If you’ve been around the design industry as long as I have, you’ll know that designing more efficient buildings is not a new concept. I got my start back in the early 1980s when solar and underground homes were becoming the rage, leveraging thermal mass walls, solar water heaters, and passive cooling strategies. But the design technology of the day didn’t do much to support the studies of these strategies, unless you were investing in high-end modeling hardware that filled up a trailer and took a small bank to finance.

As BIM became more prevalent in the market, the desire to leverage these design models for more than just coordination and better documentation has increased. The U.S. government introduced the Energy Conservation & Production Act (ECPA) that established minimum efficiency requirements for federal buildings. If you are designing new commercial and residential federal buildings, they must meet or exceed these efficiency standards, which are based on the model energy codes such as ASHRAE Standard 90.

So how do we meet these requirements while minimizing the financial impact on standard design practices? By leveraging the BIM model, you can achieve more accurate results, study different design alternatives, and make more effective decisions earlier in the design process. But it’s all based on the theory that you are creating the model as effectively as possible.

Let’s get started by taking a look at setting up your Revit model for better results using exported data and features.

Project Setup and Modeling Tips for Best Analysis Results

Prior to starting your model, you need to understand what the requirements are for the project, and then decide what the source model will be used for the analysis. It breaks down into two options — an architectural model, driving by masses or bounding elements (including walls, doors and windows, roofs, floor slabs, etc.); and the MEP model, that usually incorporates a linked MEP model, and added spaces and zones with greater detail.

Before the Project Setup

Begin with the energy model requirements. For every project, you need to establish a baseline, including the following items:

1. What type of energy modeling and what details are required?

While some projects require the entire scope of the project examine energy usage, you may only need to be examining specific portions of the load requirements, such HVAC heating and cooling load. Set these requirements at the start of the project to provide a clear set of tasks needed.

2. What are the Federal/State/Local governing agency requirements the design must meet?

These can vary widely based on the location of the project. The baseline established by the governing body should set the goals for the project. For example, the U.S. federal government has its own requirements for commercial buildings that new construction projects must comply with, in order to be approved.

3. What are the expected Energy Use Intensity (EUI) metrics?

Energy use intensity is defined as the amount of total energy consumed by a building in one year (measured in kBtu or GJ), divided by the total gross floor area of the building. The values for each measure, including lighting, power, HVAC and other loads, can be expressed in a form of charts defined by the BEM application:

4. For Heating/Cooling Load requirements, can you compare systems to determine Annual Energy Costs?

The types of building service systems have expanded over the past few decades, as more efficient technologies are developed and produced. The baseline for using these technologies must be balanced with a return on investment, or reasonable payback versus initial implementation costs.

5. Can you examine Potential Energy Savings based on structure design/conditioning systems/passive equipment?

In addition to traditional heating and cooling loads, the ability to review renewable resources, such as solar, geothermal and wind systems should be included in the overall energy usage for the project. As with newer systems, a sustainable ROI should be clearly defined.

6. Are there opportunities for Water/Sustainable Material reuse?

Other areas that can be addressed in a BIM project such as Revit include the ability to examine other areas of efficiency. For example, does the roof and site design support the collection of greywater or rainwater for site re-use? If sustainable materials are used, do they contain adequate thermal properties when compared to new materials? Is there a cost benefit for using recycled materials in the project? The key is to make sure the project files can provide the right data to measure the impact on the design.

Setting up the Project

When you are defining the project, there are several items you should consider reviewing. Some items can be predefined in your template, while others are going to be specific to the project. There are architectural and MEP settings that can make a difference on your analysis results, so let’s start by looking at a few key items.

Template Settings and Project Files

The architectural template can include several settings that save the user time when setting up a project. These settings are set as defaults, but can be adjusted based on specific project needs.

Area and Volume Calculations — this setting controls how Revit measures room information. If you are going to use spaces in conjunction with rooms, or rooms as the exported volumes, you must set the volume computations to calculate both areas and volumes:

Note: Leave this set to areas only if you are not using Revit geometry for energy modeling, as it does affect overall project performance.

