Adding a couple of inches of styrofoam under the concrete would not cost that much, and then, the heated floor would work nicely. I did that to a house we built in Arkansas.
Piped hot water from the switched dump load of the 4 KW wind turbine I built. I put 2--2KW water heater elements in a 150 Gallon butane tank that was sitting on the concrete floor of the attached greenhouse we had. Radiant heat from that tank kept plants from freezing.
We had an automotive engine water pump hooked to a DC generator as a motor. Only ran that gen/motor for 10 minutes at a time. Never had "HOT" water, but, you could feel the difference after the turbine was switched to those heater elements. We were in extreme NW Arkansas in the Ozarks, 40 miles south of Missouri state line..