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Cool roofs for passive cooling:performance in different climates andfor different insulation levels in ItalyVincenzo Costanzoa, Gianpiero Evolaa & Luigi Marlettaa

a Department of Industrial Engineering, University of Catania,Viale Andrea Doria 6, Catania 95125, ItalyPublished online: 20 Dec 2013.

To cite this article: Vincenzo Costanzo, Gianpiero Evola & Luigi Marletta (2013) Cool roofs forpassive cooling: performance in different climates and for different insulation levels in Italy,Advances in Building Energy Research, 7:2, 155-169, DOI: 10.1080/17512549.2013.865556

To link to this article: http://dx.doi.org/10.1080/17512549.2013.865556

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Cool roofs for passive cooling: performance in different climates and fordifferent insulation levels in Italy

Vincenzo Costanzo*, Gianpiero Evola and Luigi Marletta

Department of Industrial Engineering, University of Catania, Viale Andrea Doria 6, Catania 95125, Italy

Roofs are severely hit by solar radiation in summer; hence the use of cool materials on thefinishing layer provides a significant reduction in the heat flow entering the building, withsensible attenuation in the building cooling load. In this paper, a case study is presented,based on the dynamic simulation of an existing office building in Catania (southern Italy).Here, a part of the roof has been recently treated with a commercial cool paint, with the aimof improving thermal comfort in summer. Hence, the simulations represent a preliminarystudy that will allow assessing the expected effectiveness of the intervention. More in detail,the results of the simulations will be discussed in terms of both thermal comfort and energysavings, through the evaluation of parameters such as the roof surface temperature, theoperative temperature and the cooling load for both conditions, i.e. with and without thecool paint. The paper also discusses the potential increase in the energy needs for winterheating, and looks at the overall annual balance in terms of primary energy; this is made byconsidering different climatic conditions and envelope characteristics. These aspects areusually not well highlighted in the current scientific literature.

Keywords: cool roofs; thermal comfort; building energy performance; primary energyconsumption; design suggestions

1. Introduction

The roof surface represents about 20–25% of urban surfaces and 60–70% of the building envelopeon average in Italy, depending on the building typology; thus, it plays a very important role in theenergy balance of buildings, and it is important to find appropriate solutions to improve its energyperformance, also in relation to the specific climate.

In this context, cool materials represent an efficient way to cope with both the increase inenergy consumption in summer and the urban heat island effect, especially in southern regionswith mild Mediterranean climate. As an example, Boixo, Diaz-Vicente, Colmenar, and Castro(2012) have estimated a potential energy saving of 295,000 kWh per year in the whole regionof Andalusia, only considering residential buildings with flat roofs using electric heating.

In general, cool materials are characterized by high values of solar reflectance (r > 0.6), whichstrongly reduces the amount of solar radiation absorbed by the roof outer layer, and by high infra-red emissivity values (ɛ > 0.8), which contributes to dissipate the heat accumulated during the daythrough an intensive radiant heat exchange at night (Kültür & Türkeri, 2012). This enhancedcapacity of the roof in lowering the heat flux penetrating inside the building has been investigatedby De Brito Filho, Henriquez, and Dutra (2011), who developed a simplified steady-state

© 2013 Taylor & Francis

*Corresponding author. Email: vcostanzo@dii.unict.it, costanzo.vincenzo@gmail.com

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mathematical model for evaluating the effects of solar reflectance and infrared emissivity on thethermal performance of horizontal flat roofs in non-residential buildings. According to theirresearch, they state that roofs with both mass-insulation and good-reflectivity coating improveindoor conditions whatever the outdoor conditions.

On the other hand, Kolokotsa, Santamouris, and Zerefos (2013) have carried out a series ofsimulations for different cities in Europe in which the effectiveness of the cool roof technology,in terms of sensible heat flux released from the roof, is compared to that of a green roof. Theresults expected for cool roofswith high solar reflectance (about 0.9) are very similar to those poten-tially achievable with a green roof having high Leaf Area Index (defined as the ratio between theone-sided leafs area and the underlying ground area), at least in hot climates like that of Crete.

