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Cool and green roofs. An energy and comfort comparison between passive cooling and mitigation urban heat island techniques for residential buildings in the Mediterranean region
Energy and Buildings 55 (2012) 66–76
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Cool and green roofs. An energy and comfort comparison between passive cooling and mitigation urban heat island techniques for residential buildings in the Mediterranean region M. Zinzi ∗ , S. Agnoli ENEA Italian National Agency for New Technologies, Energy and Sustainable Economic Development Technical Unit for the Energy Efficiency, Via Anguillarese, 301, 00123 S. Maria di Galeria, Rome, Italy
a r t i c l e
i n f o
Article history: Received 12 May 2011 Received in revised form 29 July 2011 Accepted 12 September 2011 Keywords: Cool materials Green roof Thermal comfort Passive cooling Energy performance
a b s t r a c t The increase of peak and energy demand during the cooling season is becoming a crucial issue, as well as the intensification of the urban heat island effect. This trend is observed at several latitudes, including areas where overheating was unknown at building and urban levels. This phenomenon involves different issues: reduction of greenhouse gases, quality and comfort in outdoor and indoor environment, security of energy supply, public health. The building sector is directly involved in this change and adequate solutions can provide great benefit at energy and environmental levels. Roofs in particular are envelope components for which advanced solutions can provide significant energy savings in cooled buildings or improve indoor thermal conditions in not cooled buildings. Cool materials keep the roof cool under the sun by reflecting the incident solar radiation away from the building and radiating the heat away at night. Roofs covered with vegetation take benefits of the additional thermal insulation provided by the soil and of the evapo-transpiration to keep the roof cool under the sun. These two technologies are different in: structural requirements, initial and lifetime maintenance costs, impact on the overall energy performance of buildings. This paper presents a numerical comparative analysis between these solutions, taking into account the several parameters that affect the final energy performances. By means of dynamic simulations, the paper depicts how cool and green roofs can improve the energy performance of residential buildings in different localities at Mediterranean latitudes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The effects of global warming and climate changes are of relevant concern for environment and human activities in the Mediterranean area. The average air temperature rise of 2 ◦ C represents a critical limit beyond which dangerous climate changes should occur by 2030 [1]. More than 90 million people live in the twenty most populated Mediterranean metropolitan areas; according to the actual trend other 70 million of people are expected to move to leave the countryside towards the urban area by 2025 [2]. The global warming and the urban sprawl causes a number of environmental hazards, the urban heat island (UHI) is one of these. This phenomenon is defined as the air temperature rise in densely built environments respect to the countryside surroundings. The main cause is the modification of the land surface in the urban area, where the vegetation is replaced by exten-
∗ Corresponding author. Tel.: +39 06 3048 6256/4188; fax: +39 06 3048 3930. E-mail address: [email protected] (M. Zinzi). 0378-7788/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2011.09.024
sively built surfaces (typically paved roads and buildings surfaces), characterised by high solar absorption, high impermeability and favourable thermal properties for energy storage and heat release, as well as several anthropogenic. The UHI was first monitored in London back to the 19th century [3]; many studies were performed during the past decades [4–10], showing the quantitative effects of the phenomenon and the correlation with the previously enounced causes. Daily mean UHI typically ranges between 2 and 5 ◦ C, while UHI intensities (defined as maximum difference between urban and background rural temperatures) up to 12 ◦ C were registered under particular conditions. This UHI impacts important issues such as: the quality of life; the public health, especially for the most vulnerable population; the environmental hazards. Roof surfaces of the building accounts for the 20–25% of the total urban surfaces, hence they can successfully used to reduce the air and surface temperature of urban area [11]. Cool and green roof, widely described in the next paragraph, are used to mitigate the UHI and the impact was proved by several studies [12–15]. These techniques can also have significant benefits on the energy performance of buildings, providing passive cooling to the built environment. This topic is of special interest because of the rapid increase of the
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energy consumption and peak demand for cooling in Europe and in the Mediterranean basin [16]. This study aims at analysing and comparing how these UHI mitigation techniques can also improve the energy performance of cooled buildings and the thermal comfort of not-cooled buildings at Mediterranean latitudes. Special attention is paid to residential buildings, where accurate design concepts and technologies can strongly reduce the use and the installation of cooling systems without affecting the occupants comfort expectations and help achieving international environment targets. 2. Building applications of cool and green roofs Cool roofs are characterised by materials having: high solar reflectance (SR) and high thermal emittance (TE). The former expresses the ability of the materials of reflecting most of the incident solar radiation during daytime, keeping their surfaces cooler respect to conventional construction materials. The high thermal emittance allows the materials to radiate away the heat stored in the structure, mainly during night time. This thermal behaviour allows the roof to reduce the heat transfer to the built environment. Roofs characterised by low emittance values tend to not dissipate the stored heat at night and can be considered cool only if they have a very high solar reflectance. White mortars and plaster were widely used in ancient massive Mediterranean dwellings, in order to create a more comfortable built environment during the hot season. The coastal villages of Greece, Italy and Spain still witness this construction technique, which emerged again as an efficient solution during the recent years. Several numerical studies were carried out in the past years to assess the energy performance of buildings equipped with cool roofs. The impact of cool roofs on a single floor detached house placed in different climatic zones of the planet world was calculated for insulated and not insulated dwellings [17]. The cooling energy consumption reduction was 18% and 93% increasing the roof solar reflectance from 20% to 85%. Three typical building models were developed respectively for: a residential building, an office and retails store, differentiated by age (before and after 1980). The impact of cool roof ensured global energy savings from 7% to 25% according to the different age and building type for several US climates [18]. Other studies were focused to limited geographical sites, as Jordan or Honk Kong [19,20]. Other studies faced the cool roof positive impact evaluated as an additional thermal insulation [21]. The results of the analysis revealed that the integrated daily roof heat gain was not dependent on its thermal mass. An energy analysis run proved that the daily heat flow in a roof with SR of 0.65 and a thermal resistance (R-value) of 1.1 m2 K/W was equivalent to the flow in a roof with SR 0.3 and R-value 2.2 m2 K/W. Limited data from real building application are available. A field campaign was carried out in one house and two school bungalows
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in Sacramento, California. Cooling energy savings of 2.2 kWh/day were measured increasing the solar reflectance of the roof from 0.18 to 0.73 [22]. The energy savings in the school buildings was about 35%. A study in an experimental building in Rome, Italy, proved that the air temperature of an attic room decreased by 2 ◦ C increasing the albedo of the roof from 14% to 85% and this room was found cooler than an identical room at the floor below, which had no roof at all [23]. Green roofs, also called eco-roofs, use the foliage of plants to protect the building environment. The thermal loads due to the solar radiation and the air temperature are limited before entering the buildings by the vegetation layer. This depends on the absorption of the solar radiation by the plants to support their life-cycle, including: photosynthesis, evapo-transpiration, respiration. Moreover, the soil layer gives an added insulation to the building roof and the water content increases the thermal inertia of the structure. The vegetation characteristics affect, in addiction, the convective and radiative heat transfer through the roof surface. Green roofs were once typically used in northern climates to improve the insulation performance of the building envelope, but they are also an opportunity in warm climates, because of their thermal behaviour under the solar radiation. Several studies were produced during the past years trying to quantify the effect of green roofs on the energy performance of buildings. The noticeable impact of green roofs during the hot and the cold seasons was analysed in a nursery school in Athens, founding out that energy savings up to 49% could be obtained [24]. The energy and water issues were analysed in two experimental setups in Italy; particular attention was paid to the impact of the foliage on the radiation and the air temperature profiles insisting on the building roof, respect to the undisturbed values [25]. Combined measurements and calculation analyses were performed in order to assess and predict the 60% reduction of the heat flux through a green roof respect to a conventional roof in a hospital building in northern Italy [26]. A case study in Brasil demonstrated that a green roof in an experimental building reduced the heat flux by 92–97% compared to a ceramic and a metallic conventional roof [27]. Specific studies on the substrate materials, foliage characteristics and vegetal species demonstrated the variability of the green roof performances as a function of the adopted technical solutions [28–30]. 3. Methodology The scope of this work consists in the assessment of the energy performances of residential buildings using different roof solutions: standard, cool and green roofs. The study is focused on the Mediterranean area, a mild climatic zone with differences in rainfall levels and air temperature profiles that can lead to different choices of building technologies to achieve the opti-
Table 1 Air temperature and solar radiation data of the selected localities. Month
T (◦ C) Barcelona
H (kJ/h/m2 )
RH (%)
T (◦ C) Palermo
H (kJ/h/m2 )
RH (%)
T (◦ C) Cairo
H (kJ/h/m2 )
RH (%)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
8.2 9.4 11.1 13.1 17.0 20.9 23.5 24.1 21.6 17.3 12.1 9.9
288 409 553 744 879 873 1004 858 603 444 287 250
71 68 73 72 74 74 68 71 74 82 78 65
12.7 11.9 13.8 15.7 19.2 22.8 25.5 27.0 24.1 21.6 17.2 13.9
312 454 625 843 980 1090 1099 993 741 549 319 272
76 71 79 71 77 71 76 73 66 74 69 78
14.0 14.5 16.6 21.8 24.7 28.0 28.2 27.9 26.6 23.8 19.0 15.3
439 597 736 913 1052 1142 1118 1008 898 630 493 430
67 58 59 45 40 45 56 60 56 56 61 64
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M. Zinzi, S. Agnoli / Energy and Buildings 55 (2012) 66–76 Table 3 Thermal properties of dwellings envelope components.
