Linda in Ohio asks about wood wool insulation. I think that is the correct
name and have seen it used to insulate roofs in schools, swimming pools etc.
(Mine were offcuts, acquired for the price of a jar of honey.)

The drawing accompanying the Open Mesh Floor trials state, "Minimum 2" dense
polystyrene or Styrofoam insulation recommended".

Hope this has been of some assistance Linda but basically a good insulation
and one easily and cheaply obtained.

Ken Hoare