Material Thermal Properties — Revit automatically recognizes the wall, floor, ceiling and roof types and openings that are adjacent to a room. Over the past few releases, the addition of new materials, and assigned thermal properties, has enhanced the quality of the model properties. Including them in your template and standard wall types saves the time of adding them after the fact.

For example, you could include a brick material assigned to a standard wall type, and these properties would already be included:

Building/Space Type Settings

If you’re using the MEP spaces and zones for calculations, your template will include a list of building and space types that are applied to the energy settings. While you can’t edit the default list of types (yet), you can change default values, and re-use existing types for other room types. The building space types are located on the Manage tab, Settings panel under MEP Settings:

- The Building/Space Type Setting dialog lists the default building types and space types based on the ASHRAE standards. Each item include parameters to help determine energy loads, which in turn are used to help define HVAC loads. Selecting a type expose the parameters, which can be edited by a user for specific conditions:

• One of the newer settings includes the ability to set a building’s unoccupied cooling set point, which determines the temperature for the building outside of the opening/closing times.

The building types are applied globally, but you can edit specific space settings for the space type to refine the results.

• Occupancy, lighting and power schedules can be edited in the template for default settings, and these can be customized to add, rename and remove types:

By editing these in your MEP template, you can get better results for your EUI values.

External Files Associated with Analysis

Revit includes a Constructions.XML file, which lists the materials you can select from the conceptual constructions section of the energy settings, and defines the material name, description, and U value of the material:

The file can be edited with a text editor, by copying and pasting sections from </Layer> to </Layer>.

Note: examples in several languages are included in the program folders, under C:\Program Files\Autodesk\Revit 2017\en-US for example, for English — US standards.

Project Setup Workflow and Features

Once you get items setup for your template, and program, there are a few items you need to do when you get into a project. Following these steps will help to improve the analysis results and reduce the number of steps needed outside of Revit.

Site-Oriented Projects

When a project is located in conjunction with a site, and placed at its real location in the world, several items are address immediately. This includes items such as the North orientation of the building. Since this is specific to the project, you would define these settings during project setup. The location, coordinates and position tools help provide an accurate setting for the project:

When working with site oriented projects, you can acquire the coordinates from a linked Civil 3D CAD file, and rotate views to create orthographic layouts. But having the building correctly oriented relative to the site ensures that calculations for wall, window, door, roof and shading loads are correctly calculated.

Not all applications can leverage this information, including elements that exported via gbXML, but internal modeling tools can take this information into account.

Conceptual Versus Building Element Analysis

The first step in preparing your project is to determine what Revit objects will be used to perform the analysis. There are two approaches — using conceptual shapes and masses, and using actual bounding elements such as walls, openings, floors and roofs. The time to make this decision is at the start of the project — not halfway through! Each project will require different workflows, but using a conceptual model that can be carried forward into design model is a functional workflow within Revit.

Conceptual Masses

If you make the decision to use conceptual masses and elements for the BEM process, you need to understand the basic elements, and the best time to use these components.

The conceptual masses usually consist of core extrusions, sweeps/blends, and meshes. These items can help with initial placement of buildings and structures, and can be used for early solar studies and shadow impact. The models are most often formed as a low level of detail/development, typically LOD 100.

- Site orientation is key in these early studies, which are also helpful with new work projects. If the project is an urban settings, massing studies provide a much needed benefit. By using a mass to simulate an existing structure, the effect on a new building can be translated to the energy analysis model. In this case, you want to use masses early in the pre-design process, and use conceptual constructions to set the most common interior and exterior conditions.

• The key to making the conceptual massing approach work is to leverage the masses to develop floor plates. This is accomplished by selecting the mass, and then clicking Mass Floors:
• The mass floors are defined by the levels in the project. These help break the conceptual mass up into logical areas, resulting in a more accurate conceptual energy model:

- Mass floors can also be added to and solid geometry in the model based on the levels, providing some base level of geometry, even when the building isn’t a box.

So why use the conceptual mass? It produces the fastest early results, and has the simplest form of translation into other analysis tools.

Keep in mind that conceptual masses are best used for overall energy modeling studies. For more detailed heating/cooling loads, lighting and solar analysis, you will need to have building elements placed.