Table 2 Rainfall data of the selected localities. Month
Barcelona rainfall (mm)
Palermo rainfall (mm)
Cairo rainfall (mm)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
39.50 40.60 47.36 47.36 53.27 44.10 30.00 51.54 69.26 93.45 61.89 46.10
67.60 66.30 59.70 43.50 26.00 14.40 7.80 12.96 40.78 94.54 92.00 78.63
6.94 4.00 4.00 2.06 0.00 0.00 0.00 0.00 0.00 1.05 2.97 4.94
mal energy performances. The comparison is carried out by means of a numerical analysis performed using the Design Builder interface, which relies on the Energy Plus calculation engine, able to perform energy balance calculations with hourly time step. The tool carries out accurate thermal analyses and allows very detailed inputs, including: climatic data (including air temperature, solar radiation, relative humidity hourly profiles); construction materials and components in dedicated libraries or manually edited; energy systems’ specifications; time schedules (systems’ management, occupancy, electric lighting, ventilation, etc.). Energy Plus also features a validated mono-dimensional green roof model developed taking into account the evapotranspiration of the vegetation layer, the time dependent soil thermal properties (conduction and inertia), the radiative and convective heat exchanges [31,32]. The tool also allows a complete description of any construction material, since thermal conductance, solar reflectance and thermal emissivity can be modelled. In order to depict a wider overview of these techniques impact on the energy performances a number of variables are taken into account. A number of building variants are, hence, defined and the cooling and heating demands are calculated for each of them. The
Envelope component
Not-ins U (W/m2 K)
Ins U (W/m2 K)
Wall Roof Ground floor Window glass Window frame
1.4 1.4 1.7 2.8 5.9
0.7 0.6 0.8 1.8 4.7
comparison is carried out for the net energy, without considering the energy systems efficiencies, since the main objective is the optimisation of the envelope energy performances. The variants considered in the analysis are described in the next paragraphs.
3.1. Definition of the reference localities Three localities were selected, typical of different regional areas: Barcelona for the north rim, Cairo for the south rim and Palermo for the centre basin. This criterion also responds to a strict relationship between buildings and climate. The north rim is heating dominated for not insulated buildings, while heating and cooling are both relevant in buildings in the centre of the Mediterranean region. The south rim is cooling dominated for any building thermal characteristics. Investigating these three localities, allowed mapping with a good accuracy the roofing techniques’ efficiency according to the relevant Mediterranean climatic conditions. The climatic conditions of these three areas referred to air temperature and relative humidity, as well as the global solar horizontal radiation. These data are taken by the Meteonorm database, embedded in Design Builder. Table 1 reports the monthly daily mean air temperature and the solar radiation, expressed as global irradiance on the horizontal. The difference within the three localities can be easily inferred in this case, since air temperatures and solar radiation decreased with latitude. The relative humidity is also reported in Table 1, these parameters are important because of the impact on the energy use of the building and the comfort condi-
Fig. 1. Layout of the row house with the two thermal zones: ground level day zone and first floor night zone.
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Table 5 Characteristics of the vegetation and soil layers of the selected green roofs.
Fig. 2. Layout of the single floor detached house with the two thermal zones: day (right) and night zone.
tions of occupant, even if it does not show significant relationship with the latitude. Table 2 reports the monthly rainfall data expressed in millimetres as taken by the Meteonorm database. Rainfall is not embedded into the software; hence the data was inputted through an external schedule. The table depicts the different water availability in the three cities, critical parameter for green roofs applications. Cairo is practically dry through all the year. Palermo and Barcelona have different trends: the former has rainfall practically constant throughout the year, while Palermo has higher rainfall in winter but gets almost dry during summer months. 3.2. The reference buildings The analysis is carried out on two residential building typologies widely used in the Mediterranean region: row houses and detached single family houses. The buildings are considered with two different envelope configurations: insulated and not insulated, in order to have significant results for old and recent constructed buildings. The first is a two floors row house, whose geometry is presented in Fig. 1. The building is divided into two thermal zones: the day zone at the ground level, where almost all the daytime activities take place; the night zone at the upper floor, hosting bedrooms and services. The houses is in contact with the external environment through the roof, the ground floor and the north and south fac¸ades, while the east and west walls are adiabatic, assuming they are boundary layers between adjacent row houses. The main thermo-physical data are summarised in Table 3. The not insulated configuration considers a conventional double glazing unit, whose g-value is 0.75, this value decreases to 0.65 for the Table 4 Geometry of the selected buildings.
V – volume A – net gross area S – total external surface Sr – roof surface S/V Sr /V Sr /S
Unit
Row house
Detached house
m3 m2 m2 m2 m−1 m−1 –
427 116 211 68 0.50 0.16 0.32
369 100 364 112 0.98 0.30 0.30
Parameter
Value
Height of plant Leaf area index Leaf solar reflectance Leaf emissivity Minimum stomatal resistance Max volumetric moisture content of the soil Min volumetric moisture content of the soil Initial volumetric moisture content of the soil Density of the soil Specific heat of the soil Conductivity Soil layer thickness
<60 cm 1.2 0.25 0.9 120 m/s 0.32 0.01 0.15 960 kg/m3 1500 J/kgK 0.34 W/mK 0.12 m
insulated configuration, where a low-e double glazing units is considered. The windows are provided of an external shading device (30% solar transmittance and 10% solar absortpance). Assumptions are also made regarding the occupancy profile, the electric and the appliances loads. The natural ventilation/infiltration was fixed at 0.5 ACH. Since the net energy has to be calculated, the following hypotheses are assumed: the set-point temperature is continuously maintained constant and heating and cooling system generators of unlimited power are implemented to keep this condition. The air exchange rate is increased to 3 volumes per hour for the free floating analysis, that is carried out to assess the impact of the roof solutions on the indoor thermal comfort of not cooled buildings. The detached single family house consists of a single floor building and it is divided in two thermal zones: day (mainly facing east) and night (mainly facing west). The same thermo-physical and operational data of the row house are applied to this building. A schematic layout is presented in Fig. 2. These typologies were selected because they also define different thermal behaviours related to the building geometry, as inferred from Table 4. Both houses have similar roof surfaces compared to the total surface area, but the roof surface of the row house is about half of the detached house. These configurations imply a higher roof solar gain ratio in the detached house. Conversely this house has a wide open geometry (S/V ratio is 0.95 m−1 ) with consequent high thermal losses whenever the indoor air temperature is higher than outdoor and vice versa. The row house has a more compact geometry, hence tends to maintain the heat in the inside. These differences imply different cooling and heating loads profiles during the year. 3.3. The roof technologies Several studies proved the impact of the thermal insulation and the thermal mass on the heat balance of the roof and, more in general, of the building envelope [33–37]. Even if these properties affect the energy performance of buildings during the cooling and heating season, this study is, conversely, focused on assessing the energy performance of residential buildings when existing flat roofs are equipped with UHI mitigation techniques. It is worth noting that both green roofs and cool roofs can be implemented using different solutions, corresponding to different thermo-physical and biological parameters. In order to consider a limited number of variables, the selected roofs were defined according to good quality standards related to green and cool roofs. The following describes the selected roof solutions: ST Conventional roof – standard product with thermal insulation defined in Table 3; the external layer has solar reflectance 0.25, typical for most construction materials, and thermal emittance 0.9.
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Fig. 3. Not-insulated row house energy performance for different roof solutions in Palermo.
CR White cool roof – same layers of the previous roof finished with an elastomeric white coating having solar reflectance 0.8 and thermal emittance 0.9. CR low-e Metallic reflective coating – as above with a metallic layer with solar reflectance 0.65 and thermal emittance 0.4. GR Green roof, whose main properties are summarised in Table 5. The structure of the green roof is more complex. The adopted solution is defined according to typical values of the green roof design in Mediterranean area. It is important reminding that the green roof model implemented in Energy Plus, is mono-dimensional add based on several assumptions. To be noted that the roof insulation thickness is lower than conventional roof insulation, to take into account the insulation effect of the green roof. It is also noted that, according to typical green roof systems, the green area correspond to 80% of the total roof surface, the remaining is surface dedicated to footpaths, here assumed to be made of concrete.
4. Results Results refer to the set of simulations carried out including the variables above defined. A first set of calculation was made to evaluate the performance of the green roof under different moisture conditions. Following the energy simulation results are presented, considering the green roof performances under effective rainfall. The results presented the demand for heating, cooling and energy, the latter expressed as simple sum of the two energy uses. The calculations refer to the net energy demand, without considering the energy systems efficiencies, in order to focus on the envelope behaviour. The same simulations are presented in free floating conditions, in order to evaluate the improvements of comfort conditions using cool and green roofs.
Fig. 4. Not-insulated row house energy performance for different roof solutions in Barcelona.
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Fig. 5. Not-insulated row house energy performance for different roof solutions in Cairo.
