On 23 May 2006 18:32:48 GMT, a particular chimpanzee named Ian
Stirling randomly hit the keyboard and
produced:

I have a 1600s or so scottish cottage, that's lacking in insulation.
It's some 20m*5m, with a suspended wooden floor, with no insulation
underneath. The temperature below the suspended floor is essentially the
average house temp.

Hopefully, the void below your joists are ventilated. If so,
virtually all your heat from the floor will be lost through this path,
and the amount of heat lost through the soil will be insignificant.
If not, then you've got more problems than heat loss to worry about.
--
Hugo Nebula
"If no-one on the internet wants a piece of this,
just how far from the pack have you strayed?"