Cool Clay Tiles in Italian Residential Districts: Investigation of the Coupled Thermal-Energy and Environmental Effects

Passive strategies for environmental sustainability and energy reduction in the construction industry are becoming increasingly important in both the scientific community and the industry world. Particularly, cool roofs showed acknowledged contribution in cooling energy saving and reducing urban overheating phenomenon, such as urban heat island. Additionally, high albedo strategy has shown also promising benefits from a global scale perspective, by counteracting global warming measured by means of CO2eq emissions offset. In this view, the present research work combines experimental, numerical, and analytical analysis approaches with the purpose to estimate the environmental and energy impact of the application of cool clay tiles over the roof of a residential area with single family buildings located in central Italy, where a continuous monitoring setup has been operating since 2010. The multiscale and multipurpose investigation demonstrated a global year-round CO2 emissions compensation by more than 700 tons, of which the 15% is produced by the passive cooling contribution at building scale and the remaining part is produced by the climate mitigation technique. The work, therefore, showed that local energy saving strategies must be combined with larger scale models for performing an exhaustive environmental analysis.


Introduction
The overall energy needed and used up in buildings amounts to up to 40% of the total energy needed world-wide ("Annual Energy Outlook", 2011). A major increase of this percentage was also registered for office and residential buildings in particular given the developing technology used and requirements of indoor comfort for developing countries occupants (OECD, 2003). The data given become even more alarming in developing countries especially where the energy needed for construction was noted to have amounted to half of the global energy needed.
In this view, key research improvements are currently a work in progress in both the construction design and scientific community aimed at reducing the energy need for lighting, cooling, and heating, by keeping constant, or even improving, the indoor environmental quality (Salata, Vollaro, Lietovollaro, & Mancieri, 2015). Similarly, the whole life cycle assessment of buildings is now under deep investigation with the aim of reducing the effect pg. 1 of the whole construction chain in terms of CO 2 eq as a way of mitigating climate change and global warming trends (Pisello & Asdrubali, 2014;Synnefa, Santamouris, & Akbari, 2007). In particular, several passive strategies with great benefits were developed and optimized by highlighting their effects at building scale, in terms of energy saving potential and at larger scale such as green building envelopes, e.g. green roofs and walls (Malys, Musy, & Inard, 2016;Razzaghmanesh, Beecham, & Salemi, 2016).
In the same field, several important studies were undertaken to optimize the solar reflectance capability of building surface materials, with the purpose of minimizing the building summer overheating and the consequent cooling energy need. Such highly reflective coatings are named "cool" materials due to their considerable cooling potential within different climate boundary conditions, building features and occupancy levels as proved by research studies worldwide even in continental climate areas (Kolokotroni, Gowreesunker, & Giridharan, 2013;Pisello, Piselli, & Cotana, 2015) . Over the years, several case study buildings were monitored with the aim of quantifying the realistic energy reduction potential and thermal comfort improvement due to the application of cool envelopes. To this aim, experimental monitoring campaigns were performed in colder climate conditions (Hosseini & Akbari, 2014;Paolini, Zinzi, Poli, Carnielo, & Mainini, 2014). These studies demonstrated an annual benefit from highly reflective roofs with non-significant winter penalties. Consequently, additional studies were done to investigate the impact of aging and weathering phenomena on the passive cooling potential. Cool materials, such as coatings, membrances and painting, were recoginzed as the most commonly (Sleiman, Ban-Weiss, Gilbert, François, Berdahl, Kirchstetter, Levinson, 2011).
