hot box, opaque envelope, measurement, dynamic heat transfer
The characterization of the thermal behaviour of opaque building components is essential in early design stage to compare different alternatives. The evaluation of the dynamic thermal response to the external solicitation is necessary for an effective design study especially for the climates with important annual cooling demand, such as Southern European ones. According to EN ISO 13786, opaque elements can be characterized through some dynamic parameters (i.e., periodic thermal transmittance, time-shift, decrement factor), which can be calculated starting from the wall materialsâ€™ thermal properties. However, when either the material thermo-physical properties are unknown (e.g. in existing buildings) or the assumptions on which the method is based are not met (e.g. in platform-frame structures), the calculation method does not assure enough accuracy of results. Hence, for these components, a direct measure of these parameters would be extremely useful. Differently from the procedure to determine the steady-state thermal transmission properties, which is well established, there are no standard rules for the experimental measurement of the dynamic parameters. Nevertheless, as demonstrated by some recent works in the literature, it is possible to estimate EN ISO 13786 dynamic parameters from non-destructive measurements based on heat flow meters, HFM. Since these experimental procedures are still under development, the extent to which several aspects limit the accuracy and precision of the measurements are not yet cleared. In this framework, this work presents a theoretical and experimental analysis about the achievable reliability of the post-processing procedure for the evaluation of the dynamic characteristics of opaque constructions by means of HFM-based measurements in modified hotbox apparatus. Experimental and numerical tests have been performed in order to assess the impact of different sources of uncertainty on the estimation of EN ISO 13786 dynamic parameters. In this regard the main source of errors investigated deal with the sample and the hotbox apparatus, e.g. the boundary effects in heat conduction, the edge guarding of the HFM, the noise of eddy close to HFM surface, the stability in time and space of the surface temperature of the sample and the HFM assembly modality. Moreover, several aspects of the HFM are also investigated, such as the HFM calibration curve, the effect of HFM emissivity and the response time constant. A multi-layer timber wall construction has been tested in a modified hotbox apparatus with different boundary conditions, regarding, in particular, temperatures and convective heat transfer mechanisms imposed at the two sides of the specimen. A numerical model of specimen and apparatus has been also developed and calibrated against the experimental data. With this model, we have investigated further conditions related to the material properties and dynamic forcing solicitation, including the noise affecting the thermal field because of the HFM itself. The relative impacts of the different error sources have been quantified, in order to assess future applicability of HFM dynamic approach for in-situ measurements.