Furthermore, studies on the production of cool traditional clay tiles to be used in historical citycentres are on-going (Libbra, Tarozzi, Muscio, & Corticelli, 2011).

Another advantage of cool materials is that no sensible changes in the aesthetic feature of theroof must be expected (Synnefa, Santamouris, & Apostolakis, 2007): some materials – mainlypaints – have been recently developed whose chromatic result is as close as possible to the orig-inal colour of the untreated roof, as also discussed by Levinson et al. (2007). This is obtained bymaximizing the near infrared reflectance without affecting the behaviour of the paint in the visiblefield, which is strictly related to the perceived colour; new thermochromic coatings are currentlybeing developed with this aim (Santamouris, Synnefa, & Karlessi, 2011).

In order to discuss the suitability of the cool roof technology as a passive cooling strategy formoderate and hot climates, this paper presents the results of a case study in Catania (southernItaly), based on simulations with the software tool EnergyPlus. The simulations will allow eval-uating the results to be expected from the application of a commercial cool paint on a low-riseoffice building; this intervention has already been performed, and it will be the object of an exper-imental monitoring campaign.

The results of the simulations show the benefits of using cool roofs on existing buildings, interms of both reduction in cooling demand and decrease in the hours of thermal discomfort.However, attention is also paid to the winter condition, when the presence of the cool paintlowers the heat absorbed by the roof, with consequences in terms of heating demand; actually,this is a drawback of cool materials not always addressed in the scientific literature. Further inves-tigations in this direction are presented in the second part of the paper, also looking at sites withsevere winter conditions.

On the whole, the study highlights how the choice of the heating device for winter spaceheating might strongly influence the annual energy balance in terms of primary energy (PE) con-sumption, also depending on the insulation level of the envelope.

2. Methodology

The study has been developed in two different phases: the first stage involved the characterizationof the building envelope in terms of thermal and optical properties, whereas in the second stagethe calculation of the energy performance and the study of the thermal comfort were carried outthrough a series of dynamic simulations.

2.1. Characterization of the building envelope

The evaluation of the thermal transmittance (U-value) of the opaque envelope components wassupported by a measurement campaign of the envelope transmittance values, carried outthrough a heat flux meter according to the ISO standard 9869 (ISO, 1994); the instrumentchosen for this purpose is the TESTO 435–2 multifunction instrument.

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Furthermore, before applying the product to the roof, laboratory tests were conducted tocharacterize its spectral reflectance. With this aim the Perkin Elmer Lambda 750 UV/Vis/NIRspectrophotometer was used, according to the ASTM E 903–96 standard (ASTM, 2001). Inthis way, the global reflectance value r was calculated using the ASTM E 903–96 (2003) standardfor solar irradiance, while the same value for the untreated roof has been deduced from recentstudies by Levinson et al. (2010) and Romeo and Zinzi (2013).

Both theU-value of the envelope components and the r values of the existing clay tiles and thecool paint are summarized in Table 1. More details about such an experimental measurement cam-paign are reported in Costanzo, Evola, Gagliano, Marletta, and Nocera (2013).

2.2. Simulating the thermal and energy performance of the building

The assessment of the energy performance and the thermal comfort conditions in the samplebuilding are carried out using the software for dynamic thermal analysis EnergyPlus version7.0., provided by the US Department of Energy (2011).

In order to carry out a thermal comfort analysis of the building, the operative temperature hasbeen chosen as the index more closely related to the comfort condition perceived by the occu-pants. This means that a reduction in its value during the period of observation implies better con-ditions for the building occupants.

