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Zero- and low-energy housing for theMediterranean climateAnnarita Ferrante aa DAPT, Department of Architecture and Urban Planning,Università degli Sudi di Bologna, Viale del Risorgimento 2,Bologna, ItalyPublished online: 10 Apr 2012.

To cite this article: Annarita Ferrante (2012): Zero- and low-energy housing for the Mediterraneanclimate, Advances in Building Energy Research, 6:1, 81-118

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Zero- and low-energy housing for the Mediterranean climate

Annarita Ferrante∗

DAPT, Department of Architecture and Urban Planning, Universita degli Sudi di Bologna, Viale delRisorgimento 2, Bologna, Italy

The concept of zero-energy building (ZEB) has gained much international attention and nowrepresents the main future target for the design of buildings. ZEB development in buildingdesign construction practice needs to respond quickly to the mandatory new requirementsfor any new constructions in a few years. This chapter presents and discusses some progressin low- and zero-energy research and practice, with specific reference to the case of theMediterranean context. It reports the following: a review of the current policy background;some notes on the definition of low- and zero-energy buildings; case studies anddemonstration projects; an outlined description of the Mediterranean climate and its coolingdemand; passive strategies to reduce the energy demands of buildings; past and ongoingresearch studies and projects in the Mediterranean region. Starting from very recent researchstudies in the literature and achievements in building pilot projects, a brief discussion towiden and consolidate current technical knowledge about ZEB and its deep penetration intobuilding common practices is finally presented.

Keywords: passive house; zero-energy buildings; passive cooling techniques; Mediterraneanclimate

Introduction

Over the last decades, the concept of zero-energy building (ZEB) has gained much internationalattention and now represents the main future target for the design of new buildings. The growinginterest in ZEB is especially clear when considering the increasing number of low-energy andgreen buildings and the media attention they subsequently attract (Brown & Vergragt, 2008).

On the other hand, so far, current construction practices in the building housing sector showlimited signs of change (Ferrante & Cascella, 2011). This is particularly true within the context ofthe Mediterranean climate, where mild winter and hot summer conditions, together with differentsocial and economical factors, determine a sort of resistance to the deeper penetration of ZEBprinciples into building construction practices. Here the observed growing interest in green build-ings usually results in prototype models and experimental pilot studies whose real impact onbuilding practices and construction is still limited at present.

Current policy background on ZEBs/NZEBs

Following the European Commission’s proposal in November 2008, for an update of the 2002Energy Performance of Buildings Directive (EPBD), a recast of the same Directive wasadopted by the European Parliament and the Council of the European Union (EU) (European

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∗Email: annarita.ferrante@unibo.it

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Council for an Energy Efficient Economy, updated February 2011), in order to strengthen theenergy performance requirements, to clarify and streamline some of its provisions (EU Parlia-ment, 2009, 2010).

Among the highlights of the recast is a strengthening of the energy performance requirementsof new as well as existing buildings across the EU. For new buildings, the recast fixes the year2020 as the deadline for all new buildings to be ‘nearly zero energy’ (sooner for public buildings –by the end of 2018). For existing buildings, Member States are required to draw up national plansto increase the number of nearly ZEBs, although no specific targets have been set.

However, the Directive does not clearly define what a ‘nearly zero energy building’ is, eitherfor new build or refurbishment of existing buildings. Article 2(1a) gives a qualitative definition:

A ‘nearly zero energy building’ is a building that has a very high energy performance. The nearly zeroor very low amount of energy required should be covered to a very significant extent by energy fromrenewable sources, including energy from renewable sources produced on-site or nearby.

This qualitative definition causes a wide spectrum of additional specifications by variousauthors in the literature, discussing about the issues pertaining to terminology and definitionsaround buildings that consume very low or zero energy (or carbon), including those with netenergy production – ‘energy positive’.

Comprehensive review studies on the definitions of ZEBs, including Life Cycle Assessment(LCA), were reported by many authors (Hernandez & Kenny, 2010; Marszal et al., 2011).

A list of related documents of support measures in accordance with Article 10 Directive 2010/31/EU has been developed by different countries (Denmark, France, Greece, Lithuania,Romania, etc.). Other countries, such as the UK, have already established comparable targetsfor all new housing which will see ‘net-zero’ achieved by 2016.

The subject matter of the Directive is defined (Article 1) as the ‘Performance of buildings within theUnion, taking into account outdoor climatic and local conditions, as well as indoor climate require-ments and cost-effectiveness’. In particular, the Directive lays down requirements as regards: (a) thecommon general framework for a methodology for calculating the integrated energy performanceof buildings and building units; (b) the application of minimum requirements to the energy perform-ance of new buildings and new building units; (c) the application of minimum requirements to theenergy performance of: (i) existing buildings, building units and building elements that are subjectto major renovation; (ii) building elements that form part of the building envelope and that have a sig-nificant impact on the energy performance of the building envelope when they are retrofitted orreplaced; and (iii) technical building systems whenever they are installed, replaced, or upgraded; (d)national plans for increasing the number of nearly zero-energy buildings; (e) energy certification ofbuildings or building units; (f) regular inspection of heating and air-conditioning systems in buildings;and (g) independent control systems for energy performance certificates and inspection reports.

The Directive also states that its requirements are minimum requirements and shall notprevent any Member State from maintaining or introducing more stringent measures. Thesemeasures shall be compatible with the Treaty on the Functioning of the European Union andneed to be notified to the Commission.

On the definition of low-energy buildings/ZEBs

As previously noted, currently, a wide range of terms and descriptions is used in discussions onlow-energy buildings and ZEBs.

The definition of a net zero-energy building (NZEB) used by the Industry Committee duringthe negotiations on the recast is reported, although ultimately, the final document adopted the less

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stringent requirement of ‘nearly zero energy building’ as opposed to the original ‘net zero energybuilding’:

A net zero energy building is where, as a result of the very high level of energy efficiency of the build-ing, the overall annual primary energy consumption is equal to or less than the energy production fromrenewable energy sources on site.

The concept of net/nearly zero-energy building (Net ZEB) is often used and included as aspecific topic in all the renowned congresses involved in the energy efficiency of buildings(ASHRAE, Clima2010, Eurosun, etc.).

Despite this, there is no harmonized understanding about what is really an NZEB. An inter-national work was started in 2008 within the framework of the International Energy Agency‘Towards Net Zero Solar Energy Buildings’ (Task40/Annex 52 2008) to attempt at harmonizingthe overall understanding of the concept; thus the objective of this work is to study current net-zero, near net-zero and very low-energy buildings and to develop a shared, common and harmo-nized international framework (Sartori, Napolitano, Marszal, Pless, Torcellini, & Voss, 2010;Smith, Whiteleg, & Williams, 1998), including design tools, innovative solutions and guidelines.

This includes new requirements on comfort and energy performances (Sartori, Candanedo,et al., 2010), grid interactions, and an exhaustive benchmarking about the existing Net ZEBsalready built around the world.

There are some Net ZEBs built around the world so far. Very little work is available about thedesign of Net ZEBs in hot/tropical regions as well. This is why a 3-year programme has beenlaunched in France about this specific topic under the name ‘EnerPos’, a French acronym forPOSitive ENERgy building.

Four well-documented definitions – net-zero site energy, net-zero source energy, net-zeroenergy costs and net-zero energy emissions – have been reported by Torcellini, Pless, andDeru (2006), where pluses and minuses of each are discussed. These definitions have beenapplied and validated to a set of low-energy buildings for which extensive energy data are avail-able. This study shows the design impacts of the definition used for ZEB and the large differencebetween definitions. In summary, Torcellini et al. (2006) identified the following main definitionsof ZEBs:

(1) Net-zero site energy: A site ZEB produces at least as much energy as it uses in a year,when accounted for at the site.

(2) Net-zero source energy: A source ZEB produces at least as much energy as it uses in ayear, when accounted for at the source. Source energy refers to the primary energyused to generate and deliver the energy to the site (this definition is equivalent to theIndustry Committee’s definition).

(3) Net-zero energy costs: In a cost ZEB, the amount of money the utility pays the buildingowner for the energy the building exports to the grid is at least equal to the amount ofmoney the owner pays the utility for the energy services and energy used over the year.

(4) Net-zero energy emissions: A net-zero emissions building produces at least as much emis-sions-free renewable energy as it uses from emissions-producing energy sources (this isthe case of the ZCB, the ‘Zero Carbon Building’) (Table 1).

Zero- and low-energy buildings: case studies

The increasing interest in ZEBs, recent European and national Directives on Energy Performanceof Buildings (EPB) and easily accessible Best Available Techniques (BATs) all seem to point to

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Table 1. ZEB definitions summary (Torcellini et al., 2006).

Definition Pluses Minuses Other

Site ZEB . Easy to implement. Verifiable through on-site

measurements. Conservative approach to

achieving ZEB. No externalities affect

performance, can track successover time

. Easy for the building communityto understand and communicate

. Encourages energy-efficientbuilding designs

. Requires more PV export to offset natural gas

. Does not consider all utility costs (can have a lowload factor)

. Not able to equate fuel types

. Does not account for non-energy differencesbetween fuel types (supply availability, pollution)

Source ZEB . Able to equate energy value offuel types used at the site

. Better model for impact onnational energy system

. Easier ZEB to reach

. Does not account for non-energy differencesbetween fuel types (supply availability, pollution)

. Source calculations too broad (do not account forregional or daily variations in electricity generationheat rates)

. Source energy use accounting and fuel switching canhave a larger impact than efficiency technologies

. Does not consider all energy costs (can have a lowload factor)

. Need to develop site-to-sourceconversion factors, which requiresignificant amounts of information todefine

Cost ZEB . Easy to implement and measure. Market forces result in a good

balance between fuel types. Allows for demand-responsive

control. Verifiable from utility bills

. May not reflect impact to national grid for demand,as extra PV generation can be more valuable forreducing demand with on-site storage than exportingto the grid

. Requires net-metering agreements such thatexported electricity can offset energy and non-energy charges

. Highly volatile energy rates make up for difficulttracking over time

. Offsetting monthly service andinfrastructure charges require goingbeyond ZEB

. Net metering is not well established,often with capacity limits and atbuyback rates lower than retail rates

EmissionsZEB

. Better model for green power

. Accounts for non-energydifferences between fuel types(pollution, greenhouse gases)

. Easier ZEB to reach

. Need appropriate emission factors

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further exploitation of BAT and better penetration of ZEB into new building construction. Bysetting the European Directory objectives, at the European level the nearly ZEBs should be areality in 8 years.

It is possible to state that energy and green buildings now belong to the ‘history of architec-ture’: the first prototype buildings and their attempts to achieve zero heating in the form of solarhouses date back to the 1950s (Hernandez & Kenny, 2010), such as the very early ‘Bliss House’(Bliss, 1955) and the 1970 V. Korsgaard ZeB in Denmark. Among the recent experiences is thewell-known BedZED (Beddington Zero Emission Development), the English Urban Villagewhich won the ‘Housing and Building’ category at the prestigious Energy Awards in Linz,Austria, 2002 (Marsh, 2002).

Among the latest are the experiences aiming at setting to zero the carbon emission of newdevelopments and even a whole city: a Pilot City Plan study to set to zero the carbon emissionsin the existing cities has been developed in the framework of the Copenhagen Climate Plan (Cityof Copenhagen, 2009). The Climate Plan of Copenhagen demonstrates that CO2 emissions can beeffectively reduced without negatively impacting economic growth and city development. ThePlan Report demonstrates how to make Copenhagen the world’s first carbon neutral capital by2025 by means of using biomass in power stations, erecting windmill parks, increasing relianceon geothermal power and renovating the district heating network.

As reported by Kapsalaki and Leal (2011), nowadays many NZEBs, both residential andcommercial, have been built, tested and reported in different parts of the world since theearly 1990s. Kapsalaki and Leal (2011) also report on the experience of the Fraunhofer Institutefor Solar Energy Systems (ISE), with specific reference to a self-sufficient solar house in Frei-burg, which may be considered the first ‘modern’ ZEB. For 3 years (1992–1995), the house wasoccupied by a family and monitored. Results show that the sun can provide a house with all theenergy that it needs, even under the moderate solar radiation of the specific climatic region. Infact, during the whole period, the home was not connected to the grid and there was no otherexternal supply but solar energy; the energy supply was based on solar-generated hydrogen asthe energy storage form for electricity and heat and a fuel cell as a miniature cogeneration powerplant.