4.1. Impact of the green roof water content on the energy comparison Conventional and cool roofs are static elements, whose performances do not change according to the climatic conditions. Green roofs are different: climatic conditions affect the water content and, as a consequence, the cooling and heating performances. This first set of simulations refers to the not insulated
dwellings only. Figs. 3–5 report the results for the three localities and include the heating and cooling net demand for six different configurations: ST, CR, CR low-e, GR with actual rainfall, GR dry, GR wet. The latter condition implies that the roof is continually watered and the soil reached the maximum humidity content. This condition can be reached only under the hypothesis of a well designed irrigation system, with additional costs and resources’ uses that are not part of this investigation. The bars in the figure
Table 6 Calculation results: heating, cooling and total energy demand, energy savings compared to the standard roof (the first column includes: locality – insulation level – roof technique). Cooling (kWh/m2 /y)
Energy (kWh/m2 /y)
Sav. to ST (%)
Heating (kWh/m2 /y) Detached house
Cooling (kWh/m2 /y)
Energy (kWh/m2 /y)
Sav. to ST (%)
City and roof technique
Heating (kWh/m2 /y) Row house
Bar-ins-ST Bar-ins-CR Bar-ins-CR low-e Bar-ins-GR Bar-not ins-ST Bar-not ins-CR Bar-not ins-CR low-e Bar-not ins-GR Pal-ins-ST Pal-ins-CR Pal-ins-CR low-e Pal-ins-GR Pal-not ins-ST Pal-not ins-CR Pal-not ins-CR low-e Pal-not ins-GR Cai-ins-ST Cai-ins-CR Cai-ins-CR low-e Cai-ins-GR Cai-not ins-ST Cai-not ins-CR Cai-not ins-CR low-e Cai-not ins-GR
−23.2 −27.3 −22.3
4.2 1.5 3.2
27.4 28.9 25.5
0.0 −5.3 6.8
−33.2 −40.6 −33.3
8.4 3.2 6.5
41.6 43.8 39.8
0.0 −5.4 4.3
−21.3 −44.1 −54.2 −44.3
4.4 6.0 0.9 3.7
25.7 50.1 55.1 48.0
6.2 0.0 −10.0 4.2
−32.5 −71.9 −89.6 −71.9
5.9 7.9 1.1 4.8
38.3 79.8 90.7 76.7
7.8 0.0 −13.7 3.8
−38.9 −8.2 −10.9 −8.4
5.4 10.1 4.7 8.2
44.3 18.3 15.6 16.6
11.6 0.0 14.9 9.1
−68.5 −11.4 −16.0 −11.7
3.6 19.2 9.5 16.0
72.1 30.6 25.5 27.6
9.6 0.0 16.7 9.7
−7.0 −18.6 −25.5 −19.1
10.7 14.4 3.6 10.1
17.7 32.9 29.1 29.2
3.2 0.0 11.7 11.4
−11.4 −30.4 −42.6 −31.1
18.0 20.7 5.8 14.8
29.4 51.1 48.4 45.9
3.8 0.0 5.3 10.2
−15.6 −3.2 −4.9 −3.3
13.8 19.7 10.7 17.0
29.3 23.0 15.6 20.3
10.8 0.0 32.2 11.5
−29.9 −4.1 −6.9 −4.2
16.8 37.0 22.2 32.4
46.6 41.1 29.1 36.6
8.9 0.0 29.0 10.9
−2.6 −8.4 −13.3 −8.6
19.9 27.7 7.7 21.6
22.5 36.1 21.0 30.2
2.0 0.0 41.7 16.4
−3.2 −13.0 −21.8 −13.1
36.7 44.2 18.2 35.5
39.9 57.2 40.0 48.5
2.8 0.0 30.1 15.1
−6.6
24.8
31.4
13.0
−10.0
39.2
49.2
13.9
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Fig. 6. Heating, cooling and total energy demand of the row house. The different building configurations, specified on the x-axis as a function of: locality – insulation level – roof technique.
represent the cooling, heating and total net energy demand. This set of calculation is performed in order to show the impact of water content on the energy performance of buildings and to stress the importance of irrigation strategies in order to optimise the GR performance respect to other static cool techniques. The results are expressed as percentage reduction respect to the standard roof performances. The results obtained for Palermo, Fig. 3, show that the best performances are obtained with the green roof always wet, with heating demand comparable with the conventional roof, but with the cooling demand reduced by more than the half. The total energy savings are 24% with this configuration. CR reduces the cooling demand by 75%, but the increase of the heating demand lowers
the total energy savings close to 12%. The metallic cool roof produces global energy savings closer to CR, due to heating performances similar to ST and improved cooling performances. The green roof has similar performances in dry or actual rainfall conditions. Energy savings are around 11% and are obtained in winter, thanks to the higher insulation level of the roof, but low improvements are reached in the cooling season, because of the limited advantage of the dry vegetation layer. Barcelona has a cooler climate and this impact the performances of the different roofing systems in a different way respect to Palermo; see results in Fig. 4. The not insulated envelope induces a high heating demand. The best result is obtained by the dry GR, because of the insulation effect produced by the soil layer in winter
Fig. 7. Heating, cooling and total energy demand of the detached house. The different building configurations, specified on the x-axis as a function of: locality – insulation level – roof technique.
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Fig. 8. Operative and ambient air temperature profiles in the not insulated detached house in July. The operative temperatures are presented for the for different roof techniques: ST, CR, CR low-e, GR.
and the natural water content in summer. The total energy demand is reduced by 14%. Slightly worse are the performances of the green roof with effective rainfall or continuously wet, with energy savings between 10% and 11.6%. According to the above considerations, CR registers the worst performance, with a 10% total energy increase. The low emittance cool roof improves the overall energy performance of the building, because of the limited heating penalties and the 38% of the cooling demand reduction. The results of the simulation for Cairo, Fig. 5, show the climate dependence of the south Mediterranean area. Heating demand is low and the best solutions are cooling efficiency driven. CR lead to 40%, while moderated advantages are calculated for the metallic roof High differences are found for the green roof configurations: the wet green roof is the best performing solution, thanks to a
cooling demand similar to the cool roof and a heating demand slightly higher than the conventional roof. This configuration leads to 45% total energy savings. Actual rainfall and dry GR performances practically give the same performance with 13% energy savings respect to ST, with a 10% reduction of the cooling demand.
4.2. Building energy comparison for the different roofing techniques This section presents the results of the different roofing solutions: ST, CR, CR low-e and GR, with the water content determined by the rainfall only. The complete set of results is presented in Table 6, where heating, cooling and total net energy demand are
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Table 7 Cumulative distribution of the number of hours exceeding three reference operative temperatures. Roof system
hours > 26 ◦ C [–] Insulated
ST CR CR-low-e GR
Barcelona 794 366 659 428
ST CR CR-low-e GR
Palermo 1992 1333 1790 1571
ST CR CR-low-e GR
Cairo 3635 3133 3520 3289
hours > 28 ◦ C [–]
hours > 30 ◦ C [–]
hours >26◦ [–] Not insulated
hours >28 ◦ C [–]
hours >30 ◦ C [–]
68 0 26 5
0 0 0 0
931 246 727 495
227 0 60 9
0 0 0 0
759 281 599 428
24 0 0 0
2165 1101 1865 1487
997 177 677 345
215 0 47 0
2393 1530 2156 1713
875 299 654 420
3658 2806 3494 3403
2545 1217 2213 1858
1222 214 806 525
summarised. The relative energy savings compared to the standard roof are also presented for each building configuration. Fig. 6 presents the results of the row house for Barcelona, Palermo and Cairo. Barcelona is the cooler among the three selected cities and this affects the final results. The most efficient solution for the insulated configuration is the metallic cool roof a total energy saving close to 7%; slightly lower savings (6.2%) are calculated for GR. Green roof is the most effective technique for the not insulated configuration, with an energy demand reduction of close to 12%; due to the extra insulation of the soil and vegetation layer. Cool roof are by far the best cooling efficient technique for both configurations, with almost no need of cooling systems for the two configurations. The impact on the energy demand is negative for the two configurations, because of the increase of heating demand due to the reduced solar gains through the roof. CR is the most performing solution for the insulated row house in Palermo, with global savings close to 15%. The CR low-e application gives a 10% energy savings, while limited advantages are obtained with GR. Very close results, around 11% of energy savings, are obtained for the three roof solutions for the not insulated configuration. The energy reduction is all in cooling mode for CR, while metallic cool and green roof reduce both, the heating and the cooling demand. Cairo is cooling dominated and CR gives the best results in summer and in the whole year, with a reduction of total energy demand between 30% and 40% for the two building configurations. CR lowe and GR ensure energy savings between 11% and 18%, except for GR for the insulated configuration, since the extra insulation does not lead to cooling reduction in hot climates. The detached house presents slightly different results compared to the row house and they are shown in Fig. 7. Because of the higher surface exposed to the outdoor environment, this house typology is characterised by higher heating and cooling energy demands. The heating demand is even more predominant in Barcelona. Green roof reduce both, the heating and cooling demand, with annual savings of about 8–10% respect to ST, for the insulated and not insulated configuration. The CR low-e ensures small energy savings on annual basis, around 4% for both configurations. The impact of CR is negative on yearly basis but cooling savings between 60% and 85% are calculated. Different results are obtained for the Palermo and Cairo calculations; the high reflecting roof techniques ensure in all the situations a total net energy saving of 17% and 10% respectively for the insulated and not insulated configurations. GR reduces the heating and the cooling demand with a 10% energy savings for the not insulated configuration, while similar results are obtained by the CR low-e for the insulated configuration. Cairo is characterised by a cooling dominated climate and CR has the best performances
with a 30% reduction of the annual net energy demand. Significant savings are obtained also by CR low-e and GR, the latter only for the not insulated configuration, with annual energy savings between 10% and 15%. It is worth noting that some cross checks demonstrated that the specific results, intended as energy demand normalised to the building square meters, can be applied also to the last floor of multi-storey dwellings with good accuracy. This aspect is of relevance when assessing the energy saving potentials of this building typology, often used in the densely built urban area.
