Energy performance of opaque building elements in summer:Analysis of a simplified calculation method in force in Italy

The Italian technical standard for calculation of building energy needs in summer allows a monthlyaverage method, in which solar gains through opaque building elements are evaluated distinguishingbetween dark, intermediate or light-colored surfaces, and a fixed value of solar absorptance is assigned toeach surface category. The method is aimed at ease of operation, but its steady-state calculation approachmay not allow taking into proper account dynamic effects due to thermal inertia. Moreover, roofingsolutions typical of Italy such as roof tiles, sealing membranes or painted sheet metal panels can showsimilar colors, but significantly different values of the solar absorption coefficient. On the other hand, aneasy to apply calculation method may enable most designers of the building sector to take advantage ofsolutions for control of solar gains such as cool roofs and cool colors, thus favoring their dissemination.In this paper, the calculation method currently in use in Italy is briefly presented, with the focus onthe role of opaque building elements. Either the accuracy of steady-state calculation or the reliabilityof the color-based evaluation approach is then discussed, analyzing by numerical simulation the actualdynamic behavior of most common types of roof element.