful to inform discussions if performed prior to a design charrette. ASHRAE Standard 209-2018, Simulation Aided Design, recommends that the energy modeler participate in an energy charrette prior to completing conceptual design phase analysis.

The adjacent figure illustrates how the application of a simple box model may evolve over the course of design, from a simple sensitivity study in pre-design, to a comparison of massing alternatives in conceptual design, to a fenestration shading study in schematic design.

Related Analysis Methods During Early Stage Design

Other analysis methods and model types may also be useful during early stages of design. Refer to the pages below for additional information.

Conceptual Design Models

The terms simple box model and conceptual design model are sometimes used interchangeably. Some make the distinction that a simple box model is used for a sensitivity analysis before design concepts have been developed, while a conceptual design model is used to compare different design concepts. In either case, the models include a similar level of detail.

Team Responsibilities

For the results of a simple box modeling analysis to be useful, relevant design questions must be identified, the model inputs must be reasonable and appropriate, and the outputs need to be interpreted appropriately. Limitations of the modeling tool also need to be understood. Therefore, a successful analysis is usually a team effort.

Design team members have an important role in helping to identify appropriate design questions and in reviewing input assumptions to make sure they reflect likely design and operation as closely as possible.  

The energy modeler might be an architect, engineer or specialist consultant. Whoever performs the modeling, it is important that either the modeler has a strong understanding of the simulation tool or that the work be reviewed by someone with that experience. Another important qualification is that the modeler understands how real buildings perform relative to simulations. This understanding often comes from experience with modeling existing buildings and from calibrating energy models to actual energy consumption. This experience helps the modeler to identify realistic inputs and to judge reasonableness of results.

Simple Box Modeling Approach

This section provides suggestions for steps to follow in a simple box modeling analysis.

1. Determine initial design questions to be evaluated
2. Identify design constraints
3. Select energy modeling tool
4. Identify model inputs
5. Develop a base case simple box model
6. Create alternative models
7. Analyze the outputs
8. Present the results
9. Iterate

Determine initial design questions to be evaluated

Spend some time before creating a model to list the design questions to be explored, and if possible discuss that list with the design team. This time is well spent because it will help focus effort on the aspects of the design that are in question. It also helps in choosing an appropriate modeling tool. For example, if understanding potential daylighting savings is important, then a tool that represents daylighting performance is important.

Refer to the Learn by Design Task section of this website for numerous examples of how BEM is used to inform specific design tasks during Pre-Design and Conceptual Design.

Identify design constraints

Example Site Constraints, Including Maximum Dimensions and Adjacent Structures

Identify whether site conditions or other factors lead to constraints to consider when setting up simple box modeling alternatives: height limits, footprint constraints, structural system requirements (e.g steel vs. concrete), adjacent buildings, or owner requirements such as access to views.

Select energy modeling tool

Whole-building, annual simulation modeling tools

The appropriate type of software for simple box modeling is most often a whole-building simulation tool that runs a full year of hourly or sub-hourly calculations of thermal loads and HVAC system energy consumption. Results include estimates of energy consumption and demand for the whole building and by end use as well as estimates of heating and cooling loads.

Other calculation methods can also be useful and may provide a different perspective on building performance. One example is peak heating and cooling load calculations. These load calculations are used for sizing HVAC equipment and can be used with a simple box model to inform envelope design, but they do not provide an estimate of annual loads or energy performance. Another example is daylighting simulation software, which also provides useful design feedback at an early stage but also does not typically provide energy performance.  

Useful whole building energy modeling tool features:

• Automation to create building geometry and thermal zoning
• 3D geometry viewing for model review
• Default values for envelope constructions, internal loads and operating schedules that are visible to the user and are based on building type and the applicable energy code or building vintage
• HVAC system templates that represent standard system types and code-complying performance
• Capability to modify default inputs
• Automation to create and to run design alternatives
• Capability to calculate energy cost based on time-of-use utility rates and demand charges
• Formatted output reports
• Output data available in convenient format to import into a spreadsheet for custom reports
• Flexible hourly output capability

See also the Building Energy Software Tools directory.

Other calculation methods

Other calculation methods can also be useful and may provide a different perspective on building performance. One example is peak heating and cooling load calculations. These load calculations are used for sizing HVAC equipment and can be used with a simple box model to inform envelope design, but they do not provide an estimate of annual loads or energy performance. Another example is daylighting simulation software, which also provides useful design feedback at an early stage but also does not typically provide energy performance.  

