Defining the building geometry

The input process of defining the building geometry is similar for the different model types, but the level of detail will vary depending on the design phase. Generally, early stage analysis will use simpler geometry while later stages will increase the level of detail to more closely match the actual design. This tutorial provides a general input approach along with considerations specific to the different model types and design stages.

General approach

The model geometry is a representation of the building's design. The exterior "shell" represents what the building looks like from the outside. The shell is divided into building stories. Each story is further divided into zones which represent an area of each story that is heated and/or cooled.

Thermal zoning

Generally speaking, it is not a good idea to model the whole building as a single zone, nor is it a good idea to represent a whole building story as a single zone. This is because the exterior zones of each story are subject to greater heating/cooling loads through the building envelope than the interior zones. In many cases, the exterior zones may need heating while interior zones need cooling. If a building story is modeled as a single zone, these factors may cancel each other out and result in underestimation of heating and cooling energy.

Zones, rooms, and spaces

Some software tools make a distinction between "rooms" (or "spaces") and "zones." A room or space represents a room (or corridor, etc.) in the building whereas a zone is a part of the building that has its temperature controlled by a thermostat. A zone may be a single room, but it often consists of multiple rooms or spaces, all controlled by the thermostat located in one of the rooms/spaces.

Software tools that include both concepts will let the user create a floor plan with all of the rooms, then group a subset of rooms into larger thermal zones. However, it is often acceptable to simply draw the zones and omit the level of detail of the individual rooms/spaces, especially during early stage analysis.

Guidance for defining zones

It is important to understand when spaces can be grouped into a single zone and when they cannot. During detailed design phases, it may be clear because HVAC systems and theromostat controls may be defined. But earlier in design, some judgment is needed. If a conceptual design exists, then spaces may have been defined that have significantly different envelope conditions, internal loads or operating schedules. As an example, consider a school building, where the administrative offices operate on a different schedule and with different internal loads compared to classrooms or multipurpose rooms. In that case, it would generally be appropriate to create separate zones representing classrooms and the other space types. Some judgment is needed to determine if defining those spaces as separate zones is necessary based on the purpose of the simple box modeling exercise. In many cases, a simple perimeter/core zoning pattern is adequate for evaluating relative performance of alternatives.

Some guidance for when separate zones might be appropriate is provided in Appendix G of ASHRAE Standard 90.1.

• Peak internal loads (e.g. lighting, plugs loads and occupant heat gain) vary by more than 10 Btu/hr-ft2 (2.9 W/ft2)
• Operating hours vary by more than 40 hours per week
• Spaces are served by different types of HVAC system

3-D geometry model

From a practical standpoint, most modeling tools represent the building geometry as a collection of zones that, together, represent both the shell and zone layouts. In other words, when creating the model, users will build the building model zone-by-zone and floor-by-floor rather than drawing the shell and then subdividing it.

The approach generally involves importing a drawing of a building's floor plan, tracing the boundaries of each room or zone, and setting the floor-to-floor height for each zone. Exterior walls are generally traced at the exterior surface. Interior walls are generally traced at the center-line of the wall between an adjacent zone. The wall thickness is generally not explicitly represented, although some software tools do model the wall thicknesses so be sure to consult the software documentation. Some software tools allow an entire floor to be created at once by extruding a one-line diagram of the floor plan into 3-D zones of a specified height. Other tools require the zones to be traced one-by-one.

Be careful to ensure that adjacent zones do not have any gaps between them. Gaps may be considered "exterior walls" by the software program which would lead to unrealistic exterior envelope loads.

Adding subsurfaces (windows, doors, skylights)

Once the zone geometry is created, then exterior windows, doors, and skylights are added as sub-surfaces within exterior walls and roofs. See also: Define fenestration (glazed constructions).

2-D geometry model

It is also possible to define geometry in a more simplistic "data entry" manner. This approach requires users to input floor areas, wall areas (also orientation and tilt), and roof areas associated with each zone without actually drawing the zone geometry. This approach tends to be used by older simulation tools, or tools that specialize in load calculations but it is still fairly common. A notable limitation of this approach is that it does not have the ability to account for shading from adjacent structures, nor the ability to account for daylight controls.

Simple box model approaches

The fundamental idea of a simple box model is that it is simple: a simplified geometrical representation. An obvious reason for simplification is that the model is developed before or during conceptual design, when most details of the building geometry are still in flux. Another important reason is that a simplified model usually takes less time to create, and quick feedback is valuable in the early design stages. And additional geometry detail often does not provide a more useful result.

If a conceptual design for building massing exists, then the modeler needs to determine a reasonable level of building geometry simplification. This is clearly a case where judgment is necessary because there is usually a tradeoff between time spent creating a model and model precision. At this early stage, precision is seldom critical, and the primary goal should be to accurately represent important factors, such as those listed below, while avoiding spending time on minor details.

