The following information is obtained from Energy, Its Use and the Environment:
"A measure of the resistance of the material to heat flow is the thermal resistance R, often called its "R value" given by R=delta/k. The higher the R-value is, the better are the insulating properties of the material. Using this notation, the rate of heat flow is given by
Qc/t = 1/R x A x change in T
R is a function of both the type of material and its thickness.
L/k = R value of insulation
R-value according to Physics by Cutnell and Johnson:
- The R value expresses in a single number the combined effects of thermal conductivity and thickness.
- Larger R values reduce the heat per unit time flowing through the material and therefore mean better insulation. It also convenient to use R values to describe layered slabs formed by sandwiching toget a number of materials with different thermal conductivities and different thicknesses. The R values for the individual layers can be added to give a single R value for the entire slab. It should be noted, however, that R values are expressed using units of feet, hours, F and BTU for thickness, time, temperature, and heat, respectively.
According to the Kansas State University Extension Service the R-value of soil is about 1 per foot.