Building Element Modeling

When you need to have a more detailed analysis performed, you need to make sure you include the following items:

- Architectural model including Walls/Doors/Windows — and FLOORS, CEILINGS and ROOFS

Bounding elements are usually where most designs begin, but a common mistake is to over model. Unless you know exactly what the construction types are going to be, it’s fine to start with generic components:

Here are a couple of important tips:

• DO make sure walls are properly connected, and gaps are minimized.
• DON’T worry about column enclosures, bumps, cutouts, reveals, wall tape, etc. Too many objects within a room can actually add unneeded complexity.
• DON’T ignore the vertical design — if you have floors, ceilings and roofs, get them in the model:

t does make a difference when the energy model is defined. Missing and unenclosed elements can cause errors in the model.

- Room/Space/Area/Zone objects capable of storing data

Regardless of the project type, Revit includes all of these objects, with rooms and areas defined in an architectural model, while an MEP model uses spaces that can be associated with rooms in a linked architectural model.

Rooms should be placed in the architectural model, especially in occupied areas, and be fully enclosed — both horizontally and vertically.

The spaces that are added to the MEP model, and can also be placed in non-occupied areas that still have thermal characteristics. Zones are used to associate spaces with specific air handling systems and loads.

Key Point — When a room is placed in an area with sloping walls, set the computation height as close to the smallest horizontal area (relative to the sloped wall) to gain the most accurate volume calculation.

Key Point Two — Make sure the room extends to the bounding elements vertically, to be completed bounded by objects.

Defining Energy Model Settings and Features

Once you know what items are included, the next step is to look at the design tools you use, that are already built into Revit. Be aware of what features are included that incorporate energy modeling tools and attributes:

- Compatibility with standards, such as DOE2.2, ASHRAE 140, etc.

Currently, Revit 2017 supports data exports that comply with DOE-2.2, ASHRAE 90.1, ASHRAE 140, and ASHRAE 62.1 standards for energy modeling.

- Project geolocation is incorporated into Revit as part of the Manage > Project Information> Energy Settings tools

Climate data is obtained by determining the closest weather station based on the location.

- Project based energy modeling parameters or attributes

Each project includes options for determining default energy modeling settings.

Since these are project specific, they can be adjusted based on location, or stored as part of a project template, based on building type and construction.

- Built-in/internal direct communication with the analysis software

Most BEM applications include some form of translation between a BIM project and the analysis software. The most common format is gbXML, but Revit can also communicate directly with Green Building Studio and Insight. These are accessed from the Analysis tab.

Knowing these are the internal modeling tools, let’s take a deeper look at what you need to define in your project before performing the analysis.

Project Energy Settings

Project energy settings are a subset of project information, and are energy-modeling specific parameters needed by internal and external analysis applications. Prior to performing internal or external project energy, heat and cooling load calculations, you need to review these settings.

The following categories are defined for essential and energy analytical model settings:

- Location — identifies the city, and longitude and latitude for the building. The specific weather station and site information can also be selected.

- Mode — sets whether the model will use building elements, conceptual masses or both

- Ground Plane — specifies the level that serves as the ground level reference for the building. Surfaces below this level are considered to be underground. The default level is zero (Note: Ground plane does not affect Heating and Cooling Loads calculations. It is used with gbXML export).

- Project Phase — specifies whether existing or new construction phases are used, when they are assigned to building elements in a model.

- Analytical Space Resolution — when using an element-based analysis, specify the size of the largest gap (between two Revit elements) through which analytic spaces will not “leak.” If you run an energy simulation and a message displays that the model is too large, increase this setting and rerun the energy simulation. The default is 18 inches.

- Analytical Surface Resolution — when using an element-based analysis, this feature specifies, in combination with the Analytical Space Resolution, how accurately the boundaries of analytic surfaces match the ideal boundaries. In general, reducing the Analytical Surface Resolution results in analytic surfaces with more accurate boundaries, but this also limits how accurately analytic surfaces are modeled. The default is 1 foot (304.8 mm). If you run an energy simulation and a message displays that the model is too large, increase this setting and rerun the energy simulation. This setting determines how long an energy model can take to be defined. The maximum distance that can be ignored is two times the value defined for this parameter.