4.3. Thermal comfort in not cooled buildings This section analyse the evolution of thermal comfort conditions in not cooled buildings as a function of the adopted roof solution. The operative temperature is selected as relevant indicator, the most significant to express the indoor thermal comfort. The qualitative impact of the different roofs can be inferred from Fig. 8, reporting the operative temperature profiles of 4 days in July for the not insulated detached house in the 3 selected localities. The most effective solutions is given by CR, while CR low-e and GR improve the indoor conditions with similar thermal profiles. It is worth repeating that with actual rainfall, GR will be dry for a significant part of the cooling period. Table 7 summarises the cumulative distribution of the hours with operative temperatures above 26, 28 and 30 ◦ C. The results are presented as an average of the row and detached houses, in order to present data in a more compact way. The section presents the results of the different roofing solutions: ST, CR, CR low-e and GR, with the water content determined by the rainfall only. The impact of CR is by far the most effective for the improvement of summer thermal comfort. The hours with operative temperature higher than 26 ◦ C in Barcelona are reduced to 26% and 46% of the ST hours, for the insulated and not insulated configurations. The metallic cool roof reduce the number of hours of about 20% for both configurations; while the hours above 26 ◦ C are halved respect to ST when GR is used in both configurations. The number of hours above 28 ◦ C is negligible for CR and GR, while it is strongly reduced with the low emittance cool roof. No hours with operative temperature above 30 ◦ C are calculated. The application of the three advanced roof techniques is significant in Palermo and Cairo, even if these climatic conditions strongly increase the operative temperature levels and the number of discomfort hours. Cool roofs have a noticeable impact in reducing the number of hours with operative temperatures above 26 ◦ C, while
M. Zinzi, S. Agnoli / Energy and Buildings 55 (2012) 66–76
the metallic CR and the green roof cause a reduction lower than 10% respect to ST. The number hours with operative temperature higher than 28 ◦ C are reduced of 73% and 82% respect to ST for CR applications in Palermo; this happens for the insulated and not insulated configurations. The low emittance cool roof reduces of about the 20% the hours above 28 ◦ C, while the reduction increases to about 28% for the green roof. The three roofing systems reduce to negligible numbers the hours above 30 ◦ C. The hours with an operative temperature higher than 28 ◦ C in Cairo are reduced to 48% and 64% respect to ST when applying CR, for the insulated and not insulated configurations. Reductions around 10% and 28% are calculated for respectively CR low-e and GR. The number of hours with operative temperatures higher than 30 ◦ C is significant in Cairo and the passive techniques can improve the indoor comfort conditions. The number of hours is strongly reduced using cool roofs (between 18% and 34% respect to ST), while GR reduce to the half that numbers. Moderate advantage is achieved with the metallic cool roof, with a number of hours reducing between 66% and 75%. To be noted that being the green roof almost dry in Palermo and Cairo, most of the peculiarities of the green roof are not working. The benefits of the evapo-transpiration of the vegetation layer are party lost because of the extra insulation soil layer, which tends keeping the heat stored inside the building.
5. Conclusions This study presented a comparison among different roofing techniques able to reduce the cooling demand of residential buildings while mitigating the urban heat island. The analysis is carried out using a validated tool; hence the results’ acceptance goes along with the model accuracy. Even if energy optimisation strategies of the roof cannot prevent from taking into account the thermal insulation and thermal mass, the results show that the mitigation strategies of the urban heat island, currently planned by the metropolitan area authorities, con positively impact the energy performance of dwellings on annual basis. The upgrade of conventional hot roofing systems has net energy advantages, especially considering the new insulation standards adopted throughout the European Mediterranean countries. Cool roof are very effective for the cooling and (excluding the northern area of the basin) energy savings. Cool roofs are the most effective solutions for the centre and southern areas of the Mediterranean basin. Not insulated house might have excessive increase in heating demand but, on the other side, cool roofs practically may avoid the installation of the cooling systems, because of the very low cooling energy demand. Low emittance cool roofs perform worse than cool roofs, because of the reduced radiative losses at night time, but improve the performance of conventional roofs. For the same reason, metallic cool roofs have also limited heating penalties respect to conventional cool roofs. They might represent an acceptable compromise in the coolest Mediterranean area. Green roofs are very difficult to be modelled and correctly inputted in calculation tools, because of the high number of variables which enter into the heat transfer mechanisms and because of a general lack of information related to the input data required by the adopted model. The study highlighted a first very important issue: green roofs performances strongly depend on the water content of the systems with the adopted model. A well wet green roof has good cooling performance, but relaying on the rainfall does not ensure effective energy performances during the dry Mediterranean hot season, especially in the centre and the south east of the basin. Green roofs improve the heating performances as well, when compared with the conventional roofs. The limited water content
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avoids permanent wet conditions of the soil layer, which actively increase the thermal resistance of the structure. The dryer the roof, the lower is the heating demand. Water management need to be calibrated according to the climate conditions and the main energy use. The variability of green roofs as a function of many variable makes it clear that a definitive comparison among the selected techniques will require in-depth analyses taking into account, besides the energy issue, other important aspects: water management and demand, life cycle analysis and costs, environment impacts on urban comfort and on the urban heat island mitigation. Acknowledgments This work was carried out in the framework of the project Cool Roofs, contract number EIE/07/475/SI2.499428, supported by the Intelligent Energy Europe (IEE) program SAVE 2007. References [1] IEA, World Energy Outlook 2008–2009, International Energy Agency, Geneva, 2009. [2] L. Davì, C. Giampaglia, First medlink report: a crossed look on reports and international statistics about development, gender, freedom, conflicts and mobility in Mediterraneum, www.medlinknet.org, 2007. [3] L. Howard, The climate of London, vol. I–III, Harvey and Dorton, London, 1883. [4] H.E. Landsberg, The Urban Climate, in: International Geographic Series, vol. 28, Academic Press, New York, 1981. [5] H. Takebayashi, M. Moriyama, Surface heat budget on green roof and high reflection roof for mitigation of urban heat island, Building and Environment 42 (8) (2007) 2971–2979. [6] M. Kolokotroni, I. Giannitsaris, R. Watkins, The effect of the London urban heat island on building summer cooling demand and night ventilation strategies, Solar Energy 80 (4) (2006) 383–392. [7] M. Santamouris, Heat island research in Europe — the state of the art, Advances Building Energy Research 1 (2007) 23–150. [8] H. Taha, S.C. Chang, H. Akbari, Meteorological and air quality impacts of heat island mitigation measures in three U.S. Cities’, Lawrence Berkeley National Laboratory Report LBNL- 44222, Berkeley, CA, 2000. [9] M.L. Imhoff, P. Zhang, R.E. Wolfe, L. Bounoua, Remote sensing of the urban heat island effect across biomes in the continental USA, Remote Sensing of Environment 114 (2010) 504–513. [10] J.P. Montavez, A. Rodriguez, J.I. Jimenez, A study of the urban heat island of Granada, International Journal of Climatology 20 (2000) 899–911. [11] H. Akbari, S.L. Rose, H. Taha, Analyzing the land cover of an urban environment using high-resolution orthophotos, Landscape and Urban Planning 63 (2003) 1–14. [12] A. Synnefa, A. Dandou, M. Santamouris, M. Tombrou, N. Soulakellis, On the use of cool materials as a heat island mitigation strategy, Journal of Applied Meteorology and Climatology 47 (2008) 2846–2856. [13] H. Akbari, S. Konopacki, Calculating energy-saving potentials of heat-island reduction strategies, Energy Policy 33 (2005) 721–756. [14] H. Akbari, S. Menon, A. Rosenfeld, Global cooling: increasing world-wide urban albedos to offset CO2 , Climatic Change 94 (2009) 275–286. [15] J. Yang, Q. Yu, P. Gong, Quantifying air pollution removal by green roofs in Chicago, Atmospheric Environment 42 (31) (2008) 7266–7273. [16] W. Ishida, Overview of the world air-conditioning market, Appliances http://www.appliancemagazine.com/editorial.php?article= Magazine.com, 1812&zone=207&first=1, 2007. [17] A. Synnefa, M. Santamouris, H. Akbari, Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions, Energy and Buildings 39 (11) (2007) 1167–1174. [18] H. Akbari, S. Bretz, D. Kurn, H. Hartford, Peak power and cooling energy savings of high albedo roofs, Energy and Buildings 25 (1997) 117–126, H. [19] A. Shariah, B. Shalabi, A. Rousan, B. Tashtoush, Effects of absorptance of external surfaces on heating and cooling loads of residential buildings in Jordan, Energy Conversion and Management 39 (1998) 273–284. [20] C.K. Cheung, R.J. Fuller, M.B. Luther, Energy efficient envelope design for high rise apartments’, Energy and Buildings 37 (1) (2005) 37–48. [21] H. Suehrcke, E.L. Peterson, N. Selby, Effect of roof solar reflectance on the building heat gain in a hot climate, Energy and Buildings 40 (2008) 2224–2235. [22] H. Akbari, R. Levinson, L. Rainer, Monitoring the energy-use effects of cool roofs on California commercial buildings, Energy and Buildings 37 (2005) 1007–1016. [23] M. Zinzi, G. Fasano, Properties and performance of advanced reflective paints to reduce the cooling loads in buildings and mitigate the heat island effect in urban areas, International Journal of Sustainable Energy 28 (1) (2009) 123–139. [24] M. Santamouris, C. Pavlou, P. Doukas, G. Mihalakakou, A. Synnefa, A. Hatzibiros, Investigating and analyzing the energy and environmental performance of an
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[31] D.J Sailor, A green roof model for building energy simulation programs, Energy and Buildings 40 (2008) 1466–1478. [32] DOE, Getting Started with EnergyPlus—Essential Information You Need about Running EnergyPlus, U.S. Department of Energy, 2007. [33] R. Lollini, B. Barozzi, G. Fasano, I. Meroni, M. Zinzi, Optimisation of opaque components of the building envelope. Energy, economic and environmental issues, Building and Environment 40 (2006) 1001–1013. [34] R.U. Halwatura, M.T.R. Jayasinghe, Influence of insulated roof slabs on air conditioned spaces in tropical climatic conditions—a life cycle cost approach, Energy and Buildings 41 (2009) 678–686. [35] A. Hasan, Optimizing insulation thickness for buildings using life cycle cost, Applied Energy 63 (1999) 115–124. [36] N. Sisman, E. Kahya, N. Aras, H. Aras, Determination of optimum insulation thickness of the external walls and roof (ceiling) for Turkey’s different degree day regions, Energy Policy 35 (2007) 5151–5155. [37] M. D’Orazio, C. Di Perna, E. Di Giuseppe, The effects of roof covering on the thermal performance of highly insulated roofs in Mediterranean climates, Energy and Buildings 42 (10) (2010) 1619–1627.