The majority of NZEB case studies refer to newly conceived buildings mainly located in thecold climate regions of the world (Musal et al., 2010; Noguchi, Athienisis, Delisle, Ayoub, & Ber-neche, 2008). A larger series of NZEBs and ZEBs examples is concentrated in the Central(Germany) and Northern parts of Europe as was fully reported in the literature (Musal et al.,2010).

Although the majority of ZEBs are concentrated in the north and cold climate regions,visible and attractive demonstration ZEBs are also emerging within the hot part of the world;thanks to a massive constant increase of active energy production plants by renewablesources, a first zero-waste and zero-carbon emission city is to be constructed in Abu Dhabi,in the United Arab Emirates, UAE: ‘Masdar City’, a $22 billion complex designed byNorman Foster & Partners which is scheduled for completion by 2013. Despite its location(the oil-rich and hot part of the world) the development is designed as a huge, positive-energy building, resulting in a self-sustaining, car-free city setting. The massive solar plant(one of the world’s largest building-integrated photovoltaic arrays) is constructed first andused to generate power for continuing construction of the building, through the developmentof its solar roof pier before the underlying complex. The Masdar Headquarters building isplanned to be the world’s first large-scale positive-energy building. In addition to the largesolar roof, the building integrates in its design wind turbines for electricity generation andwind towers are to exhaust warm air and naturally ventilate the building while at the sametime drawing cool air in. Natural daylight is provided through vertical ducts, integrated into

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the structures supporting the building’s roof, which allows the creation of a shaded microclimatearound and over the building complex.

Other examples of NZEBs in a hot climate reported by Kapsalaki and Leal (2011) are in Lake-land, Florida and in Boulder, Colorado. In Lakeland (1998), an experimental residential buildingcalled PVRES consists of high levels of thermal insulation, a white reflective roof system, solarwater heating and efficient interior appliances and lighting, a high-efficiency heat pump and a PVsystem. The project, although it did not reach zero-energy annual balance (the annual energyneeds in terms of final energy were covered by 75 per cent), showed that virtually zero netutility peak coincident demand was possible and became the flagship for the programme of theUS Department of Energy, Zero Energy Homes (Parker, 2009). Solar Harvest, in Boulder, Color-ado, is a house designed and constructed by Eric Doub in 2005. The concept is a combination ofactive and passive solar design features with heavy thermal insulation, high-performance glazingand windows and highly efficient equipments. All the appliances in the house have been electricsince 2007, with all electricity needs provided by PV panels, and space heating and domestic hotwater are provided by solar thermal flat-plate collectors. It also features an extended engineeredheat recovery ventilation system and a PVC pipe buried underground for seasonal thermal pre-warming and pre-cooling of the incoming air.

Among the very recent low-energy building examples in the Southern Europe is a solar officebuilding ‘Solar XXI’. Built in Lisbon in 2006 as a demonstration project (Aelenei et al., 2011), thebuilding is currently under way to reach the net zero-energy performance. Solar XXI, whosedesign is based on a combination of passive design techniques with renewable energy technol-ogies (PV, solar collectors), may be nearly considered an NZEB, since it has been selected to par-ticipate in the SHC Task 40-ECBCS Annex 52, ‘Towards Net Zero Solar Energy Buildings’, andfor this reason, it is currently under revision for studying possible strategies for upgrading SolarXXI to the NZEB status (Goncalves, 2010).

The set of solutions adopted, such as the building envelope, the daylighting performancecharacteristics, the natural ventilation strategies, the passive heating and cooling techniques,together with the integrated renewable energy systems, qualifies the Solar XXI building forexemplary energy performance. Solar XXI building energy performance is about ten times theenergy performance of a standard new office building in Portugal. On-site measurements andthe building energy simulation model are important features in the process of assessing the build-ing energy performance. Thus, while the physical building features have been modelled with thehelp of an appropriate simulation tool, the operational characteristics can seldom be defined pre-cisely. One way to overcome the differences between the ‘real’ building and ‘predicted’ buildingbehaviour is to calibrate the simulation model through disaggregation of measured energy use andthen ‘tune’ the model to fit the measured data.

In the case of Solar XXI building, its energy performance was demonstrated by means ofexperimental data (on-site measurements) together with modelled data (using the simulationengine EnergyPlus). The Solar XXI integrates efficient solutions set and strategies, from the strat-egies to reduce building energy demands, to the architectural integration of the renewableenergies.

Figure 1 (right) shows the 2007 Solar XXI performance from an energy balance approachperspective vs the critical steps towards NZEB performance.

The building has a total habitable surface of 1,500m2 on three floors and is located inside theINETI campus in Lisbon, and it is the new office premises for the Renewable Energy Departmentof INETI. ‘Edifıcio Solar XXI’ shall operate comfortably as an office building, while it is usedalso as a demonstration project for building solar passive and active technologies. The projectin its upgraded version shows that it is possible for a building office located in Lisbon to meetsummer comfort objectives without active cooling systems, thanks to passive cooling strategies,

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such as sun protection, thermal mass, individual adaptation, earth cooling, ventilated facade andnatural ventilation. In fact, the building envelope will be definitively upgraded with additionalthermal insulation (U-value: facade: 0.5W/m2K; roof: 0.3W/m2K), double glazing with externalmovable Venetian blinds (solar factor of 0.04). With these measures, heat gains through opaquefacades and roof are (i) reduced, (ii) stored in the mass of the building and (iii) released during thenight to indoor spaces that can be sufficiently cooled down by natural ventilation.

The reduction of internal heat loads will be achieved by extended daylight use, since in thecentral part of the building there is a skylight that harnesses natural lighting for the threefloors, because of transparent elements between the central corridor and the adjacent rooms.As regards natural ventilation, two main techniques were applied:

. Ventilated facade: Using the heat generated in the rear part of the photovoltaic panels, oper-ating together with two openings in each room (at low and high height) to create a free con-vection air movement in the South facade.

. Stack effect: By openings in the skylight to allow the night cooling ventilation.

The building is provided with an earth-to-air heat exchanger, a system of 32 concrete buriedpipes, 4.6m underground, with 30cm diameter each, having a buried plenum 15m away from thesouth facade of the building. The pipes take the outside air, cool it down and conduct it into thebuilding by a vertical distribution system. In each room, there is an entrance for two pipes, whichcan be manually regulated, and a small fan for increasing the incoming air flow rate.

As far as the individual adaptation is concerned, users are allowed to change their clothes, toopen or close windows and doors, to regulate the position of the Venetian blind and to regulate theair flow rate coming into their room from the earth tubes.

The monitoring of energy performances is ongoing. The two summers of 2006 and 2007outdoor air temperatures were quite high for Lisbon, above 358C (up to 408C during daytime).In these two summers, the mean temperature inside the south-facing rooms (the hottest part ofthe building) varies between 24 and 25.48C, for maximum temperatures from 26.4 up to28.18C (Pagliano, 2008).

The Mediterranean climate

The Mediterranean climate is the less extensive of the meso-thermal climates, according to the20th century geographical classification developed by the German climatologist WladimirKoppen (1846–1940) which continues to be the authoritative map of the world climates in usetoday.

Figure 1. Solar XXI. The building (left) – the path to net zero-energy performance.Source: Goncalves (2010).

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Currently, the upgraded version of the Koppen classification uses six letters to divide theworld into six major climate regions, based on average annual precipitation, average monthly pre-cipitation and average monthly temperature. As also reported by Lavee, Imeson, and Sarah(1998), Koppen defined the Mediterranean climate (or Etesian climate) as the area where: (i)the mean temperature of the coldest month is between –3 and 188C; (ii) the summer season isgenerally dry and the rainfall amount of the wettest month is at least three times greater thanthat of the driest month; (iii) the mean temperature of the warmest month is above 228C; (iv)the mean annual rainfall amount (in mm) is higher than 20 times the mean annual temperature(in degrees Celsius). The first three conditions also refer to semiarid and arid regions adjacentto the Mediterranean climate zones. Thus, the crucial difference between the Mediterraneanand adjacent arid climate zones is the mean annual rainfall.

The Mediterranean sea contributes to the temperate warm climate, retaining heat in summerand releasing it in winter.

The majority of the regions with Mediterranean climates have relatively mild winters and hotsummers. However, winter and summer temperatures may vary greatly between different regions.In the case of winters for instance, Lisbon experiences very mild temperatures in the winter, withfrost and snow practically unknown, whereas Thessaloniki has colder winters with annual frostsand snowfall. As a further example, inland France is very different from the southern borders ofEgypt or Libya. Nonetheless, many similarities can be found for a wide number of countries bor-dering the basin: in almost all the coastline cities, the minimum yearly average temperature iswithin 5–108C and the maximum is within 27–348C, with the highest values being recordedin the Turkish coastline and Cyprus.

Other climatic recurrences in the Mediterranean climate are: the higher the maximum airtemperature, the wider the average temperature fluctuation of the hottest month. Furthermore,the climate is far more severe in inland locations than by the sea: as in every climatologicdomain, the highland locations of the Mediterranean present cooler temperatures in winter thanthe lowland areas. This is the reason why some Spanish authors opt to use the term ‘ContinentalMediterranean climate’ (translation from Clima Mediterraneo Continentalizado) for some regionswith lower temperature in winter than the coastal areas. Figure 2 shows the high variability of theaverage annual global radiation in Europe.

In this climate zone the problem of buildings’ energy containment is primarily concerned withthe excessive heat in the summer months.

The cooling demand in the Mediterranean climate

Throughout the world, human beings are moving into cities (EU Report, Brussels, 2010). Theprogressive increase of global warming will specifically raise urban temperatures and the heatisland effect.

After the Messina earthquake of 1908 (which caused about 83,000 deaths) the hot summer of2003 with �70,000 deaths, mostly in the cities, was the second heaviest natural disaster of the last100 years in Europe. As reported by Santamouris, Pavlou, Syneffa, and Niachou (2006), duringthe summer period, high ambient temperatures and heat waves cause dramatic problems to vul-nerable population living in overheated households. In France the estimated death toll of the 2003heat wave was about 15,000 deaths. According to the EuroSurveillance, an estimated 22,080excess deaths occur in England and Wales, France, Italy and Portugal during and immediatelyafter the heat waves of the summer of 2003. Additionally, 6,595–8,648 excess deaths havebeen registered in Spain, of which approximately 54 per cent occurred in August and 1,400–2,200 even in the Netherlands, of which an estimated 500 occurred during the heat wave of 31July–13 August. In parallel, it is reported that approximately 1,250 heat-related deaths occurred

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in Belgium during the summer of 2003, almost 975 excess deaths during June–August in Swit-zerland and 1,410 during the period 1–24 August in Baden-Wurttemberg, Germany. Studies inEurope and the US (Klinenberg, 2002; Michelozzi et al., 1999, 2005) show that the greatestexcess in mortality was registered for people of low socioeconomic status, living in buildingswith improper heat protection and ventilation.

As reported by many authors (Santamouris, 2001), countries in Mediterranean areas haverecorded a huge increase in summer cooling demand. Overheating is especially evident inurban areas.

The effects of global warming are of relevant concern for both the environment and humanactivities in the Mediterranean area. The average yearly air temperatures are expected to increasebetween 2.2 and 5.18C (in summer between 2.7 and 6.48C). On the northern side of the Mediter-ranean basin, the increase (about 388C) is likely to be most pronounced in winter. This is forecastto occur by 2100, although more recent research studies show that the time span may be shorter(Hanson et al., 2007). The 28C air temperature rise represents a critical limit beyond whichdangerous climate changes could occur, by 2030.

These climatic conditions in the Mediterranean area would result in one additional month ofsummer and in a longer duration of tropical nights (namely temperatures always above 208C).Another consequence is a decrease in rainfall of around 30 per cent with respect to the actualaverage standards.

All climate forecasts also converge onto similar considerations for extreme events such as thefollowing: (i) the increase of heat waves, in their frequency, intensity and duration: these phenom-ena will mainly affect the more continental zones, away from the coastlines; (ii) a clear increase incontinental drought: a drop in the number of rainfall days and an increase in the length of thelongest rainfall-free periods.

As a consequence of heat balance, air temperatures in densely built urban areas are higher thanthe temperatures of the surrounding rural zones.

Figure 2. Average annual global radiation in Europe.Source: Meteotest; database Meteonorm.

