Unless you plan to keep your shed climate controlled with heat or air conditioning, you don’t need to install a moisture or vapor barrier. If you want your shed heated, you need to put the moisture barrier on the inside of the frame. If you want your shed cooled, the moisture barrier goes on the outside of the frame. You can get away with using 6 mil poly, but a 10 mil or a 20 mil poly is better. However, you don’t need to use a moisture barrier if your shed isn’t very well insulated because the increased airflow will provide circulation and keep things dry.