Revit has about 200+ exterior wall assemblies we can select from when performing early energy analysis, which even includes a straw bale wall. Today's post will cover this workflow, one we use here at Lake|Flato, and what to watch out for when using it...
Thermal Properties Overview
First, there are three ways in which the thermal properties of a wall are assigned. The image below, from a presentation I did at the Minnesota AIA Convention in 2017, shows the hierarchy of these options. The first two options, Conceptual and Schematic, are the simplest and apply to all exterior walls in the Revit model (this defines the BIM Setting within Insight).
FYI: Revit automatically determines which walls are exterior when the energy analysis model (EAM) is created. It has nothing to do with the wall's Function property.
The third option, Detailed Elements, allows the thermal properties associated with each material to be considered. This means each exterior Revit wall type can have its own thermal properties.
Exterior Wall Hierarchy Explained
By default, the Conceptual Types are used. Since we are talking about early energy modeling, if this is very early in the process, you likely do not have a specific wall type in mind yet. Which makes sense, since that is partly why we are doing early modeling... to figure out what the wall construction should be to meet a specific goal. At this early stage the options are intentionally limited so we can focus on other elements of the design, like building shape, orientation, etc.
FYI: Although the word "Mass" is used in this dialog, it applies to Revit walls and/or mass surfaces converted to exterior walls in the EAM.
Once you have a better idea of what the exterior wall construction will be, you can use Schematic Types. When the Override option is checked, the selected wall assembly is used in place of the Conceptual Types option. Notice you can just override specific categories, like Exterior Walls... all other categories still use the Conceptual Types selection.
When Detailed Elements is selected in the Energy Settings dialog, the thermal properties of the Revit wall type are used, if they exist. If a wall does not have any thermal properties, then it will use the schematic override if one is selected, otherwise it will roll all the way back to the conceptual types.
For the Detailed Elements workflow, you need to make sure each material in a wall has a thermal property and that they match the products you are specifying. I do not use this option very much... once you start getting this detailed it is probably time for the mechanical engineer to take over and preform energy modeling that sizes equipment and such.
Exterior Wall Construction Details
When selecting an exterior wall assembly from the schematic types dialog, you will notice there is a simple description of the wall construction and a listed U value. Although this should not drastically affect the overall results for an entire building, you have to be a little carful here. The U value is not the only metric affecting performance, and some wall construction elements are not listed in the name.
Notice in the table below, the U value selected would suggest the EUI would change relative to that number. However, certain materials like brick have different Specific Heat and Density properties (see previous image for an example of these thermal properties).
FYI: moving this project from San Antonio, Texas to Northern Minnesota, within Insight, creates a much larger range in the EUI results due to micro climate conditions.
Autodesk provides a PDF, you can download, to see more detail about each exterior wall assembly and shown in the example below. Note these values are all metric.
In this example, notice it also has brick which is not mentioned in the name. It also has a layer that includes batt insulation and metal studs, which a combined/normalized R-value similar to the post linked above.
To see the full document, click here (1.1 MB PDF file)
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