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As reported by Founda (2011), the urbanization of Athens and its effect on the temporal vari-ation of air temperature have been stressed by all researchers, even in very early studies concern-ing climatic changes in the city. Most studies carried out up to the late 1980s identified the urbaneffect on the minimum rather than on the maximum temperature (Arseni-Papadimitriou, 1973;Katsoulis, 1987; Katsoulis & Theoharatos, 1985). The urban effect is reported to be more pro-nounced in winter and is related to the heat produced by human activities, especially in theevening hours, whereas mean and maximum temperatures are supposed to be less affected bythe urban effect due to the influence of the sea breeze (Katsoulis, 1987). In more recentstudies, however, it has been reported that the urban effect is manifested mainly in the dailymaximum temperature, which has increased significantly during the summer since the mid-1970s (e.g. Metaxas, Bartzokas, & Vitsas, 1991; Mihalakakou, Santamouris, Papanikolaou, &Cartalis, 2004; Philandras, Metaxas, & Nastos, 1999).

Climatic measurements from almost 30 urban and suburban stations, as well as specificmeasurements performed in 10 urban canyons in Athens, Greece, have been used to assess theimpact of the urban climate on the energy consumption of buildings (Santamouris, 2001). It isfound that for the city of Athens, where the urban heat island (UHI) intensity exceeds 108C,the cooling load of urban buildings may be doubled, the peak electricity load for cooling purposesmay be tripled, especially for higher set point temperatures. Regarding the potential of naturalventilation techniques when applied to buildings located in urban canyons, it is found that,mainly during the day, this is seriously reduced because of the important decrease of the windspeed inside the canyon. Air flow reduction may be up to 10 times the flow that correspondsto undisturbed ambient wind conditions. Thus the UHI in Athens was found to have a strongereffect on the daily maximum summer temperature (up to 10–158C for urban and rural/suburbanregions) (Livada, Santamouris, Niachou, Papanikolaou, & Mihalakakou, 2002; Santamouris,2001, 2007).

Using an ensemble of regional climatic scenarios, Founda and Giannakopoulos (2009) reportthat, by the end of the century, maximum and minimum summer air temperatures in Athens areexpected to increase by approximately 48C with respect to 1961–1990. It is also reported that theexceptionally hot summer of 2007 is likely to be the ‘normal summer’ of the period 2071–2100.

Zinzi (2010) reports interesting data about the Mediterranean region defining it as a ‘geo-graphical complex entity’ which consists of 23 seaside states with about 600 cities, 46,000kmof coastline, more than 450 million inhabitants (2005), 7.2 per cent of the world population, 9per cent of total energy supply, 10 per cent of electricity consumption and 8 per cent of CO2 emis-sion (Davı & Giampaglia, 2007). According to Zinzi (2010), in this complex area, a first simplepartition can be assessed on a socioeconomic basis: the north rim European states and the southand east rim states, with their transition economies. The Mediterranean is widely recognized asthe area of the world in which the call for sustainable development encompasses all mainissues (Plan Bleau, 2008) because: (i) it represents a fragile eco-region, where development isalready set back by environmental damage; (ii) it is an example of a common contrast betweenthe developed northern part and the developing southern part.

The demographic trend in the Mediterranean region is quite alarming: the north rim popu-lation increased by 14 per cent from 1975 to 2005, when it reached 190 million inhabitants.The south rim population has almost doubled in the same period, accounting now for morethan 258 million men and women (Zinzi, 2010). The south rim trend appears to be critical interms of environmental impact because all the issues are strictly related to the massive urbansprawl. The urban population increased from 42 per cent of the total to 62 per cent in 2005.Another consequence of urban sprawl in several countries is that more than 20 per cent of thepopulation moved into major cities; the percentage increased to 26 per cent in Turkey andabove 35 per cent in countries such as Greece and Portugal. Percentages above 60 per cent

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were recorded in smaller states, where the population is concentrated in a few urban areas, such asIsrael or Jordan. According to the actual trend, it is expected that another 70 million people willlive in metropolitan areas by 2025, with about 90 million expected to dwell in the coastal urbansettlements. Thus, the future of Mediterranean countries relies on the implementation of newdevelopment models based on a conscious integration of environmental, social and economicissues.

As was also reported by Zinzi (2010), a strong impact of the UHI is on energy use in build-ings. The outdoor air temperature increase has several implications: (i) energy consumptionincrease for cooling the building; (ii) increase of peak cooling demand and, as a consequence,chillers’ size increase; (iii) energy price increase; (iv) less ‘granted’ energy supply, especiallyin crucial periods, as in summer heat waves; (v) reduction in the efficacy of bioclimatic andpassive cooling strategies, often based on night natural ventilation techniques, when theoutdoor air temperature reaches several degrees lower than the indoor one.

According to OECD/IEA (2007), the building sector, including both residential and non-resi-dential, is experiencing an incremental trend in energy consumption, accounting for one-third ofglobal end uses (the percentage rises to more than 50 per cent considering the sole electricity con-sumption). The relative electricity consumption increase was 2.3 per cent in 2005 with respect tothe year before, but the value for the building sector was 3.7 per cent. A decisive role in thiscontext is played by room air conditioners and cooling systems mainly used in dwellings,whose market is under a continuous positive trend (Zinzi, 2010), also considering the previsionof additional increasing electricity consumption in the next few years, because of the growingenergy demand of transition economies.

Strategies to reduce the cooling demand in the Mediterranean region

As Zinzi (2010) has pointed out, there are two main challenges concerning the building sector: (i)reducing the energy consumption in European countries and (ii) preventing less developedcountries from following the same development patterns of the most developed ones, as is hap-pening in recent years. As described in the previous paragraphs, the first task is not easy to achievein the Mediterranean region, especially for inland locations, with highly variable conditionsthroughout the year. Here the buildings have to respond to such variations and be flexible bothwith hard winters and with oppressive heat in the summer.

At the territorial and urban scale it has been largely demonstrated that plants have a strongeffect on climate: trees and green spaces can help cool our cities (Buttstadt, Sachsen, Ketzler,Merbitz, & Schneider, 2010; Santamouris, 2001) and save energy (Yamashita, 1996). Treesalso help mitigate the greenhouse effect, filter pollutants, mask noise and prevent erosion (Fer-rante & Mihalakakou, 2001). Results of computer simulations aimed at studying the combinedeffect of shading and evaporative transpiration of vegetation on the energy use of severaltypical one-storey buildings in US cities have shown that by adding one tree per house, thecooling energy savings varied from 12 to 24 per cent, while adding three trees per house canreduce the cooling load between 17 and 57 per cent. According to this study, the direct effectsof shading account for only 10–35 per cent of the total cooling energy savings (Gaitani, Miha-lakakou, & Santamouris, 2007; Santamouris, 2001).

Increasing urbanization and deficiencies in development control in the urban environmenthave important consequences for the thermal degradation of urban climate and the environmentalefficiency of buildings. Furthermore, research works developed within the frame of the ResearchProject POLIS in Athens (Ferrante, Santamouris, Koronaki, Mihalakakou, & Papanikolau, 1998)have shown some appropriate procedures to design the use of natural components – such as greenroofs and pedestrian permeable surfaces in the streets – within urban canyons. The design of

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outdoor spaces – even if reduced to the envelope of the buildings because of existing urban con-straints within existing thickly built urban areas as well as the use of natural components – hasbeen regarded as a key means of improving urban conditions in relation to both the microclimateand the reduction of pollutants. By ‘making-up’ the building’s surfaces and elevation facades withgreen components or shading devices, different scenarios have been proposed in different casestudies in Athens downtown. Experimental software research models have been used to quantifythe positive effects of these selected passive techniques. Obtained results clearly indicated thatouter surfaces’ alternative design acts as a prior microclimate modifier and deeply improvesoutdoor air climate and quality (up to 2/38C reduction in ambient temperature) (Santamouris,2001).

Other significant physical factors in the thermal performance of urban environments are windflows and air circulation (Ricciardelli & Polimeno, 2006; Santamouris, Papanikolau, Koronakis,Livada, & Asimakopoulos, 1999) as well as air stratification within urban canyons. In particular,the heat island effect and the microclimatic conditions typical of urban canyons (Bitan, 1992)appear to be strongly influenced by thermal properties of the materials and components used inthe buildings and on the streets (Buttstadt et al., 2010). Comparative research studies demon-strated that the use of cool coloured materials (Synnefa, Santamouris, & Apostolakis, 2007)and thermo-chromic building coatings can contribute to energy savings in buildings, providinga thermally comfortable indoor environment, and at the same time, they can highly improvethe urban microclimatic conditions (Karlessi, Santamouris, Apostolakis, Synefa, & Livada,2009).

Thus, morphological and spatial geometry of buildings, thermal properties of surface coatingsand green surfaces have a strong potential on the energy performance and cooling demandreduction in urban areas. Thus, planning strategies to reduce the cooling demand at the citylevel of the Mediterranean region should consider green and natural components as the maintools for improving climatic conditions of urban areas.

Energy and environmental performance in the Mediterranean areas: lessons fromthe past

Many authors in the literature have reported on the importance of vernacular or traditional con-struction as a frame of reference model of bioclimatic architecture (Canas & Martın, 2004; Coch,1998).

To understand the possible strategies to be adopted for low- or zero-energy buildings it is alsoimportant to recall the basic bioclimatic principles of ancient and traditional architecture. Theresults of this study can be used in two different forms: (i) to make a proposal for the recoveryof vernacular constructions with peculiar bioclimatic strategies; (ii) to translate some of the bio-climatic strategies used in vernacular constructions to the present modern buildings (Canas &Martın, 2004).

All over the world, houses, small towns and villages of the past collectively contain some ofthe best-preserved climatic conscious and aesthetically enjoyable traditional architectural typesand techniques. Furthermore, current passive techniques used in reducing the cooling demandof buildings are based on or derive from systems and components used in the traditional and ver-nacular architecture.

In comparison with the representative architecture, popular architecture is performed by thepeople as a direct response to their needs and values (Coch, 1998). These buildings show agreater respect for the existing environment. They do not reject theoretical aesthetic pretensionsand use local materials and techniques as far as possible, repeating and slowly modifying thecourse of history models, which take the constraints imposed by the climate fully into account.

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In spite of the plurality of solutions – always limited by the basic constraints of the climate –within the different geographical contexts, it is interesting to observe how practically identicalarchitectural models are developed in similar climates with highly different cultures and verydistant locations.

Different research studies in the literature on interesting examples of vernacular and tra-ditional architecture have been selected. These examples, together with their evaluation interms of climatic response are reported below.

Canas and Martın (2004) reports and analyses 212 cases of old buildings in Spain as a guide tothe study of the bioclimatic strategies more widely used in Spanish popular architecture. Signifi-cant points are highlighted: (i) the bioclimatic strategies used in popular constructions correctlyrespond to the conditions imposed by the climate; (ii) selected strategies and technologicaltools for protection against the solar radiation may be found in buildings located in the southernmiddle part of Spain, where the solar radiation received is very high, while, as an opposite, strat-egies for the use of solar radiation appear in the northern middle part of Spain where the solarradiation is lower; (iii) strategies for protection against cold temperatures agree with theregions of Spain where the temperatures are minimum; and (iv) generally, technological toolsfor protection against rain are located mainly in areas where the averages of precipitations arehigh. In particular, passive regional strategies are: (a) Andalusian ‘patios’ whose function is toaccumulate fresh air; (b) light colour of the facade as a mechanism for the protection againstsolar radiation, since light colours reject the solar rays, reducing solar heating; (c) the use of veg-etation for the shading of the housing; and (d) orientation of the building for the collection of ahigher amount of solar radiation in winter than in summer.

Other energy-efficient design strategies have been investigated by Manioglu and Yılmaz(2008) in Mardin, a town situated in the hot-dry area of the south-eastern part of Turkey.This study examined building types and bioclimatic cooling techniques by evaluating themin terms of design criteria such as building types and forms, selection of the area, distancebetween buildings, orientation and building envelopes. Typical features of this traditionalsettlement are: (i) orientation (south-east); (ii) close distance between buildings; (iii) plantype – the courtyard houses; and (iv) compact forms to minimize the area affected by thesolar radiation.

In particular, in the traditional architecture of Mardin, the arrangement of close form typesin courtyards produced shaded areas. In the courtyards, with the help of water and plants forevaporative cooling, the floor temperature is further minimized by natural elements forcooling (channels and water poured out from the pool). ‘Eyvan’ and ‘revak’, typical spatialelements of the traditional houses, are used to create shady and cool living spaces duringthe day.

The thermally massed envelope details of these houses are very convenient for continentalclimates, where the summers are very severe with high swings in daily temperature variations.The thermal mass slows down the heat transfer through the envelope; thus lower day-time temp-eratures will be reached indoors when the outdoor air temperature is much higher. Furthermore,the high-thermal-mass building envelope, which has a higher surface temperature on the outerside, rapidly loses heating energy to the atmosphere via radiation at night to start the next dayfrom a cooler level.

