Vapor barrier in bar floor

I am looking a good way to keep moisture off the bottom row of hay. We are currently using pallets and they work good but Is time consuming. Has anyone tried or know of anyone that has tried putting down a vapor barrier, like some thick plastic wrap or a woven mat down then putting gravel over the top to eliminate moisture on the bottom row of hay. I could see the the plastic wrap working well just didn't know if it would hold moisture if it ever got in the barn. I could see the woven mat working better because it would allow some moisture to pass through but didn't know if would create a good enough barrier to keep moisture out.

Vapor barrier in bar floor