"Theres no chance of spacing opening at the posts" So the t&g must be either going on first or you are sliding it behind the posts. Either way, I think you would be safer with drywall or something like that where you can tape the edges & make sure you have a good seal. It may not be impossible to get a good seal with poly alone, but it is rare; especially if there are wires and outlets and other things in the wall. I agree with Dana about the blown in insulation - many times better than batts.