An effective way to quantify the intensity of uncomfortable thermal sensation due to overheat-ing in a living space is the measure of the difference between the room operative temperature anda threshold value; however, the duration of such overheating should also be taken into account.With this aim, in this paper we will adopt an indicator called intensity of thermal discomfort (ITD)for overheating, introduced by Sicurella, Evola, and Wurtz (2012), which is defined as the timeintegral, over the occupancy period P (from 9:00 to 18:00 for weekdays in this case), of the posi-tive differences between the current operative temperature and the upper threshold for comfort:

ITD =∫PDT+(t)dt, (1)

DT+ = Top (t)− Tlim (t) if Top (t) ≥ Tlim (t),0 if Top (t) , Tlim (t),

{(2)

Table 1. Features of the sample building.

General informationLocation Catania, Italy (LAT. 37°31′N, LONG. 15°04′E)Building type Office buildingSurface area 207 m2

Operation hours 09:00–18:00 from Monday to FridayMain orientation NE-SO

Building envelopeS/V ratio 0.47 (m−1)Walls – U-value U = 0.80 (W m−2 K−1)Roof – U-value U = 0.70 (W m−2 K−1)Floor – U-value U = 1.90 (W m−2 K−1)Windows – U-value U = 2.80 (W m−2 K−1)Shading White blindsClay tiles – r-value 0.25 [−]Cool paint – r-value 0.45 [−]

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The value of the threshold temperature Tlim depends on the choice of a specific thermalcomfort theory. In this paper, the adaptive approach is chosen, as described in UNI EN 15251(2007): hence, the threshold value is not constant in time, but it should be determined daily asa function of the running mean outdoor air temperature Trm. The formulation of the thresholdtemperature is given in Equation (3), and corresponds to the fulfilment of Category I introducedby the EN Standard (high level of expectation):

Tlim = 20.80+ 0.33 · Trm. (3)

3. The case study

3.1. Description of the building

The building considered in this case study is an existing office building inCatania (southern Italy), atown on the eastern coast of Sicily, whose main features are summarized in Table 1. The groundfloor hosts a series of offices used by the teachers of the localUniversity,while the basement is occu-pied by laboratories; the roof is walkable and hosts the air-conditioning devices.

The composition of the outer walls and the roof is reported in Table 2, from the outermost tothe innermost layer. The windows consist of double-glazing filled with Argon (4–12-4 mm),whose aluminium frame is provided with thermal cutting; the shading system consists of whitecurtains. The whole window system shows a thermal transmittance value U = 2.80 W m−2·K−1,calculated according to UNI EN ISO 10077-1 (2007). In Table 2, the most important thermalproperties of the building materials are also collected, as reported by the national standardsUNI 10351 (1994) and UNI 10355 (1994), as well as by the international standard UNI ENISO 6946 (2008).

Table 2. Characteristics of the opaque envelope.

MaterialsThickness

(cm)Density(kg·m−3)

Specific heat(J·kg−1·K−1)

Conductivity(W·m−1·K−1)

RoofClay shingles 1.2 1800 840 0.72Mortar 2 2000 840 1.4Sand 2 1700 840 0.6Polyester membrane 0.8 1120 1460 0.16Light cement screed 10 1600 880 0.65Mineral wool 3 35 840 0.044Reinforced base 6 2000 840 1.4Prefabricated slab 6 2000 840 1.16Air gap 30 1.2 1000 *False ceiling 2 900 840 0.21

Outer wallsPlates of basalt stone 3 2800 840 3.5Mortar 3 2000 840 1.4Concrete block 12 1400 880 0.43Polystyrene 3 20 1400 0.036Air gap 17 1.2 1000 **Hollow clay block 8 750 840 0.4Inner plaster 2 1400 840 0.7

*R = 0.23 (m2 K W−1)**R = 0.18 (m2 K W−1).

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3.2. Settings for the simulations

In order to simulate the dynamic energy performance of the building with EnergyPlus, the follow-ing assumptions were made:

. annual simulation period with hourly time step;

. the local weather file for the site of Catania is derived from the library available on the Ener-gyPlus weather data;

. occupancy pattern: from Monday to Friday, 09:00–18:00;

. electrical heat gains: 150 W per workstation;

. lighting systems: 6 W m−2;

. people-sensible load: 60 W per person;

. outdoor air infiltration rate: 0.5 h−1 during the occupancy period, 0.2 h−1 during the remain-ing time.

