Dana - if there is a chance of condensate on the poly to the point of saturation when the proper amount of cellulose is applied to support air conditioning, then I stand corrected.

Summer periods where outdoor dew points dwell north of 23C are common in places like Washington DC or Philadelphia (both in climate zone 4A), and many people like to air condition down to 21C (sometimes even lower- paradoxical since the some same people prefer 23C or higher indoor temps in winter). In that climate zone interior side poly sheeting (in either walls or ceilings) causes at least as many mold problems as it solves.