Conceptual Design is when the architect begins to define the form, scale, mass and general appearance of a building within its surrounding context. It can be as simple as a series of sketches, or it can be more a more detailed set of 3D models and other materials to help communicate the design vision to the client. It is typically an iterative process where the architect works with the client and other team members to refine the concepts and prepare for the latter phases of the project.
Sometimes architects will develop the conceptual designs during the proposal phase (before being selected by the owner) as a way to demonstrate their design philosophy and processes. Since this work may begin before a contract is awarded, the BEM activities in this section are often performed by the architects or, an engineering or BEM consultant on the bidding team may assist the architect "at risk" to help win the project.
BEM can provide valuable information at the earliest stages of design, even before the creation of the architect’s first “cocktail napkin” sketch. These design kickoff steps help orient the designers to a project’s unique performance opportunities:
Establish or confirm project goals/targets
At the start of conceptual design, confirm the performance goals established in pre-design. Or, if performance goals were not set during pre-design, do it now!
Assess likely energy use patterns
Identify likely patterns such as end-use energy breakdown and seasonal consumption. This information provided early in the design process can help the design team to focus efforts on high priority efficiency strategies. There are several approaches to identifying energy use patterns before a concept design has been developed.
- Develop a simple box model, then analyze the outputs to find insights about its efficiency opportunities.
- Run a prototype model
- Find performance data for similar buildings in reference sources such as ASHRAE's Advanced Energy Design Guides
- Find benchmark data for your building type
Design charrettes are interactive brainstorming sessions held early in the design or programming stages of a project. "[They] place the client and other stakeholders, as well as all or most of the professional disciplines that will contribute to the project, in the same room for, usually, one to three days." 
Goals of the charrette include "kicking-off the integrated design process, reviewing project expectations, and exploring design strategies that are most appropriate to achieve a project’s sustainable design goals."
An "energy charrette" held early during design is one of the requirements of ASHRAE Standard 209. Some of the required topics include determining the purpose of the BEM in the project, defining baselines, establishing perfromance metrics, reviewing benchmarks and generating a list of ECMs.
BEM Practitioners can assist the charrette process by helping to:
- Brainstorm and vet ECMs
- Perform pre-charrette analysis
- Perform analysis during the charrette
- Perform post-charrette analysis
Read more about Using BEM to support a charrette.
Building Form and Mass
Massing refers to the three-dimensional shape of the building, and may also be called the building's form. Decisions about the form and mass of a building can have meaningful energy impacts that are permanent (you can't easily retrofit the building's footprint), so BEM analysis in this phase is extremely important though it is often overlooked.
BEM practitioners can assist with the following tasks:
- Compare massing options - If multiple massing alternatives are being considered, a model can assess their relative performance in terms of EUI, energy cost, peak cooling load, or other metrics important to the project.
- Analyze the building's orientation - determine how the building's orientation on the site (with respect to sun exposure and prevailing wind conditions) may impact its performance.
During this phase, many design features will be evaluated for inclusion in the project. These may be structural features, features of the facade, concepts related to passive design features such as natural ventilation or daylighting, and types of active systems (e.g. HVAC, lighting) that are desirable for the project. The BEM practitioner can be instrumental in evaluating different options and demonstrating their relative performance so that the design team can make informed selections. Some options, such as HVAC system type or natural ventilation, may have some bearing on the building's form due to floor-to-floor height requirements or depth of footprint. Additionally, an assessment of thermal comfort associated with design choices can begin at this stage.
For projects that are considering multiple options of form and mass (as discussed above), some design features may perform well on one massing option but not as well on another, so it is important that design features are analyzed within that context.
BEM can be used to analyze design features in many categories:
- Constructions and thermal mass options
- Fenestration and daylighting options
- HVAC system options
- Natural ventilation design features
- Passive solar design features
- Renewable energy options
Assess Design Performance
After the project team has developed a design concept, then BEM can be used to assess its performance. This assessment can be useful at any point during the conceptual design process. For example, it can serve as a first step before evaluating alternatives, when it can help to establish a performance baseline and highlight opportunities to explore. It can also be used to track performance compared to project goals at any point along the way.
There are some important points to keep in mind when using BEM to assess performance during early design.
- There is significant unavoidable uncertainty; many inputs will be based on educated guesses about later design decisions and building operation.
- Any reporting of energy use predictions should note the uncertainty.
- Running multiple versions of the model with a range of inputs for uncertain variables (e.g. plug loads, schedules of building operation) can be useful to show how predicted energy use will vary depending on assumptions made at this stage.
- If an energy consumption prediction is desired, then extra attention from the owner and design team is needed to review model inputs and ensure that assumptions represent likely building operation.
- BEM will be most useful at this stage for comparing the relative performance of design concepts.
Compare performance to targets
BEM analysis is vital to ensuring that the design is on track to meet the project’s performance goals. Knowing the current design’s performance helps to inform further design decisions. While there will be uncertainty at this early stage, the BEM results are still useful as a preliminary check on design performance. The results will be especially useful for comparing how different concepts contribute to meeting targets.
Identify priorities for improvement
Every project is different and may have unique opportunities for improvement. Those opportunities will vary depending on factors like climate, site, building occupancy and owner preferences. BEM can be used to identify which elements of a design concept are most critical to performance and inform further conceptual design decisions. Read more about using BEM analysis to provide insights.
Assess heating and cooling loads
Many of the decisions made during the conceptual design phase affect the heating and cooling loads that need to be met by the HVAC system. These include decisions like building form, orientation, window area, and construction types. BEM can provide a preliminary estimate of the peak heating and cooling loads for a design concept. These loads are important because they affect the size and cost of the HVAC system.
Use BEM to assess a design in terms of maintaining comfort conditions when active HVAC systems are not available. BEM is most commonly used to estimate energy consumed by HVAC systems to maintain indoor comfort conditions. However, BEM can also be used to estimate indoor temperature and humidity when HVAC systems are not operating. This type of analysis could be useful to compare design concepts with, for example, different amounts of thermal mass, different levels of insulation or different natural ventilation strategies. It is important to note that this is an advanced analysis and an energy modeler performing this type of modeling should be very familiar with the calculation methods used by their simulation tool.