• Floor area
• Wall and window area by orientation
• Roof area
• Significant shading elements
• Thermal mass

Knowledge of how your simulation tool performs its thermal calculations is helpful when making decisions about how to make the model as simple as possible while still being reasonably accurate. In most tools the important geometry factors are surface area, orientation and tilt.

Window simplification

Example Window Simplification (Requires attention to window U-factor input to ensure impact of window frames is considered and attention to daylight savings calculation if automatic daylighting control is modeled)

The example in the adjacent figure shows multiple windows being simplified to a single window with the same area, orientation and tilt. In most cases, this simplification will not significantly affect accuracy, as long as the thermal impact of window frames is considered. However, some programs can estimate daylight illuminance and calculate electric lighting savings due to automatic daylighting controls. Those daylight illuminance calculations may be affected by the dimensions and location of windows, so that impact should also be considered.  

Shading simplification

Example Window Shading Simplification

Window shading features, such as louvers or overhangs, can be simplified in some cases. The adjacent figure shows an example with the impact shading louvers approximated as a single overhang with the same projection factor.

Roof geometry simplification

Example Roof Geometry Simplification

Other building geometry features can often be simplified without significantly affecting the analysis. The adjacent figure shows an example of roof geometry simplification, where the sloped roof is approximated with a flat roof. Unless the roof design is a specific subject of study, this simplification is unlikely to have much impact on analysis of the relative performance of other design features.

Massing simplification

Example Building Geometry Simplification

This example shows where some details of the building geometry are ignored and the building is represented instead as a simple rectangle with wall and window area approximately equal to the detailed model. This type of simplification might reduce the time needed to develop the model and provide more time for analysis.

Thermal zoning simplification

Perimeter/Core Thermal Zoning Example - Rectangular Footprint

Perimeter/Core Thermal Zoning Example - Other Footprints

Separate Thermal Zones for Ground, Middle and Top Floors, with Multiplier Applied within Software for Middle Floor Loads

Deciding on a thermal zoning layout is often a first step to developing an energy model, and judgment is necessary at all stages. Even in detailed energy models, some simplification is typical, and multiple HVAC control zones are often combined into single thermal zones to reduce model complexity and simulation time.

At the time of a simple box model analysis, it will rarely be the case that HVAC control zones are already defined. Therefore, simplified perimeter/core thermal zoning is typically applied. Perimeter zones are defined that extend from exterior walls to a depth of 12 to 20 feet, and separate perimeter zones are defined for each orientation. Spaces more than 12 to 20 feet from the perimeter are defined as separate core zones.

In the typical case for a simple box model, a rectangular building has 5 zones per floor: one for each of the four orientations and one for the core space as shown in the adjacent figure. For multi-story buildings, a separate set of zones should be defined for the ground floor and top floor because they experience different loads compared to middle floors due to additional ground floor heat transfer from the floor slab (or basement, or crawlspace) and top floor loads from the roof. Middle floors are all adjacent to spaces with similar temperatures so heat transfer between them is minimal. In some software tools, the middle floors can be represented by a single floor with a multiplier on results, and this approach is generally acceptable for simple box modeling.

In some cases, additional thermal zones beyond the simple perimeter/core configuration will be appropriate. If a conceptual design exists, then spaces may have been defined that have significantly different envelope conditions, internal loads or operating schedules. As an example, consider a school building, where the administrative offices operate on a different schedule and with different internal loads compared to classrooms or multipurpose rooms. In that case, it would generally be appropriate to create separate zones representing classrooms and the other space types. Some judgment is needed to determine if defining those spaces as separate zones is necessary based on the purpose of the simple box modeling exercise. In many cases, the simple perimeter/core zoning pattern is adequate for evaluating relative performance of alternatives.

Pre-design considerations

If a conceptual design does not yet exist, then further judgment is required when developing building massing. A first consideration is whether there are site conditions or constraints that limit building form. A second consideration is whether the design team or owner has preferences for building form. In the absence of other guidance, a reasonable source comes from a national laboratory project to create reference models for the commercial building stock. See the table below for suggested values from that project for aspect ratio, floor-to-floor height, and window-wall ratio (glazing fraction). Another source for window-wall ratio is Appendix G of ASHRAE Standard 90.1-2019, which includes a table of values for different building types, which are applied to the baseline building for compliance calculations.  

