keeping the fire stoaked.

[MR P-BODY]

I'm guess'n you use antifreeze in these.. as a few days with no power would destroy the system.. not understanding the "foam" under the slab.. as I'd think that help the slab crack if heavy weight was put on one spot..

Yes... I used RV antifreeze.... its 2" Styrofoam sheets on top of the sand... its
plenty solid for the concrete... that insulates the slab from the cold ground

[TheSilverBuick]

Styrofoam is amazingly strong. They use it under highways these days too. I don't recall if it was modern marvels or what, but I watched a whole show on the properties of styrofoam and it was dang interesting. It can have more properties than just the light weight packing variety.