I have a shed that has a wood floor laying on a steel frame, which is laying on some tiles on the bare earth. I would say there's roughly 30cm of clearance between the dirt and the floor boards. I want to add some insulation, and I'm wondering what put under there. I'm assuming I need some kind of plastic sheet to stop moisture, and some sort of isolating material like rock wool. Does the moisture barrier go on the bottom, with the rock wool between the floorboards and the barrier, or do I place the barrier on top, exposing the rock wool to the earth? If there's an aluminum film on the rock wool, does it go up or down, or do I need a different material entirely?
You probably don't need a separate vapor barrier. What you'll want to do (since the bottom doesn't touch the ground) is put regular faced batts in, with the kraft paper on the bottom side. That's both a vapor barrier and the easier install method.
Putting plastic on the ground under the shed can't hurt, but isn't necessary.