For a thermal conductivity barrier that's fire proof use some Micore 300 board, but that will need to be covered with something that's durable, as Micore sure isn't. My hearth is two layers of Micore topped with a layer of hardi-backer and then slate over top of that. The Micore can be sourced at Upper Canada Forest Products; it has to be carried on edge as it won't support its own weight when carried flat.
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"The trouble with people idiot-proofing things, is the resulting evolution of the idiot." Me
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