- Perimeter Zone Depth — this measure the distance from a building’s outside walls into the building, when conceptual masses are used instead of spaces and zones. It is set in conjunction with Perimeter Zone Division, which divides the model into discrete thermal zones. The zone divisions leverage the orientation of the conceptual masses in a model, allowing different areas to be analyzed independently.

Additionally, you can select Other Options to access advanced energy settings, and provide more detail for the energy model. When using the Use Conceptual Masses option only, several features become available when mass floors are included in the model.

- Target Percentage Glazing — This setting specifies the percentage of exterior walls to be glazed openings (windows). It is also known as the window-to-wall ratio (WWR). The default is 40%. For curtain walls, the maximum is 95%, which takes into account the framing area.

- Target Sill Height — Specify the distance from the floor to the bottom of the window. Window areas below task height (typically 0.75 meters or 2.5 feet) contribute to heat gain and heat loss without contributing to effective daylighting. The Target Percentage Glazing and Target Sill Height settings work together. If you specify a larger Target Percentage Glazing, Revit may use a sill height that is lower than specified to meet the requirement.

The total height of the window directly influences the shade depth required to protect the window from solar gain. Taller windows require deeper shades.

- Glazing is Shaded — Select this setting if you want light shelves to shade windows and other glazing for conceptual energy analysis. Proper shading greatly reduces cooling energy spent on a space with large areas of unprotected glazing. In the conceptual model, automatic light shelves are external only, and they are manipulated separately from their windows. However, you can manually create light shelves or other types of shades (such as awnings) for the conceptual model by using mass surfaces.

- Shade Depth — sets the default depth for shading elements in a conceptual model. The option becomes available when the Glazing is shaded option is selected.

- Target Percentage Skylights — Specify the percentage of roofs that should be skylights. This value is also known as the skylight-to-roof ratio (SRR). The default is 0%.

- Skylight Width and Depth — When you specify a value for Target Percentage Skylights, use this setting to specify the size of the skylights. Enter an average dimension defining the width and depth of the skylights. For example, enter 4’ to specify skylights that are 4’ wide by 4’ deep.

Building Data is included when the heating and cooling load tools are used. This information provides information based on ASHRAE standards, including:

- Building Type — based on the types defined in the Building / Space Types setting for the project. The type is defined according to the gbXML schema 0.37 (similar to ASHRAE).

- Building Operating Schedule — Default uses the schedule defined by the building type, but you can override the setting as needed for the project. The settings include: o Default — value determined by the Building Type defined under Building and Space Type settings.

- HVAC System — Based on ASHRAE standards, this option specifies the type of heating, ventilation and air conditioning system used in the building. Select the system once all other settings have been defined. For more detail, see the HVAC Settings section below.

- Outdoor Air Information — Based on ASHRAE 60.1 standards, this option allows you to directly set outdoor air conditions based on per person, per area and air changes per hour. There are overall default conditions for building that can be overridden at the zone level.

Export Category — select between rooms and spaces, depending on the model elements used, or the external application used to analyze the model.

Note: for programs that do not correctly support exported gbXML, try exporting IFC or DXF files to help create the geometry in the analysis modeling program.

Material Thermal Properties — this exports thermal properties of elements based on materials assigned to the components, such as the brick and block in a wall. Note that you must use materials that have thermal properties defined, so verify this using the Material Editor tool. Default Revit materials include the correct thermal properties assigned by material.

- Conceptual Types — defines what material is used based on the default constructions for conceptual masses and building elements, and do not contain specific thermal properties.

- Schematic Types — overrides conceptual settings and defines what material is used based on the constructions.xml file associated with your language version of Revit. Additional types can be defined other than the building default, when building elements are used.

- Detailed Elements — uses the assigned thermal properties included with materials, which are assigned by building element type (such as wall, door, roof, floor, window, etc.)

