Thermal comfort optimization through bioclimatic design in Mediterranean cities

2.1 Introduction

An overabundance of reviews has been carried out to evaluate the significant impact of vernacular buildings' thermal properties on occupant thermal comfort.^24–26 However, little research has been conducted to comprehend the potential energy efficiency of implementing passive-cooling design strategies in modernized buildings. In residential building retrofitting interventions, VPCs implementation led to 60% less energy required for heating and cooling, according to a pilot study by Serghides in 2007 and 2010.²⁷

2.2 Previous studies on Modern Mediterranean

Throughout the energy efficiency improvement of thermally inefficient modern buildings, an overheating risk has been most frequently observed in vulnerable social housing development estates due to climate change and the detrimental impact on occupant thermal comfort.^28,29 Therefore, to create thermally comfortable indoor environments, modern buildings require high cooling demand.³⁰ Serghides, in 2010, argued that despite the sizeable accumulation of technical information, present-day houses tend to be less comfortable than traditional structures.³¹ Considering interactions between buildings, systems, and occupants is essential to provide thermally comfortable conditions in homes.³²

2.3 Bioclimatic design approach

The concept of bioclimatic architecture is based on taking maximum benefits from the surrounding climate conditions and building placement, to meet indoor thermal comfort needs with minimum energy consumption.¹ Bioclimatic architecture is occasionally based on vernacular architecture, and attempts to analyze traditional architecture based on the climate and culture of a place, and to study the architectural and construction solutions. This type of architecture adapts to the local climate without using additional devices that consume energy and leave an ecological footprint.²

To apply bioclimatic architecture, it is necessary to consider the building's location, climate and microclimate; the next step would include the architectural skin, as one of the main elements to consider when striving for comfortable conditions.³

Different bioclimatic diagrams are used as tools with which to determine comfort levels.³ One of the most widely used tools includes the diagram developed by Baruch Givoni (Figure 1). The Givoni bioclimatic chart is mainly applied for residential scale construction, and it provides more alternatives in building design to enable thermal comfort, including natural ventilation, evaporative cooling, thermal mass, passive heating, conventional air conditioning or dehumidification.⁴

Figure 1. Psychrometric chart adapted from Givoni.³

2.4 Thermal comfort design variables

As per ASHRAE 55, thermal comfort is defined as “that condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation.” The human body is in the constant process of heat exchange with the environment. This heat balance of the human body governs the thermal comfort experience of individuals. As such, many variables affect human thermal comfort.⁵

Research on thermal comfort focuses on two main factors, namely, the thermal environment factor (including air temperature, relative humidity, air velocity, and mean radiant temperature) and the individual factor (including clothing and metabolic rate).⁶ Although people could achieve thermal comfort through self-adjustments, such as clothing, activity level, and psychological preference, the thermal environment still plays an essential role in research on thermal comfort. Improving the thermal environment by adjusting architectural forms is one of the most effective methods to achieve thermal comfort.⁷

The predicted mean vote (PMV) scale is often used to measure thermal comfort (Figure 2). The PMV, which was developed by Ole Fanger, is a seven-point scale ranging from −3 to +3 and is the most commonly used thermal comfort index.⁸ PMV is the mean vote that one would expect to obtain from averaging the thermal sensation votes of a large group of people in a given environment.

Figure 2. Predicted percentage dissatisfied (PPD) as a function of predicted mean vote (PMV).¹⁰

• Thermal comfort measurements

The PMV is a complex mathematical expression that involves the individual as well as the four environmental parameters. On the same lines, the predicted percentage dissatisfied (PPD) gives the percentage of people who are dissatisfied with the thermal environment. When PMV is zero, the PPD is of five percent, which means that when the sensational level of cold or hot is zero, five percentage votes are for discomfort.⁹

2.5 Mediterranean region climate and building features

The Mediterranean area is a distinct geographical entity, which has been inhabited since the origins of human history, with particular geomorphologic and climatic features. From Antiquity to the present, cultural trends of East and West intersect and influence each other, and in combination with the natural environment, the climate, the light and the sea, the previous natural elements have defined a very special way of life and consequently, a unique architectural style.¹¹

Mediterranean climate refers to the typical climate of the Mediterranean Basin, and is a particular variety of subtropical climate.¹¹ To state it simply, it usually consists of hot dry summers and mildly cold, wet winters with high daily thermal excursions characterize the Mediterranean climate. Traditional Mediterranean architecture evolved to produce buildings that would be in harmony with the harsh climates of its various regions. In the traditional architecture, the mechanism of indoor thermal regulation was incorporated in the building itself.¹²

In the Mediterranean region, ventilation and sun protection measures, together with appropriate materials and construction, represent the main issues of bioclimatic efficiency. Ventilation is necessary for comfort and hygiene; even on hot summer days when the outdoors are warmer than the building interior. In traditional buildings, attention was given to ventilation, especially to the pre-treatment of air. Solar shading is important, as well, to control the penetration of the sun in the summer.¹²

The paper summarizes the most common design features in Mediterranean Residential Architecture Style in Table 1, and a graphical summary in Figure 3, based on literature describing Architectural Design Guidelines and its codes; specifically, the Mediterranean Revival Style, as the style incorporated references from various Mediterranean regions such as the Spanish Renaissance, Italian Renaissance, and Arabic Andalusian Architecture.