Identify model inputs

As much as possible, identify project-specific input values rather than rely on defaults. Review important inputs with the design team.

See also these pages:

• Preparing Model Inputs - provides a detailed set of instructions for gathering input information, and how to use it in the simulation software tools. These instructions include specific guidance on assessing the "right" level of detail for early-stage simple box models.
• Presenting an Input Summary - provides tips on how to present input data and assumptions to the design team to ensure it is valid for the analysis

Develop a base case simple box model

The starting point for analysis is often a model with code-minimum inputs for envelope performance, lighting and HVAC efficiency. However, the design team may have other preferences for the level of performance in the base case mode, perhaps based on a previous design or the firm’s typical design standard. As noted earlier, this model might be a simple box shape or might represent a specific design concept.

The specific details of how to build the model are presented in the Preparing Model Inputs section of this website.

Create alternative models

During early-stage analysis, BEM models are used to compare design options. After creating the base case model, the next step is to create alternate versions of the model that represent the different design options being considered by the design team.

Example Parametric Analysis of Window Area and Overhang Shading

Electricity Impacts of Window Area and Overhang Shading Alternatives

Peak Load and Cost Impacts of Window Area and Overhang Shading Alternatives

Selecting design options

The art of simple box modeling is in defining appropriate alternatives to answer the design questions that were identified earlier. The alternatives will represent different design concepts and may cover a range of performance to understand sensitivity of results to changes in design.

Alternative design concepts might include different options for building form, such as  tall vs. short or wide vs. narrow. Concepts could also include alternative heating and cooling approaches, such as radiant or air-based systems and overhead or underfloor air delivery.

It may be appropriate to evaluate different structural systems -- such as steel, wood and concrete -- that affect heat transfer and thermal mass. Natural ventilation is another potential concept for evaluation.

Sensitivity analysis

Alternatives set up for a sensitivity analysis are also good candidates for simple box modeling. For example, models covering a range of wall insulation, perhaps including extremes of no insulation and very high insulation, provide useful information about the importance of wall performance. If other models cover ranges of window performance and roof insulation, then the results will show the relative importance of the design elements and help the design team prioritize where to invest most effectively in performance. Sensitivity analysis is also useful for evaluating the impact of window area or building orientation.

The adjacent figures show an example sensitivity analysis that includes three different window sizes, with and without overhangs. The graphs of results show that lighting energy decreases as window area increases because the model includes automatic daylighting controls. Cooling and heating energy increase, though the cooling penalty is partially mitigated with overhangs. The analysis also shows that the building’s peak cooling load increases significantly with larger windows. 

Evaluating combined impacts of "measure packages"

Example Results Showing Efficiency Measures Added Incrementally

Another element of the art of simple box modeling is creating models with packages of alternatives to estimate combined impacts. One brute force approach is to model all possible combinations, an approach that was once impractical due to the computation time required but now can be possible through use of cloud computing and tools that can run large batches of simulations. Otherwise, a useful approach is to use some judgment based on your sensitivity analysis to select appropriate levels of performance for different building elements and then add them one by one to the base case model. That incremental approach shows how each element contributes to the total savings, as illustrated in the adjacent figure.

Analyze the outputs

Analysis of the energy model outputs serves two important roles: 1) verifying the quality of the model and 2) answering design questions and providing insights.

See the section Analyzing Model Outputs for detailed guidance.

Present the results

Effective presentation of results may be the most important step in the process. Presenting the results clearly and in a manner that directly addresses a particular design question or objective will provide the most impact. The Learn by Design Task section of this site is devoted to applying BEM effectively for a wide range of design objectives and includes much more guidance on presentation approaches.

See also an overview of Presenting the Results.

Iterate

The design process is rarely linear. Plan to repeat the analysis with refined inputs or additional alternatives based on feedback from the design team.

Transitioning to later modeling phases

Much of the time that goes into a simple box modeling exercise is for tasks other than actually building the models. It takes time to become familiar with the proposed building program and with project-specific constraints and requirements. It also takes time to gather information to inform model inputs and to define the alternatives to be evaluated.

In an ideal world, the simple box model could be seamlessly transformed into a more detailed model as the design is refined. The actual process will vary among simulation tools, but often the quickest path at later stages is to start over creating the building geometry model with the more detailed design information. Fortunately, project knowledge gained during the simple box modeling exercise often helps modeling proceed faster in those later stages.