Observing traditional examples in this area, it can be seen that the transparency ratio of thebuilding envelope is chosen as low as possible and the opaque parts of the building envelopewere constructed by the materials with a high heat capacity as thick as possible. The high heatcapacity of the opaque component provides a high time lag for the transmission of the outsidetemperature to the internal area, while the low transparency ratio minimizes the direct solarradiation gained through the windows.

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Thus, in this climate, other precautions against the solar radiation are: (i) minimization of thearea and the number of windows; (ii) construction of a window at a high level to block the floorradiation; (iii) minimization of the absorptivity of the facades by white or light colours; (iv) pro-viding natural ventilation, especially at night; and (v) constructing a part of the house into ground,which is to be always cooler than the outer ambient temperature in summer.

The research study also attempts to compare the different energy performances in a traditionalhouse and a modern house (built after 1990): a simplified thermal evaluation and comparison of atraditional house with a contemporary house have been given by using data derived frommeasurements of physical data and questionnaires, which have been carried out for 100 buildings.The modern building was constructed by contemporary techniques in compliance with theinstructions of the Energy Conservation Standard of Turkey. For the measurements, roomswith similar features in modern and traditional houses were chosen. (The main facade of thesetwo rooms are south-east oriented; they present a flat roof; they are situated in the upper floorof both houses; no shading device was used on the facades of both houses.) The thickness ofthe walls is 0.8m in the traditional house and 0.25m in the modern house. Thermal propertiesof the building envelopes materials were stone wall for the traditional house, 2.33l (W/mK),and brick for the modern house, 0.46l (W/mK). For windows, the single glazed woodenwindow frame realizes a U-value of 4.5W/m2K in the traditional house and the double glazed,PVC window frame realizes a U-value of 2.6W/m2K in the modern house.

After comparing the obtained results, it has been observed that air temperature and surfacewall temperature are much lower in the traditional building than in the modern. In particular,thermal comfort temperature resulted in quite stable indoor and low values of air temperatureduring all of the reference day in the traditional building. Satisfyingly, the measurements alsomeet the user’s perception of indoor temperature in summer, which has been obtained by a ques-tionnaire carried out with 100 occupants in the selected modern and traditional buildings at thesame time with the measurements.

Another comparative investigation of passive control methods in indoor environments of thetraditional (Kerala house) and modern architecture has been performed by Dili, Naseerb, andZacharia Varghese (2011) for the hot humid climate. The study was conducted by continuouslymonitoring the indoor and outdoor conditions of both the buildings using a custom-made instru-ment called the ‘Architectural Evaluation System’. The results reveal that an efficient passive andnatural control system exists in the Kerala traditional architecture in providing a comfortableindoor environment irrespective of the outdoor climatic conditions. It was found that thediurnal variation of indoor air temperature is lower in the traditional building of Kerala as com-pared to the modern building. This result demonstrates that: (i) the traditional building has a muchhigher thermal insulation property; (ii) low mean radiant temperatures contribute much to theindoor thermal comfort; (iii) air flow in the traditional building is continuous, while in themodern house it is rare and draughty; and (iv) the characteristics of air flow in the traditionalbuilding maintain the air temperature at a lower level while providing evaporative cooling tothe occupants.

The thermal analysis confirms the thermal comfort of the traditional building, whereas themodern building is proved to be very hot and uncomfortable to live in during summer. It canthus be concluded that the traditional architecture of Kerala has an effective passive andnatural control system that is responsible for providing a comfortable thermal environmentindoors in summer.

Similar studies and similar results comparing modern and traditional architecture have beenperformed in other hot and humid parts of the world. The natural environment control systemof Korean traditional architecture is compared with a contemporary architecture (Do-Kyoung,2006). Specific traditional passive tools to ventilate and prevent humidity stagnation in the

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traditional houses are described, such as the concept of ventilation by ‘ondol’ and window system.The results of these studies show that in Korean traditional architecture the climatic environmentcould be controlled to manage the extreme differences in climate of the four seasons and moun-tainous ground. In contrast, Korean contemporary architecture ignores the natural surroundingsand relies solely on contemporary technology, which consumes a great deal of energy.

Architectural structure and environmental performance have been reported also for the casestudy of the traditional buildings in Florina, NW Greece (Oikonomou & Bougiatioti, 2011). Thisstudy was based on the documentation and analysis of the architectural and bioclimatic aspectsof a sample of 40 remaining houses of the 19th and the beginning of the 20th century. The analy-sis has been performed considering the building types and form, the materials and the construc-tion techniques, whereas the analysis of bioclimatic aspects involves the thermal behaviour ofthe building shell, the thermal and the visual comfort conditions. The aim of the study was todocument and assess, both qualitatively and quantitatively, bioclimatic principles and tools ofthe local traditional architecture, in order to draw conclusions concerning those principles thatcan be integrated into the refurbishment of existing buildings or the design of new ones in tra-ditional surroundings. Traditional architecture of Florina is characterized by proper southern oreastern orientation of the buildings and by the exploitation of the prevailing winds. To a largeextent, the buildings are oriented to achieve the best possible exploitation of solar radiation forpassive solar heating. Natural materials are used with great efficiency and according to theirphysical properties and thermal characteristics (density, heat capacity, time lag), as well astheir durability, improving the inter-seasonal thermal behaviour of the various spaces and enfor-cing the bioclimatic function of the buildings. In this way, the winter living spaces, which aresituated on the ground floor, are characterized by increased thermal mass and reducedthermal losses due to the increased wall thickness and the reduced number of openings,whereas the summer living spaces on the upper storey are characterized by reduced thermalmass and enhanced ventilation due to the light-weight walls and the increased number ofwindows. Computer-aided thermal analyses have shown that for the coldest day (26thJanuary), the main winter living spaces had slight diurnal variation and range (around 08Cwith outside temperatures below 2128C. For the hottest day (25th August), while the exteriortemperature ranged from 228C (early in the morning) to 348C (around noon), the summerliving spaces were warmer than the outdoor air temperatures in the morning and in the night(258C), but significantly fresher around noon (298C). When natural ventilation is inserted asan additional parameter (passive cooling strategy) in the computer analysis, the inside tempera-ture ranges from 23.5 to 27.58C. As a whole, the traditional buildings evaluated in this studyresulted in highly energy performing buildings both in the summer period and in the coldwinter season.

The energy and microclimatic performance of traditional buildings have been performed alsofor the well-known case of ‘Sassi’, historical hypogenous buildings in south Italy (Cardinale,Rospi, & Stazi, 2010). The ‘Sassi’ district of Matera, a UNESCO World Heritage since 1993,is described as an exceptional example of traditional bioclimatic Mediterranean architecture.The authors found that both hypogenous units in these stone buildings (the “Sassi” buildingsare partially dug into the ground and partially built above ground) optimally perform duringsummer and winter conditions. They analysed the energy performance of the hypogenous build-ing structures during 1 calendar year, finding that the huge thermal mass of the walls ensured con-stant and regular microclimate indoor conditions throughout the seasons, without relevantdifferences in the daily thermal oscillation. In spite of some air change need (thermal-hygrometriccomfort values in deep hypogenous units are not fully reached), these structures present a nullthermal balance during mid-season, while in the summer the floor loses heat, thereby coolingthe environment. The opposite occurs in winter.

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The monitoring campaign has confirmed all results obtained through the dynamic analysis. Byquantifying the total energy balance of the hypogenous structures, the authors found the follow-ing: (i) during the winter season, the heat flow loss from the walls was balanced by the positiveenergy gain through the floor, which stabilized the temperature that remains constant at about 12–138C; (ii) during mid-season, exchanged heat fluxes are essentially null, resulting in a constantevolution of temperature with values of 15–168C; and (iii) in summer, the heat flow dispersedfrom the floor counterbalances the incoming heat flow for transmission and ventilation, reducinghigh summer temperatures.

During the summer season the indoor temperature is in the range of 18–218C (especially indeep hypogea). It can be concluded that these buildings were built as ‘zero energy’ houses andthey can be used today, after conservative and light-method-based restoration processes, withlimited or null use of technology energy systems.

As a general rule, it can be stated that incorporation of traditional or traditional-based buildingtechniques into a newly designed building may help the design process to use low-cost andachievable local construction practices towards the achievement of zero energy balance andzero on-site CO2 emission within the Mediterranean climate (Ferrante et al., 1999). In the refer-ence case study (a newly designed housing development for a site located in the south of Italy) theconcurrence of different building components aims to achieve multi-purpose objectives within thesame building frame. These components mainly derive from the re-elaboration of traditionalforms and techniques (building type – a courtyard house presenting a good balance betweennatural ventilation and building compactness, high mass envelope features and constructionfrom the local practices, selected passive tools for energy saving and plants systems for energymicro-generation from renewable energy sources (RES)). With specific reference to the Mediter-ranean climate, in fact, ZEB design is essentially achievable by the synthesis of existing technol-ogies and know-how in RES into traditional building types and consolidated constructionpractices.

Design strategies and tools in the Mediterranean climate

As can be observed from the analysis of the traditional bioclimatic examples in architecture, inwarm climates, the ‘porous’ form of the building type is generally advantageous to achieveshadow areas and improve natural ventilation. This has to be compensated for by a compactform and reduced values of the S/V ratio, which is a beneficial strategy during winter, especiallyfor inland locations with a high seasonal thermal range. Thus, taking into account the need forventilation (cross ventilation and vertical air extraction provided by internal courtyard and verticalducts) the S/V values should be in a reasonable balance between natural ventilation requirementsin summer period and thermal outflow in cold winter season.

Therefore, under the assumption that sustainability in construction should be first and fore-most achieved through the passive building conception, the resulting built organism must beable to provide adequate interior comfort by taking advantage of the potential energy sub-systems building components (walls, orientation, S/V ratio, ventilation strategies, etc.) and thatof its surroundings (landscape, shape and orientation).

The best solution could be found in variable or movable systems and components: to closeparts of the building (thus resulting in a compact form) in the cold winter season and toremove or keep them open in the summer, in order to increase the surface area of heat exchangeand ensure ventilation (especially at night, when the air temperature decreases).

This behaviour may be defined as ‘adaptive’, since it is also similar to the adaptation capacityof many living organisms.

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Many of the design decisions taken in the countries of northern Europe have been adopted inour latitudes with counterproductive results. For example, large windows, or solar greenhouses,high compactness, high insulation performing materials, etc. may produce summer discomfort inthe Mediterranean region.

Generally, the building envelope design should be focused on technological solutions able toguarantee an adequate balance between the thermal insulation requirements in the winter seasonand the thermal mass in the hot summer season. In this framework, as an example, constructivetypology in masonry brick walls ensures high values of superficial mass and adequate levels of thedamping and phase displacement of the thermal wave (Ferrante et al., 1999).

Passive techniques for building cooling in the Mediterranean climate

The responsible sources of cooling demand or thermal discomfort in non-cooled buildings are inthe following order: solar radiation, relative humidity and air temperature (Coch & Serra, 1996).Because of this, to optimize buildings’ energy performance, rational design and use is crucial.Solar radiation control is the most important strategy and, although much has been writtenabout the efficacy of transparent shading systems, it is important to assess the impact ofopaque envelope solar control as well.

As reported by Santamouris (2006), passive cooling relies on the use of solar and heat controltechniques, heat amortization and heat dissipation techniques. Intensive research studies carriedout on the topic have permitted us to develop advanced and low-cost systems and techniques that,when applied, contribute highly to decreasing the cooling needs of buildings and improve indoorenvironmental quality. In particular, as reported by Santamouris (2006), the development of high-reflectivity coatings for the building envelope can considerably decrease the solar input to thebuildings, while new developments on ventilation technology permit us to dissipate successfullythe excess heat to the ambient air, improve indoor comfort and decrease indoor pollutantsconcentration.

In particular, because of the high horizontal solar radiation at Mediterranean latitudes duringthe cooling season, roofing systems may be considered the envelope components, which can beapplied strategically in solar control (Zinzi, 2010). According to this author, solar gains are, inparticular, the most important load during the cooling season, when weather conditions and thesun’s position high on the horizon determine high irradiation levels on horizontal and low-sloped surfaces for many hours. The reduced number of sunshine hours and the low incidenceangle of the sun make the contribution of solar gains in wintertime less important.

