Johns Manville Super Firetemp X 4'x4' for $10/sheet?

I'm from Maine and I've been trolling for a while, planning to build a RMH. I'm also planning to build a wood gasifier.

Someone near me in Maine is selling Johns Manville Super Firetemp X 4'x4' sheets of various thicknesses for $10/sheet.

Is this a steal for a useful resource for these projects? What thickness should I buy?

Hi Brad;
That product is good up to 1700F  J tube rockets can run 1800F, batchbox rockets can run over 2100F
It will not hold up as any interior part of the core.
It could be used as insulation behind bricks or possibly under bricks beneath your core.
It would also make a good isolator for a mass against an outside wall.
The price is great I would go for 1" thick.