I am planning on putting in a subfloor over my concrete basement floor. In the summer months the slab has a lot of moisture builup but it is probably from the humidity in the air. I was planning on painting a sealer on the floor laying 2x4 pt sleepers with rigid insulation covered by t&G 3/4" exterior grade plywood. I have gotten several opinions on the placement of a 6mil poly vapor barrier. Should it go over the slab and under the 2x4's or over th 2x4' and under the plywood? I'm afraid if I place it wrong I will get a lot of moisture, mold and mildew leading to a foul odor and rot.