A Round Robin series of tests was run to estimate the precision of ASTM C518, when used to measure the k-factor of rigid urethane modified polyiso cyanurate foam boardstock. Data on different board thicknesses was gathered [well aged 1.0 inch (2.5 cm), 2.0 inch (5.0 cm), and 3.0 inch (7 5 cm) thick, foil/kraft faced laminate board] to represent the entire product range. Thus, information will be pre sented on the precision of ASTM C518 as a function of sample thickness, as well as the between-lab and within-lab variability