Figure 3. Graphical summary of the common design features in Mediterranean Residential Architecture Style.

3.1 Lamaca, Oroklini, Cyprus

• Climate and location

Cyprus has an intense Mediterranean climate with a typical seasonal rhythm concerning temperature, rainfall and weather in general. The predominantly clear blue skies and intense sunshine periods give large seasonal and daily variations between the temperature of the coast and that at the interior of the island, that also is considerably affected by climate change, especially near the coasts. Its average hottest peak reaches 41°C in the summer and drops to an approximate of 5°C in the winter. Relative humidity ranges from 40-60%, and a large daily temperature range is noted with up to 18°C difference between day and night. Thus, Cyprus's climate calls for the need for cooling in the summer, and the large amount of solar radiation during the summer may easily be used for heating in winter. The house is located in the Larnaca District, in the Oroklini village.¹⁵ The land is located on a small hill, where neighboring buildings are located at a distance. On the eastern side, there is a road, while to the south, the plot borders a green space, which grants it more privacy.

• Building description

The building houses a family of four. It consists of three levels (Figure 4). The ground floor is divided into two individual departments, one of them being accommodated with the entrance, the living room and dining area, while the other department is accommodated with another living room with a dining area and kitchen. The first floor consists of four bedrooms and an office. The mezzanine directly communicates with these areas through an internal staircase (Figure 5). Situated in a central point on the north with southern clerestory windows which, when opened, give the advantage of direct sunlight gain and contribute to the natural ventilation and stack effect of all spaces on all floors.¹⁵

Figure 4. House in Lamaca, Oroklini, Cyprus.¹⁵

(a) South façade; (b) North and East façade.

Figure 5. Floor plans of the house in Lamaca, Oroklini, Cyprus.¹⁵

(From left to right): Ground floor plan. First floor plan. Mezzanine floor plan.

The building frame is reinforced concrete. The external walls are 25cm-thick masonry brick, 5cm-thick thermal insulation, plaster, and stone cladding. The internal walls are 10cm-thick masonry brick wall, plaster, and paint. The roof is inclined reinforced concrete, with 10cm-thick thermal insulation, water barrier, and ceramic roofing tiles. The windows are aluminium profile, double glazed, have low emissivity, and argon filled. Also, the floors are composed of ceramic tiles.¹⁵

• Bioclimatic approach

The bioclimatic approach is applied in; orientation as most spaces are South-oriented, thermal mass in floors, walls, and staircase; passive solar heating as direct gain (glass openings and clerestory windows). Solar control is achieved with external shading devices with regards to East and West. Moreover, the shading of openings is achieved with the use of overhangs, with the extension of the floors on the southern and northern sections and extension of the roof and the balcony on the southern section. There is also anticipation at the time of writing to plant trees around the building shell, but this has not been developed to a satisfactory level yet. The natural ventilation relies on night ventilation (in the summer nights all openings can be opened manually, except the clerestory windows which are electrical), cross ventilation (provisions had been made so that most spaces have openings on two sides), and stack effect (Figure 6) (At the top point of the staircase as well as in the mezzanine area, clerestory windows have been placed and are opened during the summer months).¹⁵

Figure 6. South-north cross sections showing the natural ventilation.¹⁵

3.2 Casa Pineda, Barcelona, Spain

• Climate and location

The Mediterranean coast of Spain is in the warm climate zone, where average temperatures in July and August are about 26°C. Recent results indicate that summer temperatures may be 3°C to 4°C lower. Winters are moderately cold and humid, so active heating is still necessary for residential buildings. The weather in Palau Plegamans (province of Barcelona) is similar to Barcelona, but the daily temperature oscillation in the summer is more pronounced, with typical temperatures of 14°C in the early morning hours. Located at 41° North, high solar radiation is available during the winter months.¹⁶

• Building description

The detached family house “Casa Pineda” in Palau Plegamans has one story and no basement floor. The user of the building is a two-person family (Figure 7). It is a lightweight building with a wooden beam structure; the roof is covered with rear-ventilated roof tiles. In Catalonia, it is common to combine passive houses with wood structure, which is not more expensive than the ‘traditional’ construction system, based on burnt bricks.¹⁶ The building is located in a suburban district. The L-shaped building is oriented towards the south, defining a homely open space in the garden.

