On 4/22/2015 6:58 AM, Robert Green wrote:
wrote in message

FWIW, I really doubt if foam is necessary because heat rises and even
heating the soil below will eventually cause the surface to become
warmer.

conducted heat does not rise hot air rises.

You're making a pretty big assumption that this would all be conducted heat.
It's not. Are you familiar with the thermal properties of soil and why it's
been used as an insulator in rammed earth homes and other types of
construction? It's because soil does NOT behave thermally like a liquid or
a metal, it's actually fine grains of material surrounded by millions of
tiny *air* pockets.




Compare the thermal conductivity of soil to some other materials and you'll
see it's more insulator than conductor

Thermal Conductivity
- k -
W/(m K)

Glass, wool Insulation 0.04
Gold 310
Gravel 0.7
Ground or soil, very moist area 1.4
Ground or soil, moist area 1.0
Ground or soil, dry area 0.5
Ground or soil, very dry area 0.33

That's even before we consider thermal equilibrium and which direction heat
would move in if the ground below was warmer than the surface of the
driveway, which it almost always is.

It is better than gold, but not nearly as good as insulation. There is
a reason foam board is used to insulate basement floor even though they
are over dirt. I agree it is not an even distribution, but there will
always be loss into the dirt.