Measurement of low thermal effusivity of building materials using the thermal impedance method

Thermal impedance is a way of defining the characteristics of thermal syste ms. It is a function that represents the relation between the frequency com ponents of temperature and the flux density in a plane for each frequency. Up to now, its use has been restricted to one-directional conductive system s. From the experimental point of view, it is determined simply by measurin g the flux density and temperature simultaneously in a measurement plane. I n practice, a fluxmeter in which a thermocouple has been placed is put in c ontact with the sample. The changes in flux density and temperature measure d in this way are different from those in the material access plane. The re asons for this perturbation are the presence of the sensor and the sensor/m aterial contact resistance. In the case of slow changes, due, for example, to micro-climatic variations or day/night stresses of the order of 10(-5) o r 10(-4) Hz, this perturbation is negligible. Studies in these frequency ra nges have been exploited in several works. In the present study, we show th at it is possible to use thermal impedance as a way of characterizing therm al systems for higher frequencies, taking into account the perturbation cre ated by the measuring instruments. By means of a sensitivity study, we demo nstrate several cases linked with the nature of the test material. A freque ncy range is determined where the perturbation due to the measuring instrum ents is not too great, allowing the materials to be characterized. Several common construction materials are studied. Particular emphasis was laid in this work on characterizing insulating materials, which are hard to study i n variable conditions. The tests discussed in this article were performed i n the laboratory in ambient temperature conditions close to 20 degreesC. Th e pseudo-random stresses were generated artificially. Series of 100 tests w ere run for each material. They led to the determination of thermal effusiv ity with less than 5% error. The method gives results that are reproducible and can be validated by simulation.