Thermal Conductance
The C-value (thermal conductance) represents the amount of the heat flow through a square foot of material per hour per degree of temperature difference between the inside and outside temperatures. The C-value is based on the stated construction or thickness of the material. This distinguishes it from the k-value, which is based on a 1-in thickness of a homogeneous material having no fixed thickness. For example, under “loose fill” in Table 3-1, you will see vermiculite listed and given a k-value of 0.48. Why a k-value? It is given a k– value (rather than a C-value) because vermiculite is a loose-fill insulating material that can be poured into a space to form whatever thickness is desired. It is convenient, then, to give numerical values (k-values) based on 1-in thicknesses of these materials. If, say, vermiculite were used to insulate the top-floor ceiling (that is to say, the attic floor) and poured to a depth of 4 in, the R-value (thermal resistance) would be calculated as follows:
Under “masonry units” in Table 3-1, you will find hollow clay tile listed with six different stated thicknesses (e.g., “1 cell deep, 3 in; 1 cell deep, 4 in”). Each is given a C-value rather than a k-value because the thickness of the material listed in the table is the same thickness that will be used in the construction of a structural section.
The masonry wall in Table 3-2 also contains gypsum plaster (sand aggregate) 1⁄2 in thick. The C-value is given as 11.10, and its R-value as 0.09 (Table 3-1). In other words, the R-value (thermal resistance) is calculated as follows:
Examine the data in Table 3-1 once more. You will probably note that each building material has either a k-value or a C-value, but not both. Each material is offered on the basis of either a stated thickness or an arbitrary thickness, as was just explained. Note also
that each type of building material has an R-value determined by either of the following two equations:
The R-value is simply the reciprocal of either the k-value or the C-value.