The plastic moisture barrior under my 'tin shack' (double wide) has been
directly on the ground for the past 7 years and not even the old weeds are
growing under it.

This covering isn't 100% since it is slit and fitted around the support
piers (cinder blocks) but it is very effective since I'm on a hill side and
sometimes drainage water will flow under the house, under the plastic. I
still have stuff stored under there and it remains dry.. But then I have
plenty of air circulation via the many require vent openings.

Steve