Experimental Validation of the Potential of Cross-Ventilation Strategy as a Natural Cooling Technique Integrated in a Real Historic Building

Academic Editor: Guillermo Hauke Received: 9 December 2024 Revised: 23 January 2025 Accepted: 10 February 2025 Published: 18 February 2025 Citation: Palomo Amores, T.; Ruda Sarria, F.; Medina, D.C.; Valera, T.C.; Sánchez Ramos, J.; Álvarez Domínguez, S. Experimental Validation of the Potential of Cross-Ventilation Strategy as a Natural Cooling Technique Integrated in a Real Historic Building. Appl. Sci. 2025,15, 2174. https://doi.org/10.3390/ app15042174 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Experimental Validation of the Potential of Cross-Ventilation Strategy as a Natural Cooling Technique Integrated in a Real Historic Building Teresa Palomo Amores 1, Francisco Ruda Sarria 1, Daniel Castro Medina 2, Teresa Cano Valera 3, José Sánchez Ramos 1,* and Servando Álvarez Domínguez 1 1Grupo Termotecnia, Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, 41092 Seville, Spain; [email protected] (T.P.A.); [email protected] (F.R.S.); salvar[email protected] (S.Á.D.) 2Grupo Termotecnia, Escuela Superior de Ingeniería, Universidad de Cádiz, 11519 Cádiz, Spain; [email protected] 3Universidad de Sevilla, 41004 Seville, Spain; [email protected] *Correspondence: [email protected] Abstract: Natural ventilation in hot climates represents a key strategy to reduce the dependence on mechanical cooling systems, especially in historic buildings, where it is essential to balance thermal comfort and heritage conservation. This study analyses the effectiveness of various natural ventilation strategies in a historic building located in Écija, Seville, which is characterised by a warm climate with nocturnal thermal dips. Experimental data obtained during a summer monitoring campaign were used to validate computational fluid dynamics (CFD) models and thermal simulations. In addition, an efficiency index was defined to quantify the indoor temperature reduction. The results show a cooling efficiency close to 40%, achieving an average reduction of 3 ◦ C in the indoor temperature during the summer. Simulations of different modes of operation of natural ventilation show a 30% improvement in comfort hours according to Spanish regulations and a 50% reduction in the thermal difference during non-comfort hours. This work demonstrates that natural ventilation can significantly improve indoor conditions, offering a sustainable and replicable approach for historic buildings in hot climates. Keywords: natural ventilation; cross-ventilation; historic building; indoor thermal comfort 1. Introduction 1.1. Context Nowadays, the urban climate is being extensively studied by the scientific community due to the different effects related to climate change [ 1 ]. One of the most notorious is the general rise in temperatures in urban environments caused by massive urbanization, rapid growth in the population and buildings designed with poor environmental adaptation awareness [ 2 , 3 ]. This leads to heat accumulation problems in homes during periods of high temperatures. This presents a significant challenge, as it can result in adverse conditions for occupants, including discomfort, reduced indoor air quality, and increased building energy consumption. Implementing effective ventilation strategies is essential to dissipate accumulated heat and enhance indoor thermal comfort during the cooler hours, countering the adverse effects of daytime heat accumulation. In this context, several cooling techniques have been proposed throughout the years, and some of them have become part of our daily life [ 4 , 5 ]. On the other hand, passive Appl. Sci. 2025,15, 2174 https://doi.org/10.3390/app15042174 Appl. Sci. 2025,15, 2174 2 of 32 cooling methods are gaining popularity as they take advantage of the environmental conditions to achieve the desired temperature indoors without polluting or having a major impact on the environment [ 6 , 7 ]. Among them, the ventilation of a room by means of opening windows and doors is an intuitive solution, and its impact on the thermal comfort of a room has been well documented. This approach is named natural ventilation [ 8 ]. Among these natural ventilation strategies, this work focuses on the use of courtyards, a design strategy with roots dating back over 5000 years, being one of the oldest forms of vernacular architecture. While traditionally associated with Middle Eastern cultures, courtyards have been reinterpreted and adapted across diverse regions, including Latin America, China, and Europe. Despite their long history and unique role in shaping microclimates, the impact of courtyards in diverse aspects have been undervalued in more recent centuries. More specifically, the impact of courtyards as a passive design strategy is evident in both occupant comfort and building energy performance. As noted by Amir Tabadkani [ 9 ], courtyards contribute significantly to creating a more comfortable living environment while also reducing energy consumption. In the specific context of Andalusia, courtyard architecture has played a prominent role in shaping traditional building design. As described by Feduchi [ 10 ], Andalusian vernacular architecture is often characterized by a particular form where interior spaces of a building converge towards a central courtyard. This courtyard also serves as a private, outdoor space for social gatherings. The historical significance of courtyards, coupled with their demonstrable benefits in terms of comfort and energy efficiency, underscores their enduring relevance in contemporary architectural discourse. Their ability to create microclimates, promote social interaction, and enhance the overall liveability of buildings makes them a valuable design element for architects and urban planners seeking to create sustainable and human-centred environments. 1.2. Natural Ventilation Natural ventilation can be described as the air renovation inside a building or a room caused by natural thermal, wind, or diffusion effects through windows, doors or purposebuilt ventilation openings without need for external mechanically powered systems such as fans and air conditioning systems [ 11 ]. In the context of adapting and reducing global warming, this is very desirable, as it takes advantage of natural resources without energy demand. The main factor that affects the efficiency of this technique is the design of the buildings and urban planning, because the air patterns within the target zone will change depending on them [11,12]. This topic has been deeply studied and is well documented. Its factors and consequences have been stated in some papers [ 13 ], as well as reviews of the different forms of studying this method, which can be through experimentation, wind tunnel experiments and CFD [ 14 ]. The first approach is the most precise one for knowing the real effect of the ventilation, but it is difficult, as it requires a building where the sensors are set to take good measurements. The second one provides good results as well, but it can be expensive to experiment in a wind tunnel and not everybody can have access to one. Finally, the last approach is the most accessible and fast one, but on the downside, it can have larger errors if it is not well calibrated. Furthermore, different models or types of natural ventilation have been described [15]: single-sided ventilation, buoyancy-driven stack ventilation and wind-driven cross-ventilation. Among them, the first one involves only one aperture to the exterior, for instance, one door or window open. Here, the air renewal is produced by changes in pressure due to the wind in the surroundings of the aperture. Because of this, the effectiveness of this type of ventilation is usually low and has huge variations. This is the most common one in single rooms; thus, a lot of study has been conducted in this field, more specifically when the room is connected to a courtyard [ 16 ]. However, it has been Appl. Sci. 2025,15, 2174 3 of 32 proved that this type of ventilation has the lowest ventilation potential of the three options. On the other hand, buoyancy-driven stack ventilation has its basis in the density difference of the air. It draws air with a lower temperature from apertures near the ground level and expels hotter air from the inside of the building through higher ventilation openings, such as chimneys or atriums. Although the ventilation efficiency is good, it becomes poorer when it comes to single floors or low dwellings. Finally, in wind-driven cross-ventilation, the driven forces are caused by a pressure difference between two or more opposing apertures; as a result, the flow runs through the enclosed space from the aperture with higher pressure to the lower one. As some authors highlight [ 15 , 17 ], this can be the most useful ventilation strategy as it is only needs two apertures in different rooms for it to work, and usually, the pressure difference is enough to provide an appropriate air mass exchange rate. Different studies have been conducted about this ventilation mechanism; for instance, different apertures sizes have been tested [ 18 ], different configurations of windows layout in the room [ 19 ] or even including internal obstacles [ 20 ]. Many different techniques have been proposed to improve the ventilation potential [ 21 ] but most of them rely on CFD simulations or combinations between them and wind-tunnel experiments [ 22 ]. However, these studies mostly focus on the streamline distribution inside the room and the air ventilation rate, but data about the potential temperature reduction when this approach is used are quite scarce; additionally, it is usually based on CFD experimentation, which lacks experimental validation. Although wind-driven ventilation is normally studied for buildings where each aperture faces toward different streets [ 17 , 23 ], some papers assess the potential of crossventilation when one aperture faces toward the street and the other faces toward the courtyard [ 24 ]. This situation has way more potential for cooling indoor spaces than the previous one; part of this is because of the good climatic conditions that courtyards trigger. 