This monograph presents both the theoretical foundations and practical applications of the principle behind the "instantaneous" flat heat source method, developed for measuring the thermophysical properties of insulation materials. It formulates a mathematical model of the temperature field within the tested sample and derives the key equations that form the basis of the new measurement technique. The book also identifies optimal operational parameters and rational design dimensions for the measuring device, aimed at minimizing measurement errors of the targeted thermal properties. This work will be valuable to researchers, engineers, graduate students, and undergraduates involved in the design, development, and application of methods and instruments for assessing the thermophysical characteristics of materials.