Passive cooling in Évora’s traditional architecture€¦ · 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and 341 Advanced Ventilation Technologies

3412nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

developed in a urban and building scale (Simplício, 1991).Knowing that it is three times more difficult, which means expensive, to cool a room 1ºC that warm it (Schit-tich, 2003) it’s suitable to make the question: why are ancient buildings more comfortable in summer? When Europe consumes nearly half of all energy produced in the operation of buildings, heating, cooling and light-ing, sustainability rises again as an important issue in architecture (Schittich, 2003).Évora is located geographically in a region character-ized for a period of heat extremely uncomfortable. It is suitable, due to the profuse vernacular and erudite architecture, to be an appellative case of study from which we can remove fruitful illations. Reaching an un-derstanding of the morphology of this architecture pro-vided of conclusive data, we would be able to acquire some knowledge to apply these lessons in urban and building design.

2. GEOGRAPHY AND CLIMATE

The most conditioning climate factors in Mainland Portu-gal (latitudes between 37ºN and 42ºN) are, in addition to latitude, its orography and the effect of the Atlantic Ocean.

Figure 1: Portugal map, Évora’s wind rose and Évora’s monthly cycles temperature (min - max) and precipitation. (Santos, 2002).

ABSTRACT

In this study we identify and describe the design strat-egies adapted to the climate and used in ancient con-structions. The research is focused on the information obtained from cases found in Évora’s architecture. The aim of this paper is to sensitize the rational use of ener-gy showing efficient natural cooling design strategies to achieve it. It can thus be an effective tool for attenuating the growth of energy consumption for air conditioning. The results of the study can be used to adapt some of these strategies to new constructions in this region or regions with similar climates. It will contribute to re-es-tablish the dialogue between the architecture and the en-vironmental conditioning, frequently treated separately.

1. INTRODUCTION

The aim of this study is to identify the strategies, used in traditional architecture, adapted to the local climate. Learning from the ancient construction experience we want to contribute to improve bioclimatic architecture.The use of conventional air-conditioning systems have to be considered with care due to the need of reducing the environmental impacts, which result from fossil fu-els burning and from ozone-depleting CFC refrigerants used by conventional air conditioners.The use of passive cooling techniques in summer is ad-visable either in the perspective of improving the thermal environment of non-climatised spaces as in the perspec-tive of reducing energy consumption when the spaces are climatised. It can thus be an effective tool for attenuating the growth of energy consumption for air conditioning.The results of the study can be used to adapt these an-cient construction strategies to new buildings, increas-ing the quality of design and breaking the aesthetics scepticism around bioclimatic architecture.The research is focused on the information obtained from the architecture present in Évora, Portuguese UNESCO World Heritage City.Occupied before 700 B.C., it had been influenced by many cultures through the times. The Romans - the first civilization to consecrate juridically the right to sun - and Muslims - with ages of construction experience in rough hot dry climates - marked deeply the way the city grew and

Passive cooling in Évora’s traditional architecture

J. Fernandes, J. Correia da SilvaUniversity of Évora, Portugal

PALENC 2007 - Vol 1.indd 341 3/9/2007 1:24:46 µµ

342 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

Évora is located in the southern interior part of Portu-gal (latitude 38.6oN, longitude 7o54´W), in a vast region called Alentejo. Its climate is Mediterranean temperate, hot and dry during summer, with very high tempera-tures during July and August, with highs between 35°C and 40°C, reaching sometimes highs of 45°C.In the interior of the Alentejo, average rainfall amounts are the order of 500 mm and show large interannual variability (Fig. 1).In general terms, the relief of the involving area of Évo-ra can be considered as relatively uniform; most of the area is situated between 200 and 300 meters of altitude, with some elevations rising to 400 meters. The city stands in the confluence of three hydrographic basins, Tagus, Guadiana and Sado. Strategically well located as a geographically center attracted the military interest acquiring a remarkable regional importance during the Roman occupation (Simplício, 1991).In July and August the dryness is severe and the storms are rare. The direct influence of such climate on man is evident. In the dwellings, the practice of the whitewash painting and the absence of wide apertures are concom-itant conditions of the climate.

