Situation:
Create an insulated office within a larger warehouse space. Warehouse is 18′ floor to roof deck, clients want 10′ t-bar ceiling for new office space.

Issue: how to insulate…
Existing building is poorly insulated, thus need to create insulated ceiling above office.

Solution:
Thus far, ideas include installing Owens Corning Sonobatt directly over the t-bar ceiling.

Or creating a 2′ conditioned plenum between the t-bar and a separate insulated structure (such as hanging from the existing roof OWSJ a lattice of 2″x2″ at 2′ o.c. each direction. Then laying 8″ batt insulation above it, and stapling 6 mil poly below.

My thoughts are that putting Sonobatt directly over the t-bar will not work – aside from poor service access, there will be no air-barrier between the conditioned office space below and the non-conditioned air above the t-bar. The air will pass right through the batt.
Not to mention condensation issues (warm/humid office air going up through t-bar ceiling into the batt insulation and condensing).

Is the second solution the best idea?

The project is in Toronto, Canada.

Thanks for any guidance.

Kyle