Double vapor barriers

[Alan]

If you're adding insulation to an existing space, you have to check whether there's already a vapor barrier before you put one in. Kraft faced batts, foil faced foam board, and some other materials already have vapor barriers built in. So installing plastic sheeting or another barrier over these creates a double vapor barrier that traps moisture between the layers instead of managing it and ends up causing the exact problems you're trying to prevent.
Found this doing some research