Interestingly, the first heated floors were accomplished by the Romans by building fires and routing the hot air under their stone floors. However, using hot air under a slab is very inefficient for heating a slab and the heated space above the slab and only works if the hot air is much hotter than what you want the temperature of the heated space to be. Perhaps think of it this way...heat always moves from cold to hot and the rate that the heat moves is proportional to the temperature difference. In terms of steady state conditions, if you have a constant 70F air temperature below the slab and a constant 60F air temperature in the space above the slab, the center of the slab will eventually reach 65F. The surface of the floor will only be slightly warmer than 60F. It is the temperature of the surface of the slab that heats the space above the slab and in this case, the rate of heat that will move into the heated space will be very small because of the very small temperature difference. In order to heat the space above the slab, the rate of heat that moves from the slab surface to the heated space must be greater than the rate of heat that moves from the heated space to the outdoors. The rate of heat that moves from the slab surface to the heated space is proportional to the difference of the heated space temperature and the slab surface temperature. The rate of heat that moves from the heated space to the outdoors is proportional to the difference of the heated space temperature and the outdoor temperature. So depending on the energy efficiency of the heated space (i.e., how well-insulated and well-sealed from the outdoors), the slab surface temperature needs to be anywhere from a couple degrees to many degrees above the desired heated space temperature (e.g., 72F to 85F). So bottom line, if you really want the slab to heat the building, you will want to put PEX in the slab and circulate water that is at least 85F and perhaps even higher temperature depending on the energy efficiency of the building. You will also want to put insulation under the slab so most of the heat generated by the PEX goes up into the heated space and NOT down into the crawl space below the slab. If you don't want the slab to actually heat the building and you only want the slab to not feel cold, then just condition the crawl space as you indicated. |