Insulation and R Values

It is well to mention that in some systems like that in Fig. 2- 2 two-dimensional heat flow may result if the thermal conductivities of materials B, C, and D differ by an appreciable amount. In these cases other techniques must be employed to effect a solution. 

In Chap. I we noted that the thermal conductivities for a number of insulating materials are given in Appendix A. In classifying the performance of insulation, it is a common practice in the building industry to use a term called the R value, which is defined as 

The units for R are °C m2/W or °F ft2 h/Btu. Note that this differs from the thermal-resistance concept discussed above in that a heat flow per unit area is used. 

At this point it is worthwhile to classify insulation materials ip terms of their application and allowable temperature ranges. Table 2-1 furnishes such information and may be used as a guide for the selections of insulating materials. 

Type Temperature range, C conductivity, mW/m °C Density, kg/m3 Application 

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