R value - Definition - the "resistance" of a substance to heat transfer by conduction (according to Energy, Its Use and the Environment)
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 over k. The higher the R-value is, the bettwer 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 funciton of both the type of material and its thickness.
L/k = R vlaue of insulation
R-value according to Physics by Cutnell and Johnson
R value - it expresses in a single number the combined effects of thermal conductivity and thickness. - Larger R values reduce the heat perunit 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 differen tthermal conductivities and idferenet thicknesses. The R values for the indicidual layeers can be added to give a single R value for the entire slabe. 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.