I am going to be insulating my floor in the next few days. I'm planning on two layers of 1 inch XPS. For a total of 2 inches. I am putting in two layer so I can offset the joints between the sheets. On top of which I will place 5/8" tongue and groove plywood sub floor.
I had been shopping for the foam insulating board, and the price almost everywhere it was around $20 per 4 x 8 sheet. Then a couple of days ago I saw that an upper midwestern, big box store, the Menards chain, was selling the stuff for $14.98. Waited for a day and the price seemed to have gone back up to $18.95, at least on the chain's website. I decided that I needed to get moving with the project, so I went to the store ready to buy at the higher price and lo and behold it was still $14.98, so I grabbed me some 20 sheets.
While I was in the store I saw that they have a better selection of steel framing supplies than Home Depot or Lowes. I am thinking about furring out my walls with the 2 x 2 metal studs and making a partition wall between the rear sleeping area and the rest of the bus with 2x3. I am trying to use only materials that resist mold.