Internal void spaces are not recognised by SBEM and so the room type “Internal Void or Warm Roof” should not be used. For such spaces that are within the thermal envelope of the building, you are advised to include the roof in the room below or else; do not model the roof as a room and instead add the insulation and void properties to the construction you assign as the roof/ceiling of the room below.
The iSBEM user manual provides some guidance on modelling both ceiling voids and attic spaces.
Essentially any ceiling void volume should be absorbed into the thermal zone associated with it. For the purposes of SBEM modelling, models should be built without any ceiling voids.
If you are switching from a DSM model where you modelled the voids you could use the merge function to combine the zones together.