Investigation of an equivalence criterion between periodic thermal transmittance of opaque building elements and cool roof or cool color coatings

At this time, just a few requirements have been introduced in Italy about the summer performance of buildings, one among them concerning the maximum allowed value for the periodical thermal transmittance of opaque building elements. In fact, it is generally assumed that thermal cycles induced by solar radiation at the external surfaces can be substantially damped through the thickness of roofs and walls thanks to properly low values of their periodic thermal transmittance. The result is generally achieved by giving roofs and walls an adequate levels of either thermal insulation or thermal inertia, but this is generally reflected in the adoption of a high surface mass and makes it difficult to use wooden roofs, plasterboard structures or sandwich panels with foam insulation. However, using an outer coating with high solar reflectance, identified with the terms of cool roof or cool color in the technical nomenclature used in the U.S.A., permits to dampen the thermal cycles induced by solar radiation directly at the outer surface rather than through the thickness. In the present paper, we investigate an equivalence criterion between periodic thermal transmittance and solar reflectance by means of dynamic calculation approaches. The criterion is studied in terms of containment of either thermal energy needs for air conditioning or discomfort of the built environment.