3. PASSIVE COOLING STRATEGIES

In this chapter we’ll approach different types of strate-gies found in the city to mitigate the effect of the uncom-fortable heat. Cooling strategies are deeply related to the materials and the techniques of building construction. To discern these techniques and materials is a hard task because the ethereal whitewashed coverings that protect the walls, only being possible to analyze and to identify after the Time raises, one by one, the veils that cover them.The passive cooling strategies found were: adequate ur-ban planning, high thermal inertia, ventilative cooling, evaporative cooling, protection against solar radiation, use of vegetation and earth cooling.

3.1 Adequate Urban PlanningThis strategy influences solar radiation received on fa-çades and streets. The narrow and sinuous streets form structures of “urban-patios” that promote ventilation (air crossing) between some streets. These streets re-duce the effect of strong winds and its public and pri-vate patios reduce the area’s overall surfaces exposed to solar radiation. Thus, this compact urban layout reduces the number of surfaces displayed to the sun and allows buildings to shade one another (Fig. 2).In the morning, these narrow streets walls and street pavements, due to their thermal inertia, remain colder than ambient air. Cold air is denser and therefore heavi-er than warm air so, cold air remains in the streets dur-

ing the morning while there’s no wind.The existence of patios in the buildings of the old town, from the most erudite to the more popular - appearing some as vestigial space between the diverse construc-tions - is very common, which reveals the importance given them (Fig. 2). Anyway, all of them have influence in the passive climatization of the buildings. Not only the form and the orientation of patios influence its cool-ing effect, but also Patio cooling efficiency is improved by the existence of fountains and vegetation. Evapora-tive cooling, evapotranspiration and shading effect con-tribute to the cooling efficiency of patios.

Figure 2: Évora’s old town plan.

In some places of the city we could find pedestrians covered circulation galleries to protect them from the sun and rain (Fig. 3).

Figure 3: Narrow street and galery.

3.2 High Thermal InertiaThe application of this strategy allows protecting the in-door spaces of the building from extreme temperatures. The materials and techniques of construction play an ex-



cellent role in this strategy. The more adjusted reply for a climate such as Évora, with high thermal amplitudes, it’s to build with strong thermal mass which, will cause the dampening of the temperature variation and delay the transfer of heat between the exterior and the interior. Materials and built types are chosen in order to optimize a comfort relation between the building and the exte-rior environment. The materials and techniques we find abundantly are taipa, raw clay bricks and adobe. Soil is the most used material in this region and it is, main-ly, due to it that buildings have a high thermal inertia.Taipa is undoubted the type of soil technique more ex-panded in Alentejo. Due to its permeability it is of ex-treme importance to cover it, or it will degrade easily. Processes as the periodic whitewashing are decisive to keep it in good shape. Due to this covering process it is, in a naked eye observation, virtually impossible to distinguish a wall from taipa from one of masonry of adobe or raw clay brick. Only with sophisticated tech-nology, as a sclerometer or ultrasonic waves, it is pos-sible to estimate the resistance of the wall material to compression and discern it.The option for the construction in heavy mass, beyond its functional requirement of structure, has a comfort requirement inherent to the material thermal inertia and to its capacity to regulate the degree of humidity in the interior spaces through its hygroscopicity. The white-washing covering influences this feature since it is per-meable to water vapour.During the day the low thermal conductivity allows, the building interior to keep a lower temperature than the exterior air. Due to the materials thermal time lag, the indoor surfaces are cooler than interior air, in the morn-ing, and hotter during the afternoon, when the heat pass-es from the interior mass to the indoor air. The indoor temperature, in a building with high inertia, reaches its maximum at the end of the day. Night ventilation bal-ances the irradiation of heat for the interior.

