I'm unsure how to approach this, as solid blocks of materials next to each other (e.g. steel and air) with no barrier inbetween as shown on http://en.wikipedia.org/wiki/Thermal_conductivity "x" would equal zero, this can't be correct. Is there another equation relating temperature difference, area and conductivity of two adjacent materials? Possibly also mass, volume or density? All I really want is a rate of heat loss via conduction in Watts or Watts/m². I know how to calculate Black Body ratiation via the Stefan-Boltzmann constant.