Figure 7. House in Casa Pineda, Barcelona, Spain.¹⁶

(a) The southern façade; (b) The eastern façade showing L-shaped structure and open garden.

• Bioclimatic approach

The building is a lightweight construction with low thermal inertia and there is no active cooling system. An EIFS with EPS insulation was applied to the exterior walls on the northern and eastern side; the other façades are finished with a wooden ventilated structure. The main openings are towards the South (50% of all windows) (Figure 8), so in winter, the solar gains due to windows are about 40% higher than the transmission losses of the windows. Windows were installed on the inner side of the walls. This solution has a high thermal bridge effect. Summer comfort is achieved by appropriate user behavior: during daytime, closing exterior blinds which reduces mechanical ventilation; during night times, tilting windows and wide opening in the early morning hours. All windows have efficient sun shading blinds.¹⁶

Figure 8. Floor plan and cross section of the house “Casa Pineda”, Barcelona, Spain.¹⁶

3.3 Umbertide, Umbria, Italy

• Climate and location

The project is a development of bioclimatic housing for a housing development in Umbertide, Umbria, in Italy. Umbertide lies in a Mediterranean temperate climate with rainy winters and hot summers. The local climate creates a significant requirement for energy for heating and cooling. During the summer months, light breezes (under five to 10 metres per second) from the South, East or West enter the city from the surrounding hills and are caught by the streets that separate the buildings, providing them with natural air conditioning. During the remaining seasonal periods, the prevalent direction of the wind is North-West. The most comfortable months are in the spring (May) and in autumn (September and October); the central months (from June to August) are hot and humid with light breezes.¹⁷

• Building description

The alternation of the form and function of this external space is organized according to the position of the site, in correspondence with the main pedestrian wind axis or with the secondary edible garden lines. The apartment buildings range from two to five floors, depending upon different typologies (Figure 9), and each floor is composed of four rooms (bedrooms, kitchen and living room) and two bathrooms. The construction system is as follows; a structural system for row houses and detached houses consisting of load-bearing walls of traditional brick masonry; apartments are made from reinforced concrete frames with traditional brick cladding; external walls comprising ventilated cavities, with traditional brick masonry; roofs comprising a concrete structural system, sealer, air space and traditional roof tiles.¹⁷

Figure 9. House development in Umbertide, Umbria, Italy.¹⁷

(a) Building typology showing the shading screens on the external façade; (b) The housing courtyard.

• Bioclimatic approach

At the building level, the use of the stack effect (integrated with a convective loop system and with a chimney) to realize a system of cooling and ventilation resulted in a good microclimate and adequate indoor conditions during summertime. The development of this system has been progressive and has been tested and improved by continuous fluent simulations. Incoming air is transported through the building by different chimneys, either for admission or for expulsion functions, which work by stack effect (Figure 10). In the admission chimney, during winter, the air flows into the glazed section, becomes warmer as a result of the passive greenhouse effect and is then distributed to each room. In summer, the air flows into an insulated section and provides cooling due to its speed and continuous circulation; moreover, shading systems at the windows and at the glazed chimney prevent overheating. Part of the south façade is completely glazed in order to increase direct heat gain. Sun protection in summer is provided through selective shading, which allows winter direct heat gain from the sun, but screens out summer sun.¹⁷

Figure 10. The apartment house in Umbertide, Umbria, Italy.

(a) Cross section showing predicted ventilation system; (b) Typical Floor plan.

3.4 Examples of mediterranean muilding features

The study analyzed three examples of passive housing in Mediterranean cities, which have managed to achieve sustainable design and improved building performance, along with maintaining the Mediterranean architecture style. The study concluded that the most common Mediterranean local building features utilized in passive building design based on these examples are low-pitched roofs and large sized windows, followed by a projected front porch, red clay roofs, pergolas and screens, top chimneys, open floor plan and high ceiling, as well as two-story buildings. Table 2 summarizes the achieved features in the examples, to present a framework for the selection of the examined building features in Section 4.

3.5 Results

The paper investigates the impact of local Mediterranean building features on the thermal comfort of a house in Alexandria. First, it discusses the climatic characteristics of the location, and analyzes the bioclimatic chart. Furthermore, it examines the performance of the house of the case study, with and without the tested building features.

Underlying data

Data supporting reported results are available online at https://doi.org/10.5281/zenodo.4814710.²³

Data are available under the terms of the Creative Commons Attribution 4.0 International license (CC-BY 4.0).