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Cool and green roofs. An energy and comfort comparison between passive cooling and mitigation urban heat island techniques for residential buildings in the Mediterranean region M. Zinzi ∗ , S. Agnoli ENEA Italian National Agency for New Technologies, Energy and Sustainable Economic Development Technical Unit for the Energy Efficiency, Via Anguillarese, 301, 00123 S. Maria di Galeria, Rome, Italy
a r t i c l e
i n f o
Article history: Received 12 May 2011 Received in revised form 29 July 2011 Accepted 12 September 2011 Keywords: Cool materials Green roof Thermal comfort Passive cooling Energy performance
a b s t r a c t The increase of peak and energy demand during the cooling season is becoming a crucial issue, as well as the intensification of the urban heat island effect. This trend is observed at several latitudes, including areas where overheating was unknown at building and urban levels. This phenomenon involves different issues: reduction of greenhouse gases, quality and comfort in outdoor and indoor environment, security of energy supply, public health. The building sector is directly involved in this change and adequate solutions can provide great benefit at energy and environmental levels. Roofs in particular are envelope components for which advanced solutions can provide significant energy savings in cooled buildings or improve indoor thermal conditions in not cooled buildings. Cool materials keep the roof cool under the sun by reflecting the incident solar radiation away from the building and radiating the heat away at night. Roofs covered with vegetation take benefits of the additional thermal insulation provided by the soil and of the evapo-transpiration to keep the roof cool under the sun. These two technologies are different in: structural requirements, initial and lifetime maintenance costs, impact on the overall energy performance of buildings. This paper presents a numerical comparative analysis between these solutions, taking into account the several parameters that affect the final energy performances. By means of dynamic simulations, the paper depicts how cool and green roofs can improve the energy performance of residential buildings in different localities at Mediterranean latitudes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The effects of global warming and climate changes are of relevant concern for environment and human activities in the Mediterranean area. The average air temperature rise of 2 ◦ C represents a critical limit beyond which dangerous climate changes should occur by 2030 [1]. More than 90 million people live in the twenty most populated Mediterranean metropolitan areas; according to the actual trend other 70 million of people are expected to move to leave the countryside towards the urban area by 2025 [2]. The global warming and the urban sprawl causes a number of environmental hazards, the urban heat island (UHI) is one of these. This phenomenon is defined as the air temperature rise in densely built environments respect to the countryside surroundings. The main cause is the modification of the land surface in the urban area, where the vegetation is replaced by exten-
∗ Corresponding author. Tel.: +39 06 3048 6256/4188; fax: +39 06 3048 3930. E-mail address: [email protected] (M. Zinzi). 0378-7788/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2011.09.024
sively built surfaces (typically paved roads and buildings surfaces), characterised by high solar absorption, high impermeability and favourable thermal properties for energy storage and heat release, as well as several anthropogenic. The UHI was first monitored in London back to the 19th century [3]; many studies were performed during the past decades [4–10], showing the quantitative effects of the phenomenon and the correlation with the previously enounced causes. Daily mean UHI typically ranges between 2 and 5 ◦ C, while UHI intensities (defined as maximum difference between urban and background rural temperatures) up to 12 ◦ C were registered under particular conditions. This UHI impacts important issues such as: the quality of life; the public health, especially for the most vulnerable population; the environmental hazards. Roof surfaces of the building accounts for the 20–25% of the total urban surfaces, hence they can successfully used to reduce the air and surface temperature of urban area [11]. Cool and green roof, widely described in the next paragraph, are used to mitigate the UHI and the impact was proved by several studies [12–15]. These techniques can also have significant benefits on the energy performance of buildings, providing passive cooling to the built environment. This topic is of special interest because of the rapid increase of the
M. Zinzi, S. Agnoli / Energy and Buildings 55 (2012) 66–76
energy consumption and peak demand for cooling in Europe and in the Mediterranean basin [16]. This study aims at analysing and comparing how these UHI mitigation techniques can also improve the energy performance of cooled buildings and the thermal comfort of not-cooled buildings at Mediterranean latitudes. Special attention is paid to residential buildings, where accurate design concepts and technologies can strongly reduce the use and the installation of cooling systems without affecting the occupants comfort expectations and help achieving international environment targets. 2. Building applications of cool and green roofs Cool roofs are characterised by materials having: high solar reflectance (SR) and high thermal emittance (TE). The former expresses the ability of the materials of reflecting most of the incident solar radiation during daytime, keeping their surfaces cooler respect to conventional construction materials. The high thermal emittance allows the materials to radiate away the heat stored in the structure, mainly during night time. This thermal behaviour allows the roof to reduce the heat transfer to the built environment. Roofs characterised by low emittance values tend to not dissipate the stored heat at night and can be considered cool only if they have a very high solar reflectance. White mortars and plaster were widely used in ancient massive Mediterranean dwellings, in order to create a more comfortable built environment during the hot season. The coastal villages of Greece, Italy and Spain still witness this construction technique, which emerged again as an efficient solution during the recent years. Several numerical studies were carried out in the past years to assess the energy performance of buildings equipped with cool roofs. The impact of cool roofs on a single floor detached house placed in different climatic zones of the planet world was calculated for insulated and not insulated dwellings [17]. The cooling energy consumption reduction was 18% and 93% increasing the roof solar reflectance from 20% to 85%. Three typical building models were developed respectively for: a residential building, an office and retails store, differentiated by age (before and after 1980). The impact of cool roof ensured global energy savings from 7% to 25% according to the different age and building type for several US climates [18]. Other studies were focused to limited geographical sites, as Jordan or Honk Kong [19,20]. Other studies faced the cool roof positive impact evaluated as an additional thermal insulation [21]. The results of the analysis revealed that the integrated daily roof heat gain was not dependent on its thermal mass. An energy analysis run proved that the daily heat flow in a roof with SR of 0.65 and a thermal resistance (R-value) of 1.1 m2 K/W was equivalent to the flow in a roof with SR 0.3 and R-value 2.2 m2 K/W. Limited data from real building application are available. A field campaign was carried out in one house and two school bungalows
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in Sacramento, California. Cooling energy savings of 2.2 kWh/day were measured increasing the solar reflectance of the roof from 0.18 to 0.73 [22]. The energy savings in the school buildings was about 35%. A study in an experimental building in Rome, Italy, proved that the air temperature of an attic room decreased by 2 ◦ C increasing the albedo of the roof from 14% to 85% and this room was found cooler than an identical room at the floor below, which had no roof at all [23]. Green roofs, also called eco-roofs, use the foliage of plants to protect the building environment. The thermal loads due to the solar radiation and the air temperature are limited before entering the buildings by the vegetation layer. This depends on the absorption of the solar radiation by the plants to support their life-cycle, including: photosynthesis, evapo-transpiration, respiration. Moreover, the soil layer gives an added insulation to the building roof and the water content increases the thermal inertia of the structure. The vegetation characteristics affect, in addiction, the convective and radiative heat transfer through the roof surface. Green roofs were once typically used in northern climates to improve the insulation performance of the building envelope, but they are also an opportunity in warm climates, because of their thermal behaviour under the solar radiation. Several studies were produced during the past years trying to quantify the effect of green roofs on the energy performance of buildings. The noticeable impact of green roofs during the hot and the cold seasons was analysed in a nursery school in Athens, founding out that energy savings up to 49% could be obtained [24]. The energy and water issues were analysed in two experimental setups in Italy; particular attention was paid to the impact of the foliage on the radiation and the air temperature profiles insisting on the building roof, respect to the undisturbed values [25]. Combined measurements and calculation analyses were performed in order to assess and predict the 60% reduction of the heat flux through a green roof respect to a conventional roof in a hospital building in northern Italy [26]. A case study in Brasil demonstrated that a green roof in an experimental building reduced the heat flux by 92–97% compared to a ceramic and a metallic conventional roof [27]. Specific studies on the substrate materials, foliage characteristics and vegetal species demonstrated the variability of the green roof performances as a function of the adopted technical solutions [28–30]. 3. Methodology The scope of this work consists in the assessment of the energy performances of residential buildings using different roof solutions: standard, cool and green roofs. The study is focused on the Mediterranean area, a mild climatic zone with differences in rainfall levels and air temperature profiles that can lead to different choices of building technologies to achieve the opti-
Table 1 Air temperature and solar radiation data of the selected localities. Month
T (◦ C) Barcelona
H (kJ/h/m2 )
RH (%)
T (◦ C) Palermo
H (kJ/h/m2 )
RH (%)
T (◦ C) Cairo
H (kJ/h/m2 )
RH (%)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
8.2 9.4 11.1 13.1 17.0 20.9 23.5 24.1 21.6 17.3 12.1 9.9
288 409 553 744 879 873 1004 858 603 444 287 250
71 68 73 72 74 74 68 71 74 82 78 65
12.7 11.9 13.8 15.7 19.2 22.8 25.5 27.0 24.1 21.6 17.2 13.9
312 454 625 843 980 1090 1099 993 741 549 319 272
76 71 79 71 77 71 76 73 66 74 69 78
14.0 14.5 16.6 21.8 24.7 28.0 28.2 27.9 26.6 23.8 19.0 15.3
439 597 736 913 1052 1142 1118 1008 898 630 493 430
67 58 59 45 40 45 56 60 56 56 61 64
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M. Zinzi, S. Agnoli / Energy and Buildings 55 (2012) 66–76 Table 3 Thermal properties of dwellings envelope components.
