Compare Massing Options
A BEM analysis of massing alternatives will often compare options developed by the architect, but a massing study can also be very valuable even before any specific design options have been identified. In the latter case, the analysis helps the architect understand the relative performance of typical massing alternatives and informs their early design decisions.
Impact of Massing Decisions
Early decisions on building massing can affect building performance in many ways. Many of the impacts are associated with variations in the exposure to exterior conditions. Narrow massing options, with a high ratio of perimeter space to interior space, will perform differently from massing options with more interior space. Taller buildings will also perform differently than a shorter building of the same floor area. And the magnitude of impacts will vary with climate and building occupancy type. BEM is especially useful to assess the following impacts of massing alternatives:
- Heating and cooling loads and the associated energy consumption and demand.
- Daylighting energy savings potential.
- Natural ventilation feasibility and savings potential.
BEM is sometimes also used to assess occupant thermal comfort, which can vary among massing alternatives due to variations in exposure to exterior surfaces and access to operable windows.
And while they are not typically assessed using BEM, the following impacts may also be important to consider when comparing massing alternatives.
- Roof area and other surfaces available for installation of solar energy systems.
- Views and visual comfort for occupants.
- Impact on adjacent buildings due to solar shading and reflection.
- Embodied carbon of building materials.
A BEM analysis of massing alternatives will often compare options developed by the architect, but a massing study can also be very valuable even before any specific design options have been identified. In the latter case, the analysis helps the architect understand the relative performance of typical massing alternatives and informs their early design decisions. When massing alternatives have not yet been developed, consider creating models with variations in the following:
- Aspect ratio (the ratio of floor plan length to width)
- Number of stories
- Floor-to-floor height and ceiling height
Some factors to consider when identifying massing options for evaluation:
- Site conditions may offer unique opportunities or constraints.
- Typical perimeter zone depth is 12 to 20 feet, which means that narrow floor plans with width of 50 feet or less will have a high ratio of perimeter area to interior area. (See Defining the Building Geometry for more discussion of thermal zoning)
- Features such as courtyards, atriums and narrow wings can increase the fraction of floor area with exterior or daylight exposure.
- Buildings with larger footprints and fewer stories have larger roof area, which increases potential for daylighting via skylights and increases area available for solar energy systems.
- Massing options with large north and south exposures provide more opportunities for effective solar shading than options with large east and west exposures.
Guidance on Modeling Approach
A BEM massing study typically uses a simplified approach such as that described in the Simple Box Model page. When selecting inputs for the models, refer to guidance on Preparing Model Inputs. See especially these topics:
- Geometry. Make appropriate model simplifications, including thermal zoning: Defining the building geometry.
- Fenestration. Choose appropriate window area and performance: Define fenestration.
- Opaque constructions. Define appropriate construction types and insulation levels for the walls and roof: Define opaque envelope constructions.
- Internal loads. Select reasonable values for occupancy, lighting and plug loads; and include automatic daylight controls in the model so that the potential for daylighting savings is reflected in the analysis: Define internal loads and Sources for default internal gain assumptions.
- HVAC. Select an appropriate HVAC system and consider the implications of HVAC system choice for simple box modeling.
- Ventilation. Pay attention to choosing appropriate ventilation rates, especially for occupancy types with high outdoor air ventilation requirements, such as hospitals and laboratories: Ventilation rates.
As in any BEM analysis, a model quality review is important. See Review_and_analysis_to_verify_model_quality.
Guidance on Presenting Results
For a massing study to have a useful impact, the presentation should focus on results that are important to the project team. These results could include EUI, energy cost, peak cooling load, or other metrics. BEM results that are typically found useful are described in this page: Analyzing Model Outputs. Other important considerations:
For more inspiration in developing visual presentation of massing study results, see examples on the website Project Stasio such as the following: