Posted By Bob I on 05 Oct 2015 10:54 PM
Second law of thermodynamics: Heat Flows to Cold. A cement basement slab will draw heat out of the house to the 50o earth. Constantly. 24/7/360. The idea that your oil burner will warm up the miles of earth under your house is dreaming; won't happen. Insulate the slab.
Bob,
I have a different understanding of this heat transfer problem than what you have described. First, if you want to consider heating up miles of earth under your house, you would need to consider the geothermal gradient, which is the rate of temperature increase with depth into the earth. It is about 1 degF for each 70 ft. (https://en.wikipedia.org/wiki/Geothermal_gradient), so if the ground temperature were 50 degF at the surface, it would be 125 degF at one mile, so you would not be losing heat into "miles of earth," but rather gaining heat.
So why doesn't this underground heat reach the surface? Because the soil is a pretty good insulator. In my area, the soil is about R9 per foot of dry sand. So at 10 ft. below the gravel on the bottom of the conditioned crawl space, the heat is looking at R90 of "insulation." Detailed calculations will show that the gravel or slab does not stay at 50 degF, but both the slab and the earth under it rises in temperature, with the only significant heat losses going to the air at the ground surface.
Take my house as example. For heat traveling through the floor of the crawl space floor 4 ft. from the perimeter, it must travel that 4 ft. plus another 4 ft. to reach the surface. At R9 per ft., the heat must travel through R72 worth of insulating soil, plus R5 worth of foam to reach the air. That is the reason for the recommendation that the insulation is only needed on the outside 4 ft. around the perimeter inside the edge of the foundation.
It is a bad assumption that the earth under a home is an infinite heat sink with zero thermal resistance. A good designer does not waste precious resources duplicating what is already there in the earth, in this case, good insulation provided by the soil.