Can thermal conductivity tests be performed at thicknesses greater than that of the primary standard? As a test specimen thickness increases, the edge loss effects can also increase, causing a heat flux through the specimen that may become less one dimensional. New equipment designs in thermal comparators allow for greater guard-to-metered-area ratios, which should reduce this effect and allow for measurements on thicker specimens than before for commercially available equipment. Laboratory testing of roll and batt insulations at a representative thickness of 76 mm has always been considered to be representative of the performance at full thickness. As more stringent building codes call for increasingly higher levels of insulation in homes, verification of material performance, such as loose-fill insulation, requires that it be tested at a representative thickness that is greater than that of the calibration standard. This paper describes a series of tests performed using a series of full-thickness fiberglass insulation standards that were delivered to a national laboratory for thermal characterization and tested in a guarded hot plate apparatus conforming to ASTM C177-10 from December 2008 to July 2009. Tests were conducted on a total of six light density (12 kg/m³) fiberglass standards varying in thickness from 76 mm to 203 mm of thickness for derived thermal conductivity values with associated uncertainties, characterized as calibrated transfer specimen (CTS) sections, and brought back to determine whether they could be effectively used in this “stacking exercise” study on one commercially available large-thickness opening ASTM C518-10 thermal test apparatus. From February 2011 through August 2011, a total of 40 tests were run in single and stacked configurations up to and including 305 mm (12.0 in.) in thickness, with and without septa.