Reference Materials for Thermal Conductivity Measurements: European Situation

Abstract

A reference material (RM), as defined by the International Vocabulary of Metrology (VIM 2012), must be homogeneous, stable, and suitable for use in measurements. Certified reference materials (CRMs) are RMs with documented property values, uncertainties, and traceability. ISO 17034:2018 outlines the requirements for RM producers, ensuring that CRMs meet standards for stability, uniformity, and reproducibility. In Europe, CE marking, from French “conformité Européenne”, which means European conformity, has been mandatory for thermal insulation products since 2002, ensuring their thermal performance is verified by accredited laboratories using RMs like IRMM440 and ERM FC440. Annually, European manufacturers produce over 200 million cubic meters of thermal insulation, necessitating thousands of thermal conductivity measurements daily to maintain CE marking compliance. Key characteristics of Reference Materials include long-term stability, thermal conductivity within specified ranges, and minimal dependence on density, thickness, and applied load. Sample thickness must conform to apparatus specifications, and homogeneity must be quantified. Reference Materials must also have appropriate dimensions, surface smoothness, and manufacturability. The Joint Research Centre (JRC) Geel has produced two Reference Materials, IRMM 440 and ERM FC 440, with specific characteristics to meet these requirements. Both are glass wool fibers with low thermal conductivity and specific density and thickness. The qualification of RMs involves inter-laboratory comparisons to ensure the accuracy and traceability of thermal conductivity measurements. The European market’s organization, including the use of Reference Materials and CE marking, has significantly improved measurement consistency and product quality. This system has led to lower uncertainties in thermal conductivity measurements compared to North America, highlighting the impact of standardized RMs on industry practices. Future needs include developing RMs with lower conductivity and increased thickness to accommodate market trends towards super insulation materials and bio-based components, enhancing energy performance calculations for buildings. This paper will present the process of defining a reference material and how it affects the uncertainty level of the calculation of building energy performance. This level depends on the characteristics of the materials used, their implementation, and external factors, such as the weather, as well as the reference material used for calibration of all European thermal conductivity measurement devices.