Insulating a RMH J Tube on a subfloor

We are building our first RMH in a bedroom and have run into a few roadblocks. We want to build the J tube on a vermiculite/clay insulating base on top of a sheet of rockboard, which would either rest on the subfloor or replace the subfloor. Is this practical, and safe?

No, it is not. You need to include an airspace beneath the combustion core floor so that heat can be carried away from the subfloor.

Insulation does not stop the flow of heat, it slows it down, and if a burn lasts long enough, the heat will work its way through to the wooden supports. An airspace which allows free ventilation can short-circuit this and allow the heat to be carried out to the room air. You still want some insulation under the core to slow heat transfer as much as practical.

Ok, thank you for your quick reply! So, if we put a sheet of rockboard down on the subfloor, then strips of rockboard and another sheet of rockboard on top (to create airflow) would that work? Would all of that rockboard -which I understand would conduct heat rather than insulate it- draw heat away from our burn and make it cooler?

You need about 2" or more for air circulation to happen without a fan forcing it. Rows of bricks laid down with several inches space around each brick supporting the cement board are a common method.

This would be in combination with the insulation you described in the OP.
Aluminum foil, shiny side down, placed on top of the bricks would give some added insulation, as the shiny metal surface reflects heat, in both directions, up into the foil and down out of the foil.