Why does the energy model only use room bounding elements?

[rickSFEYK]

I have been unable to find a satisfactory answer. Elements are specified as room bounding or non-room bounding for the purposes of area and volume calculation of rooms (see here). However there are many situations where an element may have been specified as non-room bounding, e.g. an interior partition to a room. I have also observed roofs commonly specified as non-room bounding in models, perhaps to avoid calculating area/volume in schedules. You may have other examples.

What is unclear to me is why the energy model is only built from room bounding elements. Why is this a requirement? I understand areas of surfaces and volumes of thermal zones are required for the energy calculation, but it seems this could be easily obtained without requiring elements to be room bounding.

[barthbradley]

What about Masses?  

...or Room/Space Separation lines?  

[rickSFEYK]

Let me clarify my question: an energy model can be constructed from other things like masses as well. I am interested in why, when it is constructed from elements, it must be from room bounding elements.

[barthbradley]

What would delineate the bounding if there were no bounding elements?  Revit would guess?  

...I'm probably missing your point again.  Sorry.  

[SteveKStafford · Accepted solution]

I'm not a Autodesk developer, IIRC initially Revit only worked on basic massing forms and space elements. Space elements depending on room boundaries to exist and spaces define the usable parts of the building. The analysis tools that predate Revit, that they used as a frame of reference or to import/export to/with, are "simple" abstractions of a building as well...some based on providing room data without any 3d geometry at all.

Later Revit started offering the option to do calculation on model elements instead of the simpler mass/space abstraction. For this to work those elements still need to define spaces to be able to express how the calculations related to each room as well as the building overall. However Revit isn't really looking at the individual walls (layers etc)...it's looking the construction settings that are defined and those have "meaningful" names/descriptions but ultimately the "u" value assigned is the more important measure.

Short answer...like @barthbradley wrote, "what would they calculate against if not room bounding?" The room bounding behavior is already a critical part of the puzzle to define rooms/spaces, which are part of the reporting function, AND it helps differentiate meaningful from irrelevant to calculations. The calculations are also an abstraction, guestimate...the more accurately the model is created the higher the probability the results are but often the return on investment for that detail isn't significantly different from a seat of the pants estimate provided by someone with experience, total area and geographical location.

If this requirement means you can't do things you want or need to for other reasons then they recommend creating/placing a simplified version on a side track to run calculations so that doesn't interfere with your other model requirements. FWIW, I don't do it much these days, but in the past I haven't encountered many models that can run calculations on the model elements successfully. It's difficult to create a truly "tight" model that generates a result that is significantly (meaning worthwhile, worth the effort) different, different in important ways, from one based on the simpler model.

[rickSFEYK]

Thanks @Anonymous for the questions and @SteveKStafford for the background and history. In line with what Steve says, in the Autodesk Revit 2023 online guide, in "About Room Volume":

Rooms store values for a variety of parameters that affect the heating and cooling for a project. An effective energy analysis can only be accomplished if all the areas in your model are defined by room components in the building model and the entire volume of the building model is included.

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If a model is built from elements, I think technically there is no reason that rooms and space couldn't be automatically "discovered" and used for energy analysis, but for historical and practical reasons (e.g. lack of a tight model), it seems this is not done. This seems to be the cause for the requirement of "room bounding" elements for the energy model.