Do you have a product called Pratleyperl in the UK? It is made from the mineral perlite. A really remarkable thermal insulator and is mostly used in bead form and laid on your ceiling for insulation. It is also used mixed with cement to plaster house walls for insulation, which I have done to my house on the Namibian West Coast and the result is great – huge reduction in heating/cooling requirements. . Raw beads weigh about 100kg/cubic meter. Also, mixed with cement, used to insulate ovens, furnaces, cryogenic storage, etc.
Basically mix, dry, with cement in 3:1 volume, then add water till a firm mix, and form into the shape you want. A test I performed was to make a 25mm thick slab, about 200mm square, and cure well. An Oxy-Acetylene hot flame played onto one face till the surface was orange/red, with the flat of my hand up against the opposite side – warm but far from uncomfortable to keep my hand there!
You can make any shape you want, and it does not seem to powder much, unless you rub two like surfaces together.
The only heat insulator that would not do that is probably in the family of ceramics.
Joe