HVAC System Types

The following sections provide details about the assumptions used during energy analysis for each of the predefined systems types included with Revit. The options include:

- Pipe Fan Coil System, Chiller 5.96 COP, Boilers 84.5 eff • Water cooled centrifugal chiller (COP 5.96)

• Open, atmospheric pressure cooling tower with variable speed fan and 5-degree Fahrenheit (2.8 degree Celsius) approach
• Forward curved constant volume fan and premium efficiency motor
• 0.25 inch of water gauge (62.3 pascals) static pressure Constant Volume duct system
• Gas-fired hot water boiler with draft fan >2500 kBtuh, 84.5% combustion efficiency
• Variable volume hot water pump
• Hot water coil
• Variable volume chilled water pump
• Chilled water coil
• Variable volume condenser water pump
• Domestic hot water unit (0.575 Energy Factor)

- 4-Pipe Fan Coil System, Chiller 5.96 COP, Boilers 84.5 eff • Water cooled centrifugal chiller (COP 5.96)

• Open, atmospheric pressure cooling tower with variable speed fan and 5-degree Fahrenheit (2.8-degree Celsius) approach
• Forward curved constant volume fan and premium efficiency motor
• 0.25 inch of water gauge (62.3 pascals) static pressure Constant Volume duct system
• Gas-fired hot water boiler with draft fan >2500 kBtuh, 84.5% combustion efficiency
• Variable volume hot water pump
• Hot water coil
• Variable volume chilled water pump
• Chilled water coil
• Variable volume condenser water pump
• Domestic hot water unit (0.575 Energy Factor)

- 11 EER Packaged VAV, 84.5% boiler heating • Efficient 11 EER, >20-ton Packaged Variable Air Volume (VAV) Unit, Water Reheat, with Variable Speed Drive (VSD)

• Forward curved fan with VSD and premium efficiency motor
• 3.5 inch of water gauge (871.8 pascals) static pressure VAV duct system
• Integrated differential dry-bulb temperature economizer
• Gas-fired hot water boiler with draft fan >2500 kBtuh, 84.5% AFUE
• Constant volume hot water pump
• Hot water coil
• Hot water reheat boxes
• Domestic hot water unit (0.575 Energy Factor)

- 12 SEER/0.9 AFUE Split/Packaged Gas, 5–11 Ton • Efficient 12 SEER, 90% AFUE furnace split system with gas heat

• Forward curved constant volume fan and premium efficiency motor
• 2.0 inch of water gauge (498 pascals) static pressure Constant Volume duct system
• Integrated differential dry-bulb temperature economizer
• Domestic hot water unit (0.575 Energy Factor)

- 12 SEER/7.7 HSPF Split Packaged Heat Pump • Efficient 12 SEER/7.7 HSPF (Heating Seasonal Performance Factor) < 11.25-ton split/packaged heat pump system

• Forward curved constant volume fan and premium efficiency motor
• 2.0 inch of water gauge (498 pascals) static pressure Constant Volume duct system
• Integrated differential dry-bulb temperature economizer
• Domestic hot water unit (0.575 Energy Factor)

- 12 SEER/8.3 HSPF Packaged Terminal Heat Pump (PTHP) • 12 SEER/8.3 HSPF (Heating Seasonal Performance Factor) packaged terminal heat pump (PTHP)

• Forward curved constant volume fan and premium efficiency motor
• 0.25 inch of water gauge (62.3 pascals) static pressure Constant Volume duct system
• Domestic hot water unit (0.575 Energy Factor)

- Central VAV, Electric Resistance Heat, Chiller 5.96 COP • Water cooled centrifugal chiller (COP 5.96)

• Open, atmospheric pressure cooling tower with variable speed fan and 5-degree Fahrenheit (2.8-degree Celsius) approach
• Forward curved fan with Variable Speed Drive (VSD) and premium efficiency motor
• 3.5 inch of water gauge (871.8 pascals) static pressure Variable Air Volume (VAV) duct system
• Integrated differential dry-bulb temperature economizer
• Resistance reheat boxes
• Variable volume chilled water pump
• Chilled water coil
• Variable volume condenser water pump
• Domestic hot water unit (0.575 Energy Factor)

- Central VAV, HW Heat, Chiller 5.96 COP, Boilers 84.5 eff (default) • Water cooled centrifugal chiller (COP 5.96)

• Open, atmospheric pressure cooling tower with variable speed fan and 5-degree Fahrenheit (2.8-degree Celsius) approach
• Forward curved fan with Variable Speed Drive (VSD) and premium efficiency motor
• 3.5 inch of water gauge (871.8 pascals) static pressure Variable Air Volume (VAV) duct system
•  Integrated differential dry-bulb temperature economizer
• Gas-fired hot water boiler with draft fan >2500 kBtuh, 84.5% combustion efficiency
• Variable volume hot water pump
• Hot water coil
• Hot water reheat boxes
• Variable volume chilled water pump
• Chilled water coil
• Variable volume condenser water pump
• Domestic hot water unit (0.575 Energy Factor)

