I work in commercial HVAC. We had a job were the engineer spec's the in floor heat built with 6 inches of sand between the insulation and concrete. So it was 2 inch foam, re-mesh, pex, 6 inches of sand, 4 inches of concrete.
Because sand is a poor conductor of heat, the heat from the pex rose through the sand and little was conducted down to the insulation.
I like that design.
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