A Comparative Test Method to Determine Thermal Resistance under Field Conditions

This paper presents the development and application of a test method to determine thermal resistance of a material under transient conditions. The method consists of testing two material slabs placed in contact with one another, one of them being a reference material with known dependence of thermal conductivity and heat capacity on temperature and the other being a test specimen whose thermal proper ties are to be determined. Thermocouples are placed on each surface of the standard and reference materials to measure temperatures during a transient heat transport process. These tempera tures are used as the boundary conditions in a series of calculations. First, the heat flux across the contact surface between the reference and the test specimen is calculated using a numerical algorithm to solve the heat transfer equation through the reference material. Then, imposing the requirement of heat flux continuity at the contact surface, corresponding values of thermal conductivity and heat capacity of the test specimen are calculated with an iterative technique. These calculations, done as a function of time, result in a set of thermal properties of the test specimen for the range of boundary conditions imposed during the transient process. This method can be used under transient conditions of either laboratory or field testing. Of particular interest is its application to examine effects of aging (effect of time) and weathering (effect of time and environmental conditions) on thermally in sulating foams. For instance, in this research, the reference and tested specimens are placed in an exposure box representing a conventional roofing assembly and the change in the thermal properties of the test specimens is determined.