Table 2 Rainfall data of the selected localities. Month
Barcelona rainfall (mm)
Palermo rainfall (mm)
Cairo rainfall (mm)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
39.50 40.60 47.36 47.36 53.27 44.10 30.00 51.54 69.26 93.45 61.89 46.10
67.60 66.30 59.70 43.50 26.00 14.40 7.80 12.96 40.78 94.54 92.00 78.63
6.94 4.00 4.00 2.06 0.00 0.00 0.00 0.00 0.00 1.05 2.97 4.94
mal energy performances. The comparison is carried out by means of a numerical analysis performed using the Design Builder interface, which relies on the Energy Plus calculation engine, able to perform energy balance calculations with hourly time step. The tool carries out accurate thermal analyses and allows very detailed inputs, including: climatic data (including air temperature, solar radiation, relative humidity hourly profiles); construction materials and components in dedicated libraries or manually edited; energy systems’ specifications; time schedules (systems’ management, occupancy, electric lighting, ventilation, etc.). Energy Plus also features a validated mono-dimensional green roof model developed taking into account the evapotranspiration of the vegetation layer, the time dependent soil thermal properties (conduction and inertia), the radiative and convective heat exchanges [31,32]. The tool also allows a complete description of any construction material, since thermal conductance, solar reflectance and thermal emissivity can be modelled. In order to depict a wider overview of these techniques impact on the energy performances a number of variables are taken into account. A number of building variants are, hence, defined and the cooling and heating demands are calculated for each of them. The
Envelope component
Not-ins U (W/m2 K)
Ins U (W/m2 K)
Wall Roof Ground floor Window glass Window frame
1.4 1.4 1.7 2.8 5.9
0.7 0.6 0.8 1.8 4.7
comparison is carried out for the net energy, without considering the energy systems efficiencies, since the main objective is the optimisation of the envelope energy performances. The variants considered in the analysis are described in the next paragraphs.
3.1. Definition of the reference localities Three localities were selected, typical of different regional areas: Barcelona for the north rim, Cairo for the south rim and Palermo for the centre basin. This criterion also responds to a strict relationship between buildings and climate. The north rim is heating dominated for not insulated buildings, while heating and cooling are both relevant in buildings in the centre of the Mediterranean region. The south rim is cooling dominated for any building thermal characteristics. Investigating these three localities, allowed mapping with a good accuracy the roofing techniques’ efficiency according to the relevant Mediterranean climatic conditions. The climatic conditions of these three areas referred to air temperature and relative humidity, as well as the global solar horizontal radiation. These data are taken by the Meteonorm database, embedded in Design Builder. Table 1 reports the monthly daily mean air temperature and the solar radiation, expressed as global irradiance on the horizontal. The difference within the three localities can be easily inferred in this case, since air temperatures and solar radiation decreased with latitude. The relative humidity is also reported in Table 1, these parameters are important because of the impact on the energy use of the building and the comfort condi-
Fig. 1. Layout of the row house with the two thermal zones: ground level day zone and first floor night zone.
M. Zinzi, S. Agnoli / Energy and Buildings 55 (2012) 66–76
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Table 5 Characteristics of the vegetation and soil layers of the selected green roofs.
Fig. 2. Layout of the single floor detached house with the two thermal zones: day (right) and night zone.
tions of occupant, even if it does not show significant relationship with the latitude. Table 2 reports the monthly rainfall data expressed in millimetres as taken by the Meteonorm database. Rainfall is not embedded into the software; hence the data was inputted through an external schedule. The table depicts the different water availability in the three cities, critical parameter for green roofs applications. Cairo is practically dry through all the year. Palermo and Barcelona have different trends: the former has rainfall practically constant throughout the year, while Palermo has higher rainfall in winter but gets almost dry during summer months. 3.2. The reference buildings The analysis is carried out on two residential building typologies widely used in the Mediterranean region: row houses and detached single family houses. The buildings are considered with two different envelope configurations: insulated and not insulated, in order to have significant results for old and recent constructed buildings. The first is a two floors row house, whose geometry is presented in Fig. 1. The building is divided into two thermal zones: the day zone at the ground level, where almost all the daytime activities take place; the night zone at the upper floor, hosting bedrooms and services. The houses is in contact with the external environment through the roof, the ground floor and the north and south fac¸ades, while the east and west walls are adiabatic, assuming they are boundary layers between adjacent row houses. The main thermo-physical data are summarised in Table 3. The not insulated configuration considers a conventional double glazing unit, whose g-value is 0.75, this value decreases to 0.65 for the Table 4 Geometry of the selected buildings.
V – volume A – net gross area S – total external surface Sr – roof surface S/V Sr /V Sr /S
Unit
Row house
Detached house
m3 m2 m2 m2 m−1 m−1 –
427 116 211 68 0.50 0.16 0.32
369 100 364 112 0.98 0.30 0.30
Parameter
Value
Height of plant Leaf area index Leaf solar reflectance Leaf emissivity Minimum stomatal resistance Max volumetric moisture content of the soil Min volumetric moisture content of the soil Initial volumetric moisture content of the soil Density of the soil Specific heat of the soil Conductivity Soil layer thickness
<60 cm 1.2 0.25 0.9 120 m/s 0.32 0.01 0.15 960 kg/m3 1500 J/kgK 0.34 W/mK 0.12 m
insulated configuration, where a low-e double glazing units is considered. The windows are provided of an external shading device (30% solar transmittance and 10% solar absortpance). Assumptions are also made regarding the occupancy profile, the electric and the appliances loads. The natural ventilation/infiltration was fixed at 0.5 ACH. Since the net energy has to be calculated, the following hypotheses are assumed: the set-point temperature is continuously maintained constant and heating and cooling system generators of unlimited power are implemented to keep this condition. The air exchange rate is increased to 3 volumes per hour for the free floating analysis, that is carried out to assess the impact of the roof solutions on the indoor thermal comfort of not cooled buildings. The detached single family house consists of a single floor building and it is divided in two thermal zones: day (mainly facing east) and night (mainly facing west). The same thermo-physical and operational data of the row house are applied to this building. A schematic layout is presented in Fig. 2. These typologies were selected because they also define different thermal behaviours related to the building geometry, as inferred from Table 4. Both houses have similar roof surfaces compared to the total surface area, but the roof surface of the row house is about half of the detached house. These configurations imply a higher roof solar gain ratio in the detached house. Conversely this house has a wide open geometry (S/V ratio is 0.95 m−1 ) with consequent high thermal losses whenever the indoor air temperature is higher than outdoor and vice versa. The row house has a more compact geometry, hence tends to maintain the heat in the inside. These differences imply different cooling and heating loads profiles during the year. 3.3. The roof technologies Several studies proved the impact of the thermal insulation and the thermal mass on the heat balance of the roof and, more in general, of the building envelope [33–37]. Even if these properties affect the energy performance of buildings during the cooling and heating season, this study is, conversely, focused on assessing the energy performance of residential buildings when existing flat roofs are equipped with UHI mitigation techniques. It is worth noting that both green roofs and cool roofs can be implemented using different solutions, corresponding to different thermo-physical and biological parameters. In order to consider a limited number of variables, the selected roofs were defined according to good quality standards related to green and cool roofs. The following describes the selected roof solutions: ST Conventional roof – standard product with thermal insulation defined in Table 3; the external layer has solar reflectance 0.25, typical for most construction materials, and thermal emittance 0.9.
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Fig. 3. Not-insulated row house energy performance for different roof solutions in Palermo.
CR White cool roof – same layers of the previous roof finished with an elastomeric white coating having solar reflectance 0.8 and thermal emittance 0.9. CR low-e Metallic reflective coating – as above with a metallic layer with solar reflectance 0.65 and thermal emittance 0.4. GR Green roof, whose main properties are summarised in Table 5. The structure of the green roof is more complex. The adopted solution is defined according to typical values of the green roof design in Mediterranean area. It is important reminding that the green roof model implemented in Energy Plus, is mono-dimensional add based on several assumptions. To be noted that the roof insulation thickness is lower than conventional roof insulation, to take into account the insulation effect of the green roof. It is also noted that, according to typical green roof systems, the green area correspond to 80% of the total roof surface, the remaining is surface dedicated to footpaths, here assumed to be made of concrete.
4. Results Results refer to the set of simulations carried out including the variables above defined. A first set of calculation was made to evaluate the performance of the green roof under different moisture conditions. Following the energy simulation results are presented, considering the green roof performances under effective rainfall. The results presented the demand for heating, cooling and energy, the latter expressed as simple sum of the two energy uses. The calculations refer to the net energy demand, without considering the energy systems efficiencies, in order to focus on the envelope behaviour. The same simulations are presented in free floating conditions, in order to evaluate the improvements of comfort conditions using cool and green roofs.
Fig. 4. Not-insulated row house energy performance for different roof solutions in Barcelona.
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Fig. 5. Not-insulated row house energy performance for different roof solutions in Cairo.
