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Breathing Walls are envelope components, based on porous materials, crossed by a natural or forced airflow. Since they behave both as recovery heat exchangers and active insulation, reducing the conductive heat flux, they represent a promising envelope technology, allowing to reduce energy consumption in buildings.From the modeling point of view, an analytical model can be found in literature, describing heat and mass transfer across Breathing Walls in steady state conditions. However, to the best of the authors' knowledge, the model lacks an exhaustive experimental validation. Therefore, in this paper, the novel laboratory apparatus named Dual Air Vented Thermal Box developed at Politecnico of Milano is presented. The apparatus is used to experimentally investigate the steady state behavior of a 1 m 2 Air Permeable Concrete sample, crossed by an airflow at different velocities up to 12 mm/s.The temperature profile inside the sample, measured in different positions, is compared with the model predictions. While in the central portion of the wall a very good agreement is found, the experimental results at the top and at the bottom of the wall suggest a non-uniform velocity field entering the sample. A qualitative confirmation of this hypothesis is provided by CFD simulations on the apparatus, clearly showing a mixed convection regime on both sides of the wall. The results lead to state the validity of the one-dimensional analytical model in literature, although a careful application should take into account adjusted boundary conditions, consisting in an airflow velocity possibly variable with height.

The suitability of night ventilation to reduce the cooling demand in buildings can be evaluated by coupling Airflow Network Models to Building Energy Simulation tools. To estimate wind-induced ventilation, pressure coefficients (Cp) on the building envelope are key inputs, as well as local wind speed and direction. Cp data obtained by primary sources such as measurements or CFD simulations are considered the most reliable but can be difficult to obtain. An easy alternative are Cp secondary sources, such as databases providing literature data correlations. Therefore an issue arises regarding the choice of the source of pressure coefficients. This paper investigates the effects of Cp from primary and secondary sources on the predicted energy saving potential of night ventilation of an isolated office building for several European climates and some relevant design conditions and simulation parameters. Different Cp sources produce a dispersion of Cp data and differences in the calculated night ventilation rates up to 15%. Contrary to what might be expected, these differences influence only marginally the resulting passive cooling effects. Overall a stronger impact is observed for the colder climates, where higher temperature differences occur between desired indoor temperature and night-averaged outdoor temperature. Finally, for the building under study, the choice of the Cp source appears less crucial than the choice of other building simulation parameters, such as the internal Convective Heat Transfer Coefficient. This study can support building designers towards accurate energy simulations of naturally ventilated buildings