My current attic has blown in fibreglass with carboard "isulation isolation" sheets between the rafters to keep the insulation from blowing down onto the soffits. Essentially it provides a physical barrier to keep the insulation away from the edges of the roof. The current insulation is pretty much up to the top of the cardboard so adding any more would cause it to spill over and down onto the soffits.

I need to extend this physical barrier vertically up about ~6-7 inches to allow me to blow in an extra ~6-7 inches of insulation. I am thinking of stapling polyethylene plastic sheeting to the rafters and existing cardboard barrier(leaving room for the airflow from the soffits of course) Any reason why I should not do this ie is it going to cause condensation if so why?

Thanks in advanced for any response!
Jack.