4.1. Impact of the green roof water content on the energy comparison Conventional and cool roofs are static elements, whose performances do not change according to the climatic conditions. Green roofs are different: climatic conditions affect the water content and, as a consequence, the cooling and heating performances. This first set of simulations refers to the not insulated
dwellings only. Figs. 3–5 report the results for the three localities and include the heating and cooling net demand for six different configurations: ST, CR, CR low-e, GR with actual rainfall, GR dry, GR wet. The latter condition implies that the roof is continually watered and the soil reached the maximum humidity content. This condition can be reached only under the hypothesis of a well designed irrigation system, with additional costs and resources’ uses that are not part of this investigation. The bars in the figure
Table 6 Calculation results: heating, cooling and total energy demand, energy savings compared to the standard roof (the first column includes: locality – insulation level – roof technique). Cooling (kWh/m2 /y)
Energy (kWh/m2 /y)
Sav. to ST (%)
Heating (kWh/m2 /y) Detached house
Cooling (kWh/m2 /y)
Energy (kWh/m2 /y)
Sav. to ST (%)
City and roof technique
Heating (kWh/m2 /y) Row house
Bar-ins-ST Bar-ins-CR Bar-ins-CR low-e Bar-ins-GR Bar-not ins-ST Bar-not ins-CR Bar-not ins-CR low-e Bar-not ins-GR Pal-ins-ST Pal-ins-CR Pal-ins-CR low-e Pal-ins-GR Pal-not ins-ST Pal-not ins-CR Pal-not ins-CR low-e Pal-not ins-GR Cai-ins-ST Cai-ins-CR Cai-ins-CR low-e Cai-ins-GR Cai-not ins-ST Cai-not ins-CR Cai-not ins-CR low-e Cai-not ins-GR
−23.2 −27.3 −22.3
4.2 1.5 3.2
27.4 28.9 25.5
0.0 −5.3 6.8
−33.2 −40.6 −33.3
8.4 3.2 6.5
41.6 43.8 39.8
0.0 −5.4 4.3
−21.3 −44.1 −54.2 −44.3
4.4 6.0 0.9 3.7
25.7 50.1 55.1 48.0
6.2 0.0 −10.0 4.2
−32.5 −71.9 −89.6 −71.9
5.9 7.9 1.1 4.8
38.3 79.8 90.7 76.7
7.8 0.0 −13.7 3.8
−38.9 −8.2 −10.9 −8.4
5.4 10.1 4.7 8.2
44.3 18.3 15.6 16.6
11.6 0.0 14.9 9.1
−68.5 −11.4 −16.0 −11.7
3.6 19.2 9.5 16.0
72.1 30.6 25.5 27.6
9.6 0.0 16.7 9.7
−7.0 −18.6 −25.5 −19.1
10.7 14.4 3.6 10.1
17.7 32.9 29.1 29.2
3.2 0.0 11.7 11.4
−11.4 −30.4 −42.6 −31.1
18.0 20.7 5.8 14.8
29.4 51.1 48.4 45.9
3.8 0.0 5.3 10.2
−15.6 −3.2 −4.9 −3.3
13.8 19.7 10.7 17.0
29.3 23.0 15.6 20.3
10.8 0.0 32.2 11.5
−29.9 −4.1 −6.9 −4.2
16.8 37.0 22.2 32.4
46.6 41.1 29.1 36.6
8.9 0.0 29.0 10.9
−2.6 −8.4 −13.3 −8.6
19.9 27.7 7.7 21.6
22.5 36.1 21.0 30.2
2.0 0.0 41.7 16.4
−3.2 −13.0 −21.8 −13.1
36.7 44.2 18.2 35.5
39.9 57.2 40.0 48.5
2.8 0.0 30.1 15.1
−6.6
24.8
31.4
13.0
−10.0
39.2
49.2
13.9
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Fig. 6. Heating, cooling and total energy demand of the row house. The different building configurations, specified on the x-axis as a function of: locality – insulation level – roof technique.
represent the cooling, heating and total net energy demand. This set of calculation is performed in order to show the impact of water content on the energy performance of buildings and to stress the importance of irrigation strategies in order to optimise the GR performance respect to other static cool techniques. The results are expressed as percentage reduction respect to the standard roof performances. The results obtained for Palermo, Fig. 3, show that the best performances are obtained with the green roof always wet, with heating demand comparable with the conventional roof, but with the cooling demand reduced by more than the half. The total energy savings are 24% with this configuration. CR reduces the cooling demand by 75%, but the increase of the heating demand lowers
the total energy savings close to 12%. The metallic cool roof produces global energy savings closer to CR, due to heating performances similar to ST and improved cooling performances. The green roof has similar performances in dry or actual rainfall conditions. Energy savings are around 11% and are obtained in winter, thanks to the higher insulation level of the roof, but low improvements are reached in the cooling season, because of the limited advantage of the dry vegetation layer. Barcelona has a cooler climate and this impact the performances of the different roofing systems in a different way respect to Palermo; see results in Fig. 4. The not insulated envelope induces a high heating demand. The best result is obtained by the dry GR, because of the insulation effect produced by the soil layer in winter
Fig. 7. Heating, cooling and total energy demand of the detached house. The different building configurations, specified on the x-axis as a function of: locality – insulation level – roof technique.
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Fig. 8. Operative and ambient air temperature profiles in the not insulated detached house in July. The operative temperatures are presented for the for different roof techniques: ST, CR, CR low-e, GR.
and the natural water content in summer. The total energy demand is reduced by 14%. Slightly worse are the performances of the green roof with effective rainfall or continuously wet, with energy savings between 10% and 11.6%. According to the above considerations, CR registers the worst performance, with a 10% total energy increase. The low emittance cool roof improves the overall energy performance of the building, because of the limited heating penalties and the 38% of the cooling demand reduction. The results of the simulation for Cairo, Fig. 5, show the climate dependence of the south Mediterranean area. Heating demand is low and the best solutions are cooling efficiency driven. CR lead to 40%, while moderated advantages are calculated for the metallic roof High differences are found for the green roof configurations: the wet green roof is the best performing solution, thanks to a
cooling demand similar to the cool roof and a heating demand slightly higher than the conventional roof. This configuration leads to 45% total energy savings. Actual rainfall and dry GR performances practically give the same performance with 13% energy savings respect to ST, with a 10% reduction of the cooling demand.
4.2. Building energy comparison for the different roofing techniques This section presents the results of the different roofing solutions: ST, CR, CR low-e and GR, with the water content determined by the rainfall only. The complete set of results is presented in Table 6, where heating, cooling and total net energy demand are
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Table 7 Cumulative distribution of the number of hours exceeding three reference operative temperatures. Roof system
hours > 26 ◦ C [–] Insulated
ST CR CR-low-e GR
Barcelona 794 366 659 428
ST CR CR-low-e GR
Palermo 1992 1333 1790 1571
ST CR CR-low-e GR
Cairo 3635 3133 3520 3289
hours > 28 ◦ C [–]
hours > 30 ◦ C [–]
hours >26◦ [–] Not insulated
hours >28 ◦ C [–]
hours >30 ◦ C [–]
68 0 26 5
0 0 0 0
931 246 727 495
227 0 60 9
0 0 0 0
759 281 599 428
24 0 0 0
2165 1101 1865 1487
997 177 677 345
215 0 47 0
2393 1530 2156 1713
875 299 654 420
3658 2806 3494 3403
2545 1217 2213 1858
1222 214 806 525
summarised. The relative energy savings compared to the standard roof are also presented for each building configuration. Fig. 6 presents the results of the row house for Barcelona, Palermo and Cairo. Barcelona is the cooler among the three selected cities and this affects the final results. The most efficient solution for the insulated configuration is the metallic cool roof a total energy saving close to 7%; slightly lower savings (6.2%) are calculated for GR. Green roof is the most effective technique for the not insulated configuration, with an energy demand reduction of close to 12%; due to the extra insulation of the soil and vegetation layer. Cool roof are by far the best cooling efficient technique for both configurations, with almost no need of cooling systems for the two configurations. The impact on the energy demand is negative for the two configurations, because of the increase of heating demand due to the reduced solar gains through the roof. CR is the most performing solution for the insulated row house in Palermo, with global savings close to 15%. The CR low-e application gives a 10% energy savings, while limited advantages are obtained with GR. Very close results, around 11% of energy savings, are obtained for the three roof solutions for the not insulated configuration. The energy reduction is all in cooling mode for CR, while metallic cool and green roof reduce both, the heating and the cooling demand. Cairo is cooling dominated and CR gives the best results in summer and in the whole year, with a reduction of total energy demand between 30% and 40% for the two building configurations. CR lowe and GR ensure energy savings between 11% and 18%, except for GR for the insulated configuration, since the extra insulation does not lead to cooling reduction in hot climates. The detached house presents slightly different results compared to the row house and they are shown in Fig. 7. Because of the higher surface exposed to the outdoor environment, this house typology is characterised by higher heating and cooling energy demands. The heating demand is even more predominant in Barcelona. Green roof reduce both, the heating and cooling demand, with annual savings of about 8–10% respect to ST, for the insulated and not insulated configuration. The CR low-e ensures small energy savings on annual basis, around 4% for both configurations. The impact of CR is negative on yearly basis but cooling savings between 60% and 85% are calculated. Different results are obtained for the Palermo and Cairo calculations; the high reflecting roof techniques ensure in all the situations a total net energy saving of 17% and 10% respectively for the insulated and not insulated configurations. GR reduces the heating and the cooling demand with a 10% energy savings for the not insulated configuration, while similar results are obtained by the CR low-e for the insulated configuration. Cairo is characterised by a cooling dominated climate and CR has the best performances
with a 30% reduction of the annual net energy demand. Significant savings are obtained also by CR low-e and GR, the latter only for the not insulated configuration, with annual energy savings between 10% and 15%. It is worth noting that some cross checks demonstrated that the specific results, intended as energy demand normalised to the building square meters, can be applied also to the last floor of multi-storey dwellings with good accuracy. This aspect is of relevance when assessing the energy saving potentials of this building typology, often used in the densely built urban area.
