Energy Analysis Using Building Elements - Model Validation

After creating an energy analytical model based on building elements, review these tips to validate the model and resolve issues before submitting it for analysis.

When you create the energy analytical model, the software first detects the geometry and checks that required information exists for the analytical model.

Model size, complexity, and quality

Revit model content and configurations vary widely. Although using building element mode for energy analysis is designed to account for many situations, there are some limitations. In terms of time and memory limits, the greater the volume as measured by a rectangular bounding box surrounding the model, the longer it takes to generate the energy model.

The current limit on volume is approximately 20,000,000 cubic feet (based on default analytical space and surface resolution values), which could take an hour or more to process. Typically, models should only take a few seconds to a few minutes to complete. Other conditions that can exist and result in an incomplete energy model are summarized in the table below.

Condition	Description	Mitigation
Elements are missing. Elements are not set to room bounding. The model contains gaps that are larger than the Analytical Space Resolution value.	Spaces comprising the energy model are determined based on a 'flood fill' type concept using building elements set to room bounding. Small gaps and overlaps can be processed. However, if any of the specified conditions exist, space will 'leak' and those parts of the building are omitted from the resulting energy model. When spaces leak, the energy model may display the following issues: The energy model is missing parts. The energy model contains many shade surfaces. Note: If a space leaks to the exterior space, it will be omitted from the energy model. Some interior spaces may leak into one another, combining into a single interior space.	View the Revit model and check for: Missing elements such as walls, roofs, or floors. Add these elements where required. Key elements such as walls, roofs, and floors that are not set to room bounding. Set to room bounding where required. Elements that appear to be room-bounding but are not, such as generic families. Replace these elements with elements from room-bounding families. Gaps between elements that are larger than the Analytical Space Resolution value. Close these gaps where required. See About Analytical Space Resolution and Analytical Surface Resolution.
In-place families	When key elements of the building envelope are composed of in-place families, these will be omitted from the energy model.	In-place families are currently unsupported.
Columns	Surfaces will be omitted from the energy model, apart from some shading.	Columns are currently unsupported and depending on size may or may not have a significant impact on validity of the energy analysis.
Small spaces and surfaces	If a space is larger than the Analytical Space Resolution but less than 2 times its value, the space may be omitted from the energy model. In some cases, small spaces may be encompassed by a larger adjacent space. If you use the default value for Analytical Space Resolution, these limits are approximately 18 inches [457.2 mm] to 36 inches [914.4 mm] wide. The same is true for surfaces that are larger than the Analytical Surface Resolution but less than 2 times its value: they may be omitted from the energy model.	Very small spaces and surfaces are currently unsupported but are unlikely to have a significant impact on the validity of the energy analysis. Use Analytical Space Resolution and Analytical Surface Resolution settings to adjust gaps and boundaries. See About Analytical Space Resolution and Analytical Surface Resolution.
Openings in curved walls	Where Revit openings are present in curved walls, that surface will be omitted from the energy model.	Openings in curved walls are currently unsupported.

Energy Analytical Model form and precision

Given the nature of automatic energy model creation from building elements and the current capabilities of the underlying algorithm, the form and precision of the resulting energy model geometry can be quite different in appearance to the typical "closed shell" type geometry created manually by traditional tools and processes. Depending on the configuration of the Revit elements, closed shells can often be created. However, gaps between energy model surfaces and jagged edges can commonly exist. While these can look unusual and be somewhat inefficient in terms of the simulation, it does not necessarily have a significant impact on the validity of the energy analysis itself, especially at an early design stage.

Furthermore, the values for the energy model's volume and surface areas do not exactly reflect the 'true' value which can itself be determined differently with different assumptions. Generally areas and volumes are within approximately +/- 10%, but the difference can sometimes be greater. While this can have a more significant impact on energy analysis accuracy, the energy model should still be valid in most cases.

The consideration here that has to be given is a balance of time required to compute the energy model and the accuracy required. Compared to manually creating energy models, the automatic process results in a significant time savings without compromising the geometric characteristics of the building that influence energy use. Note that modifying the analytical space and surface resolution parameters can improve energy analytical model form and precision, or reduce the time required to create the energy analytical model.

Other known issues

• Revit design options: Design options are unsupported when performing energy analysis using building elements. Only the main model can be used for energy analysis using building element mode. Any elements in a design option that form the overall building envelope may cause spaces to leak and will be omitted from the analysis. (Design options are supported only for energy analysis using conceptual masses.)
• Revit room/space elements: While the energy model created from building elements works independently of room or space element geometry, if present, any data (such as room name or space occupancy, lighting, and equipment) will be used in the energy analysis. However, the energy model can only use the data from one Revit room or space object per energy model space. See About Rooms and Spaces for Energy Analysis.
• Revit room/space separation lines: Where present these are used to create separate spaces in the energy model. However, no surfaces are currently created between the adjacent spaces where the separation lines exist. As a result, no heat transfer is simulated between the 2 spaces. Assuming both spaces are set to the same condition and temperature set points, this will have minimal impact on the validity of the energy analysis.
• DOE2 limits: In Green Building Studio, the underlying energy simulation engine DOE 2.2 has certain limits on the number of energy model spaces and surfaces that it can process. Very large, complex Revit models can exceed these limits.