In the next section, the results of the simulations will be presented under four differentscenarios:

(1) no paint (solar reflectance r = 0.25 for the non-treated existing roof);(2) cool paint actually applied on the roof (r = 0.45);(3) better performing paint (r = 0.65);(4) best performing paint (r = 0.85).

The simulations are focused on both thermal comfort and energy performance of the building,as well as on the thermal behaviour of the cool paint:

. the study of the thermal behaviour of the cool paint estimates the temperature reduction inthe roof outer surface;

. comfort analysis shows the hourly evolution of the operative temperature for a referenceroom of the building. In addition, an indicator for measuring the duration and the ITD iscalculated for three rooms with different exposures;

. energy analysis evaluates the hourly heating and cooling loads, the design loads for summerand winter, and the global annual energy need. In addition, a comparison between differentheating systems will show the best performing equipment for the case study.

4. Results and discussion

4.1. Roof outer temperature

One of the most noticeable aspects related to the use of a cool paint on the finishing layer of a roofis the sharp reduction in its outer surface temperature: according to Levinson et al. (2010), Romeoand Zinzi (2013) and Bozonnet, Doya, and Allard (2011), a mean reduction of 12°C is expectedwhen using a product with average quality (r = 0.45), whereas the use of a high-reflective paint (r= 0.85) can introduce a temperature reduction up to 25°C.

As concerns this case study, the results of the simulations shown in Figure 1 show that theouter surface temperature of the existing roof is always higher than that potentially reached bythe painted roof. The minimum difference pertains to the less-performing paint (r = 0.45) andranges around 5–10°C, but in the case of the best performing paint such temperature differenceactually increases up to 20–30°C.

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Furthermore, as shown in Figure 2 for the hottest days of the year, the use of a cool paint onthe roof leads to a sensible reduction in the peak outer surface temperature, while at night theeffect is more evident. In fact, when the solar irradiance is at its height (12:00–14:00) and apeak of about 60°C is reached for the untreated roof, a paint with r = 0.45 shows a reductionof 10°C and the one with r = 0.85 has a reduction of 25°C. At night, these differences amountsto 1°C and of 3°C, respectively, which lowers the risk of vapour condensation on the roof.

4.2. Comfort analysis

The sensible reduction in the roof surface temperature, described in the previous section, leads toa significant reduction in the operative temperature of the underneath rooms, as also discussed byBorge-Diez, Colmenar-Santos, Pérez-Molina, and Castro-Gil (2013) for a case study in Haiti.

The reference room for this analysis is the office n.3, placed in themiddle of the northern side ofthe building (see Figure 3). This office is representative of the whole set of offices due north-east.

Figure 1. Surface roof temperature without paint and with growing reflectance values.

Figure 2. Outer roof surface temperature during the hottest days in summer (8–10 August).

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As shown in Figure 4, during the three hottest days of the year (from 8 to 10 August) a peakvalue of 36°C for the operative temperature is expected without paint during the occupancy period(9:00–18:00); it can also be observed that a reduction of around 1°C every Δr = 0.20 is achievedwhen using cool paints with increasing reflectance, and this reduction is comparable to thatmeasured by Arumugam (2013) in an unconditioned building in India.

In this case study, three rooms are investigated from the comfort perspective: the office n.3which is representative of the north-east rooms, the office n.6 which is due south-west and theassembly room that is characterized by many glazed surfaces and by a double exposure.

The values of the ITD discomfort index, calculated as described in Section 2.2, are shown inFigure 5. Here, the expected effectiveness of the real cool paint (r = 0.45) in reducing the thermaldiscomfort of the occupants is clear, as it implies a reduction in the ITD of about 21% with respectto the case without cool paint.

However, even better results can be obtained if using the most performing paint (r = 0.85),since the expected reduction in ITD is 63%, and this is true for all the rooms considered.

Figure 3. Ground plan with building orientation and picture of the main façade.

