A critical examination of the current European and related standards for the guarded hot plate method has been carried out to address its suitability for measuring thermal performance properties of thermal insulations at elevated temperatures in the range 100 °C to 850 °C. An historical overview illustrating the significant potential problem areas is contained in part 1 of this paper. Part 2 presents a detailed discussion of the cause and effects of the various issues and associated parameters involved. These include:(a) Apparatus principles and design, including gap heat interchange and better control of radiation and edge loss effects. (b) Design parameters for the heater, including size, construction materials and heaters, adequate modelling, emittance of plates and temperature sensors. (c) Temperature measurement and control, and the effect of thermal degradation on its performance. (d) Specimen limitations. (e) Operation and performance checks, including the need for strict operation criteria and measurements on heating and cooling. (f) Needs for an adequate range of reference materials and/or transfer standards.Possible ways and means to eliminate or minimize these effects such that the necessary precision levels satisfy current European Union mandated levels of product performance are proposed. It is recommended further that as a result of these findings a new standard specifically for use of the method at high temperatures should be prepared. Also discussed are needs for additional research work believed necessary in certain areas in order to improve the precision further over the temperature range 100 °C to 850 °C.