Figure 4: Towers near to dormitories.

3.3 Ventilative CoolingThis strategy foments indoor air circulation of the build-ing aiming the salubrity and thermal comfort.

Figure 5: Apertures above windows and doors.

The natural ventilation aims the physiological comfort and implies a voluntary act of the occupants seeking a more comfortable atmosphere. However this strategy is not used indiscriminately all day long, the occupants must know which are the most propitious periods to do it, mainly in climates such as the one from Évora.The towers that we see in Figure 4 were sentinel places and usually bell towers. However they also promote natural ventilation. Strategically placed, these towers are commonly addorsed to the dormitories, allowing the ventilation of these spaces. These towers have draughts all the time, with or without wind, which is caused by the effect of differences of air density between night and day. During hot days, exterior hot air is cooled by the contact with the walls of the tower, cooled during the night; then cool air becomes denser and goes down the tower into the rest of the building.In some big buildings, such as convents, there are patios with distinct dimensions, one hotter and another cold-er, that due to the difference of pressure originate air permutes between them. The cold air contained in the small patio (positive pressure) transfers itself to the big-ger patio (negative pressure) where hot air rising leaves free space for the cold air (Fig. 6).

Figure 6: A two-patio building, air switch.

Night ventilation is the preferential strategy in regions such as Évora, with a hot and dry climate summer. Night ventilation reduces indoor temperature, during the fol-lowing day the cooled mass serves as heat sink, keeping


344 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

the indoor temperature lower than the exterior. More-over, daily thermal amplitude is high. So, traditionally, people use nocturnal ventilation as the main strategy of natural acclimatization. During daytime, windows are closed and are protected from direct solar radiation. When the ventilation is promoted through the opening of the windows, privacy and security concerns can limit the time that these are kept opened. In Évora we found several examples of apertures, as we can see in Figure 5, usually located above windows and doors which al-low natural ventilation and prevent intrusion.

3.4 Evaporative CoolingStrategy from which the environment is cooled by evaporation of the existing water in fountains and pools, usually placed in patios and cloisters, and also by evapotranspiration from plants.Water has a great importance in a climate such as Évo-ra’s. For beyond the psychological effect, the sound of the water has a relaxing effect, more significant in hu-man comfort related with the capacity to balance and to reduce environmental temperature.In the specific case of Évora we do not find the diversity of strategies, to apply this methodology, which we can find in countries in the Middle East. However this strat-egy is, probably the most spread out, easy to find, since the erudite fountain to the simple well (Fig. 8).The process is simple, air passing over water causes evaporation, and as a result of this process heat is ab-sorbed and the air is cooled, increasing air humidity. So, the aim is to channeling breezes over the water pools be-fore they enter in the building. To assure that the humidi-fied and cooled air enters the building, the pool is placed between walls such as in a cloister or courtyard (Fig. 7).

Figure 7: Fountain, evaporative cooling methology

Figures 8: Fountains, convent and private house

3.5 Use of Vegetation and Shade DevicesThe aim of this strategy is to reduce the input of the solar radiation in the building and surrounding spaces by trees, bushes, grass and creeping plants. These are elements of extreme importance in regularizing and balance the climatic conditions, reducing drastically the thermal amplitude around the buildings. Of all the stud-ied cases, in patios where is no water there’s always vegetation to increase comfort conditions to the inhabit-ants, by means of the shade and evapotranspiration.Summertime heat is reduced by using plants that shade walls and windows from direct solar radiation, as we verify in some of cases studied (Fig. 10).Creeping plants work as a thermal protector of the fa-çades, as we can see in Figures 9 and 10. When they possess thick leafy branches an immovable air layer is created between the foliage and the wall, factor that substantially reduces the exterior superficial thermal conductance coefficient. Its insulation effectiveness can surpass a double glass window (Moita, 1985). Evapo-transpiration generates a cooling effect releasing evapo-rative water to the environment, but it also works as air-filter and oxygen supply.

