When to use 'Semi-exterior unconditioned' zones
The zone type should be set to 'Semi-exterior unconditioned' when the zone is unheated, uncooled, unoccupied and there is no significant energy consuming equipment. Examples include unoccupied roofspaces, garages, sheds etc. In this case the zone is excluded from the model and the constructions between the zone and adjacent occupied zones will use the Semi exposed wall, ceiling, floor construction data. Note that using the <None> activity is the same as setting the zone type to semi-exterior unconditioned.

Note also that is in incorrect to exclude roofspaces (and other semi-exterior unconditioned zones) by unticking the 'Include zone in thermal calculations' checkbox because in this case the adjacency in the zone below the roofspace would be set to adiabatic which would exclude any heat loss through the roof.

When to use Unheated zones
You should note the rules about unheated zones very carefully before marking a zone or group of zones unheated/uncooled by unchecking the 'Has HVAC system?' checkbox on the HVAC tab. Such zones are referred to as 'unheated' in this article. 'Unheated' in SBEM refers to zones that are genuinely cold in winter but do still have some energy consuming activity and must be included in the SBEM calculation, i.e. they aren't 'Semi-exterior unconditioned'. An example of a case where unheated should be used is a large unheated warehouse storage space having some lighting. The warehouse must be included in the SBEM model because it is occupied and has energy consuming equipment in it but it is not heated and so the 'Has HVAC system?' checkbox on the HVAC tab should be unchecked. Note that there will be significant heat loss in winter between any adjacent heated and unheated zones and this is often through standard uninsulated partitions. So be careful when using unheated zones - unheated should not be used for zones such as corridors that do not have a heat emitter but are nevertheless quite warm through indirect heating. If the zone is indirectly heated by convection/conduction from other nearby zones then you should define the zone as heated using the heating system for the adjacent heated zone(s).

Note for Building regulation compliance checking: the NCM requires good levels of insulation between heated and unheated zones, so for example in the above example of the unheated warehouse with adjacent heated office, the partition walls between the warehouse and the office must be insulated.

See also article on Why building fails on Criteria 2 when it is correctly insulted