Given the capability of outdoor environmental conditions to influence the durability and passive cooling potential of such cool materials, large research efforts are currently focused on the development of experimental test procedures to reproduce microclimate boundary phenomena during different weather and environmental stresses, i.e. pollution concentration. By shifting the perspective from a single building scale to a larger settlement scale, high albedo technologies were therefore demostrated to be able to considerably contribute to the mitigation of global warming (g (Akbari, Menon, & Rosenfeld, 2008; Cotana, Rossi, Filipponi, Coccia, Pisello, Bonamente, & Cavalaglio, 2014) as well as local overheating phenomena, i.e. heatwaves and urban heat island island (Santamouris, 2015;Wang & Akbari, 2015). In this scenario, several numerical models were implemented with the purpose of quantifying the impact of highly reflective surfaces as an offset of CO 2 eq emissions (Cotana et al, 2014).
It was proven that high albedo materials can decrease the quantity of energy absorbed by the Earth's top surface layer by consequently reducing the overall warming path attributed to anthropogenic heat sources (OReskes, 2004). Such a contribution can compensate for the negative effects of the greenhouse gas emissions. Therefore, the benefits attained by applying highly reflective surface materials, such as cool pavings and roofs, can be determied in terms of offset of CO 2 eq. For this reason, a new tailored methodology to quantitavely determine the potential of high albedo surfaces in abating the CO 2 eq emissions depending on the site location (i.e. latitude), the climate zone and the weather conditions, the surface orientation and slope, i.e. building roofs and pavements, was proposed in (Cotana et Al., 2014). Such an innovative method could be used in order to quantify additional environmental benefits of highly reflective surfaces, at an urban scale, in terms of potential of CO 2 eq emission offset. Moreover, it helps to determine the mitigation capability of different buildings or whole areas by specifying the surfaces'geometry and location.
Beginning with the aforementioned research background, the present work is an attempt at bridging the gap between the analysis at single-building level and a global approach. Therefore, building upon existing research efforts on (i) innovative cool roof technologies, and (ii) the potential of highly reflective surfaces in mitigating the global warming phenomenon, this work is an assessment of environmental and energy effects of a cool clay tile tailored for application in historical architecture for its low visual impact. Therefore, a residential neighborhood situated in Italy was chosen as a case study and three main building typologies were identified to assess the energy performance in numerical simulation environments by focusing in particular on the electricity consumption for cooling.
In this paper, the impact of the highly reflective clay tiles is assessed at first by quantifying the summer cooling enpg. 2 ergy saving potential. Therefore, the energy reduction is converted into the CO 2 eq emissions avoided by reducing the summer electricity demand. Such a benefit in terms of environmental impact is finally coupled with the benefit computed by using an energy balance model developed by the authors (Cotana et Al., 2014). The same method is therefore used for a prototype residential district with an energy behaviour that is improved by applying cool clay tiles. This results in a reduction in their impact and combining the energy saving potential and the mitigation of the global warming in terms of CO 2 eq emissions offset.

