I am building a shed. The floor consists of 4x4 PT runners, 2x8 PT girders, and 3/4" PT Plywood flooring. Anytime in the past, when I have laid PT lumber next to non-PT lumber, there is mold/mildew on the non-PT lumber after a very short time. Also, anything metal seems to rust quicker when on or next to PT lumber. I wanted a heavier duty shed, so I was thinking of adding a second, non-pressure treated layer of plywood on the first. This would strengthen the floor and also separate the PT wood from the contents in my shed. Is this plan acceptable? Should I put a moisture barrier between a single ply of pressure treated plywood and a second non-PT ply?