1.3. Natural Ventilation in Historic Buildings In vernacular architecture, the main form of cross-ventilation that is established is between the exterior of the building and the interior courtyard. The definition of a courtyard can be considered as an enclosed area surrounded completely or partly by a building’s walls and is open to the sky. This topic is scarcely discussed in the literature, with some research regarding the multiple features of courtyards [ 25 ]. However, when considering the impact on the climate, one of the most valuable outcomes of courtyards is the ability to create ideal conditions for passive cooling techniques, such as natural ventilation of rooms and buildings. One of the cases that could benefit most from natural ventilation with courtyards is historic buildings. Preserving the thermal comfort of historic buildings presents a unique challenge [ 26 ]. While modern structures often rely on mechanical systems for heating and cooling, many historic buildings were designed and constructed without such technology. This absence of built-in climate control systems, coupled with the increasing severity of climate change, has led to a growing need for sustainable and sensitive solutions to maintain comfortable indoor temperatures. Nevertheless, installing modern mechanical systems in historic buildings poses significant challenges. The integration of such systems often requires substantial modifications that can disrupt the building’s original fabric, compromising its historical integrity and architectural character. Fortunately, natural ventilation using courtyards presents a viable alternative in these instances [27]. This approach leverages the natural forces of wind and air pressure to circulate fresh air through the building, effectively cooling and ventilating interior spaces without the need for mechanical systems. This approach could take advantage of the building’s own structure to ventilate it without the need for works or modifications to the structure that could endanger the integrity of Appl. Sci. 2025,15, 2174 4 of 32 the building, as well as deteriorate its historical value. To achieve this, the airflow within the structure has been the subject of extensive research [ 28 , 29 ]. Among all the studies completed, some study the airflow pattern in a given courtyard through the calculation of the pressure coefficient on the neighbour walls [ 12 ], which gives an idea of its ventilation potential. On the other hand, some studies are more general and try to identify which are the main factors altering the airflow pattern in a courtyard modifying some factors as the building height or the courtyard length, the form of the roof [ 30 ] and even including vegetation [ 31 ]. In relation to the applicability of using courtyards to enhance wind driven cross-ventilation, Abel Tablada compared in his work [ 32 ] the ventilation potential in two blocks of buildings with a bi-dimensional CFD simulation between rooms which were under single-sided ventilation and rooms between two courtyards, obtaining that the air renovation level of the second ones is much bigger than that of the others. Additionally, Daniel Micallef [ 33 ] modelled a building with a courtyard in CFD but in this case, with a 3D geometry, also calculating the airflow across a room to study the performance of cross-ventilation. In this regard, most of the studies of cross-ventilation only calculate the air inflow in terms of wind velocity across the openings without giving more results about temperature decrease or air mass flow rate. Apart from that, research usually only calculates air ventilation through CFD simulations and does not check the relationship between that and reality. While wind-driven ventilation may not be suitable for every historic building, it offers a promising approach for many, particularly those with courtyards. By carefully considering the building’s design and local climate conditions, architects and engineers can create effective natural ventilation systems that enhance thermal comfort, preserve cultural heritage, and contribute to a more sustainable future. 1.4. Objectives The principal aim of this study is to investigate the relationship between natural nocturnal cross-ventilation and indoor conditions in historic buildings. The study is based on a real historic house located in Écija, Seville (Spain), a region characterised by a warm climate with high daytime temperatures and significant drops in temperature during the night. The objective of the research is to evaluate the impact of passive ventilation on the reduction in indoor temperatures and to define a thermal efficiency index associated with this strategy. Aeraulic simulations utilising computational fluid dynamics were conducted to examine the impact of air inlet flow rates on the flow dynamics within the building, in addition to thermal simulations to ascertain the influence of these strategies on indoor temperatures. These simulations were calibrated with the data obtained from the monitoring campaign conducted throughout the summer. Once the simulation models were established, the effect of different operational configurations of natural ventilation during the summer season on the thermal comfort of the occupants was evaluated. This study seeks to advance the disciplinary discourse on reconciling occupants’ thermal comfort with the conservation of historic building values by promoting passive ventilation solutions as an alternative to conventional mechanical systems in hot climates. 2. Materials and Methods A combined approach, comprising field measurements, computational fluid dynamics (CFD) simulations and building-specific thermal simulations, has been employed for the analysis of the passive cooling efficiency of cross-ventilation in historic buildings situated in Mediterranean climates. As illustrated in Figure 1, the methodology employed in this study is outlined as follows. To this end, the case study is initially examined, with particular attention paid to the climatic conditions of the area and the structural characteristics of the Appl. Sci. 2025,15, 2174 5 of 32 ancient building (Section 2.1). Subsequently, the monitoring plan is presented in order to accurately characterise the climatic conditions to which the ancient building is subjected during the experimentation period, as well as the interior temperature that the dwelling reaches under these conditions (Section 2.2). In this same step, the experimental plan is presented, which outlines the various studies that will be conducted to examine the impact of cross-ventilation. Subsequently, the data obtained from the experimental work will be analysed, thereby providing first-hand knowledge of the thermal behaviour of the building in a normal situation (Section 2.3). Subsequently, thermal and aero-dynamic simulations are conducted, preparing the simulation models to reflect real behaviour ( Sections 2.4 and 2.5 ). Finally, the thermal and aero-dynamic results of the inclusion of night time cross-ventilation in the ancient building are analysed, as well as the effect that different types of operation would have in the short and long term (Section 3). This makes it possible to experimentally, and later after simulations, determine the effect on people’s comfort of the use of non-invasive passive techniques in historic buildings. Appl. Sci. 2025, 15, x FOR PEER REVIEW 5 of 34 characteristics of the ancient building (Section 2.1). Subsequently, the monitoring plan is presented in order to accurately characterise the climatic conditions to which the ancient building is subjected during the experimentation period, as well as the interior temperature that the dwelling reaches under these conditions (Section 2.2). In this same step, the experimental plan is presented, which outlines the various studies that will be conducted to examine the impact of cross-ventilation. Subsequently, the data obtained from the experimental work will be analysed, thereby providing first-hand knowledge of the thermal behaviour of the building in a normal situation (Section 2.3). Subsequently, thermal and aero-dynamic simulations are conducted, preparing the simulation models to reflect real behaviour (Sections 2.4 and 2.5). Finally, the thermal and aero-dynamic results of the inclusion of night time cross-ventilation in the ancient building are analysed, as well as the effect that different types of operation would have in the short and long term (Section 3). This makes it possible to experimentally, and later after simulations, determine the effect on people’s comfort of the use of non-invasive passive techniques in historic buildings. Figure 1. Methodology for intervention. 2.1. Description of Case Study In order to analyse the effect of cross-ventilation in historic buildings, an old house located in Ecija (Seville), southern Spain, is used as a case study (Figure 2). Figure 1. Methodology for intervention. 2.1. Description of Case Study In order to analyse the effect of cross-ventilation in historic buildings, an old house located in Ecija (Seville), southern Spain, is used as a case study (Figure 2). Appl. Sci. 2025,15, 2174 6 of 32 Appl. Sci. 2025, 15, x FOR PEER REVIEW 6 of 34 Figure 2. Situation of the ancient building study. The house was constructed at the end of the nineteenth century, and therefore, its architectural style reflects the prevailing local trends of the period. In order to enhance the internal environment, thick walls were employed, thereby reducing the flow of heat from the exterior during the daytime and the loss to the exterior during the night time in periods of elevated temperatures. The construction of the walls is of considerable thickness, measuring 60 cm, which markedly increases their mass in comparison to contemporary construction. The construction of the wall follows the mud wall technique, whereby the majority of the wall is constructed from rammed earth with cement facing [34]. This technique was widely utilised for the construction of houses in the southern part of Spain [35]. Consequently, the building exhibiting a high thermal inertia, thereby enhancing its passive cooling potential through thermal storage within the walls. Conversely, the building features a central stone courtyard of 22 m 2 , surrounded by arcades. The historic structure encompasses two floors, all of which are connected to the courtyard through extensive windows. In consideration of the geographical location of the structure in question, the climate designation according to the Köppen–Geiger classification is Csa [36]. The prevailing climatic conditions are characterised by high summer temperatures and low precipitation, with mild winters. More specifically, Figure 3 illustrates the mean daily and hourly temperatures for Ecija during the summer of 2023. The period under discussion extends from June to September, months which are customarily regarded as summer in southern Spain on account of the high temperatures they typically experience. The data were obtained from an existing weather station located in a rural area in close proximity to the building under study. This station belonging to the national network of climate stations is maintained by the Spanish Government [37]. The aforementioned stations are situated in the peripheries of urban areas; thus, the data obtained represent rural climate conditions. In order to accurately assess the impact of urbanisation on temperature, it is necessary to consider the urban heat island effect. It is noteworthy that daytime temperatures exceed 35 °C, with readings reaching 40 °C on the