adding more insulation on garage side of wall, and held in place with plastic sheeting has resulted in a moisture issue

I am closing the ceiling of my garage, and wanted to do a few things before that was completed. It’s a 2 story house, and there is a shared wall between the house and the garage, on the second story. On the garage side this is above the level that the ceiling will be installed at – so will remain a cold spot. I thought I would add some extra batt insulation to the garage side of the wall. This wall would already have insulation in it(from when the house was built), and a vapour barrier on the house (warm side) as the garage is unheated. THere was also a house wrap on the garage side (exposed) and I put the new insulation right against that.
I added the insulation against this wall, and used a sheet of plastic over that to keep the extra insulation in place. It now appears that I created a moisture issue, as there is moisture trapped between the extra insulation and the plastic used to keep it in place.
I’m located in a Zone 3 area (Canada, just above ND), so considered a very cold climate during winter. I think the issue is because I used the plastic to hold it in place – can anyone confirm. Then any thoughts on what else I could use to hold the insulation in place, but not trap any moisture?
Thanks