Research Framework
The present paper consists of a multifunctional and a multipurpose procedure that groups numerical analysis, analytical methods, and final environmental assessment. With the aim of assessing the performance of a case study settlement and the possible energy and environmental saving attributed to the application of cool tiles, perspective is shifted from a single-building level to the urban scale. The numerical analysis is an attempt to implement dynamic simulation models adjusted by means of continuously monitored data of the case study. The analytical tool (Cotana et Al., 2014) was implemented to investigate the contribution of a high albedo solution in terms of CO 2 eq offsets of the settlement roofs in the considered prototype village. The final environmental assessment was performed to groups the benefits of cooling and energy saving together with the ones related to the high albedo technology as climate change mitigation techniques as acknowledged in previous works (Akbari et Al. 2013). The innovative contribution of the work consists, therefore, of (i) the implementation and integration of such proposed tools and procedures, (ii) their application from settlement level to a residential district characterized by sloped roofs covered by clay tiles (iii) the replication of the study for multiple building categories that are characterized by different architectural characteristics and construction period typically affecting the Italian building environmental and energy performance.

The Cool Clay Tile Prototype
The development of the innovative cool tiles is an attempt to improve their passive cooling capability and simultaneously minimize the exterior impact of the cool coating appearance. This is inspired by the fact that the case study village consists of a suburban district characterized by traditional architectures located on a hill. Such a tile was selected according to previous works of the authors (Pisello, 2014), where a first in-lab depiction was carried out to evaluate the solar reflectance and thermal emittance of multiple clay tile samples. Such properties are considered to have a significant impact on the cooling capability of the tile which is expressed in terms of Solar Reflection Index-SRI (Akbari, 2001).
The cool clay tile optimization was performed starting from a commercialized composite material consisting of the clay tile covered by an engobe layer. The engobe represents a sort of coating that is applied over the tile before the cooking process into the industrial ovens and that is able to preserve its optic-energy characteristics also after such oven-produced thermal stress. Therefore, already commercialized tiles usually present several engobes designed for several aesthetic purposes such as face antique appearance, coloring, finishing, etc. This engobe has pg. 3 Pisello / Environmental Science and Sustainable Development, ESSD been characterized with IR-reflective pigments. Therefore, the application of optic-energy performance of tiles on residential buildings' roofs of the selected district is the same as that of lightly-colored clay tile typically applied over traditional sloped roofs. However, the latter presents a relatively higher solar reflectance capability as reported in Table 1.

Dynamic Thermal-Energy Analysis at Building Scale
The residential district selected for this work was firstly mapped by means of GIS techniques and the main technical and architectural features were identified. The preliminary GIS assessment helped the authors to consider and evaluate several building typologies as well as cluster them with respect to the period of construction as the selected main driver for the energy behaviour in Italy. In fact, the first energy efficiency regulation applied to construction in Italy was published in 1976. Therefore, all the predecessor buildings (i.e. designed after 1976 and constructed before 1980) that were not retrofitted had to be considered as non-insulated buildings, as the continuously monitored one. Therefore, a first main building category was identified, i.e. 1 st category, of non-insulated buildings constructed before 1976. Afterwards, two additional categories were identified by using the same approach, i.e. a category including buildings built between 1980 and 2000, and a third category of houses built after 2000, characterized by very high European energy standards, as indicated in recent directives and statistics ("Energy Performance of Buildings Directive", 2005).
Therefore, the three identified building categories are as follows: • was defined. Therefore, the annual calibration procedure was performed by considering a dedicated weather file elaborated during the course of 2013 to which the energy consumption were referred.

Environmental Modeling at Global Scale
The assessment of the environmental impact of the cool tiles in terms of CO 2eq offset included different steps.
Firstly, the relation between the change in the Earth albedo and the average atmospheric temperature was quantified by means of the analytical model provided in (Cotana et al., 2014), which was applied to the case study settlement sloped surfaces (Cotana et al., 2014). Such a model consists of four main phases: (i)Comparison of the modification of the mean albedo of the Earth to that of mean atmospheric temperature through a tailored analytical procedure; (ii)Assessment of the mitigation of global warming in terms of CO 2 eq offset by assuming that the measured temperature rise is generated exclusively from the observed increase of CO 2 level; (iii)Evaluation of the effect on the climate generated by the peculiar roof treatment. Measured by considering the experimental characteristics of solar reflectance of the considered tiles; (iv)Quantification of the potential mitigation of global warming in terms of offset of CO 2 eq. This effect is therefore coupled to one generated by energy reduction achieved by cool tile implementation. Additionally, to evaluate the proposed solution in mitigating global warming at district scale, the combined effect is also assessed. For this reason, the surface of the roof in every house was evaluated and the area and the inclinationorientation characteristic taken into account in the mathematical model of Cotana and others (2014). This is the first currently available model capable of considering geometrical characteristics of surfaces with varying albedo. Therefore, experimental spectrophoto meter albedo values provided by previous studies were considered. Moreover, data about the latitude and longitude, specifically referring to the real Italian residential buildings selected as case study village, were used as input for the model.

