Up there you probably can use poly as a vapor retarder layer without getting into moisture trapping problems, provided your exterior layer can dry outward. Using a sheet material for vapor retarder gives you the opportunity to use it also as the air barrier, if it is detailed properly. One thing you can do to achieve this is to install strips of poly at intersections of interior partitions and exterior walls and to drape strips also over the tops of interior partitions. You also ought to run strips of poly under the toe plate of exterior walls (the inner one), with a line of acoustical sealant (eg. Tremco) between that poly strip and the subfloor. Later, after framing is done and you are hanging the pieces of poly on the exterior walls and upper level ceiling, you can tape seal those pieces to the strips you hung earlier. For sealing around electrical boxes, you ought to use polypans or similar devices (eg. Lessco or Airfoil) so you have something you can seal the vapor retarder to. You'll find other techniques just as useful here and elsewhere. Others will comment shortly.
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