The ASTM C 201 apparatus is being evaluated to determine its suitability for measuring the thermal conductance of ceramic fiber insulation. This work is being done in support of a proposed new ASTM test method. A two-dimensional thermal model of the ASTM C 201 apparatus was developed using a finite-difference conduction program. The errors in measured thermal conductance due to lateral heat flow were predicted for several test parameters. Results of laboratory experiments are also presented for the following investigations: hot- and cold-face temperature uniformity, calorimeter surface emittance, and calorimeter heat flow measurements. Comparisons are also made between thermal conductance data obtained from an actual furnace installation and data obtained on the C 201 apparatus. The results indicate that the ASTM C 201 apparatus yields satisfactory data for designing furnaces and predicting heat loss and cold-surface temperature on furnaces.