My electrical panel is mounted on bare studs of an exterior wall in my attached garage (living space above). The garage is not insulated and it does not need to be (except for the ceiling which will happen someday soon). I would like to mount my electrical panel on 3/4" fire-treated plywood, and I was planning to keep the stud cavities empty. Location is moderate SF Bay Area.
Do I need to be concerned with adding a vapor barrier?
The key to understanding a "vapor barrier" (really, an air barrier) is to understand that it is part of the system that separates "inside" aka conditioned space from "outside" aka unconditioned/outdoor space. So, your air barrier needs to go with the other barrier layers, in particular your thermal barrier (insulation).
From this, we can determine that since your garage is unconditioned space, the air barrier should sit on the ceiling along with the insulation there, instead of being on the wall behind the panel, divorced from the insulation and letting unconditioned air get through the insulation as a result.
