Experimental investigation of the influence of convective and radiative heat transfers on thermal transmittance measurements

Walls thermal transmittance value (U-value) is essential to identify the performance of a wall and it is used for the energy labeling procedure. If on one hand the heat-flow meter can measure heat fluxes across its plate giving back the wall's U-value, on the other hand, if air temperatures and wall's inner surface temperatures are measured, it is necessary to set the total heat transfer coefficient value in order to calculate the heat fluxes. Many correlations were developed in scientific literature to quantify the convective heat transfer coefficient and it is possible to distinguish similarity based and experimentally ones. In this paper, the actual total heat transfer coefficients in different case studies were obtained by measuring the physical parameters that are needed to define them. Convective and radiative contributions were separately evaluated and, finally, actual convective heat transfer coefficients were compared with the same coefficients obtained by applying the correlations available in literature and with the constant value suggested by the Standard. The aim of this study is to analyze the differences between the various coefficients values and their influence on the thermal transmittance evaluation, in order to better understand the existing correlations and the UNI EN ISO 6946 applicability. This is an initial part of a research which aims to provide an overall more accurate representation of the building behavior, forthcoming developments will deal with the analysis of heat transfer on the outer side of walls.