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4.3. Energy analysis

Another positive aspect closely related to the use of the cool roof technology is the sensiblereduction in the cooling load of the building, thanks to the lower rate of heat flux penetratingthrough the plain roof. This effect, well studied in the last years by means of field-based analysis(Xu, Sathaye, Akbari, Garg, & Tetali, 2012), is shown in Figure 6 with reference to the hottestdays of the year; the curves represent the building sensible cooling load, determined throughthe simulations with a cooling set point temperature of 26°C during the occupancy period insummer (09:00–18:00 for weekdays, from May to September).

Figure 4. Operative temperature in office n.3 during the hottest days of the year (8–10 August).

Figure 5. ITD index for three different rooms over the whole summer season.

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As one can observe, the peak of the cooling load can be cut by 14% in comparison with thecase without cool paint if using the real paint with r = 0.45 (from 8440 to 7230 W): the result is farmore encouraging if using a high-performing paint (r = 0.85), as the peak load is reduced by 44%,i.e. from 8440 to 5206 W.

However, the reduction in the cooling load in summer is not the only effect of the cool painton the energy performance of the building. In fact, the low absorptivity of the roof also implieslower heat gains in winter, which determines a potential increase in the winter heating load, asgiven by Kolokotsa, Diakaki, Papantoniou, and Vlissidis (2012) for the laboratory building inIraklion (Crete), where an increase in the heating energy demand of about 20% is expected.

With the aim of further investigating this aspect, the simulations were repeated for the winterseason (from October to April), by imposing a heating set point temperature of 20°C during theoccupancy period.

The resulting trend of the design thermal loads as a function of the solar reflectance r isreported in Table 3 for summer and winter: as a matter of fact, the increase in the peak heatingload due to the presence of the cool paint in winter is not negligible. Indeed, the peak heatingload raises from 14.9 to 15.6 kW (+4.7%) if using a paint with r = 0.45, and from 14.9 to 17.1kW (+14.7%) if using a paint with r = 0.85.

In any case, the most important parameter from the perspective of the overall energy savings isthe annual energy need of the building for space heating and cooling, obtained by integrating overtime the curves of the sensible load for heating and cooling.

As given in Table 4, the annual energy need for space cooling Qs is strongly reduced byincreasing the value of the roof solar reflectance r: the expected reduction is around 15% ifusing a paint with r = 0.45 and around 45% for a paint with r = 0.85. On the other hand, an

Figure 6. Sensible cooling load of the building during the hottest days of the year (8–10 August).

Table 3. Design thermal loads for heating and cooling.

r = 0.25 r = 0.45 r = 0.65 r = 0.85

Sensible cooling load (W) 8442 7299 6473 5613Sensible heating load (W) 14,885 15,661 16,299 17,083

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increase in the energy need for space heatingQw should be expected (11% for a paint with r = 0.45and 31% with r = 0.85).

As a result, the total expected annual energy need is reduced by 5% (from 8787 to 8347 kWh)and by 12% (from 8787 to 7742 kWh) in comparison with the case without cool paint, respect-ively, when r = 0.45 and r = 0.85. These values are comparable to those obtained by Zinzi (2013)for the city of Palermo in case of insulated building. Thus, the results of the simulations seem to bevery encouraging, and to justify the use of highly performing cool paint for roofs in hot climates.

5. What about PE?

5.1. Methodology

From the previous analysis, one can conclude that the adoption of cool paints always allows areduction in the annual building energy needs in hot climates. However, in the authors’opinion this conclusion should also be supported by the calculation of the overall PEconsumption.

In this case, one needs to define the plant solutions adopted to provide both heating andcooling to the sample building. In fact, the PE consumption depends highly on the efficiencyof the conversion process, which is expressed by the PE ratio (PER). Now, in this study air-con-ditioning in summer is supposed to be provided through fan-coil units fed by a reversible electricair-to-water vapour-compression chiller. As concerns space heating, two different solutions willbe investigated: air-to-water electric reversible heat pump or conventional gas-fired heatgenerator.

