The thermal behavior of an external opaque building element depends on the combination of several physical characteristics related to insulation level, thermal inertia, external radiative properties. Concerning the insulation level, parameters like the R-value and its inverse, the U-value or thermal transmittance, are commonly considered in building codes, but they are defined with reference to steady-state conditions and cannot describe the behavior of the element when it is subjected to the cycle of solar radiation. On the other hand, parameters like periodic thermal transmittance, decrement factor and time shift represent the dynamic response of the element resulting from its thermal inertia, so they are often considered but do not include the capability of returning solar energy to the atmosphere. In this regard, a few additional parameters are relevant such as solar reflectance and thermal emittance of the external surface, which are unrelated to both insulation and inertia. In order to rate the overall dynamic behavior of an external opaque building element subjected to the cycle of solar radiation and constant indoor temperature, a "solar transmittance index" (STI) is proposed. STI includes in a single performance parameters the effects of both the radiative properties of the external surface and the thermo-physical properties of the materials under the surface. The utilization of such single performance parameter may be greatly helpful in defining requirements and policies to prevent building overheating, reduce cooling energy demand and mitigate the fallouts of the urban heat island effect. (C) 2017 Elsevier B.V. All rights reserved.