Figure 9. House covered of creeping plants

Figure 10: Shade devices.

The use of shading devices in the buildings becomes particularly useful when large glass surfaces are used



and we want to reduce the excess of solar radiation in-door. This is not common in Évora’s old town, where window-openings are generally small. However we find some cases with larger ones. In some of these cases we find then shading device solutions that reduces solar inci-dence impact, mainly in summer (Fig. 10). However the most common are the small window-openings, retreated into the façade to get a little of shade, aim to assure natural lighting of the indoor reducing the solar exposure. Ver-nacular architecture teaches us that small window-open-ings function better in extreme heat situations because, although being the most sensible point of the building in this matter, its small surface reduces the heat profits.The colour applied in the outdoor surfaces isn’t a matter of collective taste; it’s an ancient knowledge, the impact of the sun in the building can be reduced in function of its colour.Whitewashed walls are thus an important element against extreme solar radiation, its white color allows a reflectance of about 90% of all the radiation received (Koch-Nielsen, 2002).

3.6 Earth CoolingEarth cooling is a strategy by which the thermal mass of the soil, where the building is implanted, is used as thermal storage.In the particular case of Évora, due to its topography, it is usual to find cases where the buildings are partially embedded in the ground (Figs 11-12). Although it isn’t a deliberately applied strategy, at least in the majority of the cases, it works as a cooling source. The cooler rooms, the most embedded, are used as cellars and storerooms to preserve sensible products from high thermal ampli-tudes. As it has already been presented in the chapter of thermal inertia, the higher the mass the higher the thermal storage capacity and that will increase time lag of temperature wave and dampening of the temperature variation. The structure of the building in contact with the ground is cooled by the heat transmission by con-duction between the soil and the walls, which absorb indoor heat, by convection (Fig. 12).

Figure 11: Building embedded in the soil and his patio

Figure 12: Heat transfer between indoor environment and the soil

As we can see by the several images, the majority of the patios have vegetation covering or shading the ground. In these cases the soil’s behaviour as cooling element is improved because its moistness losses and temperature gets lower.

4. CONCLUSION

The passive cooling strategies used in Évora answer correctly to the conditions imposed by the climate in hot summer period.We verify that the adopted strategies aims, more than balance between hot and cold seasons, are to reduce the extreme summer heat.The efficiency of the strategies is proved by the huge number of cases where they’re in, from the erudite building to the popular ones. Among the most dif-fused we have the patios, which accumulate cool air and promote ventilation; whitewashed façades to reflect solar radiation and reduce heat gains; small window openings; use of vegetation as shading de-vices; fountains and pools to promote evaporative cooling. Natural measures, which don’t need power suppliers, don’t pollute and are sustainable. New con-cept buildings should integrate these strategies, con-sciously, optimize them, evolving a new architecture.Its well known that ancient architecture, particularly the popular one, has a straight relation with the involving climate, but we still have a lack of information in this matter and a lot to learn about it. This is the time to re-learn how to built, sustainable.

REFERENCES

Koch-Nielsen, H. (2002). Stay Cool, A Design Guide For The Built Environment In Hot Climates. London. James & James Ltd.Moita, F. (1985). Energia Solar Passiva. Lisboa. Imprensa Nacio-nal – Casa da Moeda.Santos, F. (Ed.). (2002). Climate Change In Portugal, Scenarios, Impacts and Adaptation Measures. Lisboa. Gradiva.Schittich, C. (Ed.). (2003). InDetail, Solar Architecture. München. Institut für Internationale Architektur – Dokumentation GmbH & Co.Simplício, M. (1991). O Espaço Urbano de Évora. Évora. Uni-versidade de Évora.


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Passive cooling in Évora’s traditional architecture€¦ · 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and 341 Advanced Ventilation Technologies