majority of days. However, temperatures decrease significantly during the night, with readings dropping to around 20 °C, or even lower. These observations highlight the potential for energy savings through the utilisation of external air flows in cooling systems, taking advantage of the low temperatures occurring at night. Figure 2. Situation of the ancient building study. The house was constructed at the end of the nineteenth century, and therefore, its architectural style reflects the prevailing local trends of the period. In order to enhance the internal environment, thick walls were employed, thereby reducing the flow of heat from the exterior during the daytime and the loss to the exterior during the night time in periods of elevated temperatures. The construction of the walls is of considerable thickness, measuring 60 cm, which markedly increases their mass in comparison to contemporary construction. The construction of the wall follows the mud wall technique, whereby the majority of the wall is constructed from rammed earth with cement facing [ 34 ]. This technique was widely utilised for the construction of houses in the southern part of Spain [ 35 ]. Consequently, the building exhibiting a high thermal inertia, thereby enhancing its passive cooling potential through thermal storage within the walls. Conversely, the building features a central stone courtyard of 22 m 2 , surrounded by arcades. The historic structure encompasses two floors, all of which are connected to the courtyard through extensive windows. In consideration of the geographical location of the structure in question, the climate designation according to the Köppen–Geiger classification is Csa [ 36 ]. The prevailing climatic conditions are characterised by high summer temperatures and low precipitation, with mild winters. More specifically, Figure 3illustrates the mean daily and hourly temperatures for Ecija during the summer of 2023. The period under discussion extends from June to September, months which are customarily regarded as summer in southern Spain on account of the high temperatures they typically experience. The data were obtained from an existing weather station located in a rural area in close proximity to the building under study. This station belonging to the national network of climate stations is maintained by the Spanish Government [ 37 ]. The aforementioned stations are situated in the peripheries of urban areas; thus, the data obtained represent rural climate conditions. In order to accurately assess the impact of urbanisation on temperature, it is necessary to consider the urban heat island effect. It is noteworthy that daytime temperatures exceed 35 ◦ C, with readings reaching 40 ◦ C on the majority of days. However, temperatures decrease significantly during the night, with readings dropping to around 20 ◦ C, or even lower. These observations highlight the potential for energy savings through the utilisation of external air flows in cooling systems, taking advantage of the low temperatures occurring at night. Appl. Sci. 2025,15, 2174 7 of 32 Appl. Sci. 2025, 15, x FOR PEER REVIEW 7 of 34 Figure 3. Average daily and hourly temperature during the summer in Ecija. In addition, the study of wind is of particular interest. The wind speed and direction values for the summer of 2023 were obtained from the same database. These data are presented in Figure 4, which illustrates the wind rise for the summer of 2023. It can be observed that the predominant wind direction ranges from the southwest to the west. The wind speeds are predominantly below 10 km/h, which equates to 2.8 m/s. A further analysis of the data indicates that wind speeds during the day vary between a minimum of 1 km/h and a maximum of 15 km/h, with an average speed of 7 km/h. During the night, wind speeds are somewhat lower, with minimum values of less than 1 km/h, which is classified as calm, maximums of 12 km/h and an average speed of 5.4 km/h. Figure 4. Wind rose for 2023 summer in Écija. Consequently, given the climatic conditions that permit nocturnal temperature reductions, coupled with the architectural design of the residence comprising high thermal inertia, and the presence of a central courtyard linked to the rooms, it is worthwhile to examine crossventilation during the night as a passive cooling system in this historic edifice. Figure 3. Average daily and hourly temperature during the summer in Ecija. In addition, the study of wind is of particular interest. The wind speed and direction values for the summer of 2023 were obtained from the same database. These data are presented in Figure 4, which illustrates the wind rise for the summer of 2023. It can be observed that the predominant wind direction ranges from the southwest to the west. The wind speeds are predominantly below 10 km/h, which equates to 2.8 m/s. A further analysis of the data indicates that wind speeds during the day vary between a minimum of 1 km/h and a maximum of 15 km/h, with an average speed of 7 km/h. During the night, wind speeds are somewhat lower, with minimum values of less than 1 km/h, which is classified as calm, maximums of 12 km/h and an average speed of 5.4 km/h. Appl. Sci. 2025, 15, x FOR PEER REVIEW 7 of 34 Figure 3. Average daily and hourly temperature during the summer in Ecija. In addition, the study of wind is of particular interest. The wind speed and direction values for the summer of 2023 were obtained from the same database. These data are presented in Figure 4, which illustrates the wind rise for the summer of 2023. It can be observed that the predominant wind direction ranges from the southwest to the west. The wind speeds are predominantly below 10 km/h, which equates to 2.8 m/s. A further analysis of the data indicates that wind speeds during the day vary between a minimum of 1 km/h and a maximum of 15 km/h, with an average speed of 7 km/h. During the night, wind speeds are somewhat lower, with minimum values of less than 1 km/h, which is classified as calm, maximums of 12 km/h and an average speed of 5.4 km/h. Figure 4. Wind rose for 2023 summer in Écija. Consequently, given the climatic conditions that permit nocturnal temperature reductions, coupled with the architectural design of the residence comprising high thermal inertia, and the presence of a central courtyard linked to the rooms, it is worthwhile to examine crossventilation during the night as a passive cooling system in this historic edifice. Figure 4. Wind rose for 2023 summer in Écija. Consequently, given the climatic conditions that permit nocturnal temperature reductions, coupled with the architectural design of the residence comprising high thermal inertia, and the presence of a central courtyard linked to the rooms, it is worthwhile to examine cross-ventilation during the night as a passive cooling system in this historic edifice. 2.2. Monitoring and Experimentation 2.2.1. Measuring Equipment The monitoring process was designed with the objective of evaluating and validating the effect of cross-ventilation on the performance of buildings. In order to achieve this, Appl. Sci. 2025,15, 2174 8 of 32 air temperature and relative humidity sensors were utilised and positioned within the various rooms. The sensors utilised were Omega’s OM-EL-USB-2-LCD, which measured indoor conditions throughout the entirety of the summer season, spanning from 1 July 2023 to 30 September 2023 . The measurements were taken at 10 min intervals. In contrast, an outdoor weather station was utilised to record meteorological data within the urban environment. This station, the PCE-FWS 20N from PCE Instruments, measured the same time period and range as the indoor sensors. In Table 1is detailed the range, resolution and accuracy specifications for each variable recorded by each measuring device. Table 1. Measurement equipment specifications. OM-EL-USB-2-LCD PCE-FWS 20N Variables Air Temperature [◦C] Relative Humidity [%] Air Temperature [◦C] Relative Humidity [%] Wind Speed [m/s] Wind Direction [◦] Range −35/80 0/100 −40/60 1/99 0/50 1/360 Resolution 0.5 0.5 0.1 1 0.1 1 Accuracy ±0.5 ±3±1±4±1±3 2.2.2. Monitorization Setup In order to identify the best area to perform the study, Figure 5provides a visual representation of the building’s elevation, serving as a crucial tool in identifying areas with the greatest potential for cross-ventilation. The analysis reveals a clear distinction in cross-ventilation potential between different sections of the building. Rooms located on the southwest side, adjacent to the ancient building’s left side in the figure, exhibit the highest potential for cross-ventilation. This is due to their strategic placement, connecting the exterior street with the central courtyard. This connection facilitates a continuous flow of air, drawing in cool air from the street and expelling warmer air through the courtyard. In contrast, rooms situated in the eastern part of the building, depicted on the figure’s right side, face significant limitations in cross-ventilation. The narrow inner courtyard and restricted air outlet to the outside create a bottleneck, hindering the free movement of air. This restricted flow limits the effectiveness of natural cooling in these areas. Appl. Sci. 2025, 15, x FOR PEER REVIEW 9 of 34 Figure 5. Building layout showing cross-ventilation possibilities. Once the vertical layout of the dwelling is clear, Figure 6 illustrates the indoor locations of the measurement equipment (the position of the temperature sensor is indicated by the red cross.). For the sake of clarity, the rooms have been assigned numbers according to a code. The designation “GF_RY” and “FF_RY” refers to the location of the measurement equipment on the ground floor and first floor, respectively. The suffix “RY” denotes the room number. To illustrate, the designation ‘FF_R1’ denotes room 1 on the ground floor. The monitored rooms are distributed across two on the ground floor and four on the first floor, situated on the west side of the building. Furthermore, the weather station is located on the roof of the building at an elevation of 10 m above ground level [38], in order to obtain representative information about temperature and windspeed locally, including the effects of the urban heat island and the disruption triggered by the presence of buildings nearby. Figure 6. Scheme of the dwelling with the equipment locations. In order to facilitate a comparative analysis of the actual behaviour of the building with and without cross-ventilation, the monitoring campaign conducted during the summer was complemented by experimental periods during which cross-ventilation was implemented in selected rooms. Consequently, nocturnal cross-ventilation was conducted over a total of 20 randomly selected nights, distributed in consecutive periods of 1 to 5 nights. The operational hours were set from 21:00 to 10:00 h. The windows utilized for the cross-ventilation