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

• We present a method allowed in Italy to evaluate energy needs for space cooling.
• It is based on a monthly average steady-state approach.
• Solar gains through opaque elements are evaluated by a color-based scheme.
• We analyze the reliability of the method by means of simulation and experiments.
• Ease of operation may enable designers to take advantage of solutions such as cool roofs.

The Italian technical standard for calculation of building energy needs in summer allows a monthly average method, in which solar gains through opaque building elements are evaluated distinguishing between dark, intermediate or light-colored surfaces, and a fixed value of solar absorptance is assigned to each surface category. The method is aimed at ease of operation, but its steady-state calculation approach may not allow taking into proper account dynamic effects due to thermal inertia. Moreover, roofing solutions typical of Italy such as roof tiles, sealing membranes or painted sheet metal panels can show similar colors, but significantly different values of the solar absorption coefficient. On the other hand, an easy to apply calculation method may enable most designers of the building sector to take advantage of solutions 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 on the role of opaque building elements. Either the accuracy of steady-state calculation or the reliability of the color-based evaluation approach is then discussed, analyzing by numerical simulation the actual dynamic behavior of most common types of roof element.