Various passive cooling systems and their applicability to different climates and buildingtypes, as well as strategies for minimizing cooling needs by building design (e.g. ventilativecooling, radiant cooling, evaporative cooling systems, ground cooling, cooling of outdoorspaces adjacent the building, etc.) have been comprehensively investigated by Givoni (1998a).Experiments on the effectiveness of mass and night ventilation in lowering the indoor time temp-erature have also been performed by the same author (Givoni, 1994, 1998a, 1998b).

More recently, Zinzi (2010) has highlighted the need to fruitfully apply passive building tech-nologies to mitigate the cooling demand increase, reducing the energy consumption in cooledbuildings and improving the thermal comfort in non-cooled buildings. The author suggestscool materials as possible strategy options for reducing building cooling loads, since they arematerial building components which stay cool under the sun because of high solar reflectanceand thermal emittance properties.

The case-study-based research reported by Zinzi (2010) demonstrates the positive impact ofthe technology in terms of cooling and total energy savings as well as on the indoor thermal con-ditions in Mediterranean buildings. In particular, the author states that the effect of cool roofs has

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an important impact on the following: (i) energy performance of insulated buildings – built in theMediterranean north rim – with the optimization of heating and cooling performance; (ii) energyperformance of existing non-insulated buildings below 408 latitude and for all the existing build-ings to be renovated with energy measures such as roof insulation; (iii) temperature profiles innon-insulated buildings and with a high external surface to volume ratio. These configurationsensure enhanced performance of cool roof technology, with a strong decrease of discomforthours. More insulated and compact structures still benefit from cool roof applications, but theadvantages may be amplified by the contextual application of other passive techniques, such asnight ventilation, more radical window shading strategies and increased thermal mass; (iv) posi-tive impact in terms of energy performance backs up other critical issues related to cool roofs: theenvironmental impact in terms of mitigation of the UHI and an effective answer to globalwarming, which is critical in large urban areas.

Various passive techniques in hot and arid regions by using roof cooling were also performedby other studies (Nahar, Sharma, & Purohit, 2003). A range of passive techniques were used overthe roof of different experimental units for cooling the environment inside test structures: (i)painting of roof with white cement; (ii) thermal insulation over the roof; (iii) nocturnalcooling, that is, a shallow pond with 100mm water column with 40mm thick movable; (iv)thermal insulation over the roof; (v) evaporative cooling, that is, the roof is provided withgunny bags soaked with water with the help of a storage; (vi) a tank and a dripper for controllingwater without any pump, (vii) broken white glazed tile pieces stuck over the roof, (viii) air-voidinsulation, that is, inverted earthen pots having 100mm diameter and 125mm height were pro-vided over the roof and (ix) roof covered with Sania, a local insulating material used over thehuts in the arid regions and the controlled unit without any treatment. The evaporative coolinghas been found to be the best solution for conventional roof, but it requires about 50l/m2

water per day; therefore, the pieces of white glazed tiles stuck over the roof (vii) can be usedas a valid alternative tool to reduce heat load from the roof and hence cool the environmentinside buildings.

Another cooling strategy by roof cooling is represented by the use of vegetated green roofs.Integrated water and energy management in the urban context is one major route towards environ-mental sustainability and the reduction of carbon dioxide emissions into the atmosphere.

This passive cooling system has been investigated, in terms of their expected benefits to thebuilding and the urban environment, by many authors in the literature. Vegetated roofs are inves-tigated due to their recognized energy and water management potential scores. A review of relatedworldwide experiences is reported for comparison purposes by Fioretti, Palla, Lanza, and Principi(2010). The investigation is here performed within the specific climatic context of the Mediterra-nean region. Full-scale experimental results are provided from two case studies, located in north-west and central Italy, consisting of two fully monitored green roofs on top of public buildings.The attenuation of solar radiation through the vegetation layer is evaluated as well as the thermalinsulation performance of the green roof structure. The daily heat flowing through the roof surfaceis quantified; the results show that the green roof outperforms the reference roof, thus reducing thedaily energy demand of the building. In fact, the most important effect created by a green roof isthe reduction of the surface temperature and the attenuation of temperature fluctuations. In a tra-ditional roof, made of high-absorption materials, the horizontal external surface can reach veryhigh temperatures due to the combination of the air temperature and the solar irradiation absorp-tion, which determines a strong thermal load. By this analysis it was possible to evaluate the timedelay effects due to the green roof rather than the reduction in temperature alone. The green roofhas a lower temperature level due to the plant shading, insulation and evapo-transpiration of thefoils apparatus. The energetic behaviour of the green roof has been investigated for its effects onthe internal environmental conditions. As to water management, it is confirmed that green roofs

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significantly mitigate storm water runoff generation – even in a Mediterranean climate – in termsof runoff volume reduction, peak attenuation and increase of concentration time.

Green roofs, high-reflection roofs, wall planting, etc. are suggested from the viewpoint ofbuilding planning, in addition to green parks, roadside trees, etc. Improvement of the surfacecover of buildings and constructions that have been covered with cement and asphalt concreteis examined as one of the measures to mitigate the UHI phenomenon (Ferrante & Mihalakakou,2001). The use of a highly reflective paint for cool roofs and road pavements has been examinedby many authors (Santamouris, 2001; Spala et al., 2008). Among others, Akbari, Kurn, Bretz, andHanford (1997) and Akbari and Konopacki (2005) have studied different solutions and techniquesfor reducing UHIs and energy consumption by different building and urban surfaces.

Surface heat budget on green roofs and high-reflection roofs for the mitigation of UHI hasbeen studied by Takebayashi and Moriyama (2007). In this study, surface temperature, net radi-ation, water content ratio, etc. of green roofs and high-reflection roofs were observed, comparingthe heat and water budget to each other. In the daytime, the temperature of the cement concretesurface, the surface with a highly reflective grey paint, bare soil surface, green surface and thesurface with a highly reflective white paint have been observed to be in descending order. Ona surface with a highly reflective white paint, the sensible heat flux was observed to be smallbecause of the low net radiation due to high solar reflectance. On the green surface, the sensibleheat flux was small, too, because of the large latent heat flux by evaporation, although the net radi-ation was large. On the cement concrete surface and the surface with a highly reflective grey paint,the sensible heat fluxes have been observed to have almost the same values because of a similarsolar reflectance. Methods to estimate the quantity of evaporation, evaporative efficiency, heatconductivity and thermal capacity are explained, and the observation data are applied to thesemethods. Green roofs and high-reflection roofs have shown to perform optimally in the mitigationof UHI and buildings cooling.

Ventilation is another strategic passive technique for building cooling. It is well known thatproper ventilation of urban buildings can contribute highly to decrease the energy consumptionfor air conditioning and improve indoor comfort, while also contributing to reduce the concen-tration of indoor pollutants and to protect public health (Santamouris, 2006). In fact, daytime ven-tilation in mild climates and night ventilation in hot climates have been proved to be very effectivecooling techniques.

The design and integration of efficient natural ventilation systems and components such aswind and solar towers, etc. can assist in improving indoor thermal comfort. Natural ventilationcan contribute to improving thermal comfort, decrease the need for air conditioning andimprove indoor air quality in the developed world. Experimental studies have shown that effectivenight ventilation in office buildings, for example, may reduce the overheating hours by half andreduce the cooling load by at least 55 per cent.

Important research studies have been carried out on appropriate and advanced ventilationtechniques (Santamouris & Wouters, 2006). The main achievements of these studies are: (i) abetter understanding of the air flow phenomena and of the expected comfort benefits, in particularin the dense urban environment, and the development of efficient and practical procedures todesign natural and hybrid ventilation systems and configurations; and (ii) technological develop-ments mainly in the field of hybrid and mechanical ventilation that contribute significantlytowards a more comfortable and healthy indoor environment (De Gids, 2006).

Proper design of windows allows to increase air speed in households and to improve comfortby cooling down the human body through the mechanisms of convection, radiation and perspira-tion. Cross ventilation and vertical ventilation increase air flow rates and lower the indoor temp-eratures. Although the appropriate levels of air velocity needed to achieve comfort is a widelydiscussed topic within the scientific community (Arens, Blyholder, & Schiller, 1984; Tanabe &

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Kimura, 1994), studies performed in hot and humid climates confirm that increased air speed,especially at higher temperature, enhances thermal comfort conditions (De Dear, 1991; Hien &Tanamas, 2002; Mallick, 1996).

With reference to the building type and envelope, apart from cross ventilation by normal ver-tical openings, among the ventilation passive techniques in building design, vertical air extractionand windcatcher are two of the most effective techniques.

In particular, the windcatcher system represents one of the best components for providingeffective natural ventilation. Once again, these systems derive from past technologies. The wind-catcher systems were employed in buildings in the Middle East for more than 3000 years. In themodern design of windcatchers, the principles of wind effect and passive stack effect are con-sidered in the design of the stack that is divided into two halves or four quadrants/segmentswith the division running the full length of the stack. Although they have different names in differ-ent parts of the south-east Mediterranean region, the basic physical principles may be investigatedaccording to similar geometrical forms (Bahadori, 1994; Elmualim, Awib, Teekaram, & Brown,2001; McCarthy, 1999).

An increasing number of windcatcher and cool-tower systems (Etzion, Pearlmutter, Erell, &Meir, 1997; Hurdle, 2001; Swainson, 1997) have been applied to commercial buildings and resi-dential buildings (i.e. the Queen’s Buildings at Demonfort University and the BRE – Office of theFuture – or the International Centre for Desert Studies in Negev, Israel) and their performance hasbeen evaluated by many authors in the literature. Among others, Liu and Mak (2007) performedthe evaluation of a windcatcher system using computational fluid dynamics, demonstrating thatthe windcatcher cooling potential is very effective, although a precise design study has to beplanned in relation to the wind conditions of the specific site, since its behaviour is greatly influ-enced by the external wind speed and direction with respect to the windcatcher quadrants.

As illustrated in the examples from the past traditional architecture, the significance of mass isof crucial importance for the reduction of cooling load in buildings. This is an important propertyof the building envelope for energy conservation, since excess heat is stored in it and dissipated ata later stage when needed. In this way, the indoor temperature fluctuations are regulated and over-heating is avoided. These temperature fluctuations could be due to: (i) diurnal fluctuations ofoutdoor temperatures; (ii) internal gains; and (iii) incident solar radiation, especially in roomswith south glazing surfaces.

The aspects relating to mass are of particular significance for countries with large diurnal fluc-tuations for the displacement and dissipating potential possessed by mass. In these contexts, it isessential to ensure high values of superficial mass and adequate levels of the damping and phasedisplacement of the thermal wave. Thus, many authors in the literature (Canas & Martın, 2004;Ferrante et al., 1999; Givoni, 1994, 1998a; Santamouris, 2001, 2006) have reported their researchand applied studies on the effectiveness of thermal mass in the bioclimatic design for buildingcooling in hot or Mediterranean areas.

On the combination of passive and active techniques for building cooling in theMediterranean climate

In the search for further energy exploitation within the building sector, recent research studies areattempting to discover the energy saving potential achievable from the combination of active andpassive techniques.

In this context, automatic control and an active system to prevent overheating in summer rep-resent crucial strategies. In a recent study, Ochoa and Capeluto (2008) have explored the influenceof incorporating intelligence in buildings in hot climates, to achieve glare and solar radiationcontrol. Through the perspective of energy consumption and user comfort, with an emphasis

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on lighting, the study shows how decisions taken in the early design stages can affect those of laterones. By means of computer energy modelling, a prototype office unit is used to evaluate energyperformance and visual comfort in three parametric series. The first one is the result of the incor-poration of active features alone, the second one is guided by intelligent passive design strategiesand the third one is the combination of both approaches. Here, to analyse the influence of activefeatures and passive design, an office module will be studied in the city of Haifa, Israel (32.51N,351E), with a regional climate usually classified as Csa (warm summers and mild winters) in theKoppen scale (also ‘meso-thermal/humid continental’, due to its location in the Mediterraneancoastal plain). The building is a free-standing structure, with similar glazed facades in all fourmain orientations, with the view portions made of clear double glazing. Results show thatenergy savings in the ‘passive design strategies only’ (sun shading elements, daylight redirectionelements, glazing elements, etc.) was among the largest – ranging from 60kWh/m2 per year(base-case) to around 24kWh/m2 per year. Finally, the combination of optimized active features(fan ventilation elements and automatic controls) with passive design strategies offers in a con-sistent manner very promising energy savings and glare control at the same time: combiningactive features and correct passive design strategies gives consistent savings of around 50–55per cent for most cases when compared with a conventional situation. By these options, intelligentbuildings offer flexibility and convenience, such as rapid temporary changes, options to openindividual windows or operate specific blinds.

