Wednesday, September 22, 2021

WWR Calculation Method in Revit + Insight Workflow

When an Energy Analysis Model (EAM) is pushed to Autodesk Insight for analysis, we find that the Revit Model's Window to Wall Ratio (WWR) is listed as the BIM Setting. Today's post will show how this number is derived... which is rather clever!

keep reading to learn more...

Having a project's WWR is super helpful, and I have seen designers spend a lot of time manually figuring this number out for each side of a building.

I touched on the WWR info Insight lists, which is the BIM Setting based on what is modeled in Revit (if anything is molded) in this post: Autodesk Insight 360 - Walls; Part 4. If you drop down to the comments, notice Ry Auburn just asked for more info on how the WWR is calculated... since I don't work for Autodesk I cannot give the full under-the-hood answer. But, through some testing I have a pretty good way to describe this...

Sample Revit Model

Here is the simple 60' x 40' model I created with different window arrangements on each side. The walls are 20' tall tilt-up precast panels.
  • North: 320 sf curtain wall with mullions
  • East: (2) 5' x 5' windows
  • South: (3) 5' x 5' windows
  • West: no openings

The South wall is 1200sf with 75sf of windows... so, is the WWR only 6% (75/1200)? As we will see soon, it is not.

Revit Energy Analysis Model

When we create the EAM in Revit, the 20' tall precast panel is divided into two or three different analytical surfaces, based on this projects conditions.
  • Exterior Wall: Portion of wall between enclosed space and exterior
  • Shade Surface: Portion of wall not adjacent to enclosed space
  • Underground Wall: Portion of wall adjacent to enclosed space, and below the Ground setting in Revit's energy settings dialog
Notice, these surfaces may be selected, and some data can be seen via properties (see next two images). In this example. because we don't have a basement (i.e. enclosed space), the foundation walls are shade surfaces, leaving them rather insignificant in this energy analysis.

Notice, the windows are 75sf and the exterior wall is 552sf. The wall is slightly smaller than the 600sf shown in the exterior elevation above... which is related to how the EAM is auto-created... which, in the big picture, is still way more accurate and efficient than a manual workflow that often over simplifies more complex geometry and spaces.

Results in Insight

When the EAM is pushed to Insight, we see the WWR from the Revit model, listed as the BIM Setting (triangle) on the graph.


The result, best I can tell, is based on the exterior wall area related to the analytical surface... which makes sense for energy modeling! But it is not purely the analytical surface numbers shown above (75/552=13%) while the actual wall area calculated above would be (75/600=12%).

Also, the glazing area is simplified to include any frames/mullions in the opening.

Fun stuff!

P.S. stay tuned for my recent deep-dive comparing workflow and results between Revit + Insight and Rhino + Climate Studio!

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