Extensive work has been done on insulants at ambient temperature: standards, reference materials, and national and international interlaboratory measurements (ISO/TC 163 round robin test). The situation is more complex at high temperatures, and this paper in an attempt to clarify this complexity, taking into account the new International Organization for Standardization (ISO) draft proposal on guarded hot plate apparatus.

Using a simple model describing heat transfer through fibrous insulants, and introducing the notion of optical thickness or opacity, the study proposes a quantitative criterion to minimize the radiative transfer, in order to obtain measurement results independent of the experimental conditions (the thickness of the specimen and emissivities of the limiting surfaces).

This criterion shows the importance of optical parameters and should, from a practical point of view, help in choosing the reference materials used for checking and in making interlaboratory comparisons of guarded hot plate apparatuses.