IF you feel you have to use a vapor barrier (as opposed to a retarder) it always goes to the warm side. If on the outside, it will be cold, and when air gets through your fiberglass (I assume) insulation, and plenty will, the moisture in the air will condense on the poly and you will have water galore if you have any moisture in the air. Your best bet, IMO, is to NOT use the poly, to use Roxul batt insulation, and to air seal the room real well. Google airtight drywall approach. Now is the time to do this.... after you remove the cedar. Sorry about that.