Influence of the convective coefficient on the determination of thermal transmittance through outdoor infrared thermography

Several studies and correlations for the convective heat transfer coefficient (CHTC) are available in literature and in handbooks, depending on type of convection, wind speed range or on the test rigs from which they are derived. The importance of accurate evaluation of CHTC, especially at the building façades, has been highlighted in the recent years, due to the need for reliable measurement of buildings' heat transfer capability. This study aims at underlining the importance of proper CHTC values for the determination of the building envelope thermal transmittance (U-value) via infrared thermography (IRT). To this scope, firstly an overview on convective heat transfer coefficient is given; then, some CHTC models, chosen from literature, are analyzed at different wind speed classes. Subsequently, such models are employed in two formulas proposed in literature for the U-value measurement via IRT, by using data from previous experimental campaign carried out in controlled environment. Results were compared, and significant deviations were found: one of the employed approaches and formulation is less sensitive to the correlation adopted for the convection expression, amongst those considered and in the wind speed range analyzed. This constitutes an advantage, since one of the weak points of the IRT method is the convection expression itself.