U-Value Measurement in Buildings. is It an Accurate Estimate?

The U-value of thermal transmission of building components expresses the insulation capacity of walls and can be used to define the energy efficiency of buildings. The value is also required by the European directive 2002/91/EC relating to the energy performance of buildings. Recent commercial devices provide compact and easy-To-use systems for measuring thermal transmittance (U-value) according to the ISO 98691:2014 standard. Such systems include a data logger, two surface temperature sensors on the external wall, one (or two) surface temperature sensor on the internal wall, and a heat flow sensor inside. Data collected from measurements are then processed using two methods: average method (ISO9869:1994 standard) and black box method. However, several requirements and precautions must be implemented to ensure an accurate measurement. Above all the temperature difference has to remains stable over time in order to obtain a stable thermal flow through the wall. The paper presents the results of an experimental campaign implemented to measure the thermal transmission coefficient of a prototype building in the Department of Mechanical, Energy and Management Engineering of the University of Calabria. The results show how variable meteorological conditions can strongly affect the measurements. The study reveals how the real uncertainty of measurement is not in the instrument precision, but in having suitable measurement conditions, and therefore careful consideration must be implemented both in the measurement phase and in the post-processing of acquired data.