experiments were those enumerated in Figure 6. For the ground floor, Figure 5. Building layout showing cross-ventilation possibilities. The air flow patterns illustrated are schematic approximations, providing a simplified visualization for comprehension. Further analysis will delve deeper into the complex interplay of air movement between the inner courtyard and the street. This detailed Appl. Sci. 2025,15, 2174 9 of 32 examination aims to quantify the potential of cross-ventilation, coupled with the strategy of night ventilation, to maintain the historic building’s temperature within acceptable ranges. By optimizing these natural cooling techniques, the study seeks to demonstrate the feasibility of achieving thermal comfort without relying on energy-intensive conventional air conditioning systems. Once the vertical layout of the dwelling is clear, Figure 6illustrates the indoor locations of the measurement equipment (the position of the temperature sensor is indicated by the red cross.). For the sake of clarity, the rooms have been assigned numbers according to a code. The designation “GF_RY” and “FF_RY” refers to the location of the measurement equipment on the ground floor and first floor, respectively. The suffix “RY” denotes the room number. To illustrate, the designation ‘FF_R1’ denotes room 1 on the ground floor. The monitored rooms are distributed across two on the ground floor and four on the first floor, situated on the west side of the building. Furthermore, the weather station is located on the roof of the building at an elevation of 10 m above ground level [ 38 ], in order to obtain representative information about temperature and windspeed locally, including the effects of the urban heat island and the disruption triggered by the presence of buildings nearby . Appl. Sci. 2025, 15, x FOR PEER REVIEW 9 of 34 Figure 5. Building layout showing cross-ventilation possibilities. Once the vertical layout of the dwelling is clear, Figure 6 illustrates the indoor locations of the measurement equipment (the position of the temperature sensor is indicated by the red cross.). For the sake of clarity, the rooms have been assigned numbers according to a code. The designation “GF_RY” and “FF_RY” refers to the location of the measurement equipment on the ground floor and first floor, respectively. The suffix “RY” denotes the room number. To illustrate, the designation ‘FF_R1’ denotes room 1 on the ground floor. The monitored rooms are distributed across two on the ground floor and four on the first floor, situated on the west side of the building. Furthermore, the weather station is located on the roof of the building at an elevation of 10 m above ground level [38], in order to obtain representative information about temperature and windspeed locally, including the effects of the urban heat island and the disruption triggered by the presence of buildings nearby. Figure 6. Scheme of the dwelling with the equipment locations. In order to facilitate a comparative analysis of the actual behaviour of the building with and without cross-ventilation, the monitoring campaign conducted during the summer was complemented by experimental periods during which cross-ventilation was implemented in selected rooms. Consequently, nocturnal cross-ventilation was conducted over a total of 20 randomly selected nights, distributed in consecutive periods of 1 to 5 nights. The operational hours were set from 21:00 to 10:00 h. The windows utilized for the cross-ventilation experiments were those enumerated in Figure 6. For the ground floor, Figure 6. Scheme of the dwelling with the equipment locations. In order to facilitate a comparative analysis of the actual behaviour of the building with and without cross-ventilation, the monitoring campaign conducted during the summer was complemented by experimental periods during which cross-ventilation was implemented in selected rooms. Consequently, nocturnal cross-ventilation was conducted over a total of 20 randomly selected nights, distributed in consecutive periods of 1 to 5 nights. The operational hours were set from 21:00 to 10:00 h. The windows utilized for the cross-ventilation experiments were those enumerated in Figure 6. For the ground floor, two windows were employed (windows 1 and 2 on the ground floor). For the first floor, the windows used are one in each room (windows 1 and 2 on the first floor) and the three windows connecting to the inner courtyard (windows 3, 4 and 5). In all cases, the experimentation was conducted with the entire window open in order to increase the incoming air flow. 2.3. Experimental Results Having outlined the experimental setup and sensor arrangement within the study ancient house, now, the collected data are analysed. The analysis of the experimental data will be crucial for establishing a baseline understanding of the building’s thermal performance before the application of any cooling strategy. The insights gained from this comprehensive analysis will contribute significantly to the overall understanding of the Appl. Sci. 2025,15, 2174 16 of 32 purpose, an exponential law is used, which can be used to model the UBL as some authors have undertaken when researching the urban canopy layer effect on street canyons [ 48 , 49 ]. U=Uref y H0.16 (1) Another important thing is the modelling of the atmospheric turbulence caused by the urban canopy layer, which causes an increase in the fluid turbulence changing the airflow pattern. To model this in Fluent, a parameter named aerodynamic roughness length (z 0 ) can be used [ 50 ] which is an empirical parameter proposed by some authors to mathematically model the roughness of typical landscape configurations for numerical studies. With this, the turbulence created by the city skyline can be modelled and used in CFD simulations. For this research, a value of z 0 = 0.1 m is considered enough to recreate the city roughness. Moreover, the boundary conditions used in the simulation are listed in Table 3. Table 3. Boundary conditions for full model simulation. Zone Boundary Condition Inlet Velocity Inlet. x-Velocity U=Vref y H0.16 Outlet Pressure Outlet Far Field Symmetry Upstream Wall. Roughness: Cs= 1; z0= 0.1 m Downstream Wall, no slip condition Walls Wall, no slip condition Finally, the configuration of Fluent environment is also very important, as it determines how the simulation is solved. The most suitable turbulent model is needed to obtain the most realistic solution. In this case, the RANS kε equation using Re-Normalisation Groput (RNG) closure scheme and standard wall function seems to be the most appropriate one when studying the fluid dynamics of flow across the urban canopy layer according to authors [ 51 ]. To solve these equations, some inherent parameters of Fluent environment must be chosen: {Cε1,Cε2,Cν,η0,β}={1.42 ;1.68, 0.0845;4.38; 0.012} [ 52 ]. Finishing the configuration, the SIMPLE scheme is used to solve the coupled equations of pressure and velocity, with least squares cell-based discretization scheme for the gradient. Additionally, a second-order discretization upwind is selected for all the magnitudes to achieve better accuracy. The convergence criterion used to stop the simulation is setting all the residuals value to 1 −5 . When this condition is met, the iterative process finishes, and the result is considered converged. When the simulation is carried out, some results will be obtained. Firstly, the pressure coefficient is calculated in four points of the left-wing part of the dwelling and then the pressure difference between the windward and leeward part of the upper floor is obtained with (2), where ρ is the density of the fluid, ∆Cp is the difference of pressure coefficients between the points mentioned before. This way is found the direction of the flow and the magnitude of the pressure difference which will cause the cross-ventilation inside the floor. ∆P=∆Cp ∗0.5 ∗ρ∗V2 ref (2) Furthermore, the flow streamlines are represented and the mass flow rate entering the courtyard is calculated, which gives an illustrative numerical result for the air renewal inside the courtyard. When all the results have been obtained, the second part of the decoupled method begins. Firstly, the 2D geometry model of the left-wing upper floor is completed, including Appl. Sci. 2025,15, 2174 17 of 32 both rooms which face towards the street and the corridor with windows connected to the courtyard. For this purpose, the dimensions of the walls, doors and windows are the same as in the floor plan provided (Figure 14). Appl. Sci. 2025, 15, x FOR PEER REVIEW 17 of 34 appropriate one when studying the fluid dynamics of flow across the urban canopy layer according to authors [51]. To solve these equations, some inherent parameters of Fluent environment must be chosen: 󰇝𝐶,𝐶,𝐶,𝜂,𝛽󰇞󰇝1.42 ;1.68,0.0845;4.38; 0.012󰇞 [52]. Finishing the configuration, the SIMPLE scheme is used to solve the coupled equations of pressure and velocity, with least squares cell-based discretization scheme for the gradient. Additionally, a second-order discretization upwind is selected for all the magnitudes to achieve better accuracy. The convergence criterion used to stop the simulation is setting all the residuals value to 1 −5 . When this condition is met, the iterative process finishes, and the result is considered converged. When the simulation is carried out, some results will be obtained. Firstly, the pressure coefficient is calculated in four points of the left-wing part of the dwelling and then the pressure difference between the windward and leeward part of the upper floor is obtained with (2), where 𝜌 is the density of the fluid, ∆𝐶𝑝 is the difference of pressure coefficients between the points mentioned before. This way is found the direction of the flow and the magnitude of the pressure difference which will cause the cross-ventilation inside the floor. ∆𝑃∆𝐶𝑝∗0.5∗𝜌∗𝑉   (2) Furthermore, the flow streamlines are represented and the mass flow rate entering the courtyard is calculated, which gives an illustrative numerical result for the air renewal inside the courtyard. When all the results have been obtained, the second part of the decoupled method begins. Firstly, the 2D geometry model of the left-wing upper floor is completed, including both rooms which face towards the street and the corridor with windows connected to the courtyard. For this purpose, the dimensions of the walls, doors and windows are the same as in the floor plan provided (Figure 14). Figure 14. Upper floor dimensions. Regarding the meshing, in this case, every zone of the model is equally important, so a cell mesh size of 0.2 m is used everywhere. In this simulation, a distinction between types of walls is considered because of their thermal properties. Load-bearing walls are usually very thick in this type of ancient dwelling, so it can be assumed that the thermal inertia is high so the boundary condition will