Radiant floor RMH

I’m getting ready to build my first RMH.  The mass top will sit at floor level, 7 ½ by 8 feet footprint.

In February and March 2017, ---an extraordinary year for rain and snow in the Boise River basin--- I had water 8” above the  base of my future heater mass.  So I’m thinking of setting an outside row of concrete blocks, in cement mortar.  Two of the four sides butt up against, or rest on,  interior footing walls, so I’m not as concerned about settling and cracking.   Or  should I lay a concrete footing under the blocks spanning that 7.5 feet?  

I’m concerned about heat transfer through ground moisture, so I think a vapor barrier will be a good idea.  I thought it’d go inside the block wall.

Then, I’m thinking of lining the ground with perlite, an inch thick?  reducing heat transfer into the Earth.   I should also line the concrete footings with perlite.   I guess, I dont know!  

What do you folks in the know think?

Well, if you want to heat a whole slab. You need to insulate the whole perimeter and bottom. And keep this insulation dry.