How to model stairs opening between floors

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@Matt Koch, a few things to be aware of if you do this is making sure the lighting for stair tower is properly accounted for as well as its contribution to the building floor area.

Thanks for pointing that out. I am assuming - perhaps falsely so - that each floor's area (minus its respective stairwell area) is added up to get the total area for the building, and then only one stairwell area is added to that? The same would apply to elevator shafts. Plus, these areas should only be added if the stair well (or elevator shaft) is conditioned? I would love to get an air flow network going, but have not been able to understand how to do that thus far. Presumably, this would take care of the possibly substantial thermal stack effect that could occur in such a stair well?

@Matt Koch, That is a nice workaround. For my house it would create some issues with daylight as my stairwell is placed a long a 5 meter long vitrine on the east faced.

an "Air Wall" in OpenStudio is mainly intended to allow daylight to pass through a zone boundary that doesn't represent a physical wall or floor when using Radiance for daylighting. When exported to EnergyPlus an "Air Wall" has thermal properties somewhat like a piece of plywood. It doesn't introduce any airflow between the two zones. There are a few measures that use air walls and interior operable windows, to add in zone mixing, but the one's I'm aware of are for very specific use cases.