include an imposed temperature equal to the mean temperature of the dwelling (T imp ), whereas thinner walls are considered to lack this property, thus using an adiabatic wall condition. On the other side, from the results of the previous simulation, it will be obtained that the direction of the flow across the Figure 14. Upper floor dimensions. Regarding the meshing, in this case, every zone of the model is equally important, so a cell mesh size of 0.2 m is used everywhere. In this simulation, a distinction between types of walls is considered because of their thermal properties. Load-bearing walls are usually very thick in this type of ancient dwelling, so it can be assumed that the thermal inertia is high so the boundary condition will include an imposed temperature equal to the mean temperature of the dwelling (T imp ), whereas thinner walls are considered to lack this property, thus using an adiabatic wall condition. On the other side, from the results of the previous simulation, it will be obtained that the direction of the flow across the dwelling is from the courtyard to the street, so the inlet boundary condition is set as “pressure inlet” in the windows facing the courtyard, where the gauge total pressure value corresponds to the value of ∆ P calculated previously while the pressure gauge in the pressure outlet is set to zero in the windows facing the street. The floor plan with boundary conditions marked is shown in Figure 15. Appl. Sci. 2025, 15, x FOR PEER REVIEW 18 of 34 dwelling is from the courtyard to the street, so the inlet boundary condition is set as “pressure inlet” in the windows facing the courtyard, where the gauge total pressure value corresponds to the value of ∆P calculated previously while the pressure gauge in the pressure outlet is set to zero in the windows facing the street. The floor plan with boundary conditions marked is shown in Figure 15. Figure 15. Upper floor plan with boundary conditions. Ultimately, the configuration of the Fluent environment is analogous to that of the previous iteration, exhibiting only a few modifications. The initial step is to enable the energy equation, which is necessary for the simulation to perform temperature calculations. It is not evident which turbulent model is the most appropriate in this context. Nevertheless, the utilisation of SST k-Ω has been evidenced to yield favourable outcomes. The remaining configuration parameters remain unchanged from the previous iteration. Upon completion of the simulation, several results of interest are obtained. Firstly, the flow streamlines within the first floor provide insight into the location of the vortex formed within the rooms, despite the two-dimensional simplification. This is crucial for a more comprehensive grasp of the flow dynamics across the rooms, thereby facilitating the pre-determination of optimal sensor placement within the rooms. It is imperative to situate the sensors in locations shielded from the primary air jets to ensure the consistency of measurements. Subsequently, the total air mass flow rate entering the floor is determined, as well as the percentage of this entering each room. Subsequently, a final three-dimensional simulation of the room of interest is conducted. The room was modelled as a cube, with the door designated as the pressure inlet and the window facing the street designated as the pressure outlet. It can be assumed that the value of the inlet is equivalent to ∆P (the pressure in the external windows) as the pressure loss on the corridor is negligible. The boundary conditions for the floor and room walls are set as temperature-imposed walls (T imp ), in accordance with the aforementioned explanation. The ceiling is set as an adiabatic wall, given that it is not thick enough to be classified as otherwise. With this, it is expected to obtain numerical results about ventilation rate and temperature reduction efficiency. The first parameter is very important for the design of outdoor acclimatization strategies, which can be defined as an approximation of the number of times the air within a given zone is completely renewed in a period of time. This is named air renewal rate (ACH) and can be calculated with the airflow mass rate (𝑚󰇗 ) entering a room following Equation (3). 𝐴 𝐶𝐻 󰇟𝑟𝑒𝑛/ℎ󰇠  𝑚󰇗 󰇟𝑘𝑔 𝑠󰇠 ∙3600󰇟𝑠 ℎ󰇠 𝜌󰇟𝑘𝑔 𝑚󰇠 ∙ 𝐴 󰇟𝑚󰇠 ∙1󰇟𝑚󰇠 (3) On the other hand, temperature reduction efficiency (ε) is a key point of this study, as it presents a novel approach to obtain the relation between the total air entering a space (𝑚󰇗 ) and the part of it that actually participates in the heat dissipation (𝑚󰇗 ) as it Figure 15. Upper floor plan with boundary conditions. Ultimately, the configuration of the Fluent environment is analogous to that of the previous iteration, exhibiting only a few modifications. The initial step is to enable the energy equation, which is necessary for the simulation to perform temperature calculations. It is not evident which turbulent model is the most appropriate in this context. Nevertheless, Appl. Sci. 2025,15, 2174 18 of 32 the utilisation of SST kΩ has been evidenced to yield favourable outcomes. The remaining configuration parameters remain unchanged from the previous iteration. Upon completion of the simulation, several results of interest are obtained. Firstly, the flow streamlines within the first floor provide insight into the location of the vortex formed within the rooms, despite the two-dimensional simplification. This is crucial for a more comprehensive grasp of the flow dynamics across the rooms, thereby facilitating the pre-determination of optimal sensor placement within the rooms. It is imperative to situate the sensors in locations shielded from the primary air jets to ensure the consistency of measurements. Subsequently, the total air mass flow rate entering the floor is determined, as well as the percentage of this entering each room. Subsequently, a final three-dimensional simulation of the room of interest is conducted. The room was modelled as a cube, with the door designated as the pressure inlet and the window facing the street designated as the pressure outlet. It can be assumed that the value of the inlet is equivalent to ∆ P (the pressure in the external windows) as the pressure loss on the corridor is negligible. The boundary conditions for the floor and room walls are set as temperature-imposed walls (T imp ), in accordance with the aforementioned explanation. The ceiling is set as an adiabatic wall, given that it is not thick enough to be classified as otherwise. With this, it is expected to obtain numerical results about ventilation rate and temperature reduction efficiency. The first parameter is very important for the design of outdoor acclimatization strategies, which can be defined as an approximation of the number of times the air within a given zone is completely renewed in a period of time. This is named air renewal rate (ACH) and can be calculated with the airflow mass rate ( . m ) entering a room following Equation (3). ACH [ren/h] = . m[kg s]·3600[s h] ρ[kg m3]·A[m2]·1[m](3) On the other hand, temperature reduction efficiency ( ε ) is a key point of this study, as it presents a novel approach to obtain the relation between the total air entering a space ( . mnom ) and the part of it that actually participates in the heat dissipation ( . mef ) as it interacts with hot surfaces and cools them, increasing its own temperature in the process. This can be obtained in two ways, as a ratio between both . m (which cannot be obtained via simulation, only by experimentation because . mef is unknown) or as a ratio between temperature reduction caused by the wind. For the second one, with a fixed external wind temperature via previous CFD simulation, the temperature of the wind after cooling down the room can be obtained as well as the new decreased mean temperature of the space, so the efficiency can be calculated with Equation (4). ε=Twind,out −Twind,in Troom −Twind,in = . mef . mnom (4) Then, . mef is obtained with the previous parameter with Equation (5). . mef =ε∗. mnom (5) This provides a low-computational and time-consuming approach to obtain the air renovation inside a room due to wind-driven ventilation with a courtyard. Then, an experimental validation is performed in order to ensure the validity of this method. Appl. Sci. 2025,15, 2174 19 of 32 3. Results 3.1. Results of Historic Building Aeraulics In order to understand the behaviour and air flows around the ancient building, as well as the effect of cross-ventilation inside the building, the results obtained in the various studies conducted with the use of computational fluid dynamics (CFD) are presented. Firstly, the results regarding the full street simulation are presented. This analysis has been conducted for a range of wind conditions and at different times of the day and night. However, the most representative case is presented here, at night and with the average air velocity obtained for the entire summer period. This case is representative because it corresponds to the period when windows are opened to facilitate cross-ventilation for cooling the rooms of the house. This is not a viable option during the day due to the high temperatures typically reached in the area. The streamline pattern shows the movement of the air around the dwelling and inside the courtyard (Figure 16). As can be seen, the previous building alters greatly the flow. In addition, for a better comprehension of the phenomenon, the . m (in kg/s) is obtained. It is noticeable that the amount of air entering the ground floor inside the courtyard is ten times smaller than that entering the upper floor. This means that significantly less air reaches the bottom of the courtyard, reassuring the previous statement. Appl. Sci. 2025, 15, x FOR PEER REVIEW 19 of 34 interacts with hot surfaces and cools them, increasing its own temperature in the process. This can be obtained in two ways, as a ratio between both 𝑚󰇗 (which cannot be obtained via simulation, only by experimentation because 𝑚󰇗 is unknown) or as a ratio between temperature reduction caused by the wind. For the second one, with a fixed external wind temperature via previous CFD simulation, the temperature of the wind after cooling down the room can be obtained as well as the new decreased mean temperature of the space, so the efficiency can be calculated with Equation (4). 𝜀𝑇, 𝑇 , 𝑇 𝑇 , 𝑚󰇗 𝑚󰇗 (4) Then, 𝑚󰇗 is obtained with the previous parameter with Equation (5). 𝑚󰇗 𝜀∗𝑚󰇗 (5) This provides a low-computational and time-consuming approach to obtain the air renovation inside a room due to wind-driven ventilation with a courtyard. Then, an experimental validation is performed in order to ensure the validity of this method. 