I have learned in the 30+ years in the building trades that the only way that insulation works efficiently is not to compress it. The air spaces in the product, no mater what it is, foam,
fiberglass (Pink Stuff) or bubbles, is the insulateing factor and makes it work.
I think if the 1/2 inch thick foam pads are used, one would have to support the plywood with 1/2 inch spacers, to allow the insulateing air bubbles in the foam to work. If compressed...it wouldn't be any more then putting a sheet of rubber down for insulation.
I have insulated water-lines in a campground, New Hampshire, that were only down 15 inches with just straw. The trick is to fluff this straw up over it to allow air-space...