Commercial Reference Buildings - Assumptions for Building Form
(Source: U.S. Department of Energy Commercial Reference Building Models of the National Building Stock, Table 13. 2011)

Building Type	Floor Area (ft2)	Aspect Ratio	No. of Floors	Floor-to -Floor Height (ft)	Floor-to- Ceiling Height (ft)	Glazing Fraction
Small Office	5,500	1.5	1	10	10	0.21
Medium Office	53,628	1.5	3	13	9	0.33
Large Office	498,588	1.5	12	13	9	0.38
Primary School	73,960	E-Shape	1	13	13	0.35
Secondary School	210,887	E-Shape	2	13	13	0.33
Stand-Alone Retail	24,962	1.3	1	20	20	0.07
Strip Mall	22,500	4	1	17	17	0.11
Supermarket	45,000	1.5	1	20	20	0.11
Quick Service Restaurant	2,500	1	1	10	10	0.14
Full Service Restaurant	5,500	1	1	10	10	0.17
Small Hotel	43,200	3	4	11* 9	11* 9	0.11
Large Hotel	122,120	3.8* 5.1	6	13* 10	13* 10	0.27
Hospital	241,351	1.3	5	14	14	0.15
Outpatient Healthcare	40,946	1.4	3	10	10	0.19
Warehouse	52,045	2.2	1	28	28	0.006
Midrise Apartment	33,740	2.7	4	10	10	0.15

* First floor

Baseline Window-Wall Ratio from ASHRAE Standard 90.1-2019, Appendix G

Building Area Type	Window Wall Ratio
Grocery store	7%
Healthcare (outpatient)	21%
Hospital 27	27%
Hotel/motel (≤75 rooms)	24%
Hotel/motel (>75 rooms)	34%
Office (≤5000 ft2)	19%
Office (5000 to 50,000 ft2)	31%
Office (>50,000 ft2)	40%
Restaurant (quick service)	34%
Restaurant (full service)	24%
Retail (stand alone)	11%
Retail (strip mall)	20%
School (primary)	22%
School (secondary and university)	22%
Warehouse (nonrefrigerated)	6%

Detailed design input data

Detailed design model geometry is intended to represent the project design in sufficient detail that represents the design in later phases such as the design development and construction documents phases. Compliance models are generally representative of the final design.

Model geometry should be based on architectural floor plans, elevations, and section drawings. Alternately, a 3D architectural model may be used.

Architectural floor plans

Example architectural floor plan. (Source: IBPSA-USA BEM Workshop)

Floor plans show us space layouts and dimensions. In some cases, they will also have notes that indicate construction types for walls. Floor plans are needed to define the thermal zone layout, which is one of the first steps to consider when setting up a model. The adjacent image is an example of an architectural floor plan, but you may also want to look at the mechanical floor plan, which also includes locations of ducts and diffusers, to identify actual thermal zones.

Note that it is usually ok to use a simplified thermal zone layout in your model. In other words, you might consider combining some of the actual zones into larger single zones in your model for sake of simplicity. See below for additional information.

Another topic to note here is that there are different ways to get this geometry information into your BEM software. Most BEM software tools have a drawing tool that allows you to draw your thermal zones. One approach is to read measurements off the plans and then use the drawing tool to create your model. A time-saving option available in some tools is that you can import the floor plan and then trace over it, which saves some time and allows for more precision.

Thermal zoning strategies

Example of combining similar spaces into thermal zones, and dividing large spaces into multiple thermal zones. (Source: IBPSA-USA BEM Workshop)

The detailed model does not need to exactly represent the architectural drawings. In some cases, similar rooms can be combined into a single thermal zone. Large spaces in the building may need to be divided into multiple, smaller thermal zones. Generally, the mechanical (HVAC) drawings can provide some guidance—thermal zones typically are the areas in the building controlled by thermostats.

Some considerations when grouping or dividing spaces for thermal zones include:

• Window area and orientation.
• Internal loads and usage schedules.
• Perimeter vs. interior location

Elevation drawings

Example architectural elevation drawing—This example has a few different wall surfaces, including some panels of glass-reinforced concrete and some metal panels. (Source: IBPSA-USA BEM Workshop)

Elevations are useful for determining several different types of information about the building facade:

• They show dimensions that indicate the floor-to-floor height
• They show the size and location of windows
• They often include notes that identify the type of glazing in each window
• And they may include notes that indicate the type of wall finish

Section drawings

Example architectural section drawing. (Source: IBPSA-USA BEM Workshop)

Section drawings show the ceiling height in addition to the floor-to-floor height.

3-Dimensional architectural model

Example architectural BIM model. (Source: IBPSA-USA BEM Workshop)

Many architectural design projects are developed using 3-D Building Information Models (BIM) as an alternative to traditional 2-D CAD drawings.

There are tools and workflows that allow for translation of geometry information from 3D BIM to BEM software.

One thing to note is that BIM includes a lot of highly-specific details about the architectural design, and much of it is not needed for the energy model.

That means that, in most cases, there needs to be some simplification of an architectural BIM model before the information is exported to be used as input to an energy model. Otherwise, geometry input errors are likely to occur.