3. Results 3.1. Results of Historic Building Aeraulics In order to understand the behaviour and air flows around the ancient building, as well as the effect of cross-ventilation inside the building, the results obtained in the various studies conducted with the use of computational fluid dynamics (CFD) are presented. Firstly, the results regarding the full street simulation are presented. This analysis has been conducted for a range of wind conditions and at different times of the day and night. However, the most representative case is presented here, at night and with the average air velocity obtained for the entire summer period. This case is representative because it corresponds to the period when windows are opened to facilitate cross-ventilation for cooling the rooms of the house. This is not a viable option during the day due to the high temperatures typically reached in the area. The streamline pattern shows the movement of the air around the dwelling and inside the courtyard (Figure 16). As can be seen, the previous building alters greatly the flow. In addition, for a better comprehension of the phenomenon, the 𝑚󰇗 (in kg/s) is obtained. It is noticeable that the amount of air entering the ground floor inside the courtyard is ten times smaller than that entering the upper floor. This means that significantly less air reaches the bottom of the courtyard, reassuring the previous statement. Figure 16. Streamlines pattern of the full street simulation. Apart from that, the pressure contour (Figure 17) gives the static pressure in the surroundings of the dwelling which is in a depression caused by the previous building. Wall pressure coefficients are obtained in the highlighted spots. Table 4 show the pressure Figure 16. Streamlines pattern of the full street simulation. Apart from that, the pressure contour (Figure 17) gives the static pressure in the surroundings of the dwelling which is in a depression caused by the previous building. Wall pressure coefficients are obtained in the highlighted spots. Table 4show the pressure coefficient on four points of the dwelling. Both analyses are of great significance as it provides insight into the direction of airflow within a building when windows are opened and cross-ventilation is initiated. In this instance, due to the configuration of the dwelling and its location, the air will enter through the windows in the courtyard and subsequently flow to the exterior through the windows in the rooms. Table 4. Pressure coefficient on four points of the dwelling. Zone Pressure Coefficient Up-windward −0.559 Down-windward −0.557 Up-leeward −0.436 Down-leeward −0.438 Appl. Sci. 2025,15, 2174 20 of 32 Appl. Sci. 2025, 15, x FOR PEER REVIEW 20 of 34 coefficient on four points of the dwelling. Both analyses are of great significance as it provides insight into the direction of airflow within a building when windows are opened and cross-ventilation is initiated. In this instance, due to the configuration of the dwelling and its location, the air will enter through the windows in the courtyard and subsequently flow to the exterior through the windows in the rooms. Figure 17. Static pressure contour of the full street simulation. Table 4. Pressure coefficient on four points of the dwelling. Zone Pressure Coefficient Up-windward −0.559 Down-windward −0.557 Up-leeward −0.436 Down-leeward −0.438 With these pressure coefficients and Equation (2), the pressure difference between both windows of the upper floor is calculated: ∆P = 0.301 Pa. Once the difference in pressure between the windows in the courtyard and the external windows has been established, it is possible to conduct a simulation to observe the airflow within the upper floor, given that the window with the highest pressure will be located in the courtyard. It can be assumed that an air flow with a temperature equal to that of the outside, will be established with a certain mass flow, which will be determined by the dimensions of the windows that will traverse the rooms, cooling them with a certain value of efficiency. The streamline pattern (Figure 18) shows the air movement through the floor and the recirculation formed in each room. As can be seen, the mass flow of the air entering through the three windows connected to the terrace is 1.08 kg/s, distributed 54% in room FF_R2 and 46% in room FF_R1. This distribution of the mass flow is caused by the architecture of the house itself, due to the position of the doors and windows. What is really important about this figure is that it clearly shows the position of the recirculations created in the rooms with sensors, with two vortices appearing in each room, one on each side of the jet of air that crosses the room. It should be noted that although different scenarios have been analysed, the figure shows the most interesting case at night, with the difference in pressures previously obtained. Figure 17. Static pressure contour of the full street simulation. With these pressure coefficients and Equation (2), the pressure difference between both windows of the upper floor is calculated: ∆ P = 0.301 Pa. Once the difference in pressure between the windows in the courtyard and the external windows has been established, it is possible to conduct a simulation to observe the airflow within the upper floor, given that the window with the highest pressure will be located in the courtyard. It can be assumed that an air flow with a temperature equal to that of the outside, will be established with a certain mass flow, which will be determined by the dimensions of the windows that will traverse the rooms, cooling them with a certain value of efficiency. The streamline pattern (Figure 18) shows the air movement through the floor and the recirculation formed in each room. As can be seen, the mass flow of the air entering through the three windows connected to the terrace is 1.08 kg/s, distributed 54% in room FF_R2 and 46% in room FF_R1. This distribution of the mass flow is caused by the architecture of the house itself, due to the position of the doors and windows. What is really important about this figure is that it clearly shows the position of the recirculations created in the rooms with sensors, with two vortices appearing in each room, one on each side of the jet of air that crosses the room. It should be noted that although different scenarios have been analysed, the figure shows the most interesting case at night, with the difference in pressures previously obtained. Appl. Sci. 2025, 15, x FOR PEER REVIEW 21 of 34 Figure 18. Streamlines pattern of the upper floor. In addition to the flow pattern, Figure 19 shows the temperature map within the first floor of the building, demonstrating the steady-state conditions that prevail when crossventilation is occurring. In order to facilitate the calculations, the temperatures defined in Table 3 have been utilised, thereby establishing a typical case in which the walls of the building are at a specific temperature due to their mass inertia, while the external air enters at a considerably lower temperature, thereby cooling the rooms. This is performed during the night, when the temperature of the external air decreases while the temperature in the interior of the house remains practically equal. It is evident that cross-ventilation results in a reduction in room temperatures, as evidenced by the fact that the average room temperature is lower than that of the walls. The reduction achieved is not significantly influenced by location, as evidenced by the homogeneous temperature profile within the vortex. This suggests that the sensor measurements in the experimental section are representative of the room-wide temperature. Moreover, it is possible to demonstrate that a portion of the air jet does not contribute to the reduction in temperature within the designated area, as its temperature remains unaltered. This observation highlights the importance of calculating the efficiency of temperature reduction. Figure 19. Temperature contour of the upper floor. In order to gain a more detailed understanding of the behaviour of night time crossventilation, as well as the calculation of the temperature reduction efficiency, the 3D simulation of the room FF_R2, and the results are obtained. The 3D representation of this specific room has been selected for analysis because it exhibits the most interesting reduction in temperature due to the cross-ventilation strategy. Once the analysis has been Figure 18. Streamlines pattern of the upper floor. Appl. Sci. 2025,15, 2174 21 of 32 In addition to the flow pattern, Figure 19 shows the temperature map within the first floor of the building, demonstrating the steady-state conditions that prevail when crossventilation is occurring. In order to facilitate the calculations, the temperatures defined in Table 3have been utilised, thereby establishing a typical case in which the walls of the building are at a specific temperature due to their mass inertia, while the external air enters at a considerably lower temperature, thereby cooling the rooms. This is performed during the night, when the temperature of the external air decreases while the temperature in the interior of the house remains practically equal. It is evident that cross-ventilation results in a reduction in room temperatures, as evidenced by the fact that the average room temperature is lower than that of the walls. The reduction achieved is not significantly influenced by location, as evidenced by the homogeneous temperature profile within the vortex. This suggests that the sensor measurements in the experimental section are representative of the room-wide temperature. Moreover, it is possible to demonstrate that a portion of the air jet does not contribute to the reduction in temperature within the designated area, as its temperature remains unaltered. This observation highlights the importance of calculating the efficiency of temperature reduction. Appl. Sci. 2025, 15, x FOR PEER REVIEW 21 of 34 Figure 18. Streamlines pattern of the upper floor. In addition to the flow pattern, Figure 19 shows the temperature map within the first floor of the building, demonstrating the steady-state conditions that prevail when crossventilation is occurring. In order to facilitate the calculations, the temperatures defined in Table 3 have been utilised, thereby establishing a typical case in which the walls of the building are at a specific temperature due to their mass inertia, while the external air enters at a considerably lower temperature, thereby cooling the rooms. This is performed during the night, when the temperature of the external air decreases while the temperature in the interior of the house remains practically equal. It is evident that cross-ventilation results in a reduction in room temperatures, as evidenced by the fact that the average room temperature is lower than that of the walls. The reduction achieved is not significantly influenced by location, as evidenced by the homogeneous temperature profile within the vortex. This suggests that the sensor measurements in the experimental