Description of the Case Study
The selected case study neighborhood is constituted by 106 villas situated in the green belt of Perugia, a city located in central Italy within the moderate climate zone (Figure 1). Such settlement corresponds to a wide architectural variety characterized by a hill where the houses are located since they were built starting from the 60s. Therefore, three main building categories, according to the time of construction, were identified and modeled for this work. The building cluster was carried out firstly using GIS techniques and then by in-situ inspection and multiple surveys to building facilities. Possible energy retrofits were considered in the building clustering. Finally, realistic pg. 5 electric and thermal energy consumption data of 30 houses, 10 per category, were collected to adjust the dynamic simulation models up to an acceptable approximation rate according to ("Ashrae Guideline", 2002). For reasons previously mentioned, category 1 (C-I), included those buildings made, designed, and constructed before 1980 without any following retrofit. This category included 52 buildings out of 106. Category 2 (i.e. C-II) included buildings constructed from 1980-2000 and was represented by 39 buildings out of 106. The final category, category 3 (C-III), included the most recent constructions built after 2000 as well as the few remaining 15 houses. In Table 2, some general information of the building architecture and categories are reported. Moreover, the main buildings' thermal and technical characteristics were differentiated within the dynamic simulation engine for energy assessment purposes. In particular, the thermal insulation capability of both the transparent and the opaque envelope components was differentiated according to statistical data of the Italian construction stock and in situ inspections. Table 4 finally reports the main boundary conditions used for the simulation on an annual base.

Cooling Energy Reduction Assessment
The electricity energy requirement during the summer was selected as the key parameter to be analyzed. Figure 2 shows the profile of the requirement for cooling of the multiple building categories in the "cool-roof" and "noncool roof" scenario. In particular, the estimated reduction of the electric energy for cooling due to the application of cool clay tiles was converted in CO 2 eq avoided emissions (Cotana et al., 2014). The main results for each building category, i.e. C-I, C-II, and C-III are reported in Table 4.
In particular, the annual energy reduction due to the application of the cool tile is equal to 11.8% for category I, 12.7% for category II, and 13.0% for category III. Therefore, C-III buildings require more electricity than C-I buildings (7.5% more). Therefore, significant drawbacks are created by the insulated envelope in the hot season in the evaluated buildings.
In Figure 3, the energy balance of the building envelope is depicted for C-I (a), C-II (b), and C-III (c), by taking into account the energy required for heating, cooling, lighting, and all the appliances and internal gains due to the occupancy level, solar gains, and envelope infiltrations. The graphs show that buildings belonging to C-III, built after 2001, are characterized by the best performance of the building envelope from an energy balance perspective since they generally present lower energy requirements. Thermal losses and gains are therefore less affected by the outdoor boundary conditions' variability. pg. 8

Assessment of the Environmental Impact
The analytical model developed in (Cotana et al., 2014) was applied to the selected case study area in order to determine the total environmental impact of the proposed cool tiles and increase of the albedo of the evaluated district. For this reason, each roof surface included in the geometrical building model used for the dynamic energy simulation was considered in the analytical methodology. Despite the fact that roof configurations were defined in the developed energy balance tool, the realistic features of each roof, such as geometry, orientation, slope, and surface extension, were taken into consideration.
The analysis results show how applying highly reflective clay tiles on village roofs leads to a CO 2 eq emission offset of up to 631 tons per year, which is generated by the additional reflected radiation. Therefore, the total emissions evaded were computed by taking into account the whole neighborhood with the different building categories as well as the emissions avoided annually given the cooling energy saving. More specifically, the database Eco-invent 3 was taken into account to evaluate the equivalent CO 2 emissions evaded (corresponding to 0.386 kgCO2eq/kWhel) (GmbH, 2013). Moreover, all the residential buildings characterized by low-reflective clay tiles require about 2,976,558.4 kWhel/year for cooling. The same buildings, yet those with the highly reflective clay tiles, require around 12.3% less. The optimized settlement configuration therefore requires 2,610,456 kWhel/year, with a consequent calculated energy saving equivalent to 366,102.7 kWhel/year at district level.
Therefore, the benefit generated by the reduction of the energy requirement for cooling from an environmental perspective is equal to 141.3 tons of CO 2 eq/year. Such a value is then coupled with the offset of CO 2 eq emissions computed by using the model for contributing to the increase of the Earth albedo. The application of the cool clay tiles can counterbalance 772.3 tons of CO 2 eq/year by taking into account the overall settlement contribution of 106 houses.
pg. 9 Figure 4 shows the comparison between the primary energy requirements for cooling for the different evaluated building categories within the "cool" and "non-cool" configuration of the building roof. Additionally, in Figure  5 (a, b) the comparison between the total CO 2 eq emissions due to the building HVAC operation and the total CO 2 eq emissions offset guaranteed by the different type of roof cover is illustrated in terms of tons of CO 2 eq. In particular, CO 2 eq emissions reduction of 354.00, 265.50, and 102.12 tons are calculated for C-I, C-II, and C-III, respectively, thanks to the application of high albedo tiles at the settlement scale. Finally, Figure 5c shows the net balance between the CO 2 eq emissions produced by the building HVAC and the CO 2 eq emissions offset for the evaluated settlement. It is evident how the net balance for the "cool" configuration of the buildings of the selected case study village is negative compared to the "non-cool" scenario since the CO 2 eq offset is greater compared to the CO 2 eq emissions due to the HVAC operation for the benefic passive cooling action produced by the application of the cool tiles, which leads to reduce cooling energy requirements.  . Net CO 2 eq emission balance (c) between the CO 2 eq emissions produced (a) and the CO 2 eq emissions offset (b) generated for the case study settlement.

