R = the measure of a material’s resistance to heat flow. It is shown by the temperature difference necessary to cause heat to flow through a given unit area of material. This is calculated at a rate of one heat unit per hour.
R T = the measure expressing the total thermal resistance for a construction assembly. This is the sum of all individual R-values of the assembly’s component materials.
U= the coefficient of the overall heat transfer of a material. This is expressed as the rate of heat transfer through a unit area of a particular building component. This is caused by a difference of 1 between air temperatures on each side of the building component. This is the reciprocal of the R-value.
1/k = is the measure of thermal property per inch of thickness.