- Residential 14 SEER/0.9 AFUE Split/Packaged Gas <5.5 ton • Efficient 14 SEER/90% AFUE furnace <5.5-ton split/packaged system with gas heat

• Residential constant volume cycling fan
• 2.0 inch of water gauge (498 pascals) static pressure Constant Volume duct system
• Domestic hot water unit (0.575 Energy Factor)

- Residential 14 SEER/8.3 HSPF Split/Packaged Heat Pump • Efficient 14 SEER/8.3 HSPF (Heating Seasonal Performance Factor) <5.5-ton split/packaged heat pump system

• Residential constant volume cycling fan
• 2.0 inch of water gauge (498 pascals) static pressure Constant Volume duct system
• Integrated differential dry-bulb temperature economizer
• Domestic hot water unit (0.575 Energy Factor)

- Residential 17 SEER/9.6 HSPF Split HP <5.5 ton • 17.4 SEER/9.6 HSPF <5.5-ton split/packaged air source heat pump, intermittent fan mode

• Residential constant volume cycling fan
• 2.0 inch of water gauge (498 pascals) static pressure Constant Volume duct system
• Premium efficiency, on-demand tankless domestic hot water heater (0.85 Energy Factor)

- Underflow Air Distribution • Packaged Variable Air Volume (PVAV) system with under floor air distribution

• Forward curved fan with Variable Speed Drive (VSD) and premium efficiency motor
• 3.5 inch of water gauge (871.8 pascals) static pressure VAV duct system
• Gas-fired hot water boiler with draft fan >2500 kBtuh, 84.5% combustion efficiency
• Integrated differential dry-bulb temperature economizer
• Variable volume hot water pump
• Hot water coil
• Hot water reheat boxes
• Improved efficiency domestic hot water heater (85% thermal efficiency)

Creating the Energy Model

Once you have reviewed all of the project settings, and you have your geometry added, the next step is to create the energy model. Actually, this is one of the easiest steps — in Revit 2017 and back to Revit 2014, when the feature was added, the energy model is defined by selecting the Create Energy model tool.

After selecting the tool, you will get a warning message, due to the amount of time it takes to define the model.

The energy model is defined based on the volumetric space of all elements used in the create process, and the Analytical Space Resolution setting explained in the previous section. To keep this as fast as possible:

- Keep the overall volume of the model containing elements as small as possible. Minimize the number of items that appear outside of the building envelope, such as floors that represent ramps or walkways. If they are placed, make sure the Room Bounding option setting is disabled for the object.

- Don’t set the analytical space resolution too low. Too many undefined spaces can cause the model to have failures.

After running the tool, you will be prompted to perform an energy simulation.

If you choose Run Energy Simulation, originally the model will be exported to Green Building Studio, a web based energy analysis application that was first directly integrated into Revit 2014. The program uses DOE 2.2 simulation methods to perform the energy analysis.

When Insight 360 is not installed, the program will prompt you to create a new energy simulation as a Green Building Studio project. Once the simulation is created, you can select the Results and Compare tool to see how the model performed.

But if Insight 360 is installed, the simulation is automatically pushed to the Insight 360 web services. And that’s where the fun begins. Learn about using Insight 360 and other Insight tools for lighting and solar studies in my article, “Discover Insight 360 for Building Energy Modeling.”

Conclusions

No matter what your political bent is on the climate and environment, it’s important to understand that regardless of the scenario, it benefits everyone to be better stewards of our resources. Simply stating that performing these analyses or reviewing these options are not cost effective is no longer the case. But leveraging your BIM model and using it to gain a better understanding of the energy usage and design options to produce more accurate and visible results, helps you state your case to your client and achieve this goal.

David Butts is an Autodesk Expert Elite Team member and Building Information Modeling (BIM) specialist for Gannett Fleming with over 30 years of experience in the architecture, engineering, and construction field.

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