Calcium silicate or ceramic fiber board

[UtahBeehiver]

Either works, similar in properties.

[yomer]

What about having one board vs small segments to make up the area under the base?

I found a warehouse which has several cal sil pieces laying around which I can join to form one "slab", as well as several 24x24in foamglas pieces.

Should I make the extra effort and find a supplier to build insulation with as little segments as possible?

[david s]

Segments ar fine. In fact they have the advantage of allowing a pathway for moisture to escape, provided you have some drain holes in the supporting slab. Just don’t make the gaps too big.Maybe nothing bigger than 1/4”. The floor bricks will easily bridge over that.