Furthermore, current studies are interestingly investigating the potential of combining bothactive energy generation systems from RES and passive techniques.

By coupling active features and correct passive design strategies, consistent savings may beachieved (Eicker & Dalibard, 2011). In particular, recent studies exploiting the radiative coolingcapacity of active energy producers have led to the production of a new photovoltaic–thermal(PVT) system, which has been developed with the goal of providing both electrical andcooling energy for buildings. Experimental studies of PVT collectors were carried out in Stuttgartto validate a simulation model, which calculates the night radiative heat exchange with the sky.Larger PVT frameless module surfaces were then implemented in a residential ZEB and testedunder the climatic conditions of Madrid. Measured cooling power levels were between 60 and65W/m2, when the PVT collector was used to cool a warm storage tank, and 40–45W/m2,when the energy was directly used to cool a ceiling. The ratio of cooling energy to electricalenergy required for pumping water through the PVT collector at night was excellent withvalues between 17 and 30. The simulated summer cooling energy production per square metreof PVT collector in the Madrid/Spain climatic conditions is 51kWh/m2/a. In addition to thethermal cooling gain, 205kWh/m2/a1 of AC electricity is produced under Spanish conditions.

Ongoing research studies and projects on ZEB in the hot and Mediteranean climate

Current research studies are attempting at evaluating the energy and comfort behaviour of build-ing models by comparing their predicted and/or measured values in different climate conditions.Simulation is employed as the main research method for these studies. For example, energysupply concepts for zero energy residential building (ZERB) in Shanghai (humid) and Madrid(dry) have been discussed by Deng et al. (2011). Two typical housing models are designedaccording to the real occupancy condition, the life schedule, the thermostats settings, etc., forthe two cities. An energy analysis considering the annual balance of input from the grid andoutput from renewable power systems is made. Indoor comfortable comparisons between thetwo models are presented to show optimal design strategies for heating, ventilation and air con-ditioning under different weather conditions. The analysis of the primary energy payback timeand the CO2 equivalent saving has been performed in order to evaluate the performance of

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novel energy systems to verify the feasibility. The energy balance simulation result shows that theelectricity generation of PV can meet the demands of two ZERB models in Shanghai and Madrid.Indoor comfortable results show that the temperature comfort can be met for two models underShanghai and Madrid’s weather; nonetheless, humidity comfort demand needs more customizedenergy concept’s design schemes for different weather conditions (dehumidification device forShanghai or humidification device for Madrid). Calculation results show that the primaryenergy payback time of ZERB in Madrid is 10.1 years and the CO2 equivalent saving is 74.4tonne during 50 years building lifetime.

By simulating the same standard passive house building (a house with heating energy demandequal to 10–16kWh/m2 per year in different locations throughout the Mediterranean and Euro-pean regions, Grove-Smith (2010) has reported that the general principle of passive housesseems to work also in the Mediterranean warm climate; this finding has been considered generallyvalid although for southern locations such as Palermo and Seville, de-humidification would berequired, as can be observed by the obtained sensible and latent values in Table 2. Therefore,it seems that some of the winter requirements (sufficient insulation of roofs, walls, doublelow-e glazing, south-facing windows, some thermal mass) may be useful for summer periodthermal comfort if solar shading/control and light cooling are also applied.

Among the recent and ongoing research studies on low- and zero-energy housing is theexperience of a regional project (Wenzel, Deutsche Gesellschaft fur Technische ZusammenarbeitGmbH, GTZ, German Agency for Technical Cooperation), financed by the EU, that supports thedesign, construction and monitoring of ten low-energy demonstration buildings (Pilot Projects) inten southern and eastern Mediterranean countries (MED-ENEC, www.med-enec.com). The studyreports that: (i) high-energy savings can be achieved in buildings with a variety of partly matureand partly innovative technologies; (ii) low- and even zero-energy houses and green buildings aretechnically feasible in the Mediterranean region; (iii) economic considerations limit the broadapplication of some of these technologies and thus reduce the technical potential for energysavings in a large-scale dissemination; (iv) the most cost-efficient technology mix, accordingto the type of building, the climate zone, energy prices and the availability of know-how and tech-nologies, needs to be identified; (v) high transaction costs such as substantial initial learning andsearch costs put at risk the profitability of low-energy buildings in the region and constrain thedevelopment of respective markets; (vi) when using a cost-efficient technology mix and if miti-gating transaction costs, low-energy buildings become attractive in most of the countries withenergy savings of 20–60 per cent, incremental costs of 10–15 per cent and short paybackperiods; (vii) government support and incentives are necessary for overcoming the initial hightransaction costs and market failures and for boosting energy efficiency in buildings; and (viii)

Table 2. Simulated results of the same passive building in different locations.

Mannheim Torino Madrid Seville Palermo

Insulation wall (cm) 25 20 10 8 6Insulation roof (cm) 35 25 25 20 20Insulation basement (cm) 20 15 6 0 0U window frames [W/(m2K)] 0.72 0.72 0.72 1.6 1.6U-value glazing [W/(m2K)] 0.7 1.2 1.2 1.2 1.2Humidity control for cooling no yes no no yesHeating demand (kWh/m2 per year) 15.6 14.8 12.7 4.6 3.1Sensible cooling (kWh/m2 per year) 0 0.8 0.4 4.2 7.2Latent cooling (kWh/m2 year) 0 2.3 0 0 7.2

Source: Grove-Smith (2010).

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subsidies on energy are the most important single constraint for broad dissemination in some ofthe countries in the Mediterranean region.

Since early 2006, the MED-ENEC project ‘Energy Efficiency in the Construction Sector inthe Mediterranean’ supports partners in Algeria, Egypt, Israel, Jordan, Lebanon, Morocco, thePalestinian Territories, Syria, Tunisia and Turkey in boosting energy efficiency measures andthe use of renewable energies in buildings. In this framework, Pilot Projects in all ten partnercountries have been supported during a period of about 3 years starting from summer 2006.These model low-energy houses represent a rich variety of building types and include newhouses as well as refurbishments. All relevant technologies have been used in order to reduce con-ventional, that is, carbon-based, energy consumption.

Results of these pilot projects show that high primary energy savings of up to 100 per cent –compared to conventional buildings in the same country – are technically possible. This energysavings corresponds to 3–307 tonnes of avoided CO2 annually, according to the different size ofthe buildings and the chosen energy efficiency concept. The average primary energy savingachieved is 57 per cent, compared to a conventional building of the same size and comfort (con-cerning heating and cooling). In Figure 3 some pilot projects are displayed.

Finally, the behaviour of the inhabitant is crucial for realizing the theoretical saving potential.When shading devices are not used properly, when windows are left open while heating orcooling, energy-efficient technologies may not have the desired effects. In some MED-ENECPilot Projects (Jordan and Israel), users of the building receive written guidelines and explanationson the handling of the installed equipment and energy-efficient procedures and behaviour. Inaddition, a part of the theoretical saving potential may be ‘lost’ in comfort increase, forexample the rooms are a bit warmer in winter and slightly colder in summer (‘reboundeffect’). This is particularly true in countries where thermal comfort is rather poor. It should benoted that significant additional energy saving potentials exist on the urban planning level. Dis-trict heating and cooling, public transport and green spaces are some examples of energy savinginstruments and policies (Cities Alliance 10/2007).

However, the 57 per cent energy saving of the MED-ENEC Pilot Projects seems to be veryattractive.

Figure 3. Pilot Projects in Israel, Egypt, Palestinian Territories, Jordan (clockwise, from top left).Source: Wenzel (2009) and ECEEE 2009 Summer Study.

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The cost-effective results have been analysed. Figure 4 gives an overview of the economicperformance indicators of the Pilot Projects. In this context, the payback periods are the moreimportant indicators. The simple payback calculation method has been used by dividing thevalue of the annual energy costs savings (assuming a 5 per cent annual increase in energyprices) by the incremental costs of the building, compared to a conventional building. When ana-lysing the economic performance figures, major differences among the Pilot Projects becomeobvious. On the one hand, the Lebanese Project is the most attractive with a payback ofroughly 1 year. The paybacks for five other projects seem to be moderate with around 10years. However, four projects are clearly unattractive with paybacks of 18 to nearly 70 years.As shown in Figure 4 on average, the new buildings (except the Tunisian project, which is aspecial and rather expensive ‘show’ case) needed incremental investments of about 30 per centand show a payback period of about 20 years.

In some of the Pilot Projects such as in Tunisia, attracting ‘green’ clients by demonstrating the‘state of the art’ and maximizing the energy saving was a major objective. Therefore, a mix ofmature and innovative but rather expensive technologies such as large photovoltaic electricitygeneration or solar cooling was chosen. This approach makes the Pilot Project an interestingplace to visit and learn from, but reduces potential for dissemination. If, however, broad dissemi-nation of low-energy buildings in the region is the first priority, economic considerations, forexample, the relation of energy savings to additional cost limits the use of available technologiesto the most cost-efficient ‘smart technology mix’. The feasible technology mix may be differentaccording to the type of building, the climate zone, the national energy prices and the availabilityof know-how and technologies.

The integrated and mixed use of different energy forms to be considered in passive and low-energy building within the hot regions is summed up in Figure 5.

This integrated approach helps in reducing incremental costs and improving the paybackperiod. Experiences from the MEDENEC pilot projects lead to more specific design-

Figure 4. Economic performance indicators of MED-ENEC Pilot Projects.Source: Wenzel (2009) and ECEEE 2009 Summer Study.

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country-related assessment, considering the ranking of technologies according to their cost-effectiveness. While some technologies, such as photovoltaic, are sensible to national frame-work conditions (e.g. they may be cost-effective in Israel, where a feed-in tariff exists, or inAfrican countries, where grid connection is significantly lower than in the Mediterraneanregion), some others may seem less attractive. Thus, passive design features, such as orien-tation of the building, shading, natural ventilation and the use of daylight, are free of costor very cheap in the design phase and therefore always cost-efficient for new buildings. Insu-lation of roofs and walls as well as efficient lighting and solar water heaters proved equally tobe in the top ranking.

In the Mediterranean region, due to the different climate, more emphasis is on cooling, whilein Europe, the highest efficiency gains are possible for heating (JRC; Nemry & Uihlein, 2008).

On the other hand, there have been several major constraints and barriers for the MED-ENECPilot Project developers, which resulted in high and often uncompetitive costs. These effects arenot specific for the MEDA-region (MGI, 05/2007). Other country groups have different frame-work conditions and additional constraints (for instance in the cases of China and Russia), meter-ing according to consumption is not common, which is a strong disincentive for energy efficiency(GDI, 2008), but the below-described constraints are quite generally applicable (IEA, 02/2007,S. 15; Plan Bleu, 2008).

Conclusions that may be derived from the MED-ENEC Pilot Project is that ‘Government mustlead’ (Civic Exchange, 2008) – this is true not only for the Mediterranean region but also for allregions and country groups. Setting regulations and standards and enforcing them is a necessarybut not sufficient condition for success. As the value chain in the building sector is long and muchdiversified, with plenty of stakeholders acting on individual perceptions of risks and benefits, it isof highest importance that economic signals such as energy tariffs, incentives and sanctions orien-tate the actors in the market. However, resistance to change, initial market failures and high trans-action costs usually slow down or even impede this reaction and development of the market forenergy-efficient products and services. An integrated package of regulation and standards (includ-ing control and enforcement), financial support and incentives, information, training, education aswell as research and development activities has to come along with improved economic frame-work conditions.

There has been some research to identify the most (cost-) effective policy mix. But there is nosuch thing as a ‘best’ policy instrument. Each country needs to work out a comprehensive analysis

Figure 5. Integrated EE approach for buildings.Source: MED-ENEC, Carsten Petersdorff, ecofys.

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of individual framework conditions and barriers and has to design an adapted policy package. Inthis process, stakeholder participation, definition and monitoring of quantitative objectives andthe synergetic combination of instruments are major general success factors (ECOFYS, 03/2007; IEA, 03/2008; UNEP, 09/2007; WEC, 2008).

The lessons learned from the MED-ENEC project seem to be applicable for most emergingand developing countries. An appropriate policy mix according to different framework conditionsis always necessary. Countries with very low energy prices need strong political will throughenforced regulations and standards and significant subsidies for energy-efficient products and ser-vices while reducing non-targeted energy subsidies and at the same time protecting the vulnerablepart of the population. Countries where framework conditions are more favourable and where theprivate sector is strong will rather focus on policy packages with information, training, qualitycontrol, technology transfer and credit programme components.

