The building envelope, or the “skin” of the building, is made up of the walls, floors, and ceilings. Among its many functions, the envelope helps control heat exchange with the exterior environment. When specifying building envelope materials or systems, specifiers look to the R-value and U-value to establish a baseline for efficiency. ASTM C518, Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus, is the most commonly used test method to measure a material’s thermal resistance, or R-value, through a single material. ASTM C1363, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus, is used to determine thermal transmittance, or U-Value, through the built-up assembly.
Home Innovation conducts both tests, and many others, to provide our clients accurate results they can use to measure and compare the thermal performance of their products. But often, there’s some confusion – what’s the difference between R-value and U-value? R-value is a material rating; it is a measure of the performance of a specific material based on its ability to reduce the transfer of heat. U-value is not a material rating; it is a measure of how well an assembly keeps heat from escaping a building. R-value is a measure of the rate at which heat is transferred and is based on conduction and resistance. U-value is a scientific calculation of the conduction properties of the various materials used in the construction of the assembly.
