On this type of foundation (like the cabin built by Rob Lemay) how is the
underside insulated?
Most of the builders here have insulated between the floor joist using fiberglass bat between followed by harware cloth for vermin protection. This is done almost universally after the cabin in dried in by crawling around under the cabin. A couple exceptions are ones who used DIY foam and mineral wool (also from under the cabin) plus at least one who use foam board under the subfloor IIRC. Yet another is planning to insulate from above by not extending his subfloor under the bottom plate of the wall.
There are as many variations as there are people and projects.
Mike
We used fiberglass batts -- installed the wires to hold it in place (will put a critter barrier up later), then put in the batts, then the vapor barrier and the lid.
That way I wouldn't have to crawl around underneath later -- that was the plan anyway but it seems I'll have to anyway :P d*