[Cob] solar radiant sand bed under cob cottage

[tabitha and karl o'melay]

my current plan is as follows:
i'd like to incorporate solar hydronic heat into my earthen floor. the 
method that has intrigued me the most is to incorporate a large sand 
bed under my earthen floor and run pex trough it heated by solar 
collectors.
a good passive solar design will take care of the first part of the 
evening/first-night depending on amount of solar. the delayed themal 
mass sand bed will extend my solar output duration. (daycreek.com) has 
an example of this in his cordwood home.

the trouble:
the problem as i understand it is heat also conducts down into the sand 
bed from the alternate source of heat an example is normal radiant heat 
located above the sand bed in the earthen-cob/cement floor.

a solution:
drycreek overcame this by not putting upper radiant tubes above the 
solar radiant sand bed.

the real problem:
i plan to build a small cob cottage & want to heat the whole floor.

the big question:
has anyone heard of any product that works like "thermal gortex" heat 
transfer through a barrier only one direction. or a sheet of thermal 
diodes?

my personal brainstorm follows-
i know of a product from the radiant industry that is, of many names, 
(bubble foil bubble-aluminum foil sandwiched between bubble wrap). the 
r value is low but the aluminum reflective barrier reflects most of the 
"radiating" heat--increasing its insulating properties.
they also make a version, usually combined with insulation batting, 
that is (bubble foil-aluminum foil on one side of the bubble wrap)
you might see where i'm going. if the single bubble version were placed 
foil side against the sand bed the foil would conduct from the sand to 
the small bubble barrier and heat could radiate upward. conversely the 
heat from above would reflect back like the normal (bubble foil bubble) 
version.

  the caveat as i see it is--the cold that could live below could also 
conduct through the foil and penetrate into the upper layer. but at the 
end of the day heat does like to rise.

  any ideas? bubble insulation

just trying to make some sense out of it all
karl