You don't want ceramic wool to spread into your cooking chamber, ever. Fibers of ceramic wool should be treated the same as asbestos, always wear respirator when handling, cutting, etc.
I would rather pour in a layer of refractory cement for thermal isolation. You can find many brands from the favorite online shopping site but my recommendation is the brand Girtech. It is non-toxic, doesn't contain harmful ingredients like many other do. It can be used as a mortar with regular firebricks and also as a pourable/castable refractory. 1/2" layer would give you 3100F heat isolating barrier. The only drawback of Girtech is that it is very gritty, but if you are pouring a layer that shouldn't be a big problem.