How can I assign different floor (slab) constructions to different rooms on the same floor, if the construction set only allows one default floor construction?

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Go through the following tabs in the OpenStudio Application (or App):

• Facility : you can assign a default construction set (DCS) for the entire building
• Facility : you can also assign a DCS per story
• Space Types: and/or assign an additional DCS to a specific space type
• Spaces : you can finally assign a DCS to a given space
• Spaces : you can, as you state, hard-assign a construction to a specific surface

The OpenStudio rules are as follows:

• a hard-assigned, surface-specific construction is never overwritten by any DCS (e.g. a space)
• a space-assigned DCS trumps all other DCS (e.g. space type)
• a space type-assigned DCS overrides all other higher-level DCS (e.g. story)
• a story-assigned DCS overrules the building-assigned DCS

So one can efficiently mix-and-match, e.g.:

• assign outdoor-facing roof & wall constructions to the building DCS (if universally applicable in your model)
• assign a default ground-facing floor construction to the building DCS (as a fallback)
• add as many alternate, ground-facing floor constructions to individual space type or space DCS (as needed)
• finally hard-assign a ground-facing floor construction to a given surface (last resort, as needed)

In the above example, the building DCS could be complete (covering all surface types in your model). The space type or space DCS can be partially completed (e.g. holding only ground-facing floor constructions). If well planned, this can be very efficient. Hope this helps.