How to add a dead-air plenum space type so that infiltration can be considered coming from it?

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Often if you have an unconditioned plenum space like a dropped ceiling, energy modelers just make it part of the space below, but it's fine to model the plenum as its own zone/space, and there are advantages to this approach at times.

Generally you model the plenum like other spaces but with the following exceptions

1. Make sure to un-check "Part of Total Floor Area" for the space. This will make sure it isn't counted towards the total building area and as a result wont affect area normalized results.
2. Make a new space type for the plenum that doesn't have any internal loads in it, except for infiltration.
3. Create a construction set with no constructions in it and apply it to the space type. If your plenum is dropped ceiling, then add a floor construction to it that is something like acoustical ceiling tile or drywall (but not a concrete slab). Or if your plenum is more like a raised floor, then do the opposite - create a construction to use for the plenum ceiling that represents the raised floor material. OpenStudio will handle construction conflicts between the plenum and the main space so that the space has a mirrored construction to what the attic has.

Now you're all set. As usual the plenum should be part of a thermal zone. You could make it a zone by itself, or combine it with the space below, but combining it with the space below defeats the point of modeling it separately, but is nice if you want to see how much of a difference it makes to the simulation results.

The approach for an attic is very similar, but you may have ventilation in addition to infiltration, and you may have insulation on the attic floor and no insulation on the attic roof/ceiling.

One last note. If you are modeling a very complex floor plan (50 +/- or more spaces) then plenum space may end up with more than 100 surfaces which can result in a slower simulation. One solution to this is to not intersect and match the attic with the spaces below but use an adiabatic boundary condition, but with that approach there would be no heat transfer across the plenum, may make more sense to do that when it is a supply or return air plenum.