Furthermore, the results of such an investigation are also likely to depend on the site-specific weather conditions, and especially on the duration of the heating and coolingseasons. For this reason, the calculation of the PE consumption will be performed – for thesame sample building discussed in the previous sections – in three different cities, placed insouthern (Catania, LAT 37°31′N), central (Rome, LAT 41°53′N) and northern Italy (Milan,LAT 45°28′N).

The overall annual PE needs for both space cooling and heating are expressed by Equation (4):

PE = Qs

PERs+ Qw

PERw. (4)

Here, the first addend is the PE consumption for cooling in summer (s), while the secondone is the PE consumption for heating in winter (w). The primary energy ratios PERs andPERw depend on the plant configuration and are summarized in Table 5, where the value0.46 represents the average efficiency for the production and the distribution of the electricenergy.

For the calculation of the system efficiency, the mean values reported in Table 6 wereassumed. As one can observe, the energy efficiency ratio (EER) of the chiller and the coefficientof performance (COP) of the heat pump are not the same, as they strongly depend on the mean

Table 4. Annual building energy needs as a function of the solar reflectance of the roof.

r = 0.25 r = 0.45 r = 0.65 r = 0.85

Summer energy need Qs (kWh) 5565 4726 3891 3042Winter energy need Qw (kWh) 3222 3621 4110 4700Total energy need (kWh) 8787 8347 8001 7742

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outdoor air temperature. In cold climates (Milan, EER = 3.89) the chiller behaves better than inhot climates (Catania, EER = 3.65). On the other hand, the heat pump is highly penalized incold climates (Milan, COP = 3.09), whereas it is very efficient in hot climates (Catania, COP =3.76). On the contrary, the efficiency of gas-fired heat generators can be considered independentof site-specific weather conditions.

5.2. Results: the role of system configuration

From the results reported in Figures 7 and 8, it emerges that the choice of the heating devicedeeply influences the annual PE consumption, and that this can modify the overall evaluationabout the actual convenience of a cool roof.

Indeed, if looking at Figure 7, when adopting an air-to-water heat pump for space heating,increasing r values of the roof covering always imply a reduction in the annual PE consumptionin both Catania (up to 11% for a white paint with r = 0.85) and Rome (about 3% by using the bestperforming paint). On the contrary, in Milan the use of a cool paint for treating the roof seems toinduce a slight increase in the annual PE consumption (around 3% for r = 0.85, if compared to theexisting covering with r = 0.25).

Table 5. PER for different heating and cooling systems.

Air-cooled vapour-compression chiller PERs = EER·0.46Air-to-water heat pump PERw= COP·0.46Gas-fired heat generator PERw= η

Table 6. Mean performance coefficients for summer and winter air-conditioning devices.

Catania Rome Milan

EER 3.65 3.86 3.89COP 3.76 3.49 3.09η 0.75 0.75 0.75

Figure 7. Annual PE consumption of the sample building (heat pump for heating).

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In fact, due to the cold climate and the long heating season, in Milan the contribution of the PEconsumption in winter is far higher than in summer, even when using a very efficient heatingsystem. Thus, the benefits produced by the cool roof in summer are overtaken by the penalizationinduced in winter.

This behaviour is even more emphasized when adopting a gas-fired heat generator for spaceheating (see Figure 8). In this case, due to the lower rate of conversion from primary to finalenergy, the weight of the winter performance is higher than for a heat pump, thus an increasein the annual PE consumption with r is always expected, whatever the city (from 5% inCatania to 9% in Milan, considering the comparison between r = 0.85 and r = 0.25).

However, it is worth underlining that the reduction in the outer surface temperature of the roof,associated with the use of cool paints with increasing r values, is very close to what is alreadyshown in Section 4.1 whatever the climate. This proves the unconditional impact of cool roofsin addressing the heat island phenomenon, i.e. when thinking at an urban scale and not at thebuilding scale.