Conclusion
The present work consists of the coupled assessment of the (i) energy and (ii) environmental impact generated by the application of a higlhy-reflective and low-visual impact clay tiles in a neighborhood in central Italy. In particular, a historic traditional settlement was selected as representative of typical Italian residential buildings, pg. 10 where cool roofs usually cannot be utilized due to the several aesthetic and visual constraints imposed by national regulations preserving the quality of local landscape and environment. Therefore, the selected case study area included 106 detouched houses characterized by sloped roofs covered by traditional red coloured tiles. Such tiles were therefore optimized with the aim of reducing the summer cooling electricity requirement and calculating the related offset of CO 2 eq emissions by using coupled (i) preliminary experimental campaign and an (ii) analytical model. Such an analytical tool, which was implemented in a previous work, was used to determine the relation between the highly reflective surface and the correspondent potentiality in terms of tons of CO 2 eq offset.
The coupled experimental and numerical methodology applied allowed to detect that the high albedo tile, in spite of its low impact in terms of exterior appereance, can guarantee around 13% of electric cooling energy saving for the case study settlement with different building categories. Such an energy reduction was calculated to be about 141.2 tons of CO 2 eq/year, by taking into account the average rate of emissions of Italian national cooling electrical systems and grid effectiveness. Such a benefit from an environmental point of view, when combined to the one produced by the implementation of the cool solution, was found to be around 772 tons of CO 2 eq/year for the case study settlement.
The achieved results indicate how cool roof solutions can be taken into consideration for application even in traditional residential areas due to their key cooling energy saving potential and coupled considerable effect from an environmental perspective.

Future Developments
The present research presents a "prototypical study" aimed at assessing the combined environmental and energy impact of cool roofs and presents many limitations. Firstly, the assessment of the residential building categories in the case study neighborhood was undertaken by evaluating all the building features and performing indoor surveys around only 40% of the houses. However, the whole neighborhood was considered into the analysis and realistic assumptions were made by directly observing the houses outdoor features. Therefore, the classification of the selected houses in building categories helped simplify the problem geometry without significant effect on the reliability of the method. Future developments for the study could consider a more realistic and a more complex configuration of the buildings. Moreover, an in-field evaluation of the building envelope technologies can permit a more precise modeling for the purpose of the dynamic simulation. Further efforts could also focus on the costbenefit analysis of the cool clay tiles implementation and on the assessment of the potential financial benefits attributable to such a proposed solution by taking into account the mechanism of EU ETS for the emission credits trading.