lew (01/09/83)
Mark Davis posted the equivalence: R-value of 1 = 1 hour * feet * feet * degrees F / BTU Note that this is not quite the reciprocal of conductivity. The R-value of a wall doubles if the thickness doubles. The conductivity is an intensive property of the material and does not depend on thickness. To get reciprocal conductivity you have to replace degrees F (a unit of temperature difference) with degrees F per inch (a unit of temperature gradient.) The R-value as it stands is obviously the unit of interest in materials of fixed thickness which are measured by the square foot. Also, Phil Karn mentioned " ... a metric unit specifying the temperature gradient in degrees Celsius ... to cause a 1 calorie/hour/square meter heat flow." This confuses the temperature difference with temperature gradient, and besides, neither the calorie nor the hour is a metric unit. The metric unit of thermal conductivity is: watts/meter/K This is one of those cases where inequivalent units cancel: the thickness unit cancels half of the area unit. Lew Mammel, Jr. ihuxr!lew