An apparatus for the routine measurement of thermal conductivity of materials for building application based on a transient hot-wire method Alessandro Franco * Dipartimento di Energetica ‘‘L. Poggi’’, Universita ` di Pisa, Via Diotisalvi, 2, 56126 Pisa, Italy Received 18 October 2006; accepted 16 February 2007 Available online 12 March 2007 Abstract An apparatus has been designed and developed for the measurement of the thermal conductivity of samples of non-metallic materials whose thermal conductivity is quite low (in the range between 0.2 and 4 W/m K) by the transient hot-wire method. It is especially con- ceived to ensure short time consuming and economic measurement of the thermal conductivity of mortar and lateritic bricks for building application. Thermal conductivity is measured by tracking the thermal pulse propagation induced in the sample by a heating source consisting of a Nickel alloy wire. The temperature is measured on the wire by means of two T type (Copper–Constantan) thermocouples. The heat impulse transferred to the wire between two observed times gives a temperature increment of 2–10 °C, depending on the thermal con- ductivity of the material tested and on the preselected level of the heating power supplied. The thermal conductivity of the materials can be obtained in a comparative way or in a semi-absolute way. In both cases a preliminary calibration of the instrument, obtained with a reference sample whose thermal conductivity previously certified by absolute methods, is in the range required. In the second case, the calibration is necessary to obtain the characteristic curves of the instrument. The paper shows the measurement obtained using materials with thermal conductivity between 0.2 and 1.5 W/m K. In good exper- imental conditions the accuracy of the measurements is within 5%. The proposed apparatus offers significant advantages, mostly in terms of economy and flexibility, over systems currently in use and over similar systems based on transient methods. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Thermal conductivity; Measurement; Transient hot-wire method; Line-source; Building material 1. Introduction The use of thermal insulator mortar and plaster and low thermal conductivity bricks is regarded as one of the most effective means of energy conservation in buildings. Many types of insulation materials are available which differ with regard to their thermal properties. Recent efforts have shown that mortar, blocks and bricks made from lateritic soil or porous concrete can reach thermal conductivity val- ues low enough to meet future building envelope standards. The need for the development of advanced materials for building applications requires a more accurate look at their thermal properties. In general, the effective conductivities of materials depend on density, temperature, moisture con- tent, as well as the constituents and voids present in their structures. The thermal conductivity of an insulation mate- rial and in particular of lateritic materials does not only depend on its density, temperature and moisture content, but it also depends on the material atomic and molecu- lar structure, porosity, anisotropy, structural faults and 1359-4311/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2007.02.008 * Tel.: +39 050 2217154; fax: +39 050 2217150. E-mail address: alessandro.franco@ing.unipi.it www.elsevier.com/locate/apthermeng Applied Thermal Engineering 27 (2007) 2495–2504