This study aims to introduce and experimentally validate a model to predict the potential of evaporative cooling as a way to mitigate unwanted heat gain in buildings' roofs. The experimental setup, which emulates a roof on a small-scale, was developed to independently measure the instantaneous heat flux permeating through two equal porous medium samples under the same environmental conditions, with the only difference that one was completely dry and other was fully wet, thus creating a reference heat gain value to which the wet, evaporative sample could be compared. The numerical model solves an energy balance within the porous sample and a surface energy balance at its upper surface while considering the environment's temperature, humidity, wind speed and solar irradiation as boundary conditions. The experimental and numerical model have their results compared for two different conditions: (i) steady-state and indoors without solar irradiation and (ii) transient outdoors under real weather. The direct comparison showed that the theoretical model developed is capable of predicting, with good accuracy, the heat flux flowing through the porous medium for most tests and that the wet porous medium can, for certain conditions, outperform its dry counter version, despite having higher thermal conductivity and absorptance due to its wetness.