section are representative of the room-wide temperature. Moreover, it is possible to demonstrate that a portion of the air jet does not contribute to the reduction in temperature within the designated area, as its temperature remains unaltered. This observation highlights the importance of calculating the efficiency of temperature reduction. Figure 19. Temperature contour of the upper floor. In order to gain a more detailed understanding of the behaviour of night time crossventilation, as well as the calculation of the temperature reduction efficiency, the 3D simulation of the room FF_R2, and the results are obtained. The 3D representation of this specific room has been selected for analysis because it exhibits the most interesting reduction in temperature due to the cross-ventilation strategy. Once the analysis has been Figure 19. Temperature contour of the upper floor. In order to gain a more detailed understanding of the behaviour of night time crossventilation, as well as the calculation of the temperature reduction efficiency, the 3D simulation of the room FF_R2, and the results are obtained. The 3D representation of this specific room has been selected for analysis because it exhibits the most interesting reduction in temperature due to the cross-ventilation strategy. Once the analysis has been completed for this room, it can be replicated for any other room. In this case, the inlet conditions are changed, ranging from V ref = 0.5 m/s to V ref = 3 m/s to obtain tendency expressions where the air renewal inside the room or the temperature reduction efficiency can be calculated from the wind velocity exterior measured. The range of speeds was selected based on the minimum and maximum values observed in night time wind speed data collected in proximity to the dwelling. When changing V ref , ∆ P is also changed in consequence following the quadratic expression (2). This range of pressure differences is set as a pressure inlet in the room door in the 3D simulation, obtaining the air mass flow rate entering the room, which can be extracted directly from Fluent (Figure 20) and then can be changed to air renewal rate in renh. It is noticeable that the relationship between V ref and . m is linear. In both instances, the linear equation relating kg/s or renh to air velocity was obtained, demonstrating an R 2 of 0.99 in both cases. It should be noted that the value of the mass flow rate of air entering the room Appl. Sci. 2025,15, 2174 22 of 32 varies from 0.20 kg/s to 1.50 kg/s for the range of velocities studied. The values obtained in this study are higher than those obtained in the two-dimensional case. This is due to the fact that in this study, the dimensions of the door through which the air enters are taken into account, which gives rise to this difference in the results. Appl. Sci. 2025, 15, x FOR PEER REVIEW 22 of 34 completed for this room, it can be replicated for any other room. In this case, the inlet conditions are changed, ranging from Vref = 0.5 m/s to Vref = 3 m/s to obtain tendency expressions where the air renewal inside the room or the temperature reduction efficiency can be calculated from the wind velocity exterior measured. The range of speeds was selected based on the minimum and maximum values observed in night time wind speed data collected in proximity to the dwelling. When changing Vref, ∆P is also changed in consequence following the quadratic expression (2). This range of pressure differences is set as a pressure inlet in the room door in the 3D simulation, obtaining the air mass flow rate entering the room, which can be extracted directly from Fluent (Figure 20) and then can be changed to air renewal rate in renh. It is noticeable that the relationship between Vref and 𝑚󰇗 is linear. In both instances, the linear equation relating kg/s or renh to air velocity was obtained, demonstrating an R2 of 0.99 in both cases. It should be noted that the value of the mass flow rate of air entering the room varies from 0.20 kg/s to 1.50 kg/s for the range of velocities studied. The values obtained in this study are higher than those obtained in the two-dimensional case. This is due to the fact that in this study, the dimensions of the door through which the air enters are taken into account, which gives rise to this difference in the results. Figure 20. Air ventilation results for different Vref for the 3D room. Then, the relationship between the temperature reduction efficiency (ε) and Vref can also be expressed (Figure 21). The tendency has a logarithmic expression, although the value of it remains practically constant, with it varying from 0.41 to 0.39. It also can be found that the higher the wind velocity is, the smaller the efficiency is. Figure 20. Air ventilation results for different Vref for the 3D room. Then, the relationship between the temperature reduction efficiency ( ε ) and V ref can also be expressed (Figure 21). The tendency has a logarithmic expression, although the value of it remains practically constant, with it varying from 0.41 to 0.39. It also can be found that the higher the wind velocity is, the smaller the efficiency is. Appl. Sci. 2025, 15, x FOR PEER REVIEW 23 of 34 Figure 21. Temperature reduction efficiency (ε) for different V ref for the 3D room. This parameter is very valuable because it means that approximately 40% of the total air entering the room participates in the temperature reduction, whereas the rest of it (60%) exits the room without dissipating heat. With these expressions, validation with the experimental and HULC data can be performed. 3.2. Thermal Effect of Cross-Ventilation Strategies The inlet flow rates obtained from the CFD studies are employed to delineate the boundary conditions for the thermal simulations conducted with HULC. Consequently, the calibrated simulation model for the building is employed once more, this time to define cross-ventilation in the rooms on the first floor. The input flows in the simulation are those defined by the aeraulic studies, which maintain the actual external climatic conditions for each of the days in question. Figure 22 illustrates the six validation periods, during which the simulation is conducted with cross-ventilation. The aforementioned periods are in alignment with the experiments conducted over the course of two or more consecutive nights. The sampling period illustrated in the figure corresponds to room FF_R2. The figure illustrates the interior temperature of the room on days with and without crossventilation throughout the summer season. It has been demonstrated that the temperature can be reduced by up to 5 °C when external temperatures are lowered and cross-ventilation is conducted for multiple nights in succession. Figure 21. Temperature reduction efficiency (ε) for different Vref for the 3D room. This parameter is very valuable because it means that approximately 40% of the total air entering the room participates in the temperature reduction, whereas the rest of it (60%) exits the room without dissipating heat. With these expressions, validation with the experimental and HULC data can be performed. Appl. Sci. 2025,15, 2174 23 of 32 3.2. Thermal Effect of Cross-Ventilation Strategies The inlet flow rates obtained from the CFD studies are employed to delineate the boundary conditions for the thermal simulations conducted with HULC. Consequently, the calibrated simulation model for the building is employed once more, this time to define cross-ventilation in the rooms on the first floor. The input flows in the simulation are those defined by the aeraulic studies, which maintain the actual external climatic conditions for each of the days in question. Figure 22 illustrates the six validation periods, during which the simulation is conducted with cross-ventilation. The aforementioned periods are in alignment with the experiments conducted over the course of two or more consecutive nights. The sampling period illustrated in the figure corresponds to room FF_R2. The figure illustrates the interior temperature of the room on days with and without cross-ventilation throughout the summer season. It has been demonstrated that the temperature can be reduced by up to 5 ◦ C when external temperatures are lowered and cross-ventilation is conducted for multiple nights in succession. Appl. Sci. 2025, 15, x FOR PEER REVIEW 24 of 34 Figure 22. Cross-ventilation validation periods. Figure 23 presents a comparison of the measured and simulated air temperature for periods one and six. The two periods depicted correspond, in the case of Figure 23a, to the cross-ventilation days between 23 July and 25 July in the initial period, and in the case of Figure 23b, between 26 August and 28 August during the sixth period. In both periods, nocturnal cross-ventilation was conducted from 21:00 to 10:00 for three consecutive nights. As can be observed, the simulated indoor air temperature is in close proximity to the actual air temperature recorded during the periods of cross-ventilation. To facilitate visualisation of the effect, the simulated indoor temperature is included for the scenario in which night ventilation had not been carried out, using the previously calibrated model. The results demonstrate a reduction in the maximum daytime temperature of approximately 1 °C on the first day and 2 °C on the second day. During the three nights with cross-ventilation in the July period, the minimum outdoor air temperatures were recorded at 20 °C, 22 °C and 18 °C, respectively. It can be observed that the third night effectively achieved a greater reduction in temperature by adding the effect of the previous night, as well as the drop in outside temperature. In the three nights of August, the initial night exhibited an outdoor temperature of 22 °C, 23 °C, and 17 °C, respectively, on the final night. This phenomenon mirrors the observations made in July. During the day, in July, temperatures reach over 35 °C, while in August, the temperature varies from over 40 °C on the first day to 32 °C on the third day. In both instances, the indoor air temperature following night time cross-ventilation remains below that which would be observed in the absence of cross-ventilation on subsequent days. Figure 22. Cross-ventilation validation periods. Figure 23 presents a comparison of the measured and simulated air temperature for periods one and six. The two periods depicted correspond, in the case of Figure 23a, to the cross-ventilation days between 23 July and 25 July in the initial period, and in the case of Figure 23b, between 26 August and 28 August during the sixth period. In both periods, nocturnal cross-ventilation was conducted from 21:00 to 10:00 for three consecutive nights. As can be observed, the simulated indoor air temperature is in close proximity to the actual air temperature recorded during the periods of cross-ventilation. To facilitate visualisation of the effect, the simulated