4.3. Thermal comfort in not cooled buildings This section analyse the evolution of thermal comfort conditions in not cooled buildings as a function of the adopted roof solution. The operative temperature is selected as relevant indicator, the most significant to express the indoor thermal comfort. The qualitative impact of the different roofs can be inferred from Fig. 8, reporting the operative temperature profiles of 4 days in July for the not insulated detached house in the 3 selected localities. The most effective solutions is given by CR, while CR low-e and GR improve the indoor conditions with similar thermal profiles. It is worth repeating that with actual rainfall, GR will be dry for a significant part of the cooling period. Table 7 summarises the cumulative distribution of the hours with operative temperatures above 26, 28 and 30 ◦ C. The results are presented as an average of the row and detached houses, in order to present data in a more compact way. The section presents the results of the different roofing solutions: ST, CR, CR low-e and GR, with the water content determined by the rainfall only. The impact of CR is by far the most effective for the improvement of summer thermal comfort. The hours with operative temperature higher than 26 ◦ C in Barcelona are reduced to 26% and 46% of the ST hours, for the insulated and not insulated configurations. The metallic cool roof reduce the number of hours of about 20% for both configurations; while the hours above 26 ◦ C are halved respect to ST when GR is used in both configurations. The number of hours above 28 ◦ C is negligible for CR and GR, while it is strongly reduced with the low emittance cool roof. No hours with operative temperature above 30 ◦ C are calculated. The application of the three advanced roof techniques is significant in Palermo and Cairo, even if these climatic conditions strongly increase the operative temperature levels and the number of discomfort hours. Cool roofs have a noticeable impact in reducing the number of hours with operative temperatures above 26 ◦ C, while
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the metallic CR and the green roof cause a reduction lower than 10% respect to ST. The number hours with operative temperature higher than 28 ◦ C are reduced of 73% and 82% respect to ST for CR applications in Palermo; this happens for the insulated and not insulated configurations. The low emittance cool roof reduces of about the 20% the hours above 28 ◦ C, while the reduction increases to about 28% for the green roof. The three roofing systems reduce to negligible numbers the hours above 30 ◦ C. The hours with an operative temperature higher than 28 ◦ C in Cairo are reduced to 48% and 64% respect to ST when applying CR, for the insulated and not insulated configurations. Reductions around 10% and 28% are calculated for respectively CR low-e and GR. The number of hours with operative temperatures higher than 30 ◦ C is significant in Cairo and the passive techniques can improve the indoor comfort conditions. The number of hours is strongly reduced using cool roofs (between 18% and 34% respect to ST), while GR reduce to the half that numbers. Moderate advantage is achieved with the metallic cool roof, with a number of hours reducing between 66% and 75%. To be noted that being the green roof almost dry in Palermo and Cairo, most of the peculiarities of the green roof are not working. The benefits of the evapo-transpiration of the vegetation layer are party lost because of the extra insulation soil layer, which tends keeping the heat stored inside the building.
5. Conclusions This study presented a comparison among different roofing techniques able to reduce the cooling demand of residential buildings while mitigating the urban heat island. The analysis is carried out using a validated tool; hence the results’ acceptance goes along with the model accuracy. Even if energy optimisation strategies of the roof cannot prevent from taking into account the thermal insulation and thermal mass, the results show that the mitigation strategies of the urban heat island, currently planned by the metropolitan area authorities, con positively impact the energy performance of dwellings on annual basis. The upgrade of conventional hot roofing systems has net energy advantages, especially considering the new insulation standards adopted throughout the European Mediterranean countries. Cool roof are very effective for the cooling and (excluding the northern area of the basin) energy savings. Cool roofs are the most effective solutions for the centre and southern areas of the Mediterranean basin. Not insulated house might have excessive increase in heating demand but, on the other side, cool roofs practically may avoid the installation of the cooling systems, because of the very low cooling energy demand. Low emittance cool roofs perform worse than cool roofs, because of the reduced radiative losses at night time, but improve the performance of conventional roofs. For the same reason, metallic cool roofs have also limited heating penalties respect to conventional cool roofs. They might represent an acceptable compromise in the coolest Mediterranean area. Green roofs are very difficult to be modelled and correctly inputted in calculation tools, because of the high number of variables which enter into the heat transfer mechanisms and because of a general lack of information related to the input data required by the adopted model. The study highlighted a first very important issue: green roofs performances strongly depend on the water content of the systems with the adopted model. A well wet green roof has good cooling performance, but relaying on the rainfall does not ensure effective energy performances during the dry Mediterranean hot season, especially in the centre and the south east of the basin. Green roofs improve the heating performances as well, when compared with the conventional roofs. The limited water content
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avoids permanent wet conditions of the soil layer, which actively increase the thermal resistance of the structure. The dryer the roof, the lower is the heating demand. Water management need to be calibrated according to the climate conditions and the main energy use. The variability of green roofs as a function of many variable makes it clear that a definitive comparison among the selected techniques will require in-depth analyses taking into account, besides the energy issue, other important aspects: water management and demand, life cycle analysis and costs, environment impacts on urban comfort and on the urban heat island mitigation. Acknowledgments This work was carried out in the framework of the project Cool Roofs, contract number EIE/07/475/SI2.499428, supported by the Intelligent Energy Europe (IEE) program SAVE 2007. References [1] IEA, World Energy Outlook 2008–2009, International Energy Agency, Geneva, 2009. [2] L. Davì, C. Giampaglia, First medlink report: a crossed look on reports and international statistics about development, gender, freedom, conflicts and mobility in Mediterraneum, www.medlinknet.org, 2007. [3] L. Howard, The climate of London, vol. I–III, Harvey and Dorton, London, 1883. [4] H.E. Landsberg, The Urban Climate, in: International Geographic Series, vol. 28, Academic Press, New York, 1981. [5] H. Takebayashi, M. Moriyama, Surface heat budget on green roof and high reflection roof for mitigation of urban heat island, Building and Environment 42 (8) (2007) 2971–2979. [6] M. Kolokotroni, I. Giannitsaris, R. Watkins, The effect of the London urban heat island on building summer cooling demand and night ventilation strategies, Solar Energy 80 (4) (2006) 383–392. [7] M. Santamouris, Heat island research in Europe — the state of the art, Advances Building Energy Research 1 (2007) 23–150. [8] H. Taha, S.C. Chang, H. Akbari, Meteorological and air quality impacts of heat island mitigation measures in three U.S. Cities’, Lawrence Berkeley National Laboratory Report LBNL- 44222, Berkeley, CA, 2000. [9] M.L. Imhoff, P. Zhang, R.E. Wolfe, L. Bounoua, Remote sensing of the urban heat island effect across biomes in the continental USA, Remote Sensing of Environment 114 (2010) 504–513. [10] J.P. Montavez, A. Rodriguez, J.I. Jimenez, A study of the urban heat island of Granada, International Journal of Climatology 20 (2000) 899–911. [11] H. Akbari, S.L. Rose, H. Taha, Analyzing the land cover of an urban environment using high-resolution orthophotos, Landscape and Urban Planning 63 (2003) 1–14. [12] A. Synnefa, A. Dandou, M. Santamouris, M. Tombrou, N. Soulakellis, On the use of cool materials as a heat island mitigation strategy, Journal of Applied Meteorology and Climatology 47 (2008) 2846–2856. [13] H. Akbari, S. Konopacki, Calculating energy-saving potentials of heat-island reduction strategies, Energy Policy 33 (2005) 721–756. [14] H. Akbari, S. Menon, A. Rosenfeld, Global cooling: increasing world-wide urban albedos to offset CO2 , Climatic Change 94 (2009) 275–286. [15] J. Yang, Q. Yu, P. Gong, Quantifying air pollution removal by green roofs in Chicago, Atmospheric Environment 42 (31) (2008) 7266–7273. [16] W. Ishida, Overview of the world air-conditioning market, Appliances http://www.appliancemagazine.com/editorial.php?article= Magazine.com, 1812&zone=207&first=1, 2007. [17] A. Synnefa, M. Santamouris, H. Akbari, Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions, Energy and Buildings 39 (11) (2007) 1167–1174. [18] H. Akbari, S. Bretz, D. Kurn, H. Hartford, Peak power and cooling energy savings of high albedo roofs, Energy and Buildings 25 (1997) 117–126, H. [19] A. Shariah, B. Shalabi, A. Rousan, B. Tashtoush, Effects of absorptance of external surfaces on heating and cooling loads of residential buildings in Jordan, Energy Conversion and Management 39 (1998) 273–284. [20] C.K. Cheung, R.J. Fuller, M.B. Luther, Energy efficient envelope design for high rise apartments’, Energy and Buildings 37 (1) (2005) 37–48. [21] H. Suehrcke, E.L. Peterson, N. Selby, Effect of roof solar reflectance on the building heat gain in a hot climate, Energy and Buildings 40 (2008) 2224–2235. [22] H. Akbari, R. Levinson, L. Rainer, Monitoring the energy-use effects of cool roofs on California commercial buildings, Energy and Buildings 37 (2005) 1007–1016. [23] M. Zinzi, G. Fasano, Properties and performance of advanced reflective paints to reduce the cooling loads in buildings and mitigate the heat island effect in urban areas, International Journal of Sustainable Energy 28 (1) (2009) 123–139. [24] M. Santamouris, C. Pavlou, P. Doukas, G. Mihalakakou, A. Synnefa, A. Hatzibiros, Investigating and analyzing the energy and environmental performance of an
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