5.3. Results: the role of the building envelope

However, one might suppose that the results described in the previous section are stronglyaffected by the insulation level of the envelope: as the sample building does not have excellentinsulation (see Section 3.1), the drawback of adopting a cool roof in winter could beoverestimated.

With this aim, the simulations were repeated by modifying the insulation level of the opaqueenvelope components, in order to get for each location U-values that comply with Italian buildingcodes (Legislative Decree 311, 2006). In particular, the original thickness of the insulation layersfor both the outer walls the and roof (2 cm of polystyrene and 3 cm of mineral wool, respectively)have been increased by a factor three in Catania and Rome and by a factor five in Milan.

The results of this new series of simulations are reported in Figures 9 and 10, respectively, forthe case with heat pump and with gas-fired heat generator. In the first case (heat pump), themessage conveyed by Figure 9 is the same as for Figure 7, apart from the different values:using cool paint implies a reduction in the annual PE consumption in both Catania and Rome

Figure 8. Annual PE consumption of the sample building (gas-fired boiler for heating).

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(8% and 4% with the most performing paint, respectively), whereas in Milan the annual balance isalmost not affected by the choice of the roof covering (only 1% increase in comparison with r =0.25). It must be underlined that, due to the over-insulation of the envelope, an increase of about10% in summer cooling needs is observed in Catania with respect to the real building.

On the other hand, as shown in Figure 10, when using a gas-fired heat generator an increase in theannual PE consumption is still expected in Milan and Rome (about 4% if comparing r = 0.85 to r =0.25),which is lower than for the actual building, thanks to the improvement in the building envelope.In Catania, the annual PE consumption is now almost independent of r. In any case, the buildingalways seems to benefit from an increase in the insulation level in terms of annual PE consumption.

Moreover, the reduction in the outer roof surface temperature due to the cool paint stillremains the same as for a building that is not well insulated (see Section 4.1), since the outer temp-erature is not significantly affected by the composition of the inner layers of the roof.

Figure 9. Annual PE consumption of the well-insulated building (heat pump for heating).

Figure 10. Annual PE consumption of the well-insulated building (gas-fired boiler for heating).

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6. Conclusions

The aim of this paper was to investigate the effectiveness of the cool roof technology for therefurbishment of an existing low-rise office building in Catania, a city in southern Italy with ahot-humid climate, in which the energy demand for space cooling in summer is predominant ifcompared to that for space heating in winter.

The simulations carried out with the software EnergyPlus have pointed out that the comfortsensation of the occupants in free running conditions in summer can be significantly improved byapplying a cool paint with an average value of solar reflectance (r = 0.45), which corresponds tothe performance of a commercial paint actually applied to the roof in the framework of an exper-imental campaign. However, further enhancements might be expected if using a high-performingcool paint (up to r = 0.85).

Moreover, the application of the cool paint leads to a noticeable reduction in the buildingenergy needs for space cooling; however, an increase in the energy needs for space heating inwinter should also be expected. Such a drawback provides a sensible increase in the PE consump-tion for heating, which may also overcome the advantages achieved in summer.

In order to further investigate this aspect, a series of simulations have been carried out for twoother cities (Rome and Milan), characterized by different weather conditions, so providing usefuldesign suggestions.

In conclusion, the adoption of cool paints for roofs is a solution that must be carefully eval-uated in regions with intense or long winter period (like Milan), where the heating energy demandis predominant if compared to that for space cooling. For such climates, the application of a coolpaint to the roof should be associated with an increase in the envelope insulation level and the useof very efficient heating devices.

As regards the hottest climates of Rome and Catania, a noticeable reduction in the annual PEconsumption has to be expected also for buildings that are not well insulated, because of the majorrole played by the summer season in the annual energy balance.

Anyway, the use of cool materials (both for roofs and pavements) always plays a significantrole – on an urban scale – in reducing the heat island effect in densely inhabited environments, aswidely discussed by Gros (2013), Cheng, Su, Kuo, Hung, and Chiang (2008) and Akbari (2013).

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