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As you say, there are two main options for modeling the air space in a ceiling.

A. Model as a separate plenum zone

Advantages:

• The plenum zone will be modeled with a full heat balance, most likely at a different temperature from the occupied space, and it will not be exposed to the solar and internal gains of the occupied zone below.

• The occupied zone below will automatically have the correct zone volume for any reporting or inputs that are in air changes per hour (ACH).

• Allows the plenum zone to serve as an unducted return or supply plenum, and as a source for powered induction units. Actually, this option is required if you have any of those.

Disadvantages and Limitations:

• More surfaces and zones to describe in the model.

• The zone air will be mixed (well-stirred) at a uniform temperature. So, while buoyancy-driven surface convection is modeled, there is no circulation or stratification within the plenum zone.

B. Include the void as part of the occupied zone below and add an air gap layer to the ceiling/roof construction

Advantages:

• Simpler input.

• Many users take this approach and find it to be adequate for standard dropped ceilings.

Disadvantages and Limitations:

• The entire exterior surface area (walls that are exposed to the occupied space plus walls exposed to the plenum space) will be exposed to the conditions of the occupied space (temperature, solar, radiant exchange, etc.).

• You need to adjust the zone volume in the Zone object, if needed, to have the correct zone volume for any reporting or inputs that are in air changes per hour (ACH).

• The air gap resistance will be constant.