Thermal insulations which permit significant heat transfer by radiation show an increase in thermal conductivity (or decrease in R-value per unit thickness) as thickness is increased. This thickness effect, for a common type of glass fiber building insulation, was determined in a large heat flow meter apparatus by comparing the R-value of a stack of pieces with the sum of the R-values of the separate pieces. Contrary to widespread belief, this effect, for a change from 38 to 152 mm (1.5 to 6.0 in.) was found to be only in the range of 1.5 to 3 percent. This percentage change will depend upon the thickness used as basis for comparison; for example, if at 152 mm (6.0 in.) the change from 38 mm (1.5 in.) were 2.5 percent, the change from 25 mm (1.0 in.) would be estimated to be about 4 percent.
At greater thicknesses, even after an extensive test program using an apparatus that is exceptionally well guarded and exceeds ASTM requirements, uncertainties exist which point to the need for more strict ASTM requirements and for even larger equipment. However, the high cost, greatly increased test times, and currently unproven accuracy of thick measurements may limit the use of extremely large equipment.
Various other data relating to these tests and to verification of the test equipment are also reported.