Other instances of recent and ongoing progress on low-energy and passive buildings are rep-resented by the research and project activity of the Passivehaus Institute, which needs to be brieflysummarized.

The last 10 years has seen increasing interest in North and Central Europe in the ‘PassivhausConstruction Standard’, particularly in Germany and Northern Europe. Homes built according tothe Passivhaus standard are buildings providing a comfortable indoor climate in winter withoutthe need for a conventional heating system. To achieve this standard, it is essential that the build-ing’s space heat load does not exceed 10W/m2 living area in order to be able to use a simple airpre-heater. Simulations and measurements have shown that for the typical German climate sucha design leads to an annual demand for space heating of 15kWh/m2per year. Passive houses there-fore require roughly 85 per cent less energy for heating than a house built to existing Germanbuilding regulations. The total primary energy demand, including household electricity, islimited to 120kWh/m2 per year. In 1991, Wolfgang Feist and Bo Adamson applied the passivedesign approach to a house in Darmstadt, with the objective of providing a showcase low-energy home at reasonable cost for the German climate. By 1995, based on the proven successboth in terms of energy consumption and comfort of the first experiment, the same passivesystems were applied again in a second construction in 1995 in Groß-Umstadt. From these experi-ences Feist had codified the Passive Design of the Darmstadt and Groß-Umstadt homes into thePassivhaus standard. The standard fundamentally consists of three elements: (i) an energy limit(heating and cooling); (ii) a quality requirement (thermal comfort); (iii) a defined set of preferredpassive systems which allow the energy limit and quality requirement to be met in a cost-effectivemanner.

The success of the Passivhaus Institute in Darmstadt naturally led to the question of whetherthis is applicable in other countries, with specific reference to the Mediterranean region. Thisquestion is central to some recent research dissemination projects funded under the IEE pro-gramme by the European Commission such as the ‘Passive-On’ project (http://www.passive-on.org/en/), which primarily addresses the question of the applicability of Passivhaus Standardsin southern Europe (Portugal, Spain and Italy). In fact, although in central Europe (e.g. Germany,Austria, Northern Italy, etc.) passive design is increasingly associated with the Passivhaus stan-dard, this is not necessarily the case in southern Europe (e.g. Spain, Italy, Portugal andGreece). The project examined how to take the passive house concept forward, especially inSouthern Europe. In these regions, the problem of household energy use is not only one of pro-viding warm houses in winter but also, and in some cases more importantly, one of providing coolhouses in summer. The Passive-On project has led to three major outcomes: (i) for architects andbuilding designers the project has developed design guidelines and enhanced the PHPP SoftwareDesign Tool for developing cost-effective all season passive houses in both heating load andcooling load climates; (ii) for policy makers the project has provided a set of policy proposals,

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examining the barriers and the solutions that the EU, national and local governments can adopt topromote more wide-scale development of passive houses.

The ‘Passive-on’ consortium has therefore formulated a revised proposal for the application ofthe Passivhaus standard in Warm European Climates, taking into account the above-mentionedclimatic issues and contents. According to Passive-on, the Passive House Standard for WarmEuropean Climate is defined in six points:

(1) Heating criterion: Useful energy demand for space heating not exceeding 15kWh/m2 ofnet habitable floor area per year.

(2) Cooling criterion: Useful, sensible energy demand for space cooling not exceeding15kWh/m2 net habitable floor area per year.

(3) Primary energy criterion: The primary energy demand for all energy services, includingheating, domestic hot water, auxiliary and household electricity, does not exceed120kWh/m2 net habitable floor area per annum.

(4) Air tightness: If good indoor air quality and high thermal comfort are achieved by meansof a mechanical ventilation system, the building envelope should have a pressurizationtest (50Pa) result according to EN 13829 of not more than 0.6ach21. For locations withwinter design ambient temperatures above 08C, a pressurization test result of 1.0h21 isusually sufficient to achieve the heating criterion.

(5) Comfort criterion room temperature winter: The operative room temperatures can be keptabove 208C in winter, using the above-mentioned amount of energy.

(6) Comfort criterion room temperature summer: In warm and hot seasons, operative roomtemperatures remain within the comfort range defined in EN 15251. Furthermore, if anactive cooling system is the major cooling device, the operative room temperature canbe kept below 268C.

To achieve the Passivhaus standard it is necessary that indoor summer temperatures, morespecifically operative temperatures, remain lower than the maximum temperatures defined bythe EN 15251 standard. According to EN 15251 standard, acceptable comfort temperaturesdepend on the type of system used to provide summer comfort. If cooling is provided by anactive system, then indoor temperatures must respect those defined by the comfort model orig-inally proposed by the Fanger or predicted mean vote model (Fanger, 1967, 1970, 1994;Fanger & Langklide, 1975; Fanger, Melikov, Hanzawa, & Ring, 1988; Fanger, Ostergaard,Olesen, & Lund-Madsen, 1974). Instead, if summer comfort is provided by passive cooling strat-egies, the higher temperature limit is set by the adaptive model, which is the model taking intoaccount the ability of occupants of buildings to adapt to the prevailing climate (the adaptivecomfort model). Compared to the Fanger model, the adaptive model considers a wider rangeof temperatures as ‘comfortable’ and therefore allows for easier integration of passive coolingtechnologies. For example, applying the adaptive algorithm defined in the EN 15251 standardto typical annual weather data predicts maximum summer neutral temperatures (in correspon-dence with a sequence of hot days) for Frankfurt, Milan, Lisbon and Seville of, respectively,26.1, 27.2, 26.7 and 28.78C. As a comparison a building cooled by an active air conditioningsystem will work to a fixed set point chosen between 23 and 268C. In more recent years, someinternational standards (e.g. the US norm ASHRAE 55 2004 and the European norm EN15251) have proposed adaptive comfort models based on in-field comfort surveys. These havethus replaced the previous Fanger-based temperature standards with ‘adaptive’ ones for indoortemperature in naturally ventilated buildings.

Prototype houses applying the Passivhaus standard have been built in five partner countries(France, Spain, Portugal, Italy and the UK). They have been performed by means of national

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‘exercises’ undertaken by the partners with the aim of applying the Passivhaus standard. Thesenational proposals were formulated with reference to the standard typology of a semidetachedthree-bedroom house. The exercises revealed that heating loads are relatively low in manysouthern European countries and generally stay below the 15kWh/m2 mark. Comparatively,however, they are marginal to other household energy requirements such as water heating, light-ing and appliances. It became clear that in many cases there are cooling loads to take into accountbut often these can be met by passive strategies alone.

For the Portugal case, for example, the starting point for the Portuguese Passivhaus proposalwas a single floor, two-bedroom house, complying with the national Building Regulation 2006.The current proposal takes into account the local climate (case study for Lisbon), the constructionstandards, and the technical and economic framework.

The level of insulation in walls and roofs exceeds the national standards and the air infiltrationis controlled (about 0.8ach at 50Pa). The three main aspects explored in the proposed house are:relation with the sun, ventilation for cooling and high thermal mass to control temperature swings.Since solar availability is quite high in Portugal, even during the heating season, a key factor inthis house is the relation with the solar radiation, captured both directly (windows) and indirectly(thermal solar system). Large windows are mainly oriented south increasing the useful solar gainsduring winter. Smaller areas are oriented east and west and minimal areas to north. Solar protec-tion is chosen according to the orientation: overhangs to the south windows, thus reducing thesolar incidence during summer, and exterior Venetian blinds in all windows. A very importantfeature of the proposal is the use of a thermal solar system. The current proposal extends thesolar installation of thermal systems from the compulsory limit use of Portuguese Thermal Regu-lation for Buildings (solar system for domestic hot water) to also cover a significant portion of theheating demand, by increasing the solar panels area and using a low-temperature hydraulic heatdistribution (radiant floor). As proposed for the Passivhaus standard, the active heating andcooling capacity is limited to 10W/m2. The extra costs of the proposed Passivhaus for Portugalis 57E/m2 with a payback period of 12 years. The house combines the ability to collect solarheat (large south windows) and the capacity to regulate inside temperature with its highthermal inertia. To further reduce heat losses and gains, 150 and 100mm of insulation are pro-posed for the roof and exterior walls, with U-values of 0.23 and 0.32W/m2K, respectively.Windows facing south correspond to about 60 per cent of the total glazed area; about 20 percent of the glazed area faces east and another 20 per cent west. Low-emission double glazingcan be very effective in colder climates of Portugal, but in most situations standard doubleglazing is more cost-effective (U-values of 2.9W/m2K for standard double glazing and 1.9W/m2K for low-emission glazing are considered). The annual heating energy demand of the Passiv-haus proposed for Portugal has been estimated as 16.9kWh/m2, of which 11kWh/m2 are suppliedby the solar system (in this analysis priority of the solar system is given to heating and the solarfraction for domestic hot water is 48 per cent). The annual cooling energy demand is 3.7kWh/m2.The sum of net heating and cooling demand is 9.6kWh/m2 per year, whereas limits of heating andcooling by regulatory tools in Lisbon are 73.5 and 32kWh/m2 per year, respectively.

The existing house, with an active cooling, has a Fanger Comfort Index of 811 (the house ispenalized by the influence of the radiant temperature of the high glazed area). If no activecooling is present, the Adaptive Comfort Index (AI2) applies (ASHRAE 55). For the proposedPassivhaus for Portugal, the AI2 was 16. For this house, the resultant temperature is kept below258C for 71 per cent of the occupied time, and below 288C during 98 per cent of the occupiedtime.

In winter, the low-power heating system of 10W/m2 is in use, resulting in only 8 per cent oftime with a resultant temperature below 19.58C (the lower resultant temperature achieved is188C).

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The previous analysis shows how the strategies adopted for the design of a Passivhaus for theheating and cooling climate of Lisbon can be successful, both regarding the energy demand limitsand regarding the comfort levels requirements. Although the specific design can be very diversefrom the simple layout presented, the applied strategies have proven effective in its relation withthe climate.

Another recent pilot study is the project KeepCool2, whose overall aim is to transform themarket to achieve good summer comfort conditions with no (or limited use of) conventionalenergy and through environmentally non-harmful materials in place of conventional air con-ditioning systems. The project proposed different actions to achieve this goal. For this it wasdivided into two phases. The first one provided analysis and technical tools to overcome themost important barriers. The second phase was addressing existing networks and policymakers on the national and the European level by providing information material with soundfacts, tools and best practice examples designed especially for the target groups.

The consortium established different analysis and guidelines to support the target group inachieving sustainable summer comfort. It reports on: rules and practices in building design andoperation including proposals of good practice, incentives, recommendations for national build-ing codes regarding summer comfort and cooling system requirements, guidelines for public pro-curement of designers and planners.

To influence and change a traditional process, such as ensuring summer comfort conditionswith conventional air conditioning systems, the project KeepCool designs regular and target-group-specific information campaigns to awake the change from ‘cooling’ to ‘sustainablesummer comfort’.

Some ‘golden rules’ to achieve sustainable summer comfort are listed by the project:

(1) Start by setting an open mind and agree to use a holistic approach to your project.(2) Select the different stakeholders in your project and gather them at the earliest possible

date to encourage co-operation.(3) Define the thermal comfort objectives, using as much as possible the adaptive comfort

model.(4) Use solar shading – a well-planned installation shuts out excessive heat during summer

and helps to keep it during winter and minimizes glare problems.(5) Install energy-efficient light – a good solution will provide good working conditions

without heating the room.(6) Only buy energy-efficient office equipment. Enable power saving modes on all.(7) Allow the availability of open spaces to ensure air circulation.(8) Remember that false ceilings prevent heating and cooling storage in the building

construction.(9) Use night cooling if needed.

(10) Use free or district cooling if the above is not enough.(11) Use, as a last resort, energy-efficient active cooling.(12) Inform and train building managers and occupants on how to use the installations,

monitor performances and adequately operate and maintain the building.

The preliminary results of the IEE project KeepCool2, together with an evaluation of buildingenvelope retrofit techniques for reducing energy needs for space cooling, are discussed inPagliano (2008). In particular, a methodology for bottom-up assessment of the energy savingsrelated to ‘sustainable summer comfort’ solutions is presented; reference base case building typol-ogies are analysed in five European climates and dynamic simulations are used to calculatethe reductions in the energy need for cooling which can be achieved by specific retrofit

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actions (e.g. additions of effective solar protections, increased thermal insulation, night venti-lation, increase of active mass by phase change material (PCM), low solar absorbance surfaces,etc.).