indoor temperature is included for the scenario in which night ventilation had not been carried out, using the previously calibrated model. The results demonstrate a reduction in the maximum daytime temperature of approximately 1 ◦ C on the first day and 2 ◦ C on the second day. During the three nights with cross-ventilation in the July period, the minimum outdoor air temperatures were recorded at 20 ◦ C, 22 ◦C and 18 ◦ C, respectively. It can be observed that the third night effectively achieved a greater reduction in temperature by adding the effect of the previous night, as well as the drop in outside temperature. In the three nights of August, the initial night exhibited an outdoor temperature of 22 ◦ C, 23 ◦ C, and 17 ◦ C, respectively, on the final night. This phenomenon mirrors the observations made in July. During the day, in July, temperatures reach over 35 ◦ C, while in August, the temperature varies from over 40 ◦ C on the first day to 32 ◦ C Appl. Sci. 2025,15, 2174 24 of 32 on the third day. In both instances, the indoor air temperature following night time crossventilation remains below that which would be observed in the absence of cross-ventilation on subsequent days. Appl. Sci. 2025, 15, x FOR PEER REVIEW 25 of 34 Figure 23. Simulation and validation of the impact of nocturnal cross-ventilation. The simulation of both models, with and without night time cross-ventilation, provides insight into the effect of passive operation over a period of more than four nights. Figure 24 presents a comparison of the impact of night time cross-ventilation on indoor temperature (depicted by the blue curve) with the impact of indoor temperature in the absence of night time cross-ventilation (illustrated by the orange curve). The area between the two curves represents the temperature difference between the two scenarios. The cross-ventilation is conducted in a constrained manner over the course of 20 consecutive nights, spanning from 19 July to 8 August. As can be observed, the indoor air temperature demonstrates notable fluctuations as a consequence of the inflow of fresh air, which markedly reduces the indoor temperatures. From 8 August onwards, the nocturnal cross-ventilation is cancelled, so that the two temperatures start to come closer together. This is important, because depending on the inertia of the building, the indoor air will take more or less time to reach the temperature it would have been without night ventilation. The figure shows this period distinguishing between four phases according to the temperature difference between the blue curve with cross-ventilation (𝑇󰇜 and the orange curve without ventilation (𝑇󰇜. The segmentation into intervals demonstrates how the thermal benefits decrease over time, reflecting a progressive equilibrium thermal dynamic. Therefore, the effect of cross-ventilation in reducing indoor temperatures is highlighted, not only during its implementation, but also in the following days thanks to the architecture of the ancient building. Figure 23. Simulation and validation of the impact of nocturnal cross-ventilation. The simulation of both models, with and without night time cross-ventilation, provides insight into the effect of passive operation over a period of more than four nights. Figure 24 presents a comparison of the impact of night time cross-ventilation on indoor temperature (depicted by the blue curve) with the impact of indoor temperature in the absence of night time cross-ventilation (illustrated by the orange curve). The area between the two curves represents the temperature difference between the two scenarios. The cross-ventilation is conducted in a constrained manner over the course of 20 consecutive nights, spanning from 19 July to 8 August. As can be observed, the indoor air temperature demonstrates notable fluctuations as a consequence of the inflow of fresh air, which markedly reduces the indoor temperatures. From 8 August onwards, the nocturnal cross-ventilation is cancelled, so that the two temperatures start to come closer together. This is important, because depending on the inertia of the building, the indoor air will take more or less time to reach the temperature it would have been without night ventilation. The figure shows this period distinguishing between four phases according to the temperature difference between the blue curve with cross-ventilation ( Tc−v) and the orange curve without ventilation ( Twv) . The segmentation into intervals demonstrates how the thermal benefits decrease over time, reflecting a progressive equilibrium thermal dynamic. Therefore, the effect of cross-ventilation in reducing indoor temperatures is highlighted, not only during its implementation, but also in the following days thanks to the architecture of the ancient building. Appl. Sci. 2025,15, 2174 25 of 32 Appl. Sci. 2025, 15, x FOR PEER REVIEW 26 of 34 Figure 24. Indoor temperature simulation with and without cross-ventilation. Following the temperature results obtained, an average temperature reduction efficiency value can be calculated. This takes a value of ε = 0.408, remembering the limited effect of temperature reduction due to air entering the room due to night time cross-ventilation. If this efficiency were higher, the temperature difference between the case with and without night ventilation would be even greater. 3.3. Thermal Comfort Analysis It is crucial to consider the issue of thermal comfort within the ancient building. Thermal comfort can be defined as a state in which the occupants of a building do not experience discomfort due to excessive heat or cold. In other words, the conditions of humidity, temperature, and air movement are perceived as pleasant and suitable for the activities taking place within the building. The PassivHaus standard [53] establishes a minimum comfort value for the average air temperature of 20 °C in winter and a maximum value of 25 °C in summer, with a humidity range of 40% to 70%. These conditions are deemed to provide a sense of comfort [54]. The concept of comfort in buildings has been largely informed by Fanger’s seminal studies [55], which posit that the human body is a complex system undergoing a series of heat exchanges. It should be noted that in Spain, as part of the energy-saving and management measures in air conditioning, the regulation of space air conditioning has been modified. The new regulation stipulates that the air temperature in heated enclosures shall not exceed 19 °C, while in refrigerated enclosures it shall not be lower than 27 °C [56]. Conversely, the initial scale most frequently employed for the assessment of adaptive comfort in relation to temperature is the number of hours the building is subjected to thermal discomfort. However, it seems reasonable to posit that hours when the temperature is significantly higher than the comfort level should be weighted less heavily than hours when the temperature variation is minimal. Although it seems self-evident, this line of reasoning, which is based on the percentage of dissatisfaction (PUP) as a function of temperature difference, as originally proposed by Fanger (1970), has not been presented by some authors until very recently [57]. Accordingly, the comfort rating scale would be modified from a summation of hours to a summation of temperature differences expressed in cumulative degree-hours, or a summation of temperature differences weighted by the percentage of dissatisfied individuals. Accordingly, three thermal indicators will be calculated: Discomfort hours [h]—Number of hours in which the indoor temperature is outside the thermal comfort range. Degree-hours of discomfort [°C·h]—The sum of temperature differences when the indoor temperature is outside the thermal comfort range. Weighted discomfort degree-hours [°C·h·ppi]—The sum of temperature differences in Figure 24. Indoor temperature simulation with and without cross-ventilation. Following the temperature results obtained, an average temperature reduction efficiency value can be calculated. This takes a value of ε = 0.408, remembering the limited effect of temperature reduction due to air entering the room due to night time crossventilation. If this efficiency were higher, the temperature difference between the case with and without night ventilation would be even greater. 3.3. Thermal Comfort Analysis It is crucial to consider the issue of thermal comfort within the ancient building. Thermal comfort can be defined as a state in which the occupants of a building do not experience discomfort due to excessive heat or cold. In other words, the conditions of humidity, temperature, and air movement are perceived as pleasant and suitable for the activities taking place within the building. The PassivHaus standard [ 53 ] establishes a minimum comfort value for the average air temperature of 20 ◦ C in winter and a maximum value of 25 ◦ C in summer, with a humidity range of 40% to 70%. These conditions are deemed to provide a sense of comfort [ 54 ]. The concept of comfort in buildings has been largely informed by Fanger’s seminal studies [ 55 ], which posit that the human body is a complex system undergoing a series of heat exchanges. It should be noted that in Spain, as part of the energy-saving and management measures in air conditioning, the regulation of space air conditioning has been modified. The new regulation stipulates that the air temperature in heated enclosures shall not exceed 19 ◦ C, while in refrigerated enclosures it shall not be lower than 27 ◦C [56]. Conversely, the initial scale most frequently employed for the assessment of adaptive comfort in relation to temperature is the number of hours the building is subjected to thermal discomfort. However, it seems reasonable to posit that hours when the temperature is significantly higher than the comfort level should be weighted less heavily than hours when the temperature variation is minimal. Although it seems self-evident, this line of reasoning, which is based on the percentage of dissatisfaction (PUP) as a function of temperature difference, as originally proposed by Fanger (1970), has not been presented by some authors until very recently [ 57 ]. Accordingly, the comfort rating scale would be modified from a summation of hours to a summation of temperature differences expressed in cumulative degree-hours, or a summation of temperature differences weighted by the percentage of dissatisfied individuals. Accordingly, three thermal indicators will be calculated: Discomfort hours [h]—Number of hours in which the indoor temperature is outside the thermal comfort range. Degree-hours of discomfort [ ◦ C · h]—The sum of temperature differences when the indoor temperature is outside the thermal comfort range. Weighted discomfort degree-hours [ ◦ C · h · ppi]—The sum of temperature differences in Appl. Sci. 2025,15, 2174 32 of 32 37. 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