## 1 Introduction  
The building envelope plays a fundamental role in energy balance, in fact, more and more strict limit values for the thermal performance parameters (such as thermal transmittance of transparent and opaque components) are required for the heat loss reduction. Natural and mechanical ventilation significantly influence the thermal loads and the thermal comfort; however, mechanical ventilation could be more effective than the natural one for indoor air quality [1; 2; 3], but it is responsible for higher energy demand. The contribution of natural ventilation is also significant, and airtight frames for glazing systems are more and more adopted, in order to reduce energy consumption [4; 5; 6; 7; 8]. [PERSON] et al. [9] showed that the windows' permeability indices can have a very wide range of variability (from 4.8 to 96.4 m\({}^{3}\)/h m\({}^{2}\)), due to the year of construction, the frame material, and the opening system. Nevertheless, if the airtightness performance is too high, it can contribute to deteriorating indoor air quality (IAQ), due to the less air exchange [10]. Furthermore, the air replacement is very important, especially in natural ventilated buildings, where it is almost exclusively granted by the infiltrations and by opening the windows. Several parameters have been proposed since the 1990s to characterize the air-flow patterns, such as local mean age of air (MAA) [11], contaminant removal effectiveness, or relative contaminant removal effectiveness [12], and air exchange efficiency [13], but the mean age of air (MAA) is widely used in evaluating the indoor air quality of buildings. The age of air is the mean time that a particle takes to travel from an inlet point (such as the outdoor air intake) to the measurement point; it can be used to calculate the air-change effectiveness of a single-zone or a multi-zone system [11] and the air distribution in buildings [14]. The amount can be measured by injecting tracer gases at the inlet and recording their concentration at the position of interest. [PERSON] et al. [14] in 2006 proposed a data-based mechanistic (DBM) modeling technique to assess ventilation performance in a forced ventilation type of building. Several models and algorithms were also proposed, in order to create real-time maps of chemical concentrations in air [15], but they were not always appropriate for building applications, especially in naturally ventilated ones, where the control of air exchange in the room is fuzzy and more difficult to be modeled [16]. In these cases, experimental campaigns are more suitable for evaluating indoor air quality performance of buildings, and different measurement techniques are available for air motion characterization in rooms [17]. The tracer gas technique, based on the decay method and in compliance with ISO 16000-8:2007 [18], is a simple measurement methodology that allows calculating MAA in several experimental configurations. Experimental data can be used to validate CFD (Computational Fluid Dynamics) models, able to simulate the CO\({}_{2}\) concentration inside the room and then to calculate the MAA in different conditions. In the literature, CFD models are most focused on mechanical ventilated buildings, because the modeling is easier and the solutions are in general convergent. In natural ventilated buildings, the air movement is more difficult to follow and in many cases, the solutions of the motion fields are not convergent. [PERSON] et al. [19] studied the ventilation effectiveness in an aircraft cabin mockup by means of both experimental campaigns (using the volumetric particle tracking velocimetry (VPTV) technique) and CFD model simulations, while [PERSON] and [PERSON] [20] applied CFD simulations in forced ventilation food plants. The numerical model was also used for studying the influence of the characteristics and the placement of a complex low-velocity air diffuser on the thermal comfort and indoor air quality [21] and the characteristics of incoming air, when it is supplied via individual or multiple inlets [22].