But such passive cooling technologies, which are already available and cost-effective (suchas the use of well-designed sun shades, efficient lighting and office equipment, passive coolingvia thermal exchange with the ground, night ventilation, etc.) are not widely used on themarket today: the most common choice for a building owner when addressing summercomfort issues is still mechanical cooling, often without previously investigating other avail-able measures regarding the optimization of envelope features (e.g. solar protections, glazingsolar factor, thermal insulation of opaque surfaces and thermal mass). Only a limitednumber of retrofit actions taking into account passive cooling options have been documentedin detail.

Preliminary results of the project KeepCool2 reported by Pagliano (2008) aim at contributingto a broad market transformation from ‘a cooling approach’ to ‘a sustainable summer comfortapproach’ which makes effective use of: (i) the most advanced knowledge and technologiesfor good design of building envelope (or redesign through retrofit actions); (ii) passive coolingtechniques and comfort responses and adaption mechanisms of occupants (according to thenew European Standard EN15251/2007. ‘Sustainable summer comfort’ is defined as ‘achievinggood summer comfort conditions with no (or limited use of) non-renewable energy and throughthe use of environmentally non-harmful materials’, according to the definition set up in the Keep-Cool project.

It has been decided to focus on existing buildings and on technical energy efficiency improve-ment actions, which are defined as technical actions taken at an end-user’s site, in order toimprove the energy efficiency of the energy end-using facilities or equipment and thereby saveenergy. A series of building reference cases have been selected and a list of possible retrofittingactions have been analysed: (i) install an external movable screen blind; (ii) install an externalmovable screen blind with radiation control; (iii) install an external window awning; (iv)install efficient windows; (v) treat wall and roofs with special paintings; (vi) insulate the roof;install PCM plasterboard; (vii) use energy-efficient office equipment; (viii) install energy-efficientlightings and ballasts; (ix) install automatic night-time operable openings; (x) install automaticdaytime and night-time operable openings; (xi) install extraction system for night-time venti-lation; (xii) install extraction system for daytime and night-time ventilation; (xiii) use an existingventilation system at full speed for night-time ventilation, etc.

As to existing buildings, this research work reports the methodology and initial efforts toevaluate the saving of a number of retrofit actions on existing buildings and to include some rel-evant occupant behaviour patterns. The evaluation aims also to deliver saving estimates to be usedin the context of the National Energy Efficiency Plans. For new buildings two examples in acentral and south European climates are presented that rely on envelope and passive strategiesin order to deliver summer comfort to occupants. Although they are new buildings and low-energy summer comfort concepts have been an integral part of the energy concept since the begin-ning of the design process, the authors argue that some of the techniques adopted here are suitableto be implemented also in retrofit actions and that well-designed and implemented techniques innew buildings enhance chances of full success elsewhere. Finally, the authors suggest examples of‘comfort codes of conduct’ that systematically take advantage of behaviour choices in order toimprove comfort and take stock of opportunities for flexibility in the use of buildings, bothnew and existing. The integrated combination of actions on existing buildings, guidelines andexperience in the design of new buildings, as well as proper behavioural choices, can lead to ‘sus-tainable summer comfort’. Further research and review work in these three directions are ongoingin the IEE project KeepCool2.

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Discussion and further research

In past years, there has been growing interest among stakeholders, architects and entrepreneurs inincorporating energy efficiency tools and techniques into buildings, as a way of achieving energy-efficient buildings that comply with stringent energy codes and national goals of reducing danger-ous emissions, together with improving the corporate image.

Previously reported case and research studies demonstrate the technical feasibility of NZEBsboth in colder and in warmer areas of the Mediterranean region.

Nonetheless, in spite of the significant interest in low, passive or NZEB buildings and recog-nized overheating increase problems emerging worldwide, there is still a risk of underestimatingthe growing energy demand for cooling purposes in the Mediterranean region. This may berelated to various factors:

. A relatively new approach to passive and low-energy buildings within the MediterraneanZEB and Current Policies European Directive, which needs to be further investigatedwith specific reference to cooling demand and summer energy performance.

. Greater attention to ZEB in the northern and colder climate areas, as compared to thesouthern and Mediterranean areas.

. Problems connected with modern architecture and the use of low-inertia building com-ponents, which limit the capacity to control the building environment without resortingto methods involving high-energy consumption.

. Lack of knowledge and understanding of the potential to use available construction systemsand techniques (which can be also derived from traditional passive cooling systems).

. Technical/economic/cultural/social barriers to integrate these techniques and RES withinnew and existing buildings.

. Less favourable social and economical conditions in the Mediterranean countries, as com-pared to the Northern and Central Europe.

Since today the reserves of conventional energy sources are drastically depleted, it becomesimperative to consider the reduction of energy demand to be the primary and first option todecrease energy consumption and achieve natural cooling within our buildings. To achieve thisfirst important objective, it could be important to look into the past for guidelines and solutions.In no way is it suggested to copy traditional building types into the present or abandon the avail-able technology to return to construction practices of the past; however, the basic principles thatcan be derived from the traditional architecture concerning the correct orientation and placementof buildings and their components (openings to promote day lighting and natural ventilationduring the cooling period, distribution of thermal mass, the use of local materials, etc.) may beconsidered as the first (and lower cost) bioclimatic options to design or re-design a low-energybuilding. In fact, the architectural design process usually moves from the conceptual features(definition of massing, orientation and form) to the specific requirements (thermal mass, solarand lighting control, and mechanical ventilation type).

It should be taken into account that, as the design process advances, earlier – and often free ofcost – decisions, which could have a strong influence on the building performance, are costly anddifficult – if not impossible – to change.

Examples from the reported analysis of pilot case studies (MED-ENEC project, Passive-Hause) have shown that passive design features, such as the orientation of the building,shading, natural ventilation and the use of daylight, are free of cost or very cheap in the designphase and therefore always cost-efficient for new buildings. Insulation of roofs and walls aswell as efficient lighting and solar water heaters proved equally to be among the top-rankingcost-effective solutions in most cases.

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However, although climatic building design strategies are important, it cannot be stated thatany one of them has a higher priority; the large number of elements existing in the market today,the large set of barriers and constraints limiting their applicability, make it necessary for designersto have tools that help them to identify the best combinations for any specific situation. In thiscontext, the cost-effective analysis, together with the proper assessment of life cycle and thecorrect incorporation of the social–cultural aspects and expectations within the design process(Peattie, 2010), may represent useful components of a strategic framework plan aimed at theincorporation of zero-energy concepts into building construction.

Thus, to achieve a correct approach towards ZEBs it is necessary to start from the radicalreduction of energy demand for cooling, heating, lighting and ventilation; the use of RES tocover the remaining energy demand should be considered as the secondary and consequent step.

Previously reported examples and studies have shown the technical feasibility of ZEBs in thecontext of the European and Mediterranean regions.

A further challenge now would be to widen technical ZEB knowledge in existing builtenvironments. We need to shift our technical understanding of energy efficiency from new devel-opments and newly conceived buildings to existing buildings, since the large amount of existingstock represents the biggest challenge in carbon terms.

In this context, studies like the preliminary results of the project KeepCool2 reported byPagliano (2008) on building energy retrofitting need to be further developed and applied incurrent building practices.

Furthermore, as briefly shown by recent research studies aimed at the combination of bothpassive and active systems (Eicker & Dalibard, 2011; Infield et al., 2004), the combination ofboth new building envelopes such as sunspaces, buffer zones and active energy systems in exist-ing buildings – such as photovoltaic plant – may drastically reduce the energy requirements,thus helping to reach the target of zero energy balance in the existing buildings as well. Infact, so far, the study of the mutual effect between passive and active systems has not beenfully explored; nonetheless it is particularly important, since the use of the potential of photo-voltaic systems in cooling, shading, increasing air stratification and vertical air extraction fromexisting buildings produces additional energy savings; similarly, for example, wind micro-turbines may behave in interesting ways in existing buildings: their passive potential toextract air from buildings and its significance in terms of energy savings have been disregardeduntil now.

In addition to the current development of zero- and low-energy pilot studies and high-per-formance new buildings, the integration of passive technologies and RES within existing buildingstock could represent a challenge for future research projects.

The context of existing buildings stock and its potential to be retrofitted according to lowpassive and active techniques and zero energy concepts should therefore be further investigated.

To reach this further aim a closer collaboration between physicians, engineers, architects,energy companies, stakeholders and house owners among others has to be developed.

Following the main issues arising from the state of the art in low-energy building and ZEBwithin the context of the warm and hot climate regions in the Mediterranean area, further researchstudies and design implementation first and foremost should turn future designers’ attention,industrial and public stakeholders’ priorities from pilot zero-energy experiments and demon-stration buildings to fully distributed interventions in existing buildings, showing this opportunityas a creative and sustainable challenge for future generations of planners, industrial designers andresearchers.

Thus, by these considerations, a record of possible next steps to be proposed for policy, legis-lation, education and research may be outlined.

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As far as policy is concerned, a carbon zero and zero-energy strategy plan, with specific refer-ence to the Mediterranean climate, should be oriented at:

. providing and reinforcing specific target strategies for open areas and public spaces withinthe urban environment, to promote the use of green and permeable surfaces, thus reducingurban cooling demand at the territorial and urban scale;

. promoting, as far as possible, the use of local and/or locally produced materials and com-ponents to control and limit the embodied energy within the frame of newly conceived andexisting building retrofitting;

. incentivizing the use of the most effective and cost-efficient technology mix in relation tolocal environments and specific sites, considering that passive design features are alwayscheaper and cost-efficient with respect to active systems; the feasible technology mixshould be different according to the type of building, the climate zone, the nationalenergy prices, the availability of know-how and technologies, and the cultural and socialattitudes;

. setting special incentives (tax reduction, volumetric building compensation, etc.) to reachfully distributed interventions of zero-energy retrofitting of existing buildingswith special reference to urban areas, thus achieving win–win perspectives in relationto urban densification schemes (against urban sprawl) and full-scale energy savingstrategies.

With reference to legislative measures in the frame of ZEBs, as pointed out by Torcellini et al.(2006) ‘A good ZEB definition should first encourage energy efficiency, and then use renewableenergy sources available on site. A building that buys all its energy from a wind farm or othercentral location has little incentive to reduce building loads, which is why we refer to this asan off-site ZEB’. Thus, taking into account that it ‘is almost always easier to save energy thanto produce energy’, it is necessary to:

. Set specific energy supply options and establish a hierarchy of them in relation to the speci-ficity of sites, energy prices and cultural and social aspects.

. Consider ZEB as an additional step with respect to the passive house principles, not as analternative to it. Since the energy saving from passive cooling and heating is the first andforemost criterion to be considered in ZEB, it would be particularly important to fix an‘internal limit’ of passive requirements within ZEBs. By this, ZEB can be considered asnot different from but integrated with the passive house concepts. A new standardshould be framed in order to encourage low-energy buildings and not only high-energy per-forming buildings.

. Suggest new forms for future standards, which are more in keeping with the objectivesof the EPBD on ZEB (European Council for an Energy Efficient Economy, February2011).

Of course, setting regulations and standards and enforcing them is a necessary but not suffi-cient condition for success. As the value chain in the building sector is complex, with plenty ofstakeholders acting on individual perceptions of risks and benefits, it is of highest importance thateconomic signals such as energy tariffs, incentives and sanctions orientate the actors in themarket. Furthermore, integrated packages of regulation/standard, financial support and incentivesshould be combined with information, training, education as well as research and developmentactivities.

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Thus, as far as education and research are concerned, care and interest should be focused at:

. Design education and training programmes aimed at improving the adaptability of build-ings within the context of Mediterranean regions, to provide flexibility and thermalcomfort in both summer and winter seasons.

. Further research investigation between active and passive synergies, which may be gainedby the combination of passive techniques (sunspaces, thermal control, shading and venti-lation) and active energy systems, since this integration may generate additional energysaving disregarded until now.

. Training activities for stakeholders, private owners and citizens (also within the frame ofparticipative processes) to inform on the potential of consumers’ behaviour in reducingcooling demand and save energy. In fact, the minimization of energy use for heating andcooling the buildings could be encouraged by the education of occupants (smart behaviour)in addition to the use of free-of-cost strategies by intelligent design (using passive architec-tural strategies). For example, training and information on dress behaviour and its potentialin relation to energy consumption reduction should be encouraged and/or reinforced, sinceseasonal flexibility in clothing may have huge consequences for energy saving in buildings.

As a general rule, further interconnections among cultural/behavioural aspects with respect togreen consumption should be researched to ensure the effective penetration of low-carbon lifestyles and passive ZEBs, within the frame of a socio-oriented human environment.

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