Insulating brick below floor?

[pjcampo]

Could one use insulating brick below the standard firebrick floor instead of FB Board, and how would this compare in the insulating properties. Are there other considerations?

[dmun]

Insulating fire brick is similar in insulating quality to vermiculite concrete. Plan on two layers, or laying the bricks on their edge.

It's not usually